A Coulometric Coulometer VIVIAN B. EHLERS and JOHN W. SEASE W e r l e y a n University, Middletown, Conn.
F
A coulometer applicable i n the region below 10 cou-
OR some time there has been a used in constant potential
lombs w-as needed for work on the constant potential electrolysis of small amounts of organic compounds. The coulometer contained platinum and copper electrodes in a copper sulfate electrolyte. The current to be measured was used to plate copper on the platinum electrode, which was then reconnected as the anode, so t h a t the copper could be stripped off a t cons t a n t current. The change i n potential drop across t h e cell when the last of the copper was removed furnished a suitable end point and the current-time product for the stripping process was a measure of t h e amount of electricity originally passed through the coulometer. Under suitably controlled conditions, the amount of electricity required for the stripping process was found to be 99.7 i 0.170 of t h a t flowing during the plating process over t h e range 0.015 to 75 coulombs. An electronic tripper and a stable electronic constant current supply were eniplo) ed to permit automatic operation. With this coulometer quhntities of electricity as small as 0.01 coulomb can be measured within 0.1%; controlled potential coulometric analyses can be carried out with much smaller amountsof material t h a n heretofore.
coulometry for an accurate, convenient coulometer for measuring quantities of electricity smaller than 10 coulombs. Conventional gravimetric coulometers are inconvenient and lack accuracy in this region-for example, 1 coulomb corresponds to only 1.18 mg. of bilver. The hydrogen-oxygen coulometer, although simple and convenient to use, is not of value here because it yields only 0.1739 nil. of gas per coulomb ( 5 ) . The acid-base coulometer of Lingane and Small ( 6 ) , while Lccurate to within 0.1 % at 10 coulombs, icl good to within only 1% at 1 coulomb and the error increases rapidlv as the amount of electricity becomes smaller than 1 coulomb. The work of Campbell and Thomas (I,,%’), Zakhar’evshiI ( 1 2 , l a ) , and Francis (5)in stripping metals anodically a t constant current suggested that a suitable coulometer might result if a metal nere plated on a platinum cathode by the current to be measured and then in a subsequent step stripped anodically at constant current. The sharp change in potential across the cell when the last of the metal is removed from the platinum anode should furnish a suitable end point and the current-time produrt for the stripping proc-
TO TIMER
IO
L
Figure 1. Constant Current Generator M = m e a o h m s . K = kilo-Jhms. W = watts. a n d H = h; :nries R1. 50K, 20W, wire-wound nz. 12K, low, wire-wound R3. 15K, low, wire-wound R4, R5. 470K, 1W R6. 10K, IOW, w i r e w o u n d R i . 5K, 10W wire-wound R8. 10K, IOW, wire-wound w i t h 2 adjustable t a p s R9. 1x5 l / * W R10. 68K, 1W ~ 1 2 m, i . 10K potentiometer (General Radio Type 371) R13. l M , lW,170 wire-wound R14, R l 5 . 680K. 1W R16, R17. 25OK, 1W m a . 390K. 1W R i g . 1.5M. I/,W RZO. 150M, ‘/IW R22. 20K potentiometer (General Radio Type 371) R23. lOOK potentiometer (General Radio Type 371) R24-R29. 100K, l W , 170wire-wound
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~ 3 0 . 750, 25W, wire-wound Cl, C?. 16 pfd., 600 volts C3-Ce. 0.1 pfd., 600 volts T . 1050 volts center tapped a t 250 m a . , 5 volts at 3 a m p . , 6.3 volts a t 3 a m p . (United Transformer Co. CG429) C H . 12H a t 250 m a . , 105 o h m s (United Tranaformer Co. CGlOZ) s 1 . 4PZT s w i t c h 5-2. 3P9T switch F. 2-amp. fuse V1. 5R4GY V 2 . 6AS7G V3, V6, Y7. 12SLiCT v4. 12SJ7 V 5 . 5651 V 8 , v 9 . 5OL6GT V10. 1 2 S H i Yll-V13. 3W 120V light bulbs (General Electric Co.. Type 56)
513
ANALYTICAL CHEMISTRY
514
