PASCO Specialty & Mfg. CI-6727 User's Manual
Contents
1. Instruction Manual and 012 06601A Experiment Guide for the dl PASCO scientific Model C1 6727 CALCIUM 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 Calcium Ion Selective Electrode 012 06601A Always use eye protection and gloves when working with chemicals PASC Oe cl fic 3 Ee 012 06601A Calcium Ion Selective Electrode Table of Contents lundi ON 1 Bina 1 Equipment eS 2 Min M EAE 2 Additional Required oce terit ees rere er re tesa FE eee Fe tadas 2 Required SOLMPLOUS xsssesssecsocssecsseessisssonsssccesedants cosniies stahss rooson Soseo SE S Sros Srs ENS SESSO EESO SSSss Spss 2 General Preparation 5a diei esci tice diia ii 3 Electrode Preparation 1e ertet rrt entero terat oen adeps e ae a aeii 3 Electrode Slope Check Using Science Workshop eese 3 MEA A 4 Measuring Hints visit an aia 4 Sample Requirements eiii liinda ede ignes edt 4 Mar NAAA A ta a Iaia Gerent tenes 4 5 Measurement Procedure eeiee erts eee etse etta aeo eb a ae o esa rro roon Fe so sao re no sa UR e soU OE Re so eR Ero EUR a 5 Direct Measurement ee diles aiii ua AS ERRRR NI a A aaie 5 Low Level Calcium Determination eese enne enne eterne nennen nn 6 H lle CA 7 Titration OF oci2. for a comparison of the theoretical response to actual response at low levels of calcium chloride pH Effects The operating range of the calcium electrode is from pH 3 to pH 10 Use at other pH values can adversely affect the membrane Hydrogen ion interferes with measurements of very low levels of calcium Hydroxide ion will complex calcium ions Electrode Life Thecalciumelectrode 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 Electrode Storage The calcium electrodes may be stored for short periods of time in 1 0 X 10M calcium standard For longer storage longerthan two weeks rinse and dry the calcium membrane and cover the tip with any protective cap shipped with theelectrodes Thereference portion ofthe combination electrode or the outer chamber ofthe reference electrode should be drained of filling solution if refillable and the rubber insert placed over the filling hole PASC Ca i scentifrec Calcium Ion Selective Electrode 012 06601A Troubleshooting Guide The goal oftroubleshooting is the isolation ofa problem through checking each ofthe system components in turn the glassware the electrodes the standards and reagents the sample and the technique GlassvvarelPlastic vvare Clea
3. 7 8 Electrode Characteristics o o epne oes p notes eee es Yo E ees ePDE anaes a SPESE Sue ea da 9 Reproducibility sestese dida eet edo pee das eibi digo 9 iu e PD 9 ieoupSclm T 10 Temperature Influences d did 10 Electrode RESPONSE ueteri petitur ett tee eiae eene epi cepe er phe nee tee bnt ee e dari cep essa pass 11 Limits of Detecta aiii ree RO FU eee E v an niente 11 siio m R 11 Electrode bi c HO 11 Electrode Storage iii A A a E A ii bt 11 TroubleshoGtio GUNG cR 12 Glassware Plastic wate eite eire p eae ite dte mt misteris ts 12 Electr nica aii 12 Standards amp Reapents nicas di dic isa 12 RR 12 Tec US te 13 Troubleshooting Hints 0020002 13 Technical SUpporti ninia iio einen SAEC LY R EEUU ES ESO SERA E i ent emet 15 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 06601A manual is copyrighted and all rights reserved However permission is granted to non profit educational institutions for reproduction of any part of the Calcium Ion Selective Electrode manual providing the reproductions are used only for their laboratories and are not sold for profit Re
4. that has been used successfully in the past be sure to check the standard and reagent solutions Ifin doubt about the credibility of any of the reagents prepare them again Errors may result from contamination ofthe ISA incorrect dilution ofthe standards poorquality 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 electrodes work 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 12 PAS 0 entific 012 06601A Calcium Ion Selective Electrode 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 Refertothe instruction manual again particularly the General Preparation and Electrode Characteristics sections If trouble still persists call PASCO Technical Support Troubleshooting Hints Symptom Possible Causes Out of Range Reading defective electrode electrodes not plugged in properly reference electrode not filled PASC Ce scientific Next Step check electrode operation unplug electrodes and reseat electrodes be sure reference electrode is filled 1
