The system should yield a well defined open circuit potential in the region of potential wftere adsorption is to be studied. The electrode process should not be subject to kinetic complications, which lead to erroneously low, or even negative, calculated values of surface concentration. LITERATURE CITED
( 1 ) h S O n , F. c . , ANA;. CHEM. 33, 1498 (1961). (2) Ibid., p. 1838. (3) Anson, F. C . , J . Am. Chent. SOC.83, 2387 (1961). (4) Bard, A.’ J., ANAL. &EM. 35, 340 (1963). (5) Bjerrum, J., “Metal Amine Forma-
tion in Aqueous Solutions,” P. Haase and Son, Copenhagen, 1941. ( 6 ) Delahay, P., “Kew Instrumental Methods in Electrochemistry,” Chap. 8, Interscience, New York, 1954.
(7) Delahay, P., Berzins, T., J . Am. Chem. SOC.75,2468 (1953). ( 8 ) Ford, J. J., M. S. thesis, Iowa State College, 1954. (91 of Chemistrv. 8th ed.. . ,N.Handbook A. Lange, ed., p. 936;‘HandbooG Publ., Sandusky, Ohio, 1952. (10) Laitinen, H. A., ANAL.CHEM.33, 1458 (1961). (11) Laitinen, H. A., Gaur, H. C., Anal. Chim. Acta 18, 1 (1958). (12) Laitinen. H. A.. Grieb. M. W..’ J. ‘ Am. Chem. $ 0 ~ .77,’5201 (1955). (13) Laitinen, H. A., Mosier, B., Ibid., 80, 2363 (1958). (14) Laitinen, H. A., Randles, J. E. B., Trans. Faraday SOC.51, 54 (1955). (15) Landsberg, R . , Kitzsche, R., Geissler, W., 2. Physik. Chem. Leipzig 222, 54 (1963). (16) Loienz, W., 2. Elektrochem. 59, 730 (19.55). \ - - - - I -
(17) Lorenz, W., Muhlberg, H., Ibid., 59, 736 (1955). (18) Mamantov, G., Delahay, P., J. Am. Chem. SOC.76, 5323 (1954). (19) Meyer, F. R., Rong, G., Angew. Chem. 52, 637 (1939).
(20) Mosier, B., Ph.D. thesis, University of Illinois, 1957. (21) Reinmuth, W., ANAL. CHEM. 33, 322 (1961). (22) Rhodes. D. R.. Ph.D. thesis. Uni. versity of Illinois, i 9 6 l . (23) Ross, J. W., DeMars, R. D., Shain, I., ANAL.CHEM.28, 1768 (1956). (24) Sand, H. J. S., Phil. Mag. 1, 45 (1901). (25) Sherman, E. O., Ph.D. thesis, University of Illinois 1963. (26) Tatwawadi, 8. V., Bard, A. J., ANAL.CHEM.36, 2 (1964). (27) Work, J. B., “Inorganic Synthesis,” Vol. 2, p. 221, PllcGraw-Hill, New York, 1946.
RECEIVEDfor review April 4, 1963. Resubmitted August 19, 1963. Accepted September 30, 1963. Abstracted from the Ph.D. thesis of L. M. Chamber8 (1963). Research supported in part by the Aeronautical Research Laboratories, Office of Aerospace Research, U. S. Air Force, under contract AF 33 (616)-5446, and in part b the National Science Foundation, un&r Grant NSF-G 21049.
Solid State Potentiostat for Controlled Pote nt ia I IEIec t roIysis FREDERICK LINDSTROM and JOE B. DAVIS Clernson College, Clemson,
S. C.
b The recent development of solid state electronic devices having electrical characteristics well suited for the job to be done in ancilytical controlled potential electrolysis has led to the design and construction of an instrument half the size of an analytical balance capable of holding the potential to within a few millivolts of any preset value up to 10 volts. The current may vary frori 0 to 5 amperes while such control i s being effected. Having no moving parts or vacuum tubes, it offers superior reliability and simplicity of operation. The instrument has been evaluated 3y using it in the analysis of several standard samples with precision and accuracy.
