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366A Manual_220893C - Princeton Applied Research

1. 6 Dummy cell resistors 7 37 ELECTRODES switch 6 FUSE TAINS rali 4 Fuse 4 Galvanostatic operation 6 iio then ee e ar RU ER 9 Jacks CBE ts ET epe nee edet uet 8 sigillo a n eas ele dt iaia 8 E2 8 ELECTRODES oy ehe det t 8 9 GA ue eid eut 8 Pu nn eC TED 5 6 8 CR E 8 Dorsi ELAR ee PORE e equ e 6 8 INN uoo deponere a e A ia Siani 8 Klinge tenere 8 10 2 e ba ett eate etae 8 10 REF selena 8 9 SWEEP VOLTAGE poi ion 8 Leaky electrodes 39 Lane volt ge a pilo cha Dia sums anh 3 LOWER controls 6 MANUAL repe ee end 6 MODE switch 6 NOISE pete dite de Gok ee oa 5 OFFSET VOLTAGE controls 6 Potentiostat mode 1 Powercotd 2 222 222 eee ete eR pera 3 Power line voltage 3 Reaction mechanisms 1 Ring disk electrode 35 Rise time 5 Rotating disk electrode 14 Rotating ring disk electrode
2. 25 34 35 Safety considerations 2 SELVICE ss re Lera TP 4 Shielding grounded 39 Shipping 2 STOP AT LIMIT switch 6 Surface controlled reactions 30 Switches AUTO ute aoe ae ee ET 6 CURRENT CONVERTER 6 8 ELECTRODES Sh ees ane 6 MANUAL ie eee Du ests 6 MODE oso ONE ONE 6 STOP AT LIMIT 6 SWEEP VOLTAGE 8 Titrations diffusion layer 5 23 24 UPPER control 5554 va Re IRS ee 6 41 42 Model 366A BI POTENTIOSTAT Hardware User s Manual
3. The Model 366A is an analog system which relies on feedback for stable operation There is the possibility on certain systems that the feedback may become phase shifted and cause the unit to oscillate It is suggested that anytime an abnormal situation occurs on the Model 366A including jumps in potential or current or the overload indicator glowing for no apparent reason an oscilloscope be used to observe the outputs If oscillations are evident and the techniques noted previously do not eliminate the problem there are six jacks on the back panel to which capacitance may be connected to slow the response of the Model 366 It is suggested that 0 001 to 0 1 uF be added to the K1 or K2 jack or to both jacks Various combinations should be tried as experience will best dictate what should be used The capacitors should be either film or ceramic 30 volts minimum If problems are experienced in the galvanostatic mode of operation there are two jacks on the rear panel for external compensation in that mode only The above values are recommended VALUES OF R ARE MATCHED TO BETTER THAN 1 ELECTRODE K1 15 NORMALLY THE DISK ELECTRODE ELECTRODE K2 IS NORMALLY THE RING ELECTRODE Fig 2 SIMPLIFIED CIRCUIT DIAGRAM 1 13 CIRCUIT DESCRIPTION The first instrument for simultaneous and independent potentiostatic control of the ring and disk electrodes of a rotating ring disk electrode was described by Napp Johnson and Bruckenstein A
4. Conversely a i o Versus W is linear for constant Albery has criticized the use of plots of equation 30 as described above for quantitative analysis because a slight misjudgment in the graphical extrapolation can lead to serious error in the value determined for 14 Trans Faraday Soc 62 1938 1966 An alternative method of treating the data is described in the original literature The curved portion of 1 1 curves obtained for diffusion layer titrations can be analyzed to obtain information regarding the rate of the homogeneous reaction Consider first the example of a diffusion controlled solution reaction k gt The observed I remains zero for increasing until the radial coordinate r of the reaction zone of B with P at x 0 just reaches the inner edge of the ring electrode r R Now imagine that is held constant for this condition and the value of k for the solution reaction is considered to decrease The reagent B penetrates beyond the region R resulting in a non zero value of 1 The original literature references 4 6 below should be consulted for application of diffusion layer titrations for kinetic studies 2 6 LITERATURE CITED AND FURTHER READING 24 1 V G Levich Physicochemical Hydrodynamics Eng transl by Scripta Technica Inc Prentice Hall Inc Englewood Cliffs NJ 1962 2 W J Albery and M L Hitchman Ring disc Electrodes Clarendon Press Oxford 1971 3 Yu V Pleskov an
5. 0 5 volts per microsecond Small signal bandwidth gt 10 KHz 3 db typical Internal dummy resistors see Figure 14 REF 100K ohms CE 2K ohms K1 1K ohms K2 1K ohms 1 10 SWITCH AND POTENTIOMETER FUNCTIONS AC POWER switch Up On Down Off MODE switch Right Potentiostatic operation Left Galvanostatic operation ELECTRODES switch normal Connects electrode jacks into circuit dummy Connects a set of internal resistors into the circuit See Specifications 1 10 1 Sweep Generator Section MANUAL switch up Sweeps in positive direction center Has no effect on sweep output down Sweeps in negative direction AUTO switch on up for automatic sweep operation hold center Sweep holds at the value at time of switching zero down Sweep output is zero SWEEP RATE potentiometer and range select switch A calibrated four digit pushbutton switch and a 2 position toggle switch for adjusting the sweep rate Chapter 1 INTRODUCTION UPPER and LOWER LIMIT switches and associated toggle switches These four digit pushbutton switches set the values of the upper limit and lower limit respectively used in conjunction with the associated switches which may be used to set the values to either polarity Note The upper value must be more positive than the lower value by at least 10 mV STOP AT LIMIT switch Up The unit will sweep to the upper limit and hold Center The unit will cycle continuously betwee
6. 00 V in the positive direction and scan between the limits of 1 00 V and 0 00 V Allow the electrode potential to cycle several times until the I E curve is reproducible Adjust the zero position of the Y axis current so the entire anodic wave is contained on the scale of the chart paper Record the I E curve for the positive potential sweep The anodic wave for oxidation of will be clearly apparent The sharp increase in anodic current at 0 90 V versus SCE corresponds to oxidation of I to IO Due to the presence of surface controlled reactions during the negative potential sweep the anodic wave for the negative sweep is quite different from the wave for the positive sweep A discussion of the surface controlled reactions for a Pt electrode in an acidic solution of is given by D C Johnson J Electrochemical Soc 119 331 1972 5 Record the anodic wave during the positive potential sweep from 0 00 V to 1 00 V for the rotation speeds given below The corresponding angular velocity and square root of angular velocity are also given Rotation Speed w w rev min rad sec rad sec 6400 670 2 25 89 4900 513 1 22 65 3600 377 0 19 42 2500 261 8 16 18 1600 167 6 12 94 900 94 2 9 71 400 41 9 6 47 Representative data for the disk electrode of an RRDE are shown in Figure 12 6 Plot values of limiting current at 0 750 V versus SCE for the disk electrode as a function of The plot is predicted to be linear The intercepts of the
7. 1 8 GENERAL DESCRIPTION 4 2 5 1 38 12 Sweep GENEFALOL 3 30 RR eee ea eee 5 19 2 PolenllOSLal a SRS ence acne d 5 1 8 3 Galvanostativosa a E EE dE dR Ra eR a ane arenas 6 18 4 2 NOMMOLC I vere heRRSRERSEESSSESSERCUET4A Rag Boag Baca RUNG ERENER 6 1 9 SPEGIFIGATIONS E RT En RiEEG BARES Reg RR aaa 6 1 10 SWITCH AND POTENTIOMETER FUNCTIONS 7 1 10 1 Sweep Generator 4 7 1 10 2 K1 and K2 Electrode Section 1 8 JACK FUNCTIONS 8 ErontParielxcsssesserkerereterecte5sUte522424 4 8 Fear a REA NISSAN 9 1 12 NOISE AND OSCILLATIONS 1 1 2 1 9 1319 NUR Sd 10 Maze rr I cc aa 13 2 12 HE DIFFUSIONILAYER ERE RANE IRE VERE EROS 13 2 2 THE ROTATING DISK EL
8. 482 4411 801 S Illinois Avenue FAX 865 483 2133 Oak Ridge TN 37831 ATTN Customer Service D CUSTOMERS OUTSIDE OF U S A To avoid delay in customs clearance of equipment being returned please contact the factory or the nearest factory distributor for complete shipping information Copyright 2002 Advanced Measurement Technology Inc All rights reserved Princeton Applied Research is a registered trademark of Advanced Measurement Technology Inc All other trademarks used herein are the property of their respective owners TABLE OF CONTENTS Safety Instructions and Symbols 4 iv Cleaning Instructions a me UR AE AA AIA E AA CR RITTER RRS iv Li GENERAL 5 a Wd WA WE AA AAT ace NU Rs 1 INTRODUGTION 25 05 1 Jig I NITIAE INSPEGTION 12 221221 EEaG RES eRe eee Ree Ree REG eRe RRA Ree LOR 2 1 4 SAFETY CONSIDERATIONS 1 2 1 5 POWER VOLTAGE SELECTION 4 3 1 6 LINE F SE REPEAGEMENT e DA RODA Ee ee ee ee RES 4 TC COE ANI GES NES RR Rid RoR Mad NE NEU NE 4
9. plots will equal the charging current which passes in the electrode as a consequence of the potential scan The charging current is independent of rotation speed and the slope of the plot should be independent of the rate of potential scan 7 Calculate the diffusion coefficient for I from the slope of the l a w plot according to Equation 17 The slope is given by Equation 31 dl dw 0 62nFAD v C 31 Model 366A BI POTENTIOSTAT Hardware User s Manual Vai lt scan direction Recorder X axis sensitivity 100 mV cm Y axis sensitivity 100 mV cm ZERO position indicated by Potential sweep rate 33 3 mV sec range 0 00 V to 1 00 V vs SCE curve recorded only during positive potential sweep Current sensitivity disk 0 5 mA V 50 A cm RRDE speeds 400 900 1600 2500 3600 4900 and 6400 rev min Solution 0 5 M H SO 0 4897 mM KI Figure 12 CURRENT POTENTIAL CURVES FOR IODIDE AT A ROTATING PLATINUM DISK ELECTRODE where 1 equiv mol F 9 65 x 10 coul equiv 2 v 2 13 cm sec for 0 5 M H SO C concentration of mol liter dl dw slope mA rad sec Chapter 3 ELEMENTARY EXPERIMENTS 31 An example calculation of D is given below for the data in Figure 12 w rad sec lia 4A 6 47 115 9 71 174 12 94 231 16 18 289 19 42 349 22 65 404 25 89 462 Least squares analysis y mx b m 17 866 slope b 0 067 intercept r 0 9999 cor