5. 07 012 06601A Calcium Ion Selective Electrode Electrode Response Plottingthe electrode potential against the calcium concentration on semi logarithmic paper results in a straight line with a slope of about 26 mV per decade Refer to Figure 3 Thetimeneeded to reach 99 ofthe stable electrode potential reading the electrode responsetime varies from one minute or less for calcium concentration above 1 0 X 10M to several minutes near the detection limit Refer to Figure 5 450 440 10 M to 10 M CaCl electrode potential 20 mY 10 0 102 M to 10 10 3 xS P 10 M to 108 M CaCl 1 2 3 4 F time minutes Figure 5 Typical electrode time response to step changes in CaCl Limits of Detection The upper limit of detection in pure calcium chloride solutions is 1M In the presence of other ions the upper limit of detection is above 1 0 X 10 M but the possibility of a liquid junction potential developing at the reference electrode and the salt extraction effect are two limiting factors Some salts may infuse into the electrode membrane at high salt concentrations causing deviation from theoretical response Calibrate the electrode at four or five intermediate points or dilute the sample to measure samples between 1 0 X 10M and 1M The lovver limit of detection is influenced by the slight vvater solubility of the ion exchanger used in the sensing portion of the electrode Referto Figure
6. 3 Calcium Ion Selective Electrode 012 06601A 14 Technical Support Feedback If you have any comments about the product or manual pleaseletusknow Ifyou have any suggestions on alternate experiments or find a problem in the manual please tell us PASCO appreciates any customer feedback Your 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 prepare 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 right away you won t lose valuable data If possible have the apparatus within reach when calling to facilitate description of individual parts 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
7. PASCO When returning equipment for repair the units must be packed properly 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 www pasco com 012 06601A Calcium Ion Selective Electrode Introduction The PASCO Calcium Ion Selective Electrode is used to quickly simply accurately and economically measure calcium concentration in aqueous solutions Theory The Calcium Ion Selective Electrode consists of an electrode body containing an ion exchanger in a sensing module This sensing module contains a liquid internal filling solution in contact with a gelled organophilic membrane containing a calcium ion selective exchanger An electrode potential develops across the membrane When the membrane is in contact with a calcium solution Measurement of this potential against a constant reference potential
8. Ton eles Lab ware made of plastic not glass for all low level Electrode fill solution measurements Required Solutions The stock solutions listed in this section may be created as described pipette for fill solution Additional Required Betiere Jupe ent in the text or ordered directly form PASCO The solutions available e PASCO CI 6738 ISE lon for order and their respective prices are listed on the ISE Working SelectiveElectrode Amplifier Solution Price List e Science Workshop 2 25 or higher Deionized or distilled water for solution and standard preparation iE Calcium Chloride solution 0 1M CaCl to Amplifier desd To prepare this solution half fill a one liter volumetric flask with distilled water and add 14 7 grams of reagent grade calcium chloride CaC1 2H 0 Swirl the flask gently to dissolve the solid Fill to the mark with distilled water cap and upend several times to mix the solution Calcium Chloride solution 1000 ppm Ca Topreparethis solution half fill a one liter volumetric flask with distilled water and add 3 67 grams of reagent grade calcium chloride CaC1 2H 0 Swirl the flask gently to dissolve the solid Fill to the mark with distilled water cap filling Son S and upend several times to mix the solution Calcium Chloride solution 100 ppm Ca as CaCO To prepare this solution half fill a one liter volumetric flask p d with distilled water and add 0 15 grams o
9. a measured sample until neither active species reagent or sample is in excess Ion selective electrodes are excellent endpoint detectors since they are not influenced by solution color or turbidity Though titration is more time consuming than direct measurement it is about 10 times more accurate Titration of Calcium The method outlined in this section makes use of the calcium ion electrode as a highly sensitive endpoint detector for calcium containing samples The titrant used is EDTA EDTA complexes calcium as well as other cations The sample pH can be adjusted to pH 10 by adding ammonia to eliminate unwanted ion complexes Masking agents can be added in some cases 1 Soal the calcium ion electrode tip in 10 M or 