PASCO Specialty & Mfg. PASCO CI-6735 User's Manual
Contents
1. 9 Temperature IC iii ta ica delicias 10 Electrode Respotse ainia ii 10 Limits Of Detection sessiossa eara HI REGE RIT NEN RSEN E R d a e Dee eos ents 11 pH Effects ini lili 11 El ctrOde Lite simio oidos alii 11 IM aiMtem ane e 12 Electrode TALE cicle 12 a NO 12 Troubleshooting Guide ooooocoonnnonnonanonanconncnnnconncconononcnncnononnnconaconnconn cono sta nono nono nooo nono ncoon oca rocaon nono nono 12 Glassware Plastic Ware rincones iras 12 Electrode iii ii aia bases 12 Standards amp Reagents i2 eei eder tete etse edo pins opened ebbe ea epus tidie phe lidad ae espe ie petens taken 13 NN 13 E B 13 Troubleshooting Hints assssssssssccscsisceessstsessssdinesostseasarsesecotetaceses bna raa Gba rocas EUEe E Po YER dep Ea caer 14 Technical Support E 17 PASC Ce i serentrfie Copyright Warranty and Equipment Return Please Feel free to duplicate this manual subject to the copyright restrictions below Copyright Notice The PASCO scientific 012 06616A manual is copyrighted and all rights reserved However permission is granted to non profit educational institutions for reproduction of any part of the Nitrate Ion Selective Electrode manual providing the reproductions are used only for their laboratories and are not sold for profit Reproduction under any other circumstances without the written consent of PASCO sci2. Instruction Manual and 012 06616A Experiment Guide for the 9 97 PASCO scientific Model Cl 6735 NITRATE ION SELECTIVE ELECTRODE O 1997 PASCO scientific D A 10101 Foothills Blvd e P O Box 619011 Roseville CA 95678 9011 USA sc f teach science tentifre Phone 916 786 3800 FAX 916 786 8905 web www pasco com Nitrate lon Selective Electrode 012 06616A Always use eye protection and gloves when working with chemicals PASC Oe cl fic 3 Ee 012 06616A Nitrate Ion Selective Electrode Table of Contents ASA ON 1 Bina O E 1 Equipment ridad 3 AA NN 3 Additional Required anidar ita elias abia ite Fee INT rte eI RS 3 P UNIFEM Soltions IM m 3 General Preparativos 4 Electrode Preparation 15er rerit reete etae diana seavaceddcanedsdcapesercbeededunaesacchneseccenienbeasene 4 Electrode Slope Check Using Science Workshop sse eee 4 Measurement f S 3 Measuring Hits tias 5 Sample Requireimients sii ioris eee edu db di 5 Bite BATES dul tdi ita dial ao 5 Measurement Procedure 6 Direct Measurement 1 ee Ie In RU alii NEAR RE A FU PARERE iaa 6 Low Level Nitrate Determination essere nre tret nr ennen etre rre eieiei 7 Electrode C hnaracteristics c eoe eDu reo aE EE 9 Reproducibility EE M E 9 Tinterferen Ces m
3. Carriers will not accept responsibility for damage caused by improper packing To be certain the unit will not be damaged in shipment observe the following rules The packing carton must be strong enough for the item shipped Make certain there are at least two inches of packing material between any point on the apparatus and the inside walls of the carton Make certain that the packing material cannot shift in the box or become compressed allowing the instrument come in contact with the packing carton Address PASCO scientific 10101 Foothills Blvd P O Box 619011 Roseville CA 95678 9011 Phone 916 786 3800 FAX 916 786 3292 email techsupp pasco com web wwwWw pasco com 012 06616A Nitrate Ion Selective Electrode Introduction The PASCO scientific Nitrate lon Selective Electrode is used to quickly simply accurately and economically measure nitrate in aqueous solutions Theory The Nitrate Ion Selective Electrode consists of an electrode body containing a liquid internal filling solution in contact with a gelled organophilic membrane containing a nitrate ion exchanger When the membrane is in contact with a solution containing free nitrate ions an electrode potential develops across the membrane This electrode potential is measured againsta constant reference potential using an ISE Amplifier and Science Workshop Interface The level of nitrate ions corresponding to the measured potential is described by th