10. previously developed describing fluid dynamics at submerged surfaces with consideration of the rate of electrolysis at electrodes of different geometries Physiocochemical Hydrodynamics Prentice Hall 1962 One of the electrode systems described by Levich is the rotating disk electrode shown in Figure 4 inert shroud disc axis of rotation shaft Fig 4 CROSS SECTIONAL DIAGRAM OF A ROTATING ELECTRODE The rotating disk electrode is constructed from a disk of the chosen electrode material mounted on the end of a metal e g stainless steel or brass shaft The shaft and disk are covered by a cylindrical shroud of inert material e g Kel F Teflon epoxy etc as shown in Figure 4 The shroud has two functions 1 It prevents contact between the electrolytic solution and the metal shaft and disk edge 2 The end surface of the disk is extended for full development of fluid flow patterns near the disk electrode In use the disk electrode is rotated in a solution of electrolyte about an axis perpendicular to and passing through the center of the disk surface The velocity of rotation must be precisely controlled to within 1 The rotator is normally constructed with a synchronous motor or a variable speed motor controlled by a feedback system monitoring the rotational velocity The complexities of constructing the electrode and rotator are sufficiently great that purchase of the equipment is rec
11. the disk electrode of the RRDE is a linear function of the square root of rotational velocity w 1 2 as described by Equation 17 In this experiment the square root dependence will be tested for the oxidation of in 0 5 M H SO diffusion coefficient of l will be calculated from the slope of a plot of l 4 versus w 1 Repeat steps 1 3 of Experiment A substituting a solution of 0 5 M H SO 0 5 mM KI for the 0 5 M H SO used in Experiment A 2 Setthe controls of the Model 366A Bi Potentiostat as indicated below Section SWEEP GENERATOR SWEEP GENERATOR SWEEP GENERATOR SWEEP GENERATOR SWEEP GENERATOR SWEEP GENERATOR K1 ELECTRODE K1 ELECTRODE K1 ELECTRODE K2 ELECTRODE K2 ELECTRODE K2 ELECTRODE 3 Repeat steps 5 8 from Experiment A for a rotation speed of 6400 rev min Chapter 3 ELEMENTARY EXPERIMENTS Control AC POWER ELECTRODES MANUAL AUTO SWEEP RATE UPPER LOWER STOP AT LIMIT OFFSET VOLTAGE SWEEP VOLTAGE CURRENT CONVERTER OFFSET VOLTAGE SWEEP VOLTAGE CURRENT CONVERTER Position OFF NORMAL MODE POT Center position off ZERO 33 3 mV per sec 41 00 V 0 00 V Center position off Center position off ON 0 5 mA per volt Center position off OFF 20 MA per volt 29 4 Place the MANUAL switch of the Model 366A momentarily in the UP position and place the AUTO switch in the ON position The scan of the electrode potential will commence from 0
12. 1 and K2 There is an interaction between the two electrode signals in the uncompensated IR drop Thus the uncompensated IR drop between the reference electrode and K1 will cause an equivalent loss of potential control in K2 Although this error may be small in many cases fairly large currents are encountered with convective electrodes and the uncompensated IR drop must be kept at the lowest possible value by positioning the tip of the reference electrode very close to K1 The potential of K1 as measured at PF1 includes the error resulting from the uncompensated drop When the Model 366A Bi Potentiostat is used with ring disk electrodes K1 is usually the disk electrode and K2 is the ring electrode Model 366A BI POTENTIOSTAT Hardware User s Manual 2 THEORY 2 1 THE DIFFUSION LAYER Transport of an analyte within a solution can occur by the mechanisms of convection diffusion and migration Convective mass transport is the result of physical displacement of the solution caused by stirring Diffusion is the transport of analyte from a location of high concentration to a location of lower concentration and occurs at a rate proportional to the concentration gradient Migration is the transport of ionic analyte in response to an electrical potential gradient within the solution The contribution of migration to the total rate of mass transport can be made negligible by addition of an inert electrolyte to the solution at a concentration much
13. 89 5 236 0 379 0 370 theoretical N 44 5 236 uA 0 189 0 179 theoretical S 89 0 44 0 89 0 0 506 0 515 theoretical 35 36 Model 366A BI POTENTIOSTAT Hardware User s Manual 4 PRELIMINARY TESTING 4 1 INTRODUCTION When a malfunction or some anomalous behavior is observed during electrochemical experiments one must first determine whether the difficulty originates within the bi potentiostat or is external to it To facilitate troubleshooting a so called dummy cell has been provided in the bi potentiostat The dummy cell is constructed from four resistors with a common connection as shown in Figure 14 When the function switch labeled ELECTRODES at the left side of the front panel on the Model 366A is in the dummy position all connections to the electrolysis cell are disconnected and internal connections of CE K1 K2 and REF are made to the dummy cell as shown in Figure 14 In effect simple resistance analogs are substituted for the functions of the normal electrodes in the electrolysis cell Fig 14 DUMMY CELL Starting with the controls of the Model 366A and the X Y recorder set as described in Experiment A steps 5 and 6 change the ELECTRODES switch from normal to dummy Change the CURRENT CONVERTER switch on the K1 ELECTRODE section from 0 2 mA V to 2 mA V Turn on the AC POWER switches for the Model 366A the X Y recorder and the digital voltmeter if needed The recorder pen s
14. ECTRODE 14 2 3 THE ROTATING RING DISK ELECTRODE RRDE 18 2 4 SURFACE CONTROLLED ELECTRODE CURRENT 20 2 5 DIFFUSION LAYER TITRATIONS 4 23 2 6 LITERATURE CITED AND FURTHER 24 3 ELEMENTARY EXPERIMENTS 4 25 3 1 EQUIPMENT AND 25 3 2 EXPERIMENT A RESIDUAL CURRENT POTENTIAL CURVE FOR Pt ELECTRODES IN ACIDIC SOLUTIONS LET 26 3 3 EXPERIMENT B MASS TRANSPORT LIMITED CURRENTS AT THE ROTATING DISK ELECTRODE M M E MUN EA NM MU EM LI 29 3 4 EXPERIMENT C COLLECTION AND SHIELDING EFFICIENCY OF A ROTATING RING DISK ELCEGIRODE satana ARR Lane EIE a 32 T PBELEIMINARY TESTINGus era 37 451 INTRODUGTIONI x ppi i RIT RI RT ATTO ALA ALARE Ra 37 4 22 TROUBLESHOOTING 522 75222 OE RR SE RR 45444 hid SR cuin nias 38 INDEX aaa ARIA aa AIAR AT 41 lii Safety Instructions and Symbols This manual contains up to three levels of safety instructions that must be observed in order to avoid personal injury and or damage to equipment or other property These are DANGER Indicates a hazard that could result
15. IAL AXIAL AND RADIAL VELOCITIES AS A FUNCTION OF THE NORMAL DISTANCE FROM THE ELECTRODE SURFACE Chapter 2 THEORY OF ROTATING ELECTRONICS 15 The Navier Stokes equations which describe the convective flow at the surface of a rotating electrode were first solved independent of any electrochemical application The tangential 0 radial and axial x flow velocities vg v and v were solved as series functions and are given in Equations 5 11 vg rwG tangential 5 v rwF radial 6 vw H E axial 7 where amp 8 G 1 bE 1 3 amp 1 2 ab 1 1 15b8 9 F 1 22 1 3083 1 1202 10 H a 1 38 1 6b 1 30b 85 11 axis of rotation Vg x 0 R R W axial velocity V radial velocity Vo angular velocity N di rectioi n of rotation Fig 7 DIAGRAM OF AXIAL RADIAL AND ANGULAR FLUID VELOCITY VECTORS NEAR A DISK ROTATING IN VICOUS FLUID In Equations 9 11 a 0 510 and b 0 616 In all of the equations v kinematic viscosity of the fluid and w angular velocity of electrode rotation The velocity functions are shown plotted versus the dimensionless function x w v in Figure 6 and are diagramed in Figure 7 16 Model 366A BI POTENTIOSTAT Hardware User s Manual For small values of 6 i e near the electrode surface all but the first terms may be dropped from the series so
16. L AUTO SWEEP RATE UPPER LOWER STOP AT LIMIT OFFSET VOLTAGE SWEEP VOLTAGE CURRENT CONVERTER OFFSET VOLTAGE SWEEP VOLTAGE CURRENT CONVERTER Position Center position off ZERO 33 3 mV per sec 41 00 V 0 00 V Center position off Center position off OFF 0 5 mA per volt Center position off OFF 0 2 mA per volt Make electrical connection from the jack for K1 on the SWEEP GENERATOR section to the disk electrode contact on the RRDE Similarly connect the jack for K2 to the contact for the ring electrode Both the ring and disk electrodes are now potentiostated at 0 00 V versus SCE Transfer the leads connected to the Y axis input of the recorder from the 11 jacks to the I2 jacks of the Model 366A The Y axis is now ready to record the ring electrode current 4 The ring electrode is at a potential of 0 00 V versus SCE which is suitable for mass transport limited reduction of 1 Record the ring current as a function of disk potential I during the positive sweep of disk potential from 0 00 V to 1 00 V versus SCE for a rotation speed of 1600 rev min The I E curve for E 0 00 V is known as the collection curve typical collection curve is shown in Figure 13 5 Adjust the potentiometer of the OFFSET VOLTAGE for K2 ELECTRODE ring electrode to a setting of 2 75 which corresponds to a potential of 0 750 V 3 75 x 0 2 V turn Set the OFFSET VOLTAGE switch to the position The ring electrode pot
17. Model 366A BI POTENTIOSTAT Hardware User s Manual 220893C 0202 Advanced Measurement Technology Inc a k a Princeton Applied Research a subsidiary of AMETEK Inc WARRANTY Princeton Applied Research warrants each instrument of its own manufacture to be free of defects in material and workmanship Obligations under this Warranty shall be limited to replacing repairing or giving credit for the purchase price at our option of any instrument returned shipment prepaid to our Service Department for that purpose within ONE year of delivery to the original purchaser provided prior authorization for such return has been given by an authorized representative of Princeton Applied Research This Warranty shall not apply to any instrument which our inspection shall disclose to our satisfaction to have become defective or unworkable due to abuse mishandling misuse accident alteration negligence improper installation or other causes beyond our control This Warranty shall not apply to any instrument or component not manufactured by Princeton Applied Research When products manufactured by others are included in Princeton Applied Research equipment the original manufacturer s warranty is extended to Princeton Applied Research customers Princeton Applied Research reserves the right to make changes in design at any time without incurring any obligation to install same on units previously purchased THERE ARE NO WARRANTIES THAT EXTEND BE