ess would give the total amount of current which had flowed through the cell originally without recourse to conventional gravimetric or volumetric methods. Both silver and copper were found to work in such a coulometer. Rather surprisingly, the silver coulometer was the less accurate and reproducible of the two and in addition was far less convenient to use. For example, the cell had to be protected from light, the electrodes had to be cleaned after two or three runs, and the electrolyte could be used for only five or six runs. The copper coulometer required no such troublesome precautions and for the range from 0.015 to 75 coulombs the amount of electricity used in the stripping process was found to be 99.70 i 0.10% of that flowing in the plating process. EXPERIMENTAL
Cell. The coulometric cell was a 250-ml. beaker fitted with a rubber stopper which served to support the two electrodes. The platinum electrode was a piece of wire, or a piece of foil welded to a platinum wire support, sealed through a glass supporting rod containing a mercury pool used as a contact. Areas of platinum as small as 0.65 sq. mm. and as large as 200 sq. mm. were used. T
VI
CH
RI
1
I
Figure 2. Tripper M = m e g o h m s , K = kilo-ohms, W = watts, a n d H R1. 4000 o h m s , l o w , wire-wound R2, R3, R10, R12. 100K, 1W R4. R7. 75K, IW R5, R6. 10K wire-wound potentiometers R8. 250K, 1W R9. 680K 1W R11. 5M, :/lW R13. 510, 1W R14 500K 1W R15: 2M, ;/iW C1, C2. 8 rfd, 6OOV T. 650 volts center tapped a t 50 m a . , 6.3 volts C H . 4.5H a t 50 m a . , 300 o h m s (Stancor C1706) SI. 1P2T switch F. 1-amp. f u s e V1. 6X5GT V 2 , V3. VR150 V4. 6SL7GT V5. 6SN7GT RY1. 1P2T relay, 10K coil (Potter a n d Brumfield M. 0-50 microamperes
henries
2 amp.
'pe L55)
Care was taken to see that all of the platinum was well below the surface of the electrolyte. No deterioration of the platinum electrodes was observed even on repeated use over a period of more than a year. The copper electrode, supported by a length of copper wire, was a piece of J. T. Baker electrolytic copper foil, 0.008 inch thick and approximately 2 X 2 em. in size. After installation in the cell and before use the copper electrode was plated with a fresh deposit of copper a t 5 ma. for 24 hours. With a good grade of copper this preliminary plating process was not absolutely necessary, but whenever time permitted a fresh layer of copper was deposited on each new electrode to ensure good results. The cop er electrode was replaced whenever it began to show signs of feterioration, usually after about 1 month's immersion in the electrolyte. Electrolyte. The electrolyte was prepared by dissolving 150 grams of Mallinkrodt A. R. copper sulfate pentahydrate, 28 ml. of Baker Q.P. concentrated sulfuric acid, 50 ml. of Commercial Solvents undenatured 95% ethyl alcohol, 50 grams of Mallinckrodt
CONSTANT CURRENT GENERATOR
TRIPPER
_SIB
CONSTANT CURRENT CIRCUIT
~
Figure 3.
Stripping Circuit
RY2.
2P2T, 115VAC coil (Potter a n d Brumfield Type MR11) Clock i s Standard Electric T i m e Co. Model SI0
analytical reagent grade sodium tartrate dihydrate. and 1 gram of Eimer and Amend C.P. hydrazine sulfate in enough distilled water to make 1 liter of solution. This solution (100 ml.) was placed in the cell and used until white crystals began to separate from the solution, a t which time the coulometer began to give erratic results. This usually required a t least 48 hours. Replacement by fresh electrolyte a t this point is desirable but not absolutely essential, since the addition of enough 95% ethanol to bring the solution back to its original volume caused the precipitate to redissolve and the error to drop to its original, low value. Constant Current Generator. This constant current unit (Figure l), while similar to that described by Reilley, Adams, and Furman (9),differs from it in several important respects. It contains no reference batteries, which require replacement and have a temperature coefficient of approximately 0 . 