4. To prepare this solution half fill a 1 000 ml volumetric flask with distilled water and add 264 grams of reagent grade ammonium sulfate NH S0 Swirl the flask gently to dissolve the solid Fill the flask to the mark with distilled water cap and upend several times to mix the contents ISA is added at the rate of 2 ml of ISA to each 100 ml of standard or sample to adjust the ionic strength to about 0 12 M Reference Filling Solution 0 1 M NH SO 0 01 M KCl Nitrate Standard 0 1 M NaNO To prepare this solution add 8 50 grams of reagent grade sodium nitrate to a one liter volumetric flask about half full of distilled water Swirl the flask gently to dissolve the solid Fill to the mark with distilled water cap and upend several times to mix the solution Nitrate Standard 1 000 ppm NO E To prepare this solution add 1 37 grams of reagent grade sodium nitrate to aone liter volumetric flask about half full with distilled water Swirl the flask gently to dissolve the solid Fill to the mark with distilled water cap and upend several times to mix the solution Nitrate Standard 100 ppm NO E To prepare this solution add 0 61 grams of reagent grade sodium nitrate to a one liter volumetric flask about half full with distilled water Swirl the flask gently to dissolve the solid Fill to the mark with distilled water cap and upend several times to mix the solution PASCO screntiftc Nitrate Ion Selective Electrode 012 06616A
5. in the ambient temperature will necessitate the repetition of steps 2 6 above A new calibration curve should be prepared daily Low Level Nitrate Measurements This procedure is recommended for solutions with ionic strengths less than 10X10 M If the solution is high in ionic strength but low in nitrate use the same procedure but prepare a calibration solution with a composition similar to the sample 1 Using 20 ml of standard ISA dilute to 100 ml with distilled water This low level ISA 0 4 M NH 4 7804 is added at the rate of 1 ml low level ISA to each 100 ml of sample The background ionic strength will be 4 0X 10 M PASC Ce 7 screntitrec Nitrate Ion Selective Electrode 012 06616A 2 Dilute 1 ml of 0 1 M standard to 100 ml to prepare a 1 0X10 MN p solution for measurements in moles per liter Use the 1 000 ppm standard for preparing a 100 ppm NO standard by diluting 10 ml of the 1 000 ppm standard to 100 ml Standards should be prepared fresh daily 3 Add 1 ml of the low level ISA to a 100 ml volumetric flask and fill to the mark with distilled water Pour this solution into a 150 ml beaker and place the beaker on the magnetic stirrer Begin stirring at a constant rate 4 Place the electrode tip in the solution Assure that ScienceWorkshop is operating 5 Add increments of the 1 0X10 M or 100 ppm standards as given in Table 2 below 6 After the reading has stabilized record the Voltage reading in
6. not interfere with the electrode s response to nitrate ions Strongly acidic pH 0 2 and strongly basic pH 12 14 solutions are particularly troublesome to measure The high mobility of hydrogen and hydroxide ions in samples make it impossible to mask their effect on the junction potential with any concentration of equitransferent salt One must either calibrate the electrode in the same pH range as the sample or use a known incremental method for ion measurement 012 06616A Nitrate Ion Selective Electrode Equipment Additional Required Nitrate Ion Selective Electrode Required Equipment Nitrate Ion Selective Electrode fill solution PASCO CI 6738 ISE Ion Selective Electrode Pipette for fill solution Amplifier e ScienceWorkshop 2 2 5 or higher Sodium lon Selective PASCO ScienceWorkshop Computer Electrode Interface e Semi logarithmic 4 cycle graph paper to ISE for preparing calibration curves RUPEE Magnetic stir plate filling NS e Lab ware made of plastic not glass for all low level measurements Required Solutions OCA CO The stock solutions listed in this section may be created as describedinthetextorordered directly p d ne from PASCO The solutions available for order Figure 1 and their respective prices are listed on the ISE Included Equipment Working Solution Price List Deionized or distilled water for solution and standard preparation Tonic Strength Adjuster ISA 2 M NH 4 SO 4
7. 10 ppm nitrate standards Add 2 ml of ISA to each 100 ml of standard Prepare standards with a composition similar to the samples if the samples have an ionic strength above 0 1 M 2 Place the most dilute solution 10 M or 10 ppm in a 150 ml beaker on the magnetic stirrer and begin stirring at a constant rate After assuring that Science Workshop is operating lower the electrode tip into the solution After the reading has stabilized record the Voltage reading in the Digits Display 3 Place the mid range solution IO N or 100 ppm in a 150 ml beaker on the magnetic stirrer and begin stirring After rinsing the electrode with distilled water blot dry and immerse the electrode tip in the solution When the reading has stabilized record the Voltage reading in the Digits Display 4 Place the most concentrated solution 10 M or 1 000 ppm in a 150ml beaker on the magnetic stirrer and begin stirring After rinsing the electrode in distilled water blot dry and immerse the electrode tip in the solution When the reading has stabilized record the Voltage reading in the Digits Display 5 Using the semi logarithmic graph paper plot the Voltage reading in the Digits Display linear axis againstthe concentration log axis Extrapolate the calibration curve down to about 1 0X10 M 1 0 ppm NO 5 A typical calibration curve can be found in Figure 3 012 06616A Nitrate Ion Selective Electrode 30 70 110 10 fold chan