I
PRINCIPLE, cor trolled potential electrolysis or controlled cathode electrodeposition (5) is one of the most attractive methods For the absolute gravimetric determincttion of metals in nonferrous alloys. The problem has always been that some automatic means of controlling the electrode potential is needed to make the numerous published methods practical. Even before the electronic art had reached the stage where the design and construction of a machine for effecting control was likely t o prove fruitful, numerous more or less successful designs were advanced, as shown in recent N
reviews (2, 3). If the newer solid state devices were used, a much more satisfactory design superior to those of early workers on this problem could be developed. The current design of a simple, practical, and inexpensive potentiostat is easier, for de.cices such as transistors and diodes are natural components for such service. They are capable of handling rather large amounts of current a t low voltage. This is not so with vacuum tubes. Solid state devices are small, highly efficient and, if used within their ratings, offer unsurpassed reliability. Wadsworth (6) has dawribed an instrument employing transistors for series regulation of an 8-ampere diode power supply. It was quite stable and offered rapid response, but to control near zero current, a reverse polarity power supply had to be used to cancel the high zero current of the instrument. A potentiostat satisfactory for most published controlled cathode electrodeposition procedures must fulfill a number of requirements. It must supply about 6 volts and a current of several amperes. Accuracy of control need not be greater than about 50 millivolts, for most separations involve reduction potential differences of more than 0.1 volt. Response time must be fast enough to follow the electrode processes. A response time of several
seconds is sufficient, for changes in the electrolyte are slow. The measuring circuit must not draw an appreciable current from the reference electrode or its potential will be affected. A practical maximum current for a commercial fiber-type calomel electrode is said to be 10 microamperes (6). Regulation should be done in such a manner that minimum power lo?-5 occurs within the instrument. Shunt or series regulators, such as the Wadsworth instrument, dissipate large amounts of power relative to the output power. They are essentially variable resistors which control by wasting the excess power as heat. The Wadsworth machine mas water cooled. The motor driven variable autotransformers used in several of the early electromechanical instruments did not dissipate power as heat but did require considerable power t o drive the autotransformer. A potentiostat should offer simplicity of operation or its value as a labor saving device would be lost. As with all instruments, a potentiostat should offer reliability and loiv maintenance cost. Its first cost, maintenance cost, and operation cost must be considered in any practical design. The unit described here works on an entirely different principle. This circuit rectifies and delivers to the electrolysis cell only the amount of current needed a t a particular time in the VOL. 36, NO. 1, JANUARY 1 9 6 4
11
..
The circuit diagram of the complete unit shown in Figure 1 is given in Figure 2. ClRCUlT DESCRIPTION
Figure 1. Potentiostot for controlled potential electrolysis Top row Voltmeter to monitor potential between working electrodes, 0-10 volts. Diol for setting control potential. Ammeter to monitor cell current, 0-10 amperes, 0-1 ampere, and 0-0.1 ampere Bottom row On-off switch. Pilot light. Socket for electrode lemd plug. Current limitation control. Ammeter range switch for 10- end l - o m pere ranges. Ammeter range switch for 0.1ampere range
electrolysis process. The unit draws only that amount of power from the power lines which is needed a t the cell and the small amount needed to maintain control potentials within the instrument. The heart of this type of control system is a new .semiconductor device known as a silicon controlled rectifier (4). This device is a rectifier which will normally conduct in the forward direction only after current is applied to a gate terminal; a short pulse is sufficient. Once current is applied to the gate the rectifier continues to conduct until the forward current drops to a very low value when it ceases to conduct, just as an ordinary diode. Couductiou in the forward direction must now await a new pulse to the gate terminal. The current pulses used to initiate conduction hy the silicon controlled rectifier are conveniently obtained from a circuit incorporating another new device known as a uuijunction transistor. For a discussion of the operation of this device aud further information on silicon controlled rectifiers, see the reference cited above. The assembled potentiost.at effects control of electrode potential by testing the difference between a set internal reference potential and the potential between the cathode and a saturated calomel electrode. This potential difference is used to vary the time along the 60-cycle wave that a pulse will be generated and conducted to a circuit employing the silicon eontrolled rectifier to supply the electrolysis power. The pulse allows the rectifier to conduct for the proper fraction of the half cycle and then, as the line voltage drops to zero, the circuit resets itself. The whole process is repeated 120 times per second. This results in rapid, accnrate, and efficient control of the applied potential. 12
ANALYTICAL CHEMISTRY
Reference Potential Source. The operator of the poteutiostat sets the desired control potential with the 10-turn potentiometer dial on the front panel (Figure 1). It is connected to R Min Figure 2. The 6.3-volt winding on TI is connected to a voltage quadrupler circuit, CRrCRI2and C,-C;. After filtering by Lz and Csand regulation by the Zener diode, CRIs,about 15 volts d.c. is obtained. Rn was selected to obtain a 10-milliampere current through the Zener diode; the potential across RIB is preset a t exactly 10.0 volts by Ris. Differential Amplifier and Pulse Generator. The differential amplifier consistine of R.rR>.. (2%.B. Q2. and 0.