0 2 5 per degree centigrade ( 4 ) . The heaters of the direct-coupled aniplifiers are operated in series across the output of the regulated highvoltage supply so that the generating current is constant to within much less than 0.170 even when the alternating current line voltage varies as much as *lo%. Connecting the coulometric cell in the plate circuit of the regulator triodes makes it possible for the generator to operate satisfactorily with solutions which give a voltage drop of as much as 50 to 100 volts across the cell; the upper limit depends upon the characteristics of the regulator triodes and varies with the current being drawn. The power supply consists of a choke-input rectifier ( V I ) and a voltage regulator (V4, V3A, V 2 ) ; the latter circuit is essentially that given in the RCA "Tube Handbook" in the data sheet for the Type 5651 tube (8). The voltage reference tube ( V 5 ) furnishes a reference voltage for both the voltage regulator amplifier and the current regulator amplifier ( 8 6 , Vi, V3B). The output of the latter amplifier is applied to the grids of the series control tubes (V8-VlO) so as to hold the current through standard resistors R"bR29 constant. By suitable adjustment of R21, R22, R23, and selector switch S2 this current may be varied continuously from 0 11 to 35 ma.; currents up to 90 ma. may be obtained by connecting small incandescent lamps (V11-Vl3) in parallel with R21. R12 serves as a fine adjustment for the generating current The lower tap on R8 was adjusted so that the voltage regulator gave approximately 255 volts output; the upper tap was set so that the voltages at the anodes of T'6 were approximately equal. Tripper. This unit (Figure 2) utilizes the Schmidt trigger circuit employed by Muller and Lingane ( 7 ) with the input stage altered to eliminate batteries; a similar modification has been made by Wise, Gillies, and Reynolds (IO). The circuit is arranged so that increasing negative input voltage causes RYl to close. The tripper was set by adjusting R5 until voltmeter M read the desired tripping potential and then. with SI in the adiust oosition. varvine R6 until RY1 just closed. The trimer wis then ready for"op&ation as soon as S1 was thrown t d ?,he operate position. Stripping Circuit. The connections between t.he tripper, the constant current generator, and the cell are shown in Figure 3. The voltmeter is not necessary in routine operation but may be used if it is desired to follow the voltage drop across t,he cell during operation. Both an electronic voltmeter with an input resistance of 10 megohms and a chart recorder with an input re-
V O L U M E 2 6 , NO. 3, M A R C H 1 9 5 4
515
sistance of 300,000 ohms were used in developmental work. The generating current was standardized by means of a standard resistor (Leeds and Northrup T y e 4775 resistance box set to a suitable value) and a Rubicon &pe 2710 potentiometer which was used to measure the voltage drop across the standard resistor. Operation. The platinum electrode of a size such that the current density during the plating process would not exceed 1.0 ma.per sq. mm. and the total amount of electricity passed through the cell would not exceed 1.2 coulombs per sq. mm., was cleaned by stripping for 60 to 100 seconds after all copper had apparently been removed; switch S2 of the tripper was opened during this process to keep RY2 from operating and cutting off the current through the cell. The coulometric cell was then disconnected from the stripping circuit and placed in series with the circuit where the quantity of electricity flowing was to be measured. The copper electrode was made the anode and the platinum, the cathode. I n development work the constant current generator was used to furnish accurate1 known amounts of electricity by installing a double pole-douhe throw switch to reverse the polarities of the cell electrodes.
process the same upper limit of 1.0 ma per sq. mm. at the platinum electrode was found to apply, although current densities as low as 0.0015 ma. per sq. mm. still yielded a satisfactory copper plate. When the above-mentioned current density limits were not exceeded, the voltage drop across the cell during plating or stripping was 0.1 to 0.2 volt, rising sharply to approximately 2.2 volts when stripping was complete. d typical chart recorder plot of the potential break is shown in Figure 5.
u
15.28 ma., 16.6 mm'
40 1
1
1
0.3
Figure 5.
W
I
0.50 0.25
0 . 6 5 m m ? Pt electrode
... ........ I....
0.25
I
0.1
0.2
1.0
I
I
1.1
I
I
I
1.2
I
1.3
I
I
CURRENT DENSITY IN MA. PER MM.'
Figure 4.