8. General Preparation Electrode Preparation 1 Electrode Slope Check Using Science Workshop check electrodes each day T Remove the rubber cap covering the electrode tip Slide the rubber sleeve down away from the filling hole of the Nitrate Ion Selective Electrode Fill the electrode with the included filling solution to a level just below the fill hole Slide the rubber sleeve back over the filling hole Figure 2a Gently shake the electrode downward in the same manner as aclinical thermometerto remove any air bubbles that may betrappedbehindthenitrate membrane Prior to first usage or after long term storage immerse thenitrate membrane in nitrate standard for thirty minutes P Insert DIN The electrode is now ready for use A connector into analog channel Connect the Nitrate Ion Selective Electrode to the ISE A or B Amplifier and insert the DIN connector of the ISE Amplifierinto analog port A or B onaPASCO Computer Interface Figures 2b and 2c Rotate ring one quarter ae D c To a 150 ml glass beaker add 100 ml of distilled water turn to secure gi p A f and 2 ml of ISA Place the beaker on a magnetic stirrer and begin stirring at a constant rate Start the ScienceWorkshop software select the Ion Selective ISE Amplifier Electrode sensor open a Digital display change the electrode connector number of digits to the right of the decimal from 1 to 3 and begin monitoring data Lower the elec
9. cal Support PASC screntitrec Nitrate Ion Selective Electrode 012 06616A Troubleshooting Hints Symptom Out of Range Reading Noisy or Unstable Readings readings continuously or rapidly changing Drift reading slowly changing in one direction 14 Possible Causes defective electrode electrodes not plugged in properly reference electrode not filled air bubble on membrane electrode not in solution electrode exposed to interferences defective electrode ISA not used stirrer not grounded air bubble on membrane outer filling solution level too low samples and standards at different temperatures electrode exposed to interferences incorrect reference filling solution Next Step check electrode operation unplug electrodes and reseat electrodes be sure reference electrode is filled remove bubble by re dipping electrode put electrodes in solution soak electrode in nitrate standard replace electrode use recommended ISA ground stirrer remove bubble by re dipping electrode fill electrode to level just below the fill hole allow solutions to come to room temperature before measurement soak electrode in nitrate standard use recommended filling solution 012 06616A Nitrate Ion Selective Electrode Symptom Low Slope or No Slope Incorrect Answer but calibration curve 1s good PASC Ce scientific Possible Causes standards contaminated or incorre
10. ctly made ISA not used standard used as ISA electrode exposed to interferences defective electrode air bubble on membrane incorrect scaling of semilog paper incorrect sign incorrect standards wrong units used sample carryover Next Step prepare fresh standards use recommended ISA use ISA soak electrode in nitrate standard check electrode operation remove bubble by re dipping probe plot voltage potential on the linear axis On the log axis be sure concentration numbers within each decade are increasing with increasing concentration be sure to note sign of millivolt reading correctly prepare fresh standards apply correct conversion factor 10 M 62 0 ppm as NO 10 M 85 0 ppm as NaNO rinse electrode thoroughly between samples 15 Nitrate lon Selective Electrode 012 06616A 16 Technical Support Feedback If you have any comments about the product or manual please let us know If you have any suggestions on alternate experiments orfinda problem in the manual please tell us PASCO appreciates any customer feedback Y our input helps us evaluate and improve our product To Reach PASCO For technical support call us at 1 800 772 8700 toll free within the U S or 916 786 3800 fax 916 786 3292 e mail techsupp pasco com web WWW pasco com Contacting Technical Support Before you call the PASCO Technical Support staff 1t would be helpful to pre