electrolysis current also is desired, the input signal to Q, may he switched by Sa from the lead terminating directly a t the cathode to Rs; Rs may then be set to add an additional negative potential to the cathode voltage. This additional signal depends on the position of the current range switch S, and the setting of Ra and is a function of the cell current. As the cell current decreases near the end of the electrolysis, this effect and its need vanish. The amplifier is balanced by disconnecting the power line plug and turning switch S, on so that only the ampliier is operating and adjusting R,, and Rls for zero output with the amplifier shorted and open. This is a semipermanent adiustment. ---, The amplifier output is fed through R9 and RMto Q,. The base current of
Figure 2. Top to bowom2 Plating power supply. supply
Table I. Size
Potentiostot Specifications 19 em. high X 33.5 ern. wide X 22.8 cm.
Weight Output current
10.9 Kg. 5 amperes nominal-7 amperee maximum Output voltage 10 volts maximum Input resistance 10' ohms differential Sensitivity Approx. 2 mv. input/ volt output Line regulation Approx. 0.1% a t cathode (1 mv./volt) Response time Approx. 40 millisec. Power required 117vo!tsa.c., IOOwatb maximum Overall efficiency Up to 60%
QI controls the current passing through its collector circuit to charge C, (4). When the capacitor has charged to about 10 volts, the nnijunction transistor Q%abruptly conducts and the pulse developed is coupled by transformer Ts to the gate terminal of the silicon controlled rectifier, SCR. Since both the trigger circuit and the silicon controlled rectifier circuit operate from power line transformer secondaries, they are alwap in phase. Pulsating direct current for the trigger circuit is supplied by the 125-volt winding on Ta. This current is rectified by CR,-CR,, dropped hy resistor R8, and clipped by the 20-volt Zener diode CRe. Plating Power Supply. TIfurnishes low voltage to the following rectifier assembly at 12 volts each side of center tap. CR, and CR, conduct on alternate half cycles. The silicon controlled rectifier, SCR, completes the circuit when triggered by a pulse from TJ,supplying current to the filter L,C1and the electrolysis cell for a fraction of each half cycle. The filter
Circuit diagram of potentiostat Pulse generotor and differential omplifier.