Error as a Function of Current Density during Stripping Process
At the conclusion of the plating process the tripper was set to operate a t -1.2 volts, the cell was reconnected in the stripping circuit, and the platinum electrode was stripped a t a current density of 0.05 to 1.0 ma. per sq. mm. until the tripper operated. The amount of electricity used in the stripping process was given by the current-time product and was multiplied by 1.003 to obtain the amount of current flowing through the cell during the plating portion of the cycle. DISCUSSION
A large number of experiments were carried out, using plating and stripping currents from 113 pa. to 85 ma., total quantities of electricity from 0.015 to 75 coulombs, and platinum electrode sizes'froni 0.65 to 200 sq. mm. In 62 runs where the current drnrity :it the platinum electrode during the stripping process W:P kept between 0.05 and 1.0 ma. per sq. mm. the current-time product for the stripping process was found to be 99.7% of that for the plating process, with a standard deviation of 0.096%. When the current density a t the platinum electrode during the stripping process was less than 0.05 ma. per sq. mm., sharp, re1)rotiucible potential breaks a t the end point were not always obtained and the error increased rapidly (Figure 4). At current densities appreciably greater than 1.0 ma. per fig. mm. potential breaks were obtained while some copper still remained on the platinurn electrode and the error again increased. I n the plating
1
1
0.6
1
1
1
1
P t electrode 1
1
1
1
0.9 1.2 1.5 1.E VOLTS A C R O S S CELL
1
1
1
1
2.1
Chart Recorder Plot of Potential Break
Another factor, in addition to current density, which was found to be important in wlecting a platinum electrode of suitable size was the total amount of electricity to be measured. If more than 1.2 coulombs per sq. mm. of electrode area were passed through the cell, low results were obtained, presumably because of mechanical losses of copper from the surface of the platinum electrode. The tripper setting of - 1.2 volts was selected because it gave a highly reproducible ratio of 99.7% between the amounts of electricity passing through the cell during the stripping and plating portions of the cycle. Lower and higher settings of the tripper gave much poorer precision, although they did not affect the 99.7% ratio noticeably. The amount of copper left on the platinum electrode a t the -1.2-volt tripper setting is apparently negligible, since the results are not altered by omitting the 100second post-end point stripping between determinations. This, plus the fact that the 0.3% difference is not a function of the amount of electricity being measured, indicates that the 99.7% ratio is determined by the over-all electrolytic efficiency of the coulometer rather than by an end-point error in the stripping process. The stripping was usually carried out immediately after the end of the plating step, although it was found that, when dis. connected, the platinum electrode could stand in the electrolyte for as much as 1 hour without adverse effect on the accuracy of the determination. Longer periods of standing, however, led to losses of copper from the platinum electrode and correspondingly low results. Temperature was found to be unimportant, provided that the electrolyte was kept between 10" and 20" C. At 0" C. solid material slowly separated from solution and erratic, inaccurate results mere obtained. At 30' C. the difference between the amounts of electricity required for plating and stripping rose to 0.6% from its normal value of 0.3% and the precision decreased somewhat. ilt 50" C. this difference was sometimes as large as 4% and results were so erratic as to be useless. The coulometer hae been used in this laboratory for some time as a part of apparatus for constant potential electrolysis and has proven thoroughly satisfactory in everyday use. It is accurate and easy to employ, since no attention is required during either the plating or stripping process and all determinations are made in terms of semipermanent electrical standards. Although it has been used only in the range 0 015 to 75 coulombs, there is no reason to suppose that it cannot be used for larger, or even somewhat smaller, amounts of electricity, provided only that the specified limits of current density and maximum charge per unit area of platinum electrode surface are not exceeded. i2t the
ANALYTICAL CHEMISTRY
516 lower part of its range. in particular, the coulometer should be of great usefulness, since accurate and convenient methods have not hitherto been available for work in this region. ACKNOWLEDGMENT
The authors would like to express their appreciation to the Surdna Foundation, whose financial support has made this work possible. LITERATURE CITED
(1) Campbell, 11938).
IT. E., and Thomas, U. B., Suture, 142, 253-4
(4) Greenwood, 1. A., Jr.. Holdam. J. V.. Jr., and AIacRrre, D., Jr., “Electronic Instruments,” h1.I.T. Radiation Laboratory Series, Vol. 21, New York, hlcGraw-Hill Book Co., 1946. (5) Lingane, J. J., J . Am. Chem. Soc.. 67, 1916-22 (1945). (6) Lingane, J. J., and Small, L. A,, ASAL. CHEM.,21, 1119-22 (1949). (7) Muller, R. H., and Lingane, J. J., Ibid.,20, 795-7 (1948). (8) Radio Corp. of America, “Tube Handbook,” HB-3, Vol. 2, Type 5651, Harrison, N. J., Radio Corp. of America. (9) Reilley. C. N., Bdams, R. N., and Furman, X . H., A s . 4 ~ . CHEM.,24, 1044-5 (1952). (10) Wise, E. N., Gillies, P. W., and Reynolds, C. A., J r . , Ibid., 25, 1344-8 (1953). (11) Zakhar’evski:, AI. S.,Khim. Referat. Zhur., 2, Xo. 4, 84 (1939). (12) Zakhar’evskii, M. S., Voprosy Pitaniyu, 7, 415, 180-8 (1938).