11. e Nernst equation E E S log X where E measured electrode potential E reference potential a constant decade X level of nitrate ions in solution S electrode slope 55 The activity X represents the effective concentration of the ions in solution The total nitrate ion concentration C is the sum of free nitrate ion C p and complexed or bound perchlorate ion C The electrode is able to respond to only the free ions whose concentration is Since nitrate ions form very few stable complexes the free ion concentration may be equated to the total ion concentration The activity is related to the free ion concentration C p by the activity coefficient y by x YyC Activity coefficients vary depending on total ions strength I defined as I 5 3625 PASC Ce screntitrec Nitrate Ion Selective Electrode 012 06616A where C concentration of ion X Zy charge of ion X Y sum of all of the types of ions in the solution In the case of high and constant ionic strength relative to the sensed ion concentration the activity coefficient y is constant and the activity X is directly proportional to the concentration To adjust the background ionic strength to a high end constant value ionic strength adjuster ISA is added to samples and standards The recommended ISA for nitrate is NH 50 Solutions other than this may be used as ionic strength adjusters as long as ions that they contain do
12. entific is prohibited Limited Warranty PASCO scientific warrants the product to be free from defects in materials and workmanship for a period of one year from the date of shipment to the customer PASCO will repair or replace at its option any part of the product which is deemed to be defective in material or workmanship The warranty does not cover damage to the product caused by abuse or improper use Determination of whether a product failure is the result of a manufacturing defect or improper use by the customer shall be made solely by PASCO scientific Responsibility for the return of equipment for warranty repair belongs to the customer Equipment must be properly packed to prevent damage and shipped postage or freight prepaid Damage caused by improper packing of the equipment for return shipment will not be covered by the warranty Shipping costs for returning the equipment after repair will be paid by PASCO scientific Credits Author Peter Boyle Editor Steve Miller 11 Equipment Return O Should the product have to be returned to PASCO scientific for any reason notify PASCO scientific by letter phone or fax BEFORE returning the product Upon notification the return authorization and shipping instructions will be promptly issued NOTE NO EQUIPMENT WILL BE ACCEPTED FOR RETURN WITHOUT AN AUTHORIZATION FROM PASCO When returning equipment for repair the units must be packed properly
13. ge electrode potential 150 56mV mV 190 230 lt ppm as N 0 14 1 10 100 1 000 _ _ _ te ppm as NO a 0 1 1 10 100 1 000 10 6 10 5 10 4 10 3 10 2 10 1 NO concentration M Figure 3 Typical nitrate electrode calibration curve Note A calibration curve is constructed on semi logarithmic paper The measured electrode potential in linear axis is plotted against the standard concentration log axis In the linear region of the curve only three standards are necessary to determine a calibration curve In the non linear region additional points must be measured The direct measurement procedures given are for the linear portion of the curve The non linear portion of the curve requires the use of low level procedures 6 Toaclean dry 150 ml beaker add 100 ml ofthe sample and 2 ml of ISA Place the beaker on the magnetic stirrer and begin stirring at a constant rate Rinse the electrode tip with distilled water blot dry and lower the electrode tip into the solution When thereading has stabilized record the Voltage reading in the Digits Display Using the calibration curve determine the sample concentration 7 The calibration should be checked every two hours Assuming no change in ambient temperature place the electrode tip in the mid range standard After the reading has stabilized compare it to the original reading recorded in step 3 above A reading differing by more than 0 5mV or a change
14. h 1 m Troubleshooting Guide 12 The goal of troubleshooting is the isolation of a problem through checking each of the system components in turn the meter the plastic ware the electrode the standards amp reagents the sample and the technique Glassware Plastic ware Clean glassware is essential for good measurement Be sure to wash the glassware plastic ware well with a mild detergent and rinse very well with distilled or deionized water Electrode The electrode may be checked by using the procedure found in the sections entitled Electrode Slope Check 1 Be sure to use distilled or deionized water when following the procedures given in Electrode Slope Check 2 If the electrode fails to respond as expected