18. TOR UPPER 1 40 V SWEEP GENERATOR LOWER 0 25 V SWEEP GENERATOR STOP AT LIMIT Center position off K1 ELECTRODE OFFSET VOLTAGE Center position off K1 ELECTRODE SWEEP VOLTAGE ON Model 366A BI POTENTIOSTAT Hardware User s Manual Section Control Position K1 ELECTRODE CURRENT CONVERTER 0 2 mA per volt K2 ELECTRODE OFFSET VOLTAGE Center position off K2 ELECTRODE SWEEP VOLTAGE OFF K2 ELECTRODE CURRENT CONVERTER 20 MA per volt 5 Suggested settings for the X Y recorder are given below X axis sensitivity 200 mV in 100 mV cm Y axis sensitivity 200 mV in 100 mV cm X axis zero as appropriate so entire scan of electrode voltage from 0 25 V to 1 40 V is accommodated on scale Y axis zero center of scale 6 Make connections from the Model 366A Bi Potentiostat to the RRDE and the X Y recorder as indicated below From Model 366A Connect to CE ELECTRODE Pt counter electrode REF ELECTRODE Saturated calomel electrode K1 ELECTRODE Contact for disk electrode of RRDE K2 ELECTRODE is not used in this experiment E1 jacks of K1 section X axis input on recorder 11 jacks of K1 section Y axis input on recorder The black plug represents common potential on the Model 366A Digital voltmeter if used Parallel with X axis 7 When the solution in the electrolysis cell has been deaerated by the dispersion of N change the flow of to pass over the solution rather than through the solution An atmosphere of N must
19. YOND THE DESCRIPTION ON THE FACE HEREOF THIS WARRANTY IS IN LIEU OF AND EXCLUDES ANY AND ALL OTHER WARRANTIES OR REPRESENTATIONS EXPRESSED IMPLIED OR STATUTORY INCLUDING MERCHANTABILITY AND FITNESS AS WELL AS ANY AND ALL OTHER OBLIGATIONS OR LIABILITIES OF PRINCETON APPLIED RESEARCH INCLUDING BUT NOT LIMITED TO SPECIAL OR CONSEQUENTIAL DAMAGES NO PERSON FIRM OR CORPORATION IS AUTHORIZED TO ASSUME FOR PRINCETON APPLIED RESEARCH ANY ADDITIONAL OBLIGATION OR LIABILITY NOT EXPRESSLY PROVIDED FOR HEREIN EXCEPT IN WRITING DULY EXECUTED BY AN OFFICER OF PRINCETON APPLIED RESEARCH SHOULD YOUR EQUIPMENT REQUIRE SERVICE A Contact the Customer Service Department 865 482 4411 or your local representative to discuss the problem In many cases it will be possible to expedite servicing by localizing the problem B If itis necessary to send any equipment back for service we need the following information 1 Model number and serial number 5 Your telephone number and extension 2 Your name instrument user 6 Symptoms in detail including control settings 3 Your address 7 Your purchase order number for repair charges does not apply to repairs in warranty 4 Address to which the instrument should be returned 8 Shipping instructions if you wish to authorize shipment by any method other than normal surface transportation C U S CUSTOMERS Ship the equipment being returned to Advanced Measurement Technology Inc PHONE 865
20. ace the MANUAL switch of the Model 366A momentarily in the UP position and place the AUTO switch in the ON position The scan of the electrode potential will commence from 0 00 V in the positive direction and continue to scan between the limits of 1 40 V and 0 25 V so long as the AUTO switch is in the ON position Allow the electrode potential to cycle several times 5 to 10 until the I E curve is reproducible before recording the curve on the X Y recorder Adjustment of the sensitivity of the Y axis may be necessary to obtain the entire I E curve within the recorder scale A typical I E curve for the Pt disk electrode of a RRDE is shown in Figure 11 If the curve obtained differs substantially from that in Figure 11 refer to the section on troubleshooting in Chapter 4 Model 366A BI POTENTIOSTAT Hardware User s Manual 10 The procedures in step 9 may be repeated for the ring electrode of a RRDE Set the AUTO switch of the Model 366A to the ZERO position and transfer the lead from the disk electrode contact to the ring electrode contact The area of the ring electrode on a RRDE is usually less than the area of the disk electrode and the sensitivity of the Y axis may be increased to enlarge the l E curve for the ring electrode A typical 1 curve for the ring electrode of a RRDE is also shown in Figure 11 3 3 EXPERIMENT B MASS TRANSPORT LIMITED CURRENTS AT THE ROTATING DISK ELECTRODE The mass transport limited faradaic current in
21. and control amplifier CA1 whose output is connected to the counter electrode CE and the current to voltage amplifier C V Amplifier C V maintains the potential of K1 at virtual ground by means of its current feedback loop Rz Amplifier CA1 maintains the difference in the potential between electrode K1 E and the reference electrode potential equal to the signal potential E Ex 3 Ener si E In practice the potential of K1 is measured directly at the output of PF1 The circuitry required for potentiostatic control of the second working electrode K2 consists of the potential follower PF1 the inverter amplifier IA the control amplifier CA2 and the potential follower PF2 Amplifier CA2 functions by means of the current loop Rz maintaining the value of the potential difference between electrode K2 and the potential of the reference electrode Ege equal to minus the signal potential E Ex ER E Hence the potential of the electrodes K1 and K2 may be controlled independently of each other Because the current loop for amplifier CA2 includes electrode K1 it appears at first glance that the independent control of the current through electrode K1 has been destroyed However amplifier CA1 operates to maintain the equality E Ex E44 and the counter electrode CE serves to complete the current loop for both indicating electrodes In effect the current through CE is the algebraic sum of the currents through K
22. antity of charge Q corresponding to a surface controlled reaction is a constant for a given electrode area 22 Model 366A BI POTENTIOSTAT Hardware User s Manual Diagnostic evidence that a reaction is surface controlled is the observation that Q is independent of variations in the rate of solution stirring Students of electroanalysis should be warned that there are many electrode reactions which have mixed control a behavior which is surface controlled for some conditions and mass transport controlled for other conditions The fact that surface controlled faradaic reactions are so common for noble metal electrodes places an interesting requirement on the methodology of electroanalysis Surface controlled reactions are seldom utilized for quantitative analysis and they constitute an interference in analytical voltammetry The mass transport controlled faradaic current for the analyte must be measured in a manner which minimizes or accurately corrects for the contribution of the surface controlled reaction One method of minimizing the surface controlled current is the recording of l E curves at a very slow potential scan rate Since the total charge passed between two potential limits is fixed for a surface controlled reaction the corresponding current will be inversely proportional to the rate at which the electrode potential is scanned over the potential range of the reaction 2 5 DIFFUSION LAYER TITRATIONS Galvanostatic control of the dis
23. be maintained over the solution during experimentation to prevent entry of O into the solution The flow of N over the solution should be at a rate so that the sound of the flowing gas is audible Start the electrode rotating at 1600 rev min If N bubbles adhere to the Pt surface of the RRDE momentarily lower the electrolysis cell so the contact between the RRDE and solution is broken then return the cell to its original position Repeat this process if necessary until no bubbles adhere to the surface of the RRDE 8 Turn the AC POWER switch of the Model 366A to the ON position Also turn power switches for the digital voltmeter and X Y recorder to the ON positions The voltmeter should read 0 00 0 01 V and the recorder pen should be in the zero position on both the X and Y axis If these values are not observed refer to the section on troubleshooting in Chapter 4 Chapter 3 ELEMENTARY EXPERIMENTS 27 28 PIO amp reduction Hz evolution scan direction scan direction formation Ring electrode Recorder X axis sensitivity 100 mV cm Y axis sensitivity 100 mV cm ZERO position indicated by Potential sweep rate 33 3 mV sec range 0 25 V to 1 40 V vs SCE sweep direction indicated by arrows Current sensitivity disk 0 2 mA V 20 A cm ring 50 4A V 5 A cm RRDE speed 1600 rev min Solution 0 5 M H SO Figure 11 RESIDUAL CURRENT POTENTIAL CURVES AT PLATINUM DISK AND RING ELECTRODES Pl
24. carrier and Princeton Applied Research Save the shipping container for possible inspection by the carrier 1 4 SAFETY CONSIDERATIONS The Model 366A Bi Potentiostat has been supplied in a safe condition This manual contains some information and warnings that have to be followed by the user to ensure safe operation and to keep the apparatus in a safe condition The bi potentiostat has been designed for indoor use Warning The protective grounding could be rendered ineffective in damaged apparatus Damaged apparatus should not be operated until its safety has been verified by qualified service personnel Damaged apparatus should be tagged to indicate to a potential user that it may be unsafe and that it should not be operated Model 366A BI POTENTIOSTAT Hardware User s Manual As defined in IEC Publication 348 Safety Requirements for Electronic Measuring Apparatus the Model 366A Bi Potentiostat is Class apparatus that is apparatus that depends on connection to a protective conductor to earth ground for equipment and operator safety Before any other connection is made to the apparatus the protective earth terminal must be connected to a protective conductor The protective connection is made via the earth ground prong of the Model 366 s power cord plug This plug must only be inserted into a socket outlet provided with the required earth ground contact The protective action must not be negated by the use of an extension cord without a pro