100 ppm calcium standard solution for a minimum of one hour prior to use PASCI scentifre Calcium Ion Selective Electrode 012 06601A 2 Prepare the stock EDTA titrant as given in the section Required Solution Dilute the EDTA to 10 to 20 times as concentrated as the suspected sample concentration The sample should contain at least L0XIO M calcium for a good detection of the endpoint 3 Filla 50 ml buret with the EDTA solution Pipette 100 ml ofthe sample into a 150 ml beaker place the beaker on the magnetic stirrer and begin stirring at a constant rate Adjust the sample to pH 10 by adding ammonia 4 Position the buret tip in the beaker slightly above the liquid level in the beaker and slightly off cente
10. ading has stabilized record the voltage reading indicated in the Digits display 4 Determine the difference between the two readings The electrode is operating correctly if the potential has changed by 26 2 mV assuming the temperature is between 20 C and 25 C Seethe 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 Measurement Measuring Hints All samples and standards should be at the same temperature for precise measurement A difference of 1 C in temperature will result in a 4 measurement error Thesensing 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 counteract this effect place a piece of insulation material such as a styrofoam sheet between the stirrer and beaker Alwaysrinsethe electrodes with distilled water and blot dry between measurements Use a clean dry tissue to prevent cross contamination Forsamples with high ionic strength prepare standards whose composition is similar to the sample e Always check to see that the membrane is free from air bubbles after immersion into the standard or sample A slow responding electrode may be caused by interfere
11. asurement is a simple procedure for measuring a large number of samples A single meter reading is all that is required for each sample The ionic strength of samples and standards should be made approximately the same by adjustment with ISA The temperature of both sample solution and standard solution should be the same Direct Measurement of Calcium Note A calibration curve is constructed on semi logarithmic 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 standards close to the anticipated value of the unknown should be chosen 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 1 By serial dilution prepare three standard solutions from the 0 1 M 100 ppm or 100 ppm standard The resultant concentrations should be 10 M 10 M and 10 M or 100 10 and 1 ppm standards Add 2 ml of ISA to each 100 ml of standard 2 Place the most dilute solution 1 0 X 10M or 1 ppm ina 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 When the reading has stabilized record the voltage read
12. er blot dry and lower the electrode tip into the solution When the reading has stabilized record the voltage reading indicated 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 immerse 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 5 mV or a change in the ambient temperature will necessitate the repetition of steps 2 5 above A new calibration curve should be prepared daily Low Level Calcium Determination This procedure is recommended for solutions with ionic strengths less than 1 0 X 10 M Ifthe solution is high in ionic strength but low in calcium use the same procedure but prepare a calibration solution with a composition similar to the sample 1 Dilute 10mlofthe 0 1M standardto 1000 mlto preparea 1 0 X 1 0 M standard solution for measurements in moles per liter Dilute 10 ml ofthe 1000 ppm or 1 ml ofthe 100 ppm standard to 1000 ml to prepare a 10 ppm standard solution for measurements in ppm 2 Soak the calcium electrode for at least 1 hour in 1 0 X 10 or 100 ppm calcium standard solution 3 Toa 150 ml beaker add 100 ml of distilled water Place the beaker on the magnetic stirrer and begin stirring at a constant rate 4 Place the electrode tips in the solution Ass
13. f reagent grade calcium chloride CaCL 2H 0 Swirl the flask gently to dissolvethe solid Filltothe mark with distilled water cap filling pipette Figure 1 Included Equipment 2 FASC Qu scientific and upend several times to mix the solution 012 06601A Calcium Ion Selective Electrode lonic Strength Adjuster ISE 4 M KCL To prepare this solution half fill a 1000 ml volumetric flask with distilled water and add 298 grams of reagent grade potassium chloride KCI Swirl the flask gently to dissolve the solid Fill to the mark with distilled water cap and upend several times to mix the solution EDTA titrant 1 M stock solution for the titration of calcium To prepare this titrant add 37 2 grams of reagent grade Na EDTA 2HL0 ethylenediaminetetraacetic acid dihydrate disodium salt to a 100 ml volumetric flask add about 75 ml ofdistilled water and swirl the flask gently to dissolve the solid Fill to the mark with distilled water cap and upend several times to mix the solution General Preparation Electrode Preparation 1 Remove the rubber cap covering the electrode tip Slide yo SEE the rubber sleeve down away from the filling hole ofthe N Calcium Selective Ion 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 sleeve rubber cap Figure 2a 2 Gentlyshaketheelectrode downwardin the same manner a