see the section Measuring Hints Repeat the slope check 3 If the electrode still fails to respond as expected substitute another Nitrate Ion Selective Electrode if available that is known to be in good working order for the questionable electrode Ifthe problem persists and you are using an electrode pair try the same routine with a working reference electrode PASC entific 012 06616A Nitrate Ion Selective Electrode 4 Ifthe problem persists the standards or reagent may be of poor quality interferences in the sample may be present or the technique may be faulty See Standards Reagents Sample and Technique sections below 5 If another electrode is not available for test purposes or if the electrode in u
15. ndependent of concentration within the electrode s operating range Interferences Certain anions are electrode interferences and will cause electrode malfunction drift or measurement errors if present in high enough levels The level of interfering common anions that will cause a 10 error at three levels of nitrate is given in Table 3 TABLE 3 Concentration of Possible Interference s Causing a 10 Error at Various Levels of Nitrate Background lonic Strength of 0 12 M NH SO Interference moles liter 10 M 10 M 10 M B 3 0X10 3 0X10 3 0X10 NO 7 0X107 7 0X10 7 0X107 Br TOXIQ 7 0X10 7 0X10 CN 1 0X107 1 0X10 LXI CIO 5 0X10 5 0X10 5 0X10 y 5 0X10 5 0X10 5 0X10 CIO 1 0X10 1 0X10 1 0X10 Interference moles liter 100 ppm N 10 ppm N 1 ppm N Gr 7 600 760 76 0 NO 230 23 0 2 30 Br 400 40 0 4 00 CN 20 0 2 00 0 20 clo 30 0 3 00 0 30 p 4 00 0 40 0 04 CIO 0 07 0 007 0 0007 Interferences such as chloride bromide iodide and cyanide can be removed by precipitation with 0 5 grams of silver sulfate added to 100 ml of sample Nitrite interference can be removed by adding 0 3 grams of sulfamic acid to 100 ml of sample Carbonate and bicarbonate which are weak interferences can be removed by acidifying the sample to pH 4 5 with sulfuric acid Organic carboxylic anions hinder the nitrate electrode response and can be removed by adding 10 grams of aluminum sulfate to 100 ml of sample PASCO 9
16. oncentration unit Table 1 indicates some of the concentration units TABLE 1 Concentration Unit Conversion Factors ppm NaNO ppm NO moles liter 850 00 620 00 1 0 X 107 85 0 62 0 1 0 X 10 8 5 6 2 1 0X 101 FAS C Ce 5 Sorentrfrne Nitrate Ion Selective Electrode 012 06616A Measurement Procedure Direct Measurement Direct measurement is a simple procedure for measuring a large number of samples A single reading is all that is required for each sample The ionic strength of samples and standards should be made the same by adjustment with ISA for all nitrate solutions The temperature of both sample solution and standard solutions should be the same Direct Measurement gt Note A calibration curve is constructed on semilogarithmic paper The measured electrode potential linear axis is plotted against the standard concentration log axis In the linear region of the curve only two standards are necessary to determine a calibration curve Calibration solutions close to the anticipated value of the unknown should be chosen In the nonlinear region additional points must be measured The direct measurement procedures given are for the linear portion of the curve The nonlinear portion of the curve requires the use of low level procedures 1 By serial dilution prepare three standard solutions from the 0 1 M or 1 000 ppm stock standard The resultant concentrations should be 10 10 and 107M or 1 000 100 and
17. otential against the nitrate concentration on semi logarithmic paper results in a straight line with a slope of about 55 mV decade Refer to Figure 3 The time needed to reach 99 of the stable electrode potential reading the electrode response time varies from one minute or less in highly concentrated solutions to several minutes near the detection limit Refer to Figure 4 10 IPAS Cg 012 06616A Nitrate Ion Selective Electrode 50 103M to 102M NaNO 75 100 electrode 125 potential 159 A mV 10 M to 104M NaNO 175 200 103M to 10M NaNO 4225 103M to 10M NaNO 1 2 3 4 time minutes Figure 4 Typical electrode time response to step changes in NaNO3 Limits of Detection The upper limit of detection in pure sodium nitrate solutions is 1 M In the presence of other ions the upper limit of detection is above 10 M nitrate but two factors influence this upper limit Both the possibility ofa liquid junction potential developing at the reference electrode and the malt extraction effect influence this upper limit Some salts may infuse into the electrode membrane at high salt concentrations causing deviation from the theoretical response Either dilute samples between 1 M and 10 M or calibrate the electrode at 4 or 5 intermediate points The lower limit of detection is influenced by the slight water solubility of the ion exchanger used in the sensing portion of the electrode Refer to Fig