Reference potential
reduces the ripple to a maximum of about 80 mv., whicki may be further reduced if necessary by adding conventional filter elemeiits or a transistor filter. Since the supply circuit is effectively open for a part of each half
Table II. F'arts List 1 ohm, 5 watts 10 ohms, 1 watt 0 1 phm, 10 watts, Sichrome
Xi Ri Rs
wire
R4 Rs Re
Ri
Ra Rs, Rio Rii Biz
Ria, Ru Rib RIE R17 Ria Ri B C1 C:
c3
c,-c7 cs
CRl-CRa CR4-CR7 CRB CRrCRi2
CR,, I SCR
Q2
Ti
Tz TI
trolytic
50 Mf., 50 volts electrolytic 250 pf., 5C volts electrolytic Diodes, hl otorola PrIR322 Diodes, Motorola 1x1695 Zener diode, 20 volts, Hoffman 1Y1778 Diodes, Allied Radio ME200 Zener diode, 15 volts, Pacific lX718P Pilot lighi, 117 volts
Silicon controlled rectifier, GE 2N1770A
Transistol, Texas Instruments TI495 Unijunction transistor, GE
Qi
Qr,
100 ohms, */2 watt 500 ohms potentiometer 3.3K ohmai, 5 watts 330 ohms, watt 4.7K ohmrr, watt 51K ohms watt 500K ohms potentiometer, Bourns 3068-S 1K ohms potentiometer, Bourns 3067-S 2.5M ohms, watt 2.2K ohmci, watt 25 ohms, watt 15K ohms potentiometer 2.OK ohnis, 10-turn potentiometei., Borg Model 205 10,000pf., 20volts electrolytic, Mallory 2050-50 0 1 pf., 200 volts paper 4 pf., 50 17olts tantalum elec-
2N1671A
Q4
Transistow, GE 2N336A Transformer, 24 volts, 4 ampere 1, Stancor P-6378 Transformer, 125 voltg, 6.3 volte, Knight 61G410 Pulse transformer, 12 mh., 1.1 rat 0 , Gudeman G1057-
in
-_
Li L Y
H
Choke, 0.131 henry, 8 amperes, Stancoi C-2687 Choke, t;.5 henry, 50 ma., Knight 620136 Battery, 30 volts, Burgess T-nn
1 LU
A
PAG quick-acting instrument fuse, 5 amperes 3iiG fuse 1 ampere SPST SC: switch, C-H 8411K7 DPDT svitch, C-H ST52N SPDT mitch, Ganged to Rb DPST sxitch, C-H ST52K Meter, 1 ma., Emico RF-2
V
Emico RF-
Fi
F*
S, SZ SI
SC
c
Cabinet Chassis Socket Plug Dial Model RB Terminal strip Battery Keystone S o . 184 holder Miscellaneoushardware obtained locally
cycle before the SCR is triggered, a "free-wheeling" diode, CR2, was added to allow the induced current in L1 to continue to flow. The shunt resistors, R1, Rz, and Rl, are switched in as desired to change the meter ranges to l, 0.1, or 10 amperes. The 0.1-ampere range is selected by a push button. Meter V monitors the voltage applied to the electrolysis cell. Operation. The amplified error signal from the reference supply and the SCE-cathode circuit is fed in opposite phase through isolating resistors Rg and R1o to the base of Q1. It may be seen that when the cathode becomes more negative than the reference voltage, a negative signal is fed to the base of Q1, reducing its collector current to cause capacitor C2 to charge more slowly, thereby firing the unijunction transistor Q2 and the SCR later in the cycle and reducing the current and voltage applied to the electrolysis cell. As a result, the cathode potential is less negative. If the cathode potential becomes less negative than the reference potential, the opposite occurs-Le., the current and voltage applied to the electrolysis cell are increased. Electrical Performance. An error signal of about 10 mv. is sufficient t o advance the firing angle of the SCR 90°, which corresponds t o 50% power output from the electrolysis supply. For lower power output, a smaller error signal is required. At the end of most electrodepositions the applied voltage is small, thus the error signal is small and the cathode potential is within a few millivolts of the desired value. In the electrolysis of brass samples, the cathode us. SCE potential has been measured with a vacuum tube voltmeter and no error greater than a few millivolts was observed. Although the d.c. amplifier has an input resistance of only 0.1 megohm, the voltage applied to it is not the cathode us. SCE potential, but an error signal of a few millivolts. It would require an error signal of 1 volt to draw 10 microamperes from the reference electrode. The specifications of the completed potentiostat shown in Figure 1 are given in Table I. The nominal maximum current of 5 amperes and the absolute maximum current of 7 amperes was
Table 111.