25. ce with electronic problems in the Model 366A Bi Potentiostat 4 2 TROUBLESHOOTING 38 The troubleshooting guide below will help localize problems that are commonly encountered in the use of rotating disk and ring disk electrodes It is assumed that the dummy cell test described above has been successfully completed Problem Possible Explanation 1 No K1 current recorded over entire potential scan i e Y axis of recorder remains at ZERO X axis scans normally Discontinuity in K1 circuit Examples electrode not contacting solution electrode surface covered with gas bubble break in lead between K1 jack on Model 366A and electrode contact on rotator discontinuity in contact Suggestion Check discontinuities in lead and contact with ohmmeter 2 Y axis of recorder is off scale over entire scan i e potentiostat output is saturated Reference or counter electrodes not connected to potentiostat Examples chambers for counter electrode and or reference electrode not filled with electrolyte solution Luggin capillary not filled with electrolyte solution bubble in Luggin capillary ground glass stopcock of Luggin capillary not wetted with electrolyte solution defective reference electrode Problem Possible Explanation Suggestions Check filling solution of reference electrode A crystal in the tip of the reference electrode can terminate ionic continuity replace reference electrode as a temporary test 3 Electrical noise observe
26. ck reaction for the solution reaction is negligible the surface concentration of B at the ring electrode is zero i e limiting ring current for detection of excess B Of experimental importance are plots of l versus l typical curves for the diffusion layer titration of As III in 1 0 M H SO by electrogenerated Br are shown in Figure 9 For CP 0 the slope of the l l4 plot is as predicted by equation 22 see curve a in Figure 9 For C gt 0 0 for all values of for which the flux of B does not exceed the flux of P to the region defined by the area 1R With increasing l increases from a zero value with a nonlinear dependence on This curved portion of the I plot corresponds to the situation wherein the Chapter 2 THEORY OF ROTATING ELECTRONICS 23 reaction between B and P at x 0 is occurring in the region of the ring electrode For large ly when the flux of B far exceeds the flux of P the 1 1 curve is described by equation 28 1 Nola MB 28 where M is related to the flux of P as given by equation 29 M 1 95R nFD w v Cp 29 Note that the slope of the I I plot is N see curve b of Figure 9 for large Extrapolation of the linear portion of the I 1j curve to l 0 defines a value of which is a linear function of the flux of P as given by equation 30 lu o T MB 9 N 30 A plot of 14 plot of 14 i o Versus Cp is predicted to be linear for constant rotational velocity
27. d V Yu Filinovskii The Rotating Disc Electrode Eng transl by H S Wroblowa Consultants Bureau New York 1976 4 W J Albery and S Bruckenstein Trans Faraday Soc 62 2584 1966 5 W J Albery M L Hitchman and J Ulstrup Trans Faraday Soc 65 1101 1969 6 S Bruckenstein and D C Johnson J Am Chem Soc 90 6592 1968 7 D T Napp D C Johnson and S Bruckenstein Anal Chem 39 481 1967 8 V Levich Acta Physiochem URSS 17 257 9 D T Napp Ph D thesis University of Minnesota 1967 10 D F Unterecker Ph D thesis State University of New York at Buffalo 1973 11 D F Unterecker W G Sherwood G A Martinchek T M Reidhammer and S Bruckenstein Chem Inst 6 3 259 66 1975 Model 366A B POTENTIOSTAT Hardware User s Manual 3 ELEMENTARY EXPERIMENTS 3 1 EQUIPMENT AND SOLUTIONS 1 Model 366A Bi Potentiostat 2 Rotating platinum ring disk electrode disk electrode only used for Experiments A and B 3 Electrode rotator such as Princeton Applied Research Model 636 4 X Y recorder 5 Digital voltmeter optional 6 Saturated calomel reference electrode SCE 7 Electrolysis cell with Luggin capillary cover and facilities for deaeration with dispersed nitrogen see Figure 10 rotating electrode Pt counter electrode ort for saturated Electrode calomel electrode cell cover machined from Kel F or Teflon sealing tube used as compa
28. d on Y axis of residual current potential curve Experiment A in region of adsorbed hydrogen 0 0 V to 0 2 V vs SCE Dirty electrical contacts on rotator Suggestion Gently clean contacting surfaces with abrasive paper Bubbles impinging on electrode surface during potential scan Suggestion N should not be dispersed into electrolyte solution during electrochemical experiments Intermittent discontinuity in leads from Model 366A to electrolysis cell Suggestion Wiggle each lead to determine source System not grounded or ground loops exist between bi potentiostat and recorder and voltmeter if used Model 366A BI POTENTIOSTAT Hardware User s Manual Pickup of electrostatic noise by reference electrode Suggestion When operating the bi potentiostat at a very high current sensitivity it may be necessary to place a grounded shield constructed from copper screen around the reference electrode to minimize the pickup of electrostatic noise 4 Current at E 0 3 V vs SCE on residual current potential curve is larger than normal for properly functioning electrode of comparable area at same rotation speed See Figure 15 Dissolved O remains in solution Suggestion Continue deaeration for 10 15 minutes If proper character of I E curve is not obtained in the region of E 0 3 V proceed as follows Problem Possible Explanation a Increase rate of N flow b Improve cell lid to decrease chance of dissolution of air from room atmos
29. e Diffusion and convection both axial and radial near the surface of a rotating ring disk electrode RRDE results in products of a reaction at disk surface being transported to the vicinity of the ring electrode where they may undergo further reaction The rate of mass transport is surprisingly large for a 0 005 to 0 010 cm separation of the ring and disk electrodes unstable disk reaction products with half lives as short as 1 msec can be studied at the ring electrode if the electrode rotational velocity is approximately 10 000 rev min The fraction of species produced at the disk electrode that is capable of reacting at the ring electrode is called the theoretical collection efficiency No is a function only of the geometry of the RRDE and is independent of rotation velocity as given by Equation 22 N 1 F a B B 1 F a 1 a By 1 F o B 1 a B 22 where X n F x 3 27 dx px 1 x 3 4 In 1 1 3 2m arctan 2x 1 3 2 1 4 23 Values of N for typical values of radius ratios are given in Table 1 The limiting current in the ring of a RRDE when no reaction occurs at the disk electrode is given by Equation 21 In Equation 24 is the limiting current which could flow in the disk electrode as predicted by Equation 17 lh a 24 Chapter 2 THEORY OF ROTATING ELECTRONICS 19 Since a portion of the species reacting at the ring e
30. e The free energy of adsorption of atomic H and therefore the potential for the corresponding current peak is different for each crystal plane Measurements have revealed that three crystal planes are present at the surface of polycrystalline Pt but the values of the hydrogen adsorption energy for two of the three are very similar Hence only two peaks are clearly visible Two anodic peaks are obtained on the positive potential sweep for a Pt electrode bearing adsorbed H corresponding to the anodic dissolution of the atomic H The current potential curve obtained for a Pt electrode in H SO is shown in Figure 11 Chapter 2 THEORY OF ROTATING ELECTRONICS 21 I VA Disk electrode reaction controlled current 2Br 2 Br 2e Solution reaction Br 111 2 2Br As V Ring electrode reaction controlled potential Br 2e 2 2Br 0 3886 cm R 0 3986 cm R 0 4443 w 21 1 rad sec 1 0 M H SO 0 2 M KBr supporting electrolyte ring potential of 0 3 V vs SCE Concentration of 1 a 0 0 uM b 11 7 uM 23 4 47 0 uM e 70 5 f 94 0 uM g 117 0 uM Plot of 14 vs Cash is linear with zero intercept and slope of 5 21 x 10 uA mole L Figure 9 DIFFUSION LAYER TITRATION CURVES By their nature surface controlled reactions proceed until the reaction of the electrode surface is complete and then the faradaic current drops to zero Hence the qu
31. e power cord from the power entry module on the rear panel 2 Slide the clear plastic door to the left 3 Use pliers to carefully remove the the voltage selector card 4 With the card removed both numbers 120 and 240 are visible on it Turn the card so the number indicating the correct operating voltage 120 for 100 VAC to 125 VAC 240 for 200 VAC to 250 VAC will remain visible when the card is replaced 5 Reinsert the card into its connector with the desired operating voltage number visible Be sure the board is securely seated 6 Remove the fuse and check its rating When the lever labeled FUSE PULL is rotated out and towards the left the fuse will lift so that it can be easily removed Use a slow blow fuse rated at 3A Make sure that only fuses with the required current rating and of the specified type are used for replacement 7 When the proper fuse has been installed slide the plastic door back over the fuse compartment and reconnect the power cord 1 6 LINE FUSE REPLACEMENT If your equipment will not operate first check the fuse Remove the power cord slide the clear plastic door to the left and use the PULL FUSE lever to remove the fuse If the fuse is bad replace it with a 3 amp slow blow fuse after checking for the cause of the blown fuse Slide the clear door back over the fuse compartment and plug the power cord back into the module 1 7 SERVICE This is an operating manual only and contains no service informatio
32. ential is now at 0 750 V versus SCE for which the oxidation of occurs at a rate limited by mass transport Record the I E curve for the positive sweep of the disk potential from 0 00 V to 1 00 V versus SCE at a rotation speed of 1600 rev min The I E curve for E 0 750 V is known as the shielding curve A typical shielding curve is shown in Figure 13 Chapter 3 ELEMENTARY EXPERIMENTS 33 34 Collection Curve k Eg Shielding Curve I Eg Recorder X axis sensitivity 100 mV cm Y axis sensitivity 100 mV cm ZERO position indicated by Potential sweep rate 33 3 mV sec range 0 00 V to 1 00 V vs SCE curves recorded only during positive potential sweep Current sensitivity E curve 0 5 mA V 50 A cm I E curve 0 2 mA V 20 uA cm shielding curve 0 2 mA V 20 A cm collection curve 0 2 20 uA cm RRDE Speed 1600 rev min Solution 0 5 M 5 0 4897 mM KI Figure 13 CURVES FOR IODIDE AT A PLATINUM RING DISK ELECTRODE When experimentation is completed shut down the instrumentation by switching off the AC POWER to the recorder and then the Model 366A Remove the solution from the electrolysis cell and clean all glass surfaces by thorough rinsing with distilled water Be certain to drain and rinse the Luggin capillary so the contents will not be a source of contamination in later experiments The fritted glass of the gas dispersion tube and the counter e