18. pare the following information gt If your problem is with the PASCO apparatus note Title and model number usually listed on the label Approximate age of apparatus A detailed description of the problem sequence of events in case you can t call PASCO rightaway you won tlose valuable data If possible have the apparatus within reach when calling to facilitate description of individual parts gt If your problem relates to the instruction manual note Part number and revision listed by month and year on the front cover Have the manual at hand to discuss your questions
19. screntiftec Nitrate Ion Selective Electrode 012 06616A The above interference removal procedures require similar treatment of standards as well as samples If the electrode is exposed to high levels of interfering ions which cannot be removed the electrode reading may drift and the response may become sluggish Restore performance by soaking in distilled water for 30 minutes followed by soaking in nitrate standard for 30 minutes Temperature Influences Samples and standards should be at the same temperature since electrode potentials are influenced by changes in temperature A 1 C difference in temperature results in a 2 error at the 10 M level Because of the solubility equilibria on which the electrode depends the absolute potential of the reference electrode changes slowly with temperature The slope of the nitrate electrode as indicated by the factor S in the Nernst equation also varies with temperature Table 4 gives values for the S factor in the Nernst equation for the nitrate ion The operating range of the nitrate ion electrode is 0 C 40 C provided that temperature equilibrium has occurred Ifthetemperature varies substantially from room temperature equilibrium times up to one hour are recommended TABLE 4 Temperature vs Theoretical Values for the Electrode Slope Temperature C Sg 0 5420 10 56 18 20 58 16 25 59 16 30 60 15 40 62 13 50 64 11 Electrode Response Plotting the electrode p
20. se is suspect review the instruction manual and be sure to Clean and rinse the electrode thoroughly Prepare the electrode properly Usethe proper filling solution Adjust the pH and the ionic strength of the solution by the use of the proper ISA Measure correctly and accurately Review Troubleshooting Hints Standards amp Reagents Whenever problems arise with the measuring procedure that has been used successfully in the past be sure to check the standard and reagent solutions Ifin doubt about the credibility of any ofthe solutions prepare them again Errors may result from contamination of the ISA incorrect dilution of standards poor quality distilled deionized water or a simple mathematical miscalculation Sample Look for possible interferences complexing agents or substances which could affect the response or physically damage the sensing electrode if the electrode works perfectly in the standard but not in the sample Try to determine the composition of the samples prior to testing to eliminate a problem before it starts See Measuring Hints Sample Requirements and Interferences Technique Be sure that the electrode s limit of detection has not been exceeded Be sure that the analysis method is clearly understood and is compatible with the sample Refer to the instruction manual again Reread General Preparation and Electrode Characteristics If trouble still persists call PASCO scientific Techni
21. the Digits Display after each addition TABLE 2 Step wise Calibration for Low Level Nitrate Measurements Added Step Pipette Volume mL 1 A 0 1 2 A 0 1 3 A 0 2 4 A 0 2 5 A 0 4 6 B 2 0 7 B 2 0 Pipette A 1 ml graduated pipette Pipette 8 2 ml pipette Concentration MNO ppmNO 10X 10 0 1 2 0 X 10 0 2 4 0 X 10 0 4 6 0 X 10 0 6 9 9 X 10 1 0 29X10 2 9 4 8 X 10 4 8 Solutions additions of 1 0X10 M or 100 ppm standard to 100 ml of ISA as prepared in step 3 above 7 Onsemi logarithmic graph paper plot the voltage reading on the Digits display linear axis against the concentration log axis as in Figure 3 8 Rinse the electrode and blot dry 9 Measure out 100 ml of the sample into a 150 ml beaker add 1 ml of low level ISA Place the beaker on the magnetic stirrer and begin stirring Lower the electrode tip into the solution After the reading has stabilized record the Voltage reading indicated in the Digits Display and determine the concentration from the low level calibration curve 10 Prepare anew low level calibration curve daily Check the calibration curve every two hours by repeating steps 2 7 012 06616A Nitrate Ion Selective Electrode Electrode Characteristics Reproducibility Electrode measurements reproducible to 2 can be obtained if the electrode is calibrated every hour Factors such as temperature fluctuations drift and noise limit reproducibility Reproducibility is i