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(2) Campbell, W.E., and Thomas, U. B., Trans. Electrochem. Soc., 76, 303-24 (1939). (3) Francis, H. T., J . Electrochem. Soc., 93,79-83 (1948).
RECEIVED for review September 4, 1953. Accepted December 19, 1953 Presented before the Division of Analytical Chemistry a t the 124th Meeting of the .4XERICAS C H E ~ I I C A SOCIETT, L Chicauo, 111.
Electronic Controlled Potential Reduction or Oxidation Apparatus FREDERICK KAUFMAN, ELI OSSOFSKY, end HELEN J. COOK Ballistic Research laboratories, Aberdeen Proving Ground, M d . In the course of work on the reduction of nitrate esters, the need arose for a controlled potential device of rapid response and large possible electrolysis currents and cell potentials. An electronic instrument is described which fulfills these requirements. It is capable of delivering currents up to 7 amperes, cell potentials up to 150 volts, and has fast response, high stability, and sensitivity. Its principle of operation is as follows: The amplified difference between the desired and obtained cathode-calomel potentials controls the grids of a bank of 6AS7G power tubes. The tubes are in the electrolysis circuit and control the current. An unbalance potential of 1 to 2 mv. corresponds to a change of up to 1 ampere in electrolysis current. The instrument is also capable of controlling very small currents. Reductions of several inorganic and organic compounds at low concentrations are described.
E
LECTROLYTIC reduction a t controlled cathode potential has been the subject of a considerable number of investigations in recent years. Much of this interest aroae from polarographic work through efforts to reduce chemical species at high concentrations in the same specific and selective manner in which they are reduced at low concentrations a t the dropping mercury electrode. When successful, this technique is of great usefulness in preparative organic chemistry, in controlled reduction of metal ions, and in many other problems. Moreover, a basic understanding of the rate of large scale electrode processes is still very incomplete. Instruments of the type described here permit a study of the factors that affect the over-all rate of the electrolytic oxidation or reduction process, such as the electrode reaction, transport of solute by diffusion and convection, rate of stirring, etc. During the past few years, interest in this field has increased rapidly and a number of instruments for controlled potential electrolysis have been described (1-4, 6-11, 13). Even though several of these instruments provide good control, it seemed worth while to build another version of such an apparatus with the following properties in mind: 1. Completely electronic design, making possible instantaneous response 2. High possible cell potentials and currents large change of current per unit 3. High sensitivity-i.e.,
unbalance between desired arid obtained cathode-referenre electrode potential 4. Stability, ease of operation, and reliability Some entirely electronic instruments (3, 4 ) are among those described in the literature. The one described here is closely related to Hickling’s ( 4 ) in its basic regulating principle, but differs from i t in various ways as is shown. Its main disadvantages are low power efficiency and requirement of alternating and direct current power. Of these, the former is not serious in a research instrument and the latter is not too restrictive, since any direct current line voltage between 20 and 250 volts is suitable. DESCRIPTIOY AVD OPERATION O F I\STRU.MENT
A block diagram of the apparatus is shown in Figure 1. The small unbalance signal, e , represents the difference between the desired, eo, and obtained, ec, cathode-reference electrode potentials. It is fed into a direct current amplifier whose output, eo, controls the grids of a bank of power tubes (Type GASiG). The power tubes are in series with the electrolypis cell and control ~ electrolysis current. the f l o of Figure 2 shows the apparatus in greater detail as a controlledpotential reduction instrument, though it is easily modified for other uses as described below. The maximum obtainable electrolysis current is determined by the number of power tubes in parallel. This instrument was constructed with 27 tubes and could deliver about i amperes (0.25 ampere per tube). -4240volt direct current line eerved as power source in most of the work. While this ip well within maximum plate voltage of the
”,
CURRENT COWTROL TUBES
Q GRID
Figure 1.
-
Block Diagram of Apparatus