33. hould be at the ZERO position on both axes and the voltmeter should read 0 00 0 01 V Start the potential scan in the positive direction as in Experiment A step 11 The recorder should display a straight line of slope 1 2 If the recorder does not display a straight line of slope 1 2 check the leads from the Model 366A to the recorder also check all control settings Many electrochemical experiments with ring disk electrodes proceed with potentiostatic control of the ring electrode at a constant potential while the potential of the disk electrode is scanned within certain prescribed limits Check that the SWEEP VOLTAGE switch for K1 ELECTRODE disk is ON and the SWEEP VOLTAGE switch for K2 ELECTRODE ring is OFF Set OFFSET VOLTAGE of K2 ELECTRODE to 0 10 V The output at the E2 jack should read 0 10 V through the potential scan of the K1 electrode 37 Successful completion of these tests indicates that the Model 366A Bi Potentiostat is operating correctly and that the bi potentiostat and recorder are properly interconnected The cause of the malfunction is thus located external to the bi potentiostat in the connections to the electrolysis cell or within the cell itself A troubleshooting guide is provided below to assist in localizing several commonly encountered problems related to the electrolysis cell Failure of these tests indicates a malfunction of the Model 366A or the X Y recorder Consult Princeton Applied Research for assistan
34. in death or serious bodily harm if the safety instruction is not observed WARNING Indicates a hazard that could result in bodily harm if the safety instruction is not observed CAUTION Indicates a hazard that could result in property damage if the safety instruction is not observed Please read all safety instructions carefully and make sure you understand them fully before attempting to use this product Cleaning Instructions WARNING Using this instrument in a manner not specified by the manufacturer may impair the protection provided by the instrument To clean the instrument exterior e Unplug the instrument from all voltage sources e Remove loose dust on the outside of the instrument with a lint free cloth e Remove remaining dirt with a lint free cloth dampened in a general purpose detergent and water solution Do not use abrasive cleaners CAUTION To prevent moisture inside of the instrument during external cleaning use only enough liquid to dampen the cloth or applicator e Allow the instrument to dry before reconnecting the power cord iv Model 366A BI POTENTIOSTAT Hardware User s Manual 1 GENERAL necmaooe etto Fig 1 MODEL 366A BI POTENTIOSTAT 1 1 INTRODUCTION The Princeton Applied Research Model 366A Bi Potentiostat is a four electrode potentiostat designed to be used with a ring disk electrode It can control two working electrodes at different potentials and monitor the curre
35. ing the faradaic current in the electrode is independent of time steady state A semiquantitative relationship between electrode current and 6 was given by Nernst in 1904 According to the law of Faraday l is related to the flux of analyte N by Equation 1 2 nFN 1 where n equiv mole F 96486 6 coul equiv N moles sec The flux at the electrode surface is related to the concentration gradient of the analyte measured at the electrode surface as given by Fick s law of diffusion in Equation 2 N AD dC dx 2 where A area cm D diffusion coefficient cm sec C concentration mole cm x distance cm 13 Nernst assumed a linear concentration gradient across the diffusion layer and approximated the gradient as given by Equation 3 Cres C 19 6 3 The concentration of analyte at x is virtually equal to the analyte concentration in the solution bulk C When the electrode potential is made sufficiently large relative to the standard reduction potential so C approaches zero approaches a limiting value as given by Equation 4 limit I nFADC 5 4 rot 0 The numerical value of in Equation 4 is a kind of calibrating factor and is observed to be inversely dependent on the rate of electrode rotation 2 2 THE ROTATING DISK ELECTRODE 14 In the third decade of this century V G Levich and coworkers made great progress in combining the mathematical equations
36. is scanned in a positive direction a wave corresponding to formation of PtOH and PIO is observed at E gt 0 5 V versus SCE for a solution pH of 1 The oxide film is very dense and protects the metal from further oxidation The effectiveness of this film is apparent in the fact that Pt is known as a noble metal The film of PtO is cathodically reduced by a surface controlled reaction during the negative potential sweep Much controversy exists regarding the electrochemistry of the oxide film Because the film corresponds to more than one layer of Pt atoms the oxidation state of Pt in the film is not definitely known There is also disagreement whether the platinum oxide is reduced all the way to pure Pt during the negative potential sweep or to a semi stable state such as PtOH Surface controlled current peaks are also observed for Pt electrodes resulting from the reduction of H to produce atomic H which is adsorbed to the Pt surface Because there is energy gained by adsorption of H these cathodic surface controlled peaks are observed on the negative potential scan before the evolution of gaseous 20 Model 366A BI POTENTIOSTAT Hardware User s Manual Table 1 VALUES OF COLLECTION EFFICIENCY FOR TYPICAL RADIUS RATIOS The reason that two peaks rather than one peak are observed for the formation of adsorbed H is a rather fascinating story The Pt used for fabrication of electrodes whether in the form of a wire or plate is polycrystallin
37. k electrode of a rotating ring disk electrode with potentiostatic control of the ring electrode can be applied for quantitative analytical determinations of electroinactive species using diffusion layer titrations In the galvanostatic mode the current in the disk electrode is proportional to the signal voltage input and can be monitored as a potential by the Model 366A Galvanostatic operation of this unit is described in Chapter 1 pages 2 and 3 The name diffusion layer titration is given to a sensitive analytical technique for the quantitative determination of electroinactive analytes S Bruckenstein and D C Johnson Anal Chem 36 2186 1964 In this technique a reagent is generated under controlled current at the disk of a rotating ring disk electrode The reagent reacts homogeneously with the analyte in the region of the diffusion layer and any excess unreacted reagent is detected amperometrically at the ring electrode The theory of diffusion layer titrations was developed by Albery W J Albery S Bruckenstein and D C Johnson Trans Faraday Soc 62 1938 1966 and will be only briefly summarized here Consider the sequence of reactions shown below Disk electrode controlled current A ne B Solution reaction diffusion layer B Q Ring electrode controlled potential B ne A The boundary conditions for these reactions are summarized as follows The bulk concentration of A is large compared to P the ba
38. larger gt 100 X than that of the electroactive analyte Migration will be assumed negligible for the remainder of this discussion The region adjacent to the surface of an electrode rotating in a solution can be considered as divided into two regions for the purpose of a qualitative description of mass transport to the electrode surface For practical fluids having a finite viscosity the solution contacting the electrode surface adheres and is described by the same velocity vectors as the electrode Hence convection is negligible as a mechanism of mass transport within the adherent layer and diffusion alone is responsible for mass transport within this region The layer of adherent solution wherein diffusion is the predominant mechanism of mass transport is called the diffusion layer The thickness of the diffusion layer measured perpendicular to the electrode surface is given the symbol In the region of the bulk solution beyond the diffusion layer agitation mixing of the solution is eminent and convection predominates as the mechanism for mass transport This model usefully describes the mass transport mechanism even though there is not a sharp dividing line between the diffusion layer and the bulk convective region In summary Convection is responsible for bringing analyte from the bulk of the solution to the diffusion layer and diffusion brings the analyte across the diffusion layer to the electrode surface For a constant rate of stirr
39. lectrode compartment should be flushed several times with distilled water to remove all traces of acid and KI Store all glassware in a clean environment Remove the electrode and carefully rinse it with distilled water Store the electrode in a clean place Model 366A BI POTENTIOSTAT Hardware User s Manual 7 Calculate the shielding coefficient S and the collection efficiency from the current potential curves obtained in this experiment Refer to Equations 24 26 and Figure 13 for assistance 8 Determine the characteristic radii of the RRDE using an optical micrometer Calculate the collection efficiency for the RRDE Also calculate the shielding efficiency Compare the experimental and theoretical values of B N and S The values should agree within 10 relative error Example calculations of 8 N and S are given below for the data in Figure 13 RRDE dimensions R 0 3822 cm R 0 4010 cm R 0 4255 cm Area of disk electrode A R 0 459 Theoretical 29 p Ry R f Ry R 0 370 Collection efficiency predicted from Table 2 R R 1 049 R R 1 061 N 0 179 17 9 Shielding efficiency predicted S 1 2 0 515 51 5 Experimental results I E 0 750 V 236 I E 0 750 V 89 5 Ey 0 0 V E 0 0 V 0 0 vA Ey 0 750 V E 0 0 V 44 5 vA I E 0 000 V E 0 750 V 89 0 LA Ey 0 750 V E 0 750 V 44 0 2