22. trode tip into Figure 2 the solution Equipment Setup a filling the electrode with filling solution b amp c connecting the Using a pipette add 1 ml of 0 1 M or 1 000 ppm nitrate electrode to the ISE Amplifier and to the standard to the beaker When the reading has stabilized computer interface record the voltage reading indicated in the Digits display Using a pipette add 10 ml of the same nitrate standard used above to the beaker When the reading has stabilized record the voltage reading indicated in the Digits display Determine the difference between the two readings A difference of 55 2 mV indicates correct electrode operation assuming the solution temperature is between 20 C and 25 C See the Troubleshooting sections if the potential change is not within this range gt Note Slope is defined as the change in potential observed when the concentration changes by a factor of 10 PASCO entific 012 06616A Nitrate Ion Selective Electrode Measurement Measuring Hints e All samples and standards should be at the same temperature for precise measurement A difference of 1 C in temperature will result in a 2 measurement error The sensing membrane is normally subject to water uptake and might appear milky This has no effect on performance Constant but not violent stirring is necessary for accurate measurement Magnetic stirrers can generate sufficient heat to change the solution temperature To co
23. unteract this effect place a piece of insulating material such as Styrofoam sheet between the stirrer and beaker Alwaysrinse the electrode with distilled water and blot dry between measurements Use a clean dry tissue to prevent cross contamination e For samples with high ionic strength prepare standards whose composition is similar to the sample Always check to see that the membrane is free from air bubbles after immersion into standard or sample A slow responding electrode may be caused by interferences to the electrode To restore proper performance soak the electrode in distilled waterfor about 5 minutes to clean the membrane rinse and soak in standard solution for about 5 minutes Sample Requirements All samples must be aqueous and not contain organics which can dissolve in the membrane or extract out the liquid ion exchanger The temperature ofthe standard solutions and of the sample solutions should be the same and below 40 C About a 2 error will be introduced for a 1 C difference in temperature nterference s should be absent If they are present use the procedures found in the Interference s section to remove them e The pH range for the nitrate ion electrode is 2 5 11 Neutralize samples outside this range with acid or base to bring them in range Units of Measurement Nitrate concentrations are measured in units of ppm as sodium nitrate ppm as nitrate moles per liter or any other convenient c
24. ure 3 for a comparison of the theoretical response to the actual response at low levels of nitrate Nitrate measurements below 10 MN 0 0 6 ppm as NO should employ low level procedures pH Effects The operating range of the nitrate electrode is from pH 2 5 to pH 11 Electrode Life The nitrate electrode will last six months in normal laboratory use On line measurement might shorten operational lifetime to several months In time the response time will increase and the calibration slope will decrease to the point calibration is difficult and electrode replacement is required PASC Ca screntifttec Nitrate Ion Selective Electrode 012 06616A Maintenance Electrode Storage The nitrate electrode may be stored for short periods of time in 10 M nitrate solution For longer storage longer than two weeks rinse and dry the nitrate membrane and cover the tip with any protective cap shipped with the electrode The reference portion of the combination electrode or the outer chamber of the reference electrode should be drained of filling solution if refillable and the rubber insert placed over the filling hole Specifications Concentration Range 1 M to 7X10 M 6 2X10 to 0 5 ppm as N 0 7 pH Range 2 5 to 11 Temperature Range 0 C 40 C Resistance 100M ohms Reproducibility 2 Samples aqueous solutions only no organic solvents Storage store in dilute nitrate solution Size 110 mm length 12 mm diameter cable lengt
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