14. ing indicated in the Digits display 3 Place the mid range solution 1 0 X 1 0 Mor 10 ppm in a 150 ml beaker on the magnetic stirrer and begin stirring Add 2 ml of ISA After rinsing the electrodes with distilled water blot dry and immerse the electrode tips in the solution When the reading has stabilized record the voltage reading indicated in the Digits display 4 Placethe most concentrated solution 1 0 X 10 M or 100 ppm ina 150 ml beaker on the magnetic stirrer and begin stirring After rinsing the electrodes with distilled water blot dry and immerse the electrode tip in the solution When the reading has stabilized record the voltage reading indicated in the Digits display 5 Using the semi logarithmic graph paper plot the voltage reading linear axis against the concentration log axis Extrapolate the calibration curve down to about 2 0 X 10 M A typical calibration curve can be found in Figure 3 FAS C Qu 5 screntifttec Calcium Ion Selective Electrode 012 06601A 100 10 fold change 60 PIPPCITPTPPPPEPTPITE z 126 mV electrode 20 potential mV 20 60 ooo 1 10 100 1000 10 5 10 4 10 3 10 2 10 1 Ca concentration M Figure 3 Typical calcium electrode calibration curve 6 Toaclean dry 150 ml beaker add 100 ml ofthe sample 2 ml of ISA and 2 ml of ISA Place the beaker on the magnetic stirrer and begin stirring at a constant rate Rinse the electrode with distilled wat
15. n glassware is essential for good measurement Be sure to wash the glassware plasticware well with a mild detergent and rinse very well with distilled or deionized water Clean glassware will drain without leaving water droplets behind Electrode The electrodes 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 Iftheelectrode fails to respond as expected see the sections Measuring Hints and Electrode Response Repeat the slope check 3 Ifthe electrode still fails to respond as expected substitute another calcium electrode that is known to be in good working order for the questionable electrode 4 Ifthe problem persists the reagent may be of poor quality interferences in the sample may be present or the technique may be faulty See Standards amp Reagents Sample and Technique sections below 5 If another electrode is not available for test purposes or if the electrode in use is suspect review the instruction manual and be sure to Clean and rinse the electrodes thoroughly Prepare the electrodes properly Use the 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
16. nces to the electrode To restore proper performance soak the electrode in distilled water for about 5 minutes to clean the membrane rinse and soak in diluted standard solution for abut 5 minutes Dilute concentrated samples over 0 1M before measurement Recalibrate every few hours for routine measurement Sample Requirements All samples must be aqueous and not contain organics which can dissolve the membrane or extract out the liquid ion exchanger Thetemperature of the standard solutions and ofthe sample solutions should be the same and below 40 C About a 2 error in the slope will occur for each 1 C difference in temperature The pH range for the calcium ion electrode is 3 10 Neutralize samples outside this range with NaOH or HCI to bring them in range Interferences should be absent If they are present use the procedure found in the Interferences and Electrode Response sections to remove them Units of Measurement Calcium concentrations are measured in units of ppm as calcium ppm as CaCO moles per liter or any other convenient concentration unit Table 1 indicates some concentration units and conversion factors 4 IPASC Ci crentific 012 06601A Calcium Ion Selective Electrode TABLE 1 Concentration Unit Conversion Factors ppm Ca ppm CaCO moles liter 4 01 10 0 ix 10 10 00 24 9 2 5 X 10 40 10 100 1 1 0X10 400 80 1000 9 1 0 X 10 Measurement Procedure Direct Measurement Direct me