40. lectrode is transported to the ring by a path which approaches the surface of the disk potentiostating the disk electrode in the region of limiting current results in a decrease of the ring current from the value The extent of the decrease is Nol a I m 7 Nol a Bh z Noli a 629 5 25 A shielding efficiency S is defined as the ratio of the ring electrode current observed when the disk electrode is at a potential for the mass transported limited reaction and the ring electrode current when the disk electrode current is zero Sz 629 2 No li Bh a 1 N B 26 2 4 SURFACE CONTROLLED ELECTRODE CURRENT Current potential I E curves obtained for noble metal electrodes such as platinum are characterized by waves resulting from faradaic reactions which involve direct participation of the metal surface These faradaic reactions are in fact limited in their extent by the number of metal atoms at the surface of the electrode and are described as being surface controlled reactions Surface controlled reactions are much different than mass transport controlled reactions which occur at a rate limited by the flux of the electroactive species at the electrode surface The most familiar surface controlled reactions for platinum electrodes in aqueous solution are the anodic formation and cathodic dissolution of a thin film of platinum oxide at the electrode surface When the potential of a Pt electrode in an aqueous solution
41. lutions Thus fg rw 12 v 0 510 rx w v 13 v 0 510 2 14 The thickness of the solution layer near the electrode in which the axial flow has reached 80 of its maximum value is called the hydrodynamic layer 5 Essentially beyond 6 the angular and radial velocity components are nonexistent 3 6 v w 15 The equation of convective diffusional mass transport as it applies to the rotating disk electrode is given below 8C 0t D c C ox 16 Solution of the equation above for a rotating electrode is greatly simplified if the term cC ct is negligible This is valid if the rate of rotation is constant and steady state conditions apply Further simplification results because the electrode material is an excellent conductor and the electrical potential across the surface is uniform Furthermore the value of C at a distance x 6 is uniform and as a result 6C er 0 This is frequently called the uniform accessibility condition Equation 16 is solved with the use of the following boundary conditions i limiting disk current i e C 0 and lia ii for x C iii from Faraday s and Fick s laws _ l nFAD The mathematical solution is lg 0 62 nFAD 9y w 17 where la limiting disk current mA bulk concentration of electroactive species M A area of disk electrode cm D diffusion coefficient of electroactive
42. lyte to maintain ionic contact between the solution in the electrolysis cell and the SCE but should be in the closed position to prevent rapid mixing of the solutions between the two chambers The Luggin capillary tubing should be free of any entrapped gas bubbles as their presence will disrupt proper functioning of the bi potentiostat Mount the RRDE securely in the rotator Excessive force should not be applied to the Teflon shroud of the electrode as this may result in the slipping of the shroud along the metal shaft Mount the electrolysis cell in a position so that the RRDE enters the cell through the electrode port and the end of the RRDE is submerged approximately 5 mm below the surface of the solution The RRDE should be centered in the electrode port so the shroud of the RRDE does not rub against the cell cover Attach tubing from the N cylinder to the gas dispersion tube Pass N through the dispersion tube for approximately 10 min to deaerate the solution in the electrolysis cell The flow rate of the gas should be such that the solution is agitated vigorously by the gas bubbles but excessive splashing does not occur Proceed with steps 4 6 during deaeration 4 Setthe controls of the Model 366A Bi Potentiostat as indicated below Section Control Position AC POWER OFF ELECTRODES NORMAL MODE POT SWEEP GENERATOR MANUAL Center position off SWEEP GENERATOR AUTO ZERO SWEEP GENERATOR SWEEP RATE 33 3 mV per sec SWEEP GENERA
43. n Users should note that the Model 366A is very difficult to service in the field special fixtures and services are required that are not readily obtainable except at the factory or at certain affiliate facilities Contact the factory service department or the affiliate in your area for additional service information There are no operator serviceable parts inside Refer servicing to qualified personnel Chapter 4 describes troubleshooting procedures to determine whether problems that could be encountered during experiments are caused by the test setup or by a possible failure in the bi potentiostat 4 Model 366A BI POTENTIOSTAT Hardware User s Manual 1 8 GENERAL DESCRIPTION 1 8 1 Sweep Generator The dual range sweep generator may be used to supply a sweep or scan signal to the electrodes Particular attention was given to the design so that an operator will have complete flexibility in adjusting sweeping or stopping the sweep at any point in a particular cycle Figure 1 shows the front panel jacks and controls of the Model 366A Bi Potentiostat This is a true sweep generator in that period or frequency is not controlled but rather the rate of sweep and the upper and lower limit of the sweep are the controlled factors The two limits can be a few millivolts apart or up to four volts apart these limits can be set both positive both negative or one positive and one negative In repetitive sweep the upper and lower limits of sweep a
44. n the upper and lower limits if AUTO switch is on Bottom The unit will sweep to the lower limit and hold 1 10 2 K1 and K2 Electrode Section OFFSET VOLTAGE switch and associated toggle switch The four digit pushbutton switch sets the magnitude of the applied voltage Toggle switch in off center position no effect on electrode Toggle switch in 4 up position Positive polarity on electrode Toggle switch in down position Negative polarity on electrode SWEEP VOLTAGE toggle switch on up Output of sweep generator is applied to the electrode off down No effect on electrode voltage CURRENT CONVERTER rotary switch Sets the scale factor for converting volts on Jack I to actual current in the electrode 1 11 JACK FUNCTIONS Front Panel black jacks DC common not frame or earth ground connected SWEEP VOLTAGE Output of the Sweep Generator may be used independently of the electrodes section ELECTRODES group CE Counter electrode connection low impedance output K1 K1 disk or working electrode connection low impedance held at virtual DC common voltage potential 8 Model 366A BI POTENTIOSTAT Hardware User s Manual K2 K2 ring or second working electrode connection low impedance REF Reference electrode connection high impedance input which measures the REF voltage without loading the system Binding Post Case earth ground connection a convenient point for connecting DC common to earth ground if s
45. nal Chem 39 481 1967 The circuit of the Model 366A Bi Potentiostat represents substantial improvement over the first circuit in terms of capacity response and convenience A brief analysis of the circuit is given here based on the simplified circuit diagram shown in Figure 2 Figure 3 shows the component layout of the Model 366A Bi Potentiostat 10 Model 366A BI POTENTIOSTAT Hardware User s Manual x CET CE Sf 6 Idi Sal i 0 I 3oNvard101 5170 3uv sl B 9497 8 1 4 01 ct 2 CrisHi CrIGHI SOOPNI 40151534 9 v 1 Gai MOLSISIE 46 A 2 1 CT om 2 366v v 8 dit 3 g Cape 100 4 80151534 X4 8 1 Bra 9t 19 84 D e Crea Q QD ce zg oot aL irae ci Lidl 5 AQL 91 01 E DI 141 T 019 012 3 So i 2 8 a ora no iS e Bj dit tzia 2 Gap 60 ciam A E uus Q a I 5 T t n 5 Szia 8 cou zi Gan c2 ezionz 10 1z2 090 amp ge 20 107 C t9u XOU GO JO E e X 01 0 GEST LITT Lr QED i 9 000 GO Col T Ga 11 Fig 3 COMPONENT LAYOUT DIAGRAM Chapter 1 INTRODUCTION 12 The circuitry for the potentiostatic control of working electrode 1 K1 consists of the potential follower PF1
46. ndicator lights to indicate a possible measurement error The circuits continue to function even though the indicator is on To use the Model 366A as a 3 electrode potentiostat it is not necessary to make any connections to the K2 electrode circuit 1 8 3 Galvanostat The K1 or disk electrode can be set to a galvanostat mode In this mode the signal voltage applied to the K1 input is converted to a current determined by the setting of the CURRENT CONVERTER switch When the MODE switch of the Model 366A is switched from the position for potentiostatic operation POT to the position for galvanostatic operation GAL the operation of electrode K1 is switched from potential control to current control The operation of electrode K2 remains as potential control regardless of the mode selected for K1 Galvanostatic control of the disk electrode of a rotating ring disk electrode RRDE with potentiostatic control of the ring electrode can be applied for quantitative analytical determinations of electroinactive species by way of the so called diffusion layer titrations discussed in Chapter 2 and for the measurement of rate constants for pseudo second order homogenous reactions In the galvanostatic mode the current in the disk electrode is proportional to the signal voltage input and can be monitored as a potential at terminal 11 just as is the case for potentiostatic operation The factor for conversion of the measured potential to current value