limited by the design selection of the silicon controlled rectifier. Unless very efficient stirring were employed in the cell, a current capability of 5 amperes seems sufficient for the purpose. Construction and Maintenance. Construction of the unit was straightforward, the major problems, in common with most electronic construction, being layout, drilling, and sawing of the panel and chassis. The transformers were positioned so that there was no undesirable coupling between them as determined by an oscilloscope. Capacitor C1, a large unit 5 em. in diameter and 18 em. long, was mounted on top of the large choke L1 to save space. The amplifier, pulse generator, and reference supply were constructed on 5 x 7 cm. terminal boards which were stacked a t one corner of the chassis. The power diodes CR1-CRs and the SCR were mounted on aluminum plates with care taken to ensure good thermal contact. Parts list for the unit is given in Table 11. Little or no maintenance should be required except for the periodic replacement of the small battery which should last for its shelf life. To provide a power supply in its place would require enough additional components to make the battery economically attractive. The unit operated well at ambient temperatures and is designed for operation up to 50' C. with proper derating. ANALYTICAL APPLICATION
Apparatus and Materials. The unit described above was connected to the working electrodes through a Cinch-Jonas plug with No. 12 stranded copper wire t o minimize resistance losses. The platinum gauze cathode was 4.2 cm. in diameter and 5 em. high [Englehard Industries (EI) No. 6081. The anode was 1.4 em. in diameter and 5 em. high (E1 No. 613). The saturated calomel electrode was Beckman fiber-type No. 39970. Weighings were made with a Christian Becker AB-1 Projectomatic Balance; balance and weights were checked t o ensure weighing errors not exceeding h0.2 mg. Reagent chemicals and distilled water were used throughout. Brass Analyses. The procedure of Alfonsi (1) was employed for the anal-
Successive Determinations of Copper, Lead, and Tin
(NBS standard sample 124d, ounce metal) Sample wt., g. 0.9708 0,6273 1.0283 0.7711 1.0409 0.7247 0,9090 0.9526
a
Copper, 75
Lead, %
Tin, 70
Total, %
83.59 83.33" 83.57 83.49 83.55 83.68 83.56 83.54 83.57 0.058
5.19 5.29" 5.13 5.26 5.21 5.11 5.25 5.24 5.18 0.062
4.56 4.45" 4.64 4.46 4.43 4.49 4.47 4.41 4.49 0.081
93.34 93.07" 93.34 93.21 93.19 93.28 93.28 93.19 93.26 0.066
Means Std. dev. Not included in means.
VOL. 36 NO. 1, JANUARY 1964
13
'
O
-
C
-
-
0
%
t
- . 3 5 r . Limit
-
-
--- ------i
- . 4 t
-
.2
-
I 10
15
20
25
30
,
1 35
, 4c
Mlnulw
Figure 4.
Output current during electrodeposition of copper
Electrodeposition of 0.3890 gram of Cu from 0.4658 gram of NBS No. 124d. limiting potential; -0.35 volt VI. SCE. Current limit, 1 ampere
ysis of National Bureau of Standards standard samples: sheet brass No. 37b, ounce metal No. 124d, and coppernickel-zinc alloy No. 157a. One-gram
Table IV. Determination of Copper (NBS standard sample 37b, sheet brass) Sample Copper Found, g. % wt., g. 0.4399 70.41 0.6248 0.8269 0.5815 70.32 0,7034 0.4950 70.37 0.8039 0.5657 70.37 1.0493 0.7376 70.30 0.7082 0.4985 70.39 Mean 70.36 NBS value 70.36 0.041 Std. dev. Table V.