17. omplexation Sulfate bicarbonate and carbonate are the most common species that complex calcium ions The level of calcium ions the level of the complexing ion the pH of the solution and the total ionic strength of the solution determine the extent of the complexation Complexation reduces the free calcium ion concentration and since the electrode responds only to free calcium ions a false reading results To avoid formation of CaSO the sulfate concentrations must be less than 5X 1 0 M 50 ppm Toavoid formation of CaCO or formation ofthe CaHCO complex the pH ofthe solution should be less than 7 and the total carbonate bicarbonate concentration should be less than 3X10 M 280 ppm carbonate Temperature Influences Samples and standards should be at the same temperature since electrode potentials are influenced by changes in temperature A I C difference in temperature results in a 4 error at the 1 0 X 10 M level Provided that temperature equilibria has occurred the calcium electrodes can be used at temperatures from 0 40 C Room temperature measurements are recommended since measurements at temperatures quite different from room temperature may require equilibrium times up to one hour Table 4 indicates the variation of the theoretical slope with temperature TABLE 4 Temperature vs Theoretical Values for the Electrode Slope Temp C 0 10 20 25 30 40 S 27 10 28 09 29 08 29 58 30 07 31
18. on cautions that if present in high enough levels will cause electrode interferences and measurement errors or electrode drift when using the calcium electrodes Electrode drift and slow response could indicate the presence of high interferences from the ions listed Soak the electrode in distilled water for five minutes then for five minutes in calcium standard solution to restore proper response Table 3 Concentration of Possible Interferences causing a 1070 Error at Various Levels of Calcium Interferences moles liter 10 M ca 10 M ca 107M ca Mg 1ox10 1 0X10 1 0X10 Zn 1 0x10 1 0X10 1 0X107 Ba 7 0X10 7 0x10 7 0X10 E 4 0X10 4 0XIO 4 0X107 Na 2 0x10 2 0x10 2 0X10 Ni 5 0X107 5 0XI07 5 0X10 Cu 4 0x10 4 0X107 4 0X10 Fe 2 0X107 PX SUE 2 0X107 sr 6 0X107 6 0X107 6 0X104 H 4 0X107 4 0X10 4 0X107 Hg 4 0X10 4 0X10 4 0X10 Pb 1 0x10 1 0Xx10 1 0X10 PASC Ce 9 scientific Calcium Ion Selective Electrode 012 06601A 10 Interferences ppm 1000 ppm CaCO 100 ppm CaCO 10 ppm CaCO Mg 2 43X10 243X10 243X10 Zi 6 53X10 6 53X10 6 53X10 Ba 9 60X10 9 60X104 9 60X10 K L56X10 156X10 L56X10 Na 4 60X104 4 60X10 4 60X10 Ni 2 94X10 2 94X10 2 94x10 Cu 2 54X104 2 54X10 2 54X10 Fe LIXIO 1 11X10 1 11X10 Sr 520X10 520X10 5 20X10 H 1 4 pH 2 4 pH 3 4 pH Hg 8 0X10 8 0X10 80x10 Pb 2 0x10 2 0 2 0X10 C
19. production under any other circumstances without the written consent of PASCO scientific 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 2 8 Should the produet 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
20. r Position the electrode tips in the solution about half way between the center ofthe beaker and the beaker wall 5 Begin adding the EDTA in 0 5 ml to 1 0 ml increments and about 0 1 ml to 0 2 ml increments as the potential begins to change more rapidly Record the mV potential after each addition Continue the additions several milliliters past the endpoint 6 Plot the milliliters of EDTA added against the mV potential on standard coordinate graph paper See Figure 4 The point of greatest potential change is the endpoint Figure 4 Typical titration of 100 ml of 5 X 10 3M CaCl ph adjusted to 10 with ammonia with 0 1 M Na EDTA 440 430 20 electrode o potential mV 0 4 6 8 10 ml of 0 1 M Na EDTA added 7 The calcium ion concentration from the unknown is calculated as follows VM M 42 Ca Vos Ca where M concentration of calcium ion in the unknown moles liter V volume of EDTA added at endpoint M EDTA concentration moles liter V 2 volume of unknown sample 012 06601A Calcium Ion Selective Electrode Electrode Characteristics Reproducibility Electrode measurements reproducible to 4 can be obtained if the electrode is calibrated every hour Factors such as temperature fluctuations drift and noise limit reproducibility Reproducibility is independent of concentration within the electrode s operating range Interferences Table 3 lists some comm