47. nts generated at these electrodes Although it has been engineered specifically for ring disk electrodes it can also be used with nonrotating electrodes and a variety of cell configurations The Model 366A can be used in potentiostat mode for both working electrodes or one of the electrodes can be controlled galvanostatically while the other is controlled potentiostatically The Model 366A includes a built in sweep generator which is set by specifying sweep rate rather than frequency or period The potential of one or both electrodes may be swept Additionally an externally generated waveform may be applied to the electrodes The bi potentiostat can also be used in the conventional three electrode arrangement The bi potentiostat is particularly useful to researchers interested in reaction kinetics and reaction mechanisms In one use the disk of a rotating ring disk electrode can be cycled between potentials while the ring is held at constant potential If the appropriate choices are made for these values materials formed at the disk can be detected at the ring This allows the researcher to detect reaction intermediates and by varying the speed of the rotator and hence the time it takes for a product to move from the disk to the ring determine the half life of the intermediates The corrosion researcher may be interested in following oxidation reactions occurring at the disk by setting the ring to a potential that allows selection of specific s
48. o desired K1 K2 ELECTRODE group IN IN A voltage input jack connected through an internal 10K ohm resistor to the summing point of the K1 K2 section This allows a voltage to be applied from an external source such as a function generator This voltage is summed with the voltages applied by the offset system and the sweep voltage system E1 E2 A voltage output jack which is a voltage indication of the actual K1 K2 electrode voltage with respect to the REF electrode This is a buffered output I1 12 A voltage output jack which is a voltage indication of the actual K1 K2 electrode current it is used in conjunction with the current converter switch to produce current values For example if 2 0 volts is measured at the 11 I2 jack and the K1 K2 current converter switch is set to the 0 5 mA per volt range the actual K1 K2 current is 1 0 mA 2V x 0 5 mA V I1 12 A voltage output jack which is a voltage indication of the difference in currents flowing in the K1 and K2 electrodes The current converter switches for K1 and K2 must be in the same range for this voltage to be a valid value Rear Panel 1 K2 GAL These jacks operate as pairs and allow capacitors to be inserted into the circuit to slow the response and eliminate oscillations which may occur under certain conditions See the following section on noise and oscillations 1 12 NOISE AND OSCILLATIONS The eight black banana jacks on the Model 366A front
49. oefficient is 0 62 as predicted by Levich 2 3 THE ROTATING RING DISK ELECTRODE RRDE 18 A ring disk electrode is shown in Figure 8 In the ring disk electrode an annular electrode ring is positioned symmetrically about the disk and the surfaces of the ring and the disk electrodes are coplanar The current in the ring electrode is influenced by reactions occurring at the disk electrode which increase or decrease the concentration of electroactive species in the radial flow stream Disk Radius 1 Ring Thickness R 3R 2 Ring Inside Diameter R 2 TEFLON GAP Fig 8 ROTATING RING DISK ELECTRODE Model 366A BI POTENTIOSTAT Hardware User s Manual To obtain explicit equations predicting the ring current the equation of mass transport must be solved including the term for radial convection v 0C ar In the absence of any reaction at the disk electrode the mass transport limited faradaic current in the ring electrode is given by Equation 20 1 0 62nFm R 225 29 2 6 20 3 The ratio 1 1 is a constant for any given electrode design and is determined solely by the geometry of the electrode Rj R R R R R Ry T p 21 There is no particular advantage in the use of a ring electrode rather than a disk electrode for direct electroanalytical determinations The opportunities provided by the ring electrode design are only the result of its use in conjunction with a disk electrod
50. ommended over in house construction As a result of electrode rotation a layer of solution at the disk surface adheres and rotates at an angular velocity equal to that of the electrode Solution at some finite distance from the disk Ax is also rotating though not at the velocity of the electrode The centrifugal force upon this fluid causes it to move in a radial direction The radial fluid displacement is compensated by an axial fluid flow as shown in the pictorial representation of the flow patterns in Figure 5 Model 366A BI POTENTIOSTAT Hardware User s Manual FLOW OF SOLUTION AXIS OF ROTATION pa BS FLOW OF SOLUTION EDGE VIEW OF DISK END VIEW OF DISK Fig 5 FLUID FLOW PATTERNS AT ROTATING DISK ELECTRODE The rate of electrode rotation can be easily maintained at a constant value and consequently the fluid velocity in the vicinity of the electrode surface is constant The result is that the electrochemical current resulting from the mass transport of electroactive material from the bulk of the solution to the electrode and the subsequent electrode reaction is constant The exact functional expression describing and its relationship to the rate of electrode rotation can only be obtained from a consideration of the hydrodynamic patterns of fluid flow around the electrode A brief account of that consideration is given here tangential radial F 0 5 1 0 1 5 2 0 2 5 3 0 3 5 f x Fig 6 TANGENT
51. panel are circuit common and are floating with respect to the Model 366A case On some systems it may be necessary to connect one of the black banana jacks to earth ground to reduce noise in the system A banana jack connected to the instrument s case is provided on the front panel for convenience The bi potentiostat is equipped with a three wire power cord The case of the unit is electrically connected to the green wire in the power cord As the green wire is also connected to the normal ground pin on the 3 prong plug the case is connected to earth ground when the bi potentiostat is plugged into a three prong outlet with a good quality earth ground connection All cables connected to the Model 366A front panel especially the electrode connections should be kept as short as possible The REF electrode jack presents a very high input impedance and is therefore susceptible to picking up unwanted signals or noise The Model 366A incorporates an insulated BNC connector for the REF electrode connection allowing complete shielding of the REF input lead The shield of the REF electrode connector is driven to the same voltage as the REF signal Therefore do not ground the shield The shield should not be connected to any other point at the electrode end so no current can flow in the shield It is suggested that no connections other than to the actual electrodes be made to the ELECTRODE jacks particularly the REF jack Chapter 1 INTRODUCTION 9
52. pecies and oxidation states The presence of a surface interaction such as a filming amine inhibitor may be observed using a bi potentiostat and a ring disk electrode The electrochemical mechanisms occurring during corrosion can also be studied using a bi potentiostat 1 2 FEATURES 1 2 e gt e N 13 Four electrode potentiostat Can be used as a three electrode potentiostat May be used in galvanostat mode Current to voltage converter circuits to give output signal voltages proportional to the electrode currents F E T amplifiers for measuring electrode voltages Set of internal resistors can be switched in place of the external electrodes Outputs protected against short circuits Overload indicator Auxiliary input jacks for applying an external voltage or program to either or both working electrodes Calibrated adjustable offset voltages which can be applied independently to the two working electrodes Summing circuits which will sum all electrode input signal voltages apply sum to the electrode and provide an output voltage signal for recording Dual range controlled scan sweep generator whose output voltage can be applied to either or both working electrodes Provision to set sweep to stop at a given voltage 1 3 INITIAL INSPECTION The bi potentiostat should be inspected for shipping damage immediately after unpacking If any damage is found please notify the
53. phere Suggestion Use of an unnecessarily long piece of plastic tubing to connect the N tank and the gas dispersion tube of the cell is not advised because of the permeability of plastic by oxygen In the absence of dissolved O the residual current observed at E 0 3 V vs SCE on the positive potential sweep is approximately equal in magnitude but opposite in polarity to the current for the negative potential sweep see Figure 11 5 Current at E 0 7 V vs SCE residual current potential curve is larger than normal See l E curve in Figure 15 for ring electrode that has developed a capillary leak Probability that the seal surrounding the electrode has developed a capillary leak Suggestion Replace electrolyte solution with fresh solution to be certain that electroactive impurities are not present in the cell Disk Electrode no leak Ring Electrode leak All parameters as in Figure 11 Anomalous regions indicating leak are circled on l E curve Figure 15 CURRENT POTENTIAL CURVES FOR NORMAL DISK ELECTRODE AND LEAKY RING ELECTRODE Chapter 4 PRELIMINARY TESTING 39 40 Model 366A BI POTENTIOSTAT Hardware User s Manual Amplifier specifications 7 AUTO Switch ce Rt des 6 Circuit description 10 Compliance voltage 5 Corrosion research 1 CURRENT CONVERTER switch
54. re independently adjustable between 9 999 V The output signal sweeps between the high and low limits at the set sweep rate The sweep can be stopped and held at any point then started in either direction from the stop point at the discretion of the operator In normal operation the sweep generator is free running If desired the sweep generator can be set to automatically stop at either end of the sweep The voltage will hold at that point until the sweep circuit is started again 1 8 2 Potentiostat The potential control or potentiostat section is designed to be versatile and reliable while maintaining simplicity of operation The potential is controlled independently of the current flowing in the cell the potential applied to one electrode will not affect the potential or current of the other electrode The rise time of the amplifiers in the bi potentiostat is 0 5 volts per microsecond The Model 366A has a current output capability of 1 amp with a compliance voltage of 10 volts The outputs are current limited to prevent damage to the bi potentiostat in the event of a short circuit to DC common The bi potentiostat features a current to voltage converting system for each working electrode This circuitry produces a voltage output proportional to the electrode current to facilitate connection to external instruments such as X Y recorders or the A D interfaces in a computer controlled system The scale factor of each current to voltage conve
