Edward F. Beadel, Jr., Frank V. Lovecchio, Melvyn C. Minot, and Daniel J. Macero Syracuse University
Syracuse, New York 13210
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An Operational Amplifier-Controlled Chemical Coulometer for Determining n-Values
B y placing a chemical coulometer in the feedback loop of an operational amplifier, instead of a capacitor as is commonly done (I-,$), a novel approach is realized for the determination of n-values (or analytical concentration) (5). Experimental
The constant potential electrolysis circuit used is shown in the figure. The potential difference between the mercury pool working electrode and a saturated calomel reference electrode is maintained at a preset constant voltage, either positive or negative, by the combination of amplifier 1, booster amplifier 1, and voltage divider, R, and R2. The combination of amplifier 2 and booster amplifier 2 comprises the current following and charge summation operations. Since the output voltage of booster 2 must be such that the summing point of amplifier 2, i.e., the working electrode, is always a t virtual ground, then all the current that flows into the working electrode flows through the feedback resistor, R3, and the chemical coulometer. This current is monitored by measuring the voltage drop across the 1-ohm 0.02% precision resistor, R3, with a differential Digital Voltmeter. By using Philbrick type P66A Booster amplifiers with recommended external "boost" resistors (6) electrolysis currents of up to 100 mA are obtainable. The coulometer design used was similar to those described by Page (7) and Lingane (7, 9); while the electrolysis cell was constructed after a design by Meites (8). A pool of triply distilled Bethlehem Instrument Mercury was used as the working electrode, a standard saturated calomel electrode as the reference electrode, and a platinum gauze as the counter electrode. Reagent grade chemicals were used throughout this work.
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known amount of time and determining the volume of gas a t STP liberated per coulomb of electricity used. For this measurement the tandem combination of amplifier 1 and booster 1 was placed in the voltage follower configuration, a 0.1% precision resistor replaced the electrolysis cell, and the voltage divider set to give a constant output voltage. The rest of the circuit remained the same. Current values between 1 and SO mA were used. The average conversion factor calculated from this experimental data was found to be 0.1738 ml of gas a t STP per coulomb of electricity passed. This value compares very well with the theoretically determined value of 0.1742 ml per coulomb, i.e., a -0.3% relative error. A determination of the amount of an electroactive substance deposited in an actual controlled potential electrolysis under typical conditions was also made. The substance run was cadmium chloride in a 1.0 M NaNOa solution. A precisely measured amount of Cd(II), in the range of 0.1-0.5 mmoles, was added to the cell. The theoretical value for the number of equivalents per mole for the reduction of Cd(I1) to Cd(0) is 2.00. The accuracy of this determination, expressed as mean error, is -0.01 equivalents per mole; while the precision of the determination, expressed as standard deviation is 0.025 equivalents per mole. While the above results indicate that the operational amplifier-controlled coulometer has considerable merit for determining the concentration of electroactive species, its chief utilization in our laboratory is for the
Procedure An appropriate volume of a 1.0 M NaNO. background solution is placed in the electrolysis cell, deaerated with Linde High Purity Dry Nitrogen which is saturated with water at 2 5 T , and preelectrolyaed at -0.800 V versus S.C.E. When the residual current drops to a sufficiently low value (-0.5 mA) the gas coulometer is read. Next a stock cadmium chloride solution is quantitatively added to the cell and electrolysis at -0.750 V versos S.C.E. is carried oat. When the current has again dropped to about.0.5 mA the final reading of the gas eoulomet,eris taken and the electrolysis terminated.
Results and Discussion
The operation of the instrument was tested by passing a known constant current through the system for a
Circuit diagram for an operotion01 wnpliflor contmlled pas coulometer. St Eiectrolysis potentiol polarity switch S2,.s Electrolyris and coubmeter "ON-OFP" switch RI Resi~tor, 13K, 1% R, Potentiometer, 10-turn, 2K Ra Resistor, 1.00 zt 0.02y0 A-1 Potentid Control operational ampliner Researches, I n 4 IType P - 2 5 ~ Philbrick . A-2 Current follower operational amplifier ITyp= P-25A. Philbrick Researcher, Inc.) 8-1.2 Current booster operational amplifiers (Type P-66A, Philbrick Rereorches, I n 4
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determination of cou~ometricn-values for which the results have excellent accuracy. Literature Ciled M, T,, JONEB, H, C,,AND FrshER, 488. 956 (1959).
KELLEI,
D,
J,,
(2) T o m s . E. C.. *No D R I ~ C OC. LL P., , A w l . Chcm.. 36, 1809 (1963).
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(3) MALUBTADT. H. V., ENXE,C. G., AND TORNB. E. C.. JR.. "Eieotronios far Scientists," Benjamin. Nev York. 1961. (4) BOOMAW. C. L..m o HOLBROOK. W. 11.. A"o~. them., 36, 1793 (1963). (5) See olao: Vmcenr. C. A.. A N D WARD,J . G., I. CHEI. EDUC..46. 613 (1969). (6) G . A. Philbriok Resesrches, Inc.. Data Sheet on P-66A. (7) PAGE. J. A.. A N D LINO*N%J. J.. A n d Chirn. Ado, 16. 175 11957). (8) MEITBB.L.. Anal. Chcm.. 17, 1116 (1955). ~ .J., J . A m . Chrm. Soc., 67, 1916 (1945). (9) L r n o * ~ J.