21. s a clinical thermometer to remove any air bubbles which might be trapped behind the calcium membrane Prior to first usage or after lon term storage immerse the calcium membrane in calcium standard for thirty minutes Insert DIN 3 Connect the Calcium Selective lon Electrode to the ISE an Amplifier and insert the DIN connector of the ISE analog channel Amplifier into analog channel A or B on a PASCO nom Computer Interface Figures 2b and 2c Electrode Slope Check Using ScienceWorkshop check electrodes each day 1 Toal150 ml beaker add 100 ml of distilled water Place Rotate ring one quarter the beaker on a magnetic stirrer and begin stirring at a turn to secure constant rate Start Science Workshop select the Ion Selective Electrode sensor open a Digital display and begin monitoring data Lower the electrode tip into the ISE Amplifier electrode connector solution 2 Usingapipette add 1 ml of0 1 M 1000 ppm or 100 ppm as calcium carbonate standard and 2 ml of ISA to the Figure 2 beaker When the reading has stabilized record the Equipment Setup a filling the electrode voltage reading indicated in the Digits display with filling solution b amp c connecting the electrode to the ISE Amplifier and to the computer interface PASC Ce 3 Screntrtre Calcium Ion Selective Electrode 012 06601A 3 Usinga pipette add 10 ml ofthe same calcium standard used above to the beaker When the re
22. s that they contain do not interfere with the electrode s response to calcium ions PASCO l screntitrc Calcium Ion Selective Electrode 012 06601A Thereference electrode must also be considered When two solutions of different composition are brought into contact with one another liquid junction potentials arise Millivolt potentials occur from the inter diffusion of ions into the two solutions Electrode charge will be carried unequally across the solution boundary resulting in a potential difference between thetwo solutions since ions diffuse at different rates When making measurements itis important toremember that this potential be the same when the reference is in the standardizing solution as well as in the sample solution or the change in liquid junction potential will appear as an error in the measured electrode potential Thecomposition ofthe liquid junction filling solution in the reference electrode is most important The speed with which the positive and negative ions in the filling solution diffuse into the sample should be equiltransferent No junction potential can result if the rate at which positive and negative charge carried into the sample is equal Equipment PASCO Science Workshop Computer Interface Included Semi logarithmic 4 cycle graph paper for preparing calibration curves Linear graph paper is recommended for low level Calcium Ion Selective Electrode measurements magnetic stir plate gt Pan
23. ure that the meter is in the mV mode 6 PASCO entific 012 06601A Calcium Ion Selective Electrode 5 Add increments ofthe 1 0 X 10 M or 10 ppm standard as given in Table 2 below 6 Afterthereading has stabilized record the voltage reading indicated in the Digits display after each addition TABLE 2 Step wise Calibration For Low Level Calcium 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 Pipet A 1 ml graduated pipette Pipet B 2 ml pipette Concentration M ppm 1 0 X 10 1 0 X 10 2 0 X 10 2 0 X 10 4 0 X 10 4 0 X 10 6 0 X 10 6 0 X 10 9 9 X 10 1 0 X 107 2 9 X 10 2 9 X 10 4 8 X 10 4 8 X 10 Solutions additions of 10 ppm or 1 0 X 10 M standard to 100 ml of distilled water 7 Onsemi logarithmic graph paper plot the mV reading linear axis against the concentration log axis as in Figure 1 8 Rinse the electrodes in distilled water and blot dry 9 Measure out 100 ml ofthe sample into a 150 ml beaker and place the beaker on the magnetic stirrer and begin stirring Lower the electrode tips into the solution After the reading has stabilized record the mV reading and determine the concentration from the low level calibration curve 10 Prepare anew low level calibration curve daily Check the calibration curve every 1 2 hours by repeating Steps 2 7 above Titration The progressive and quantitative addition of a reagent to
24. using an ISE Amplifier and Science Workshop interface depends on the level of free calcium ion in solution The level of calcium ions corresponding to the measured potential is described by the Nernst equation E E S log X where E measured electrode potential E reference potential a constant S electrode slope 26 mv decade X level of calcium ions in solution The activity X represents the effective concentration of the calcium ions in the solution Total calcium concentration C includes free calcium ions C Plus bound or complexed calcium ions C since the calcium electrodes only respond to free ion the free ion concentration is C C C The activity is related to the free ion concentration C by the activity coefficient y by X YC Activity coefficients vary depending on total ionic strength I defined as I l5xc z where C x concentration of ion X Z charge on 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 and constant value ionic strength adjuster ISA is added to samples and standards The recommended ISA solution for calcium is potassium chloride KCl Solutions other than this may be used as ionic strength adjusters as long as ion
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