55. relation coefficient 17 866 0 6205 nFA D y CP n 1 eq mol A mR 0 459 cm v 2 13 cm sec C 0 4897 mM Calculation D 17 866 28636 8 6 24 x 10 1 56 x 10 cm sec 3 4 EXPERIMENT C COLLECTION AND SHIELDING EFFICIENCY OF A ROTATING RING DISK ELECTRODE In this experiment the theoretical and experimental values of the collection efficiency and shielding will be compared for a RRDE 1 Repeat steps 1 8 of Experiment A as amended in steps 1 3 of Experiment B If Experiment C is performed immediately after Experiment B there is no need to replace the solution in the electrolysis cell and one can forego the lengthy deaeration process 32 Model 366A BI POTENTIOSTAT Hardware User s Manual 2 Record the l E curves during the positive potential sweep at the disk electrode and the ring electrode for a rotation speed of 1600 rev min These curves are obtained by procedures given in step 4 of Experiment B The I E curve for the ring electrode is obtained exactly as for the disk electrode except the lead from the K1 jack on the SWEEP GENERATOR is connected to the contact for the ring electrode 3 Prepare the Model 366A for simultaneous potentiostatic control of the ring and disk electrodes Section SWEEP GENERATOR SWEEP GENERATOR SWEEP GENERATOR SWEEP GENERATOR SWEEP GENERATOR SWEEP GENERATOR K1 ELECTRODE K1 ELECTRODE K1 ELECTRODE K2 ELECTRODE K2 ELECTRODE K2 ELECTRODE Control MANUA
56. rter is set on a 16 position rotary switch providing ranges from 1 microamp per volt to 100 milliamps per volt for each electrode Applying an offset voltage to an electrode is easily accomplished by use of a toggle switch and a four digit pushbutton switch allowing a range of 9 999 V accurate to 0 05 The offsets are independently adjustable for each electrode The sweep generator output may be applied to either or both electrodes by actuation of the appropriate toggle switch s A signal from an external source such as a function generator or a computer controller may be applied to either or both electrodes by use of the input jacks in each electrode control section All front panel connections are made to the Model 366A via banana jacks except the reference electrode input which utilizes a BNC shielded connector There is also a ground jack on the front panel which is connected to the Model 366A case and to the ground connection of the power cord This permits easy connection of DC common to the case and in turn to earth ground which in some systems may be advantageous to reduce electrical noise Output jack 11 12 provides a voltage proportional to the difference between the currents flowing in the two electrodes allowing measurements where one can subtract out background signals in identical electrodes Chapter 1 INTRODUCTION 5 Whenever one of the amplifiers in the 366A is nearing saturation output 10 V the overload i
57. rtment for counter electrode medium porosity fritted glass disc liquid level gas dispersion tube Luggin capillary gt coarse porosity Pyrex beaker Fig 10 DESIGN OF SIMPLE ELECTROLYSIS CELL 8 Platinum counter electrode 15 cm length of 22 ga wire is sufficient 9 0 5MH SO 28 mL of reagent grade concentrated acid per liter of solution 10 0 5 mM KI 0 5 M H SO 0 0830 g KI per liter acidic solutions should be deaerated before addition of KI to prevent oxidation of to by dissolved oxygen 11 Cylinder of compressed purified nitrogen for deaeration of solutions 25 3 2 EXPERIMENT A RESIDUAL CURRENT POTENTIAL CURVE FOR Pt ELECTRODES IN ACIDIC SOLUTIONS In Experiment A the current potential I E behavior of a Pt electrode will be observed in an acidic solution containing no added electroactive analyte The observation of the residual I E curve is the customary basis of testing rotating disk and ring disk electrodes for capillary leakage of solution into the seal between the electrode and the surrounding insulating shroud 1 Assemble the electrolysis cell and fill with 0 5 M H SO to the appropriate level Note All glass surfaces should be cleaned prior to use by soaking in hot HNO followed by thorough rinsing with distilled water Fill the counter electrode and reference electrode chambers with 0 5 M H SO The stopcock of the Luggin reference chamber should be wetted with the electro
58. s is indicated by the setting on CURRENT CONVERTER The magnitude and polarity of the disk current can be adjusted by way of the OFFSET VOLTAGE and or the sweep generator keeping in mind the conversion factor indicated by the setting on CURRENT CONVERTER 1 8 4 Voltmeter In the upper left corner of the front panel is 31 digit voltmeter It may be used to display the Sweep Generator output or the E1 or E2 voltages applied to the K1 and K2 electrodes Alternatively it can be used to display the current readings 11 and 12 measured at the K1 and K2 electrodes Potentials are displayed directly in volts while current readings display as a fraction of the Current Converter setting e g a reading of 0 5 on the 50 4A setting corresponds to 25 uA 1 9 SPECIFICATIONS Power 100 125 VAC or 200 250 VAC selectable by voltage selection card 50 60 Hz Weight 12 Ib 5 5 kg Operating temperature 10 deg C to 40 deg C Dimensions 17 in W x 13 in D x 7 1 2 in H 43 cm W x 33 cm D x 19 cm H Sweep rate dual range 0 99 99 mV sec 0 999 9 mV sec set on four digit pushbutton switches Electrode current 1 amp maximum 6 Model 366A BI POTENTIOSTAT Hardware User s Manual Offset voltage 59 999 V set on four digit pushbutton switches Electrode voltage 10 volts Current converters 20 ranges from 0 1 microamp per volt to 200 milliamps per volt Input impedance of reference potential follower 10 ohms Amplifier response
59. species cm sec v kinematic viscosity of solution cm sec w angular velocity of electrode rotation rad sec Equation 17 is known as the Levich equation Chapter 2 THEORY OF ROTATING ELECTRONICS 17 Equation 17 is not valid for w gt because the derivation assumes laminar i e nonturbulent fluid flow at the electrode surface The onset of turbulence occurs for rotational velocities characterized by a Reynolds number Nps of approximately 10 Na is calculated by Equation 18 Na w v 18 where r is the total radius of the electrode plus the non active shroud For a typical electrode with 1 cm and v 10 cm sec the onset of turbulence is expected at w 6 x 104 rad sec 10 rev min Gregory and Riddiford J Chem Soc 3756 1956 compared experimental values of l for systems with electroactive species of known D to values predicted by the Levich equation The experimental values were approximately 5 greater for D v 4 x 10 The Levich derivation was examined and the error concluded to result from the omission of terms other than the first in Equation 11 describing the axial fluid velocity Gregory and Riddiford rederived the equation for including the second term in Equation 11 with the result given by Equation 19 lg 1 1 61 0 57 D v nFAD 9y 205 19 d This correction brings the predicted value to within 1 for D v as large as 4 x 10 Note that for small D v the numerical c
60. tective conductor by use of a plug adapter that doesn t maintain earth ground continuity or by any other means In many parts of the world commonly used power plugs and sockets differ from those in general use in the United States If the power cord plug is not compatible with the available power Sockets the power cord should be replaced with an approved type of compatible design SSCS Warning If it is necessary to replace the power cord the replacement cord must have the same polarity as the original Otherwise a safety hazard from electrical shock which could result in personnel injury or death might result The wires in the supplied power cord are color coded to maintain the correct polarity relationships Whatever the actual plug configuration the black wire should be the line or active conductor also called live or hot the white wire should be neutral and the green wire should be earth ground TNT a ae 1 5 POWER VOLTAGE SELECTION The Model 366A bi potentiostat can operate from either of two different voltage ranges Instruments are ordinarily shipped set up for operation with the power line voltage available in the country in which they are to be used Before plugging in the power cord make sure that the instrument is set for the voltage of the ac power line supply Caution The bi potentiostat may be damaged if it is set for operation from 120 V ac and turned on with 240 V ac applied to the power input connec
61. tor The bi potentiostat is set for use with the line voltage available by changing the position of the removable voltage selector card inside the power entry module on the rear panel If necessary the change from one input voltage range to another can be accomplished in the field Changing the voltage range or changing the line fuse should only be done by a qualified service technician and then only with the instrument disconnected from all sources of power A detailed discussion of how to check and if necessary change the input voltage setting follows Note the clear plastic door next to the power cord connector at the rear of the instrument When the power cord is disconnected from the rear panel connector the plastic door is free to slide to the left giving access to the fuse and to the voltage selector circuit card Chapter 1 INTRODUCTION The voltage selector card can be inserted in either of two different positions according to the power line voltage available In either position a number is visible on the upper surface of the card This number indicates the selected nominal line voltage When the number 120 is visible the bi potentiostat is set for power input voltages between 100 and 125 VAC A setting of 240 is used for voltages between 200 and 250 VAC If the number showing on the card is incorrect for the prevailing line voltage the card will have to be repositioned To change the voltage setting 1 Disconnect th

366A Manual_220893C - Princeton Applied Research

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