Determination of Copper
(NBS standard sample 157a, coppernickel-zinc alloy) Sample Copper wt., g. Found, g. % 0.7010 0.4108 58.60 0.6520 0.3819 58.57 0.7877 0.4619 58.64 1,0106 0.5924 58.61 0.8622 0.5059 58.67 0.8074 0.4730 58.58 Mean 58.61 NBS value 58.61 Std. dev. 0.037 Table VI. Zinc Recovery (NBS standard sample 43d, zinc freezing point standard) Zinc, g. Difference, Taken Found g. 0.5094 0.5096 $0.0002 0.2785 0.2784 - 0.0001 0.2141 0.2140 -0.0001 0.2700 0.2697 - 0.0003 0.2333 0.2327 - 0.0006 0.2000 0.1998 - 0.0002 Mean difference - 0.0002
14 *
ANALYTICAL CHEMISTRY
samples were dissolved in 61M hydrochloric acid with the aid of several drops of 30% hydrogen peroxide (or 16M nitric acid for the No. 157a alloy), followed by boiling briefly. After transfer to 300-ml. Berzelius beakers, 5 grams of tartaric acid, 3 grams of succinic acid, and 2 grams of hydrazine dihydrochloride were added. The sohtion was diluted to about 200 ml. and adjusted to pH 5 with 15M ammonia. The solutions were electrolyzed for copper with the cathode potential set a t -0.35 volt us. SCE. The SCE was placed in solution between the cathode and the beaker wall. To obtain more adherent deposits, the current was limited to about 1 ampere and vigorous magnetic stirring was employed. All the copper was deposited in 35 minutes with no attention by the operator. To determine lead, 20 drops of fresh 0.2% gelatin were added to the solutions after copper electrodepositions, and the lead was deposited on copper plate cathodes a t -0.65 volt vs. SCE at pH 5.4. Plating over copper simplified lead removal after weighing. After the cathodes were cleaned with dilute nitric acid, 20 ml. of 1 2 X hydrochloric acid was added and current applied to the electrodes until gassing almost ceased. Then 1 gram of sulfamic acid was added and tin deposited a t -0.70 volt. Tin results were low and erratic without the use of gelatin. Final currents were about 10 milliamperes for copper and lead and about 50 milliamperes for tin. Results of these determinations are shown in Tables 111, IV, and V. Zinc Recoveries. T o test the unit under considerably different conditions, weighed samples of NBS standard sample 43d, Zinc Freezing Point Standard, were dissolved in 5 ml. of 6 M hydrochloric acid and diluted to 200 ml.; 25 ml. of 15M ammonia and 2 grams of hydrazine dihydrochloride were added and the zinc was deposited on a copper plated cathode at -1.52 volt us. SCE. Final currents were about 20 milliamperes. Results are shown in Table VI.
Electrolysis Recordings. To follow the action of the unit during typical electrodepositions, a Varian G-11A strip chart recorder was connected to the electrode circuitry through suitable voltage dividers. By this means the smooth, stepless change of cathode os. SCE potential with time and of current with time have been observed and recorded (Figures 3 and 4). DISCUSSION
The potentiostat developed here has performed very well in all determinations attempted so far. Its output current of 5 amperes and voltage of 10 volts are sufficient for the great majority of analytical electrodepositions. Since silicon controlled rectifiers are currently available with ratings up to 235 amperes, the power output may be readily increased by the proper choicc of heavier duty components in the electrolysis power supply circuit. The output ripple, although seemingly high, has caused no difficulty and may be reduced as already mentioned. Control at positive cathode potentials may be effected by reversing the reference supply terminals. Control of anode potentials may be obtained by interchanging the proper connections. Controls are at a minimum. To place the instrument in operation, it is only necessary to set the desired reference voltage, select the proper current range and current limit, if desired, and turn on the power switch. Preset, nonoperating adjustments include only setting the voltage across the reference potentiometer to 10.0 volts and zeroing the differential amplifier. Operation under widely divergent conditions encountered in copper and zinc electrodepositions revealed no deficiencies. The instrument may also be adapted to constant voltage or con-
stant current applications. The unit is so simple to operate rind so dependable that it will serve well :in both the control and research laboratories for analytical electrolyses and other applications. LITERATURE CITED
(1) Alfonsi, B., Anai. Chim. Acla 19,
276, 389, 569 (1958).
(2) Bard, A. J., ANAL. CHEM.34, 57R (1962). (3) De Ford, D. D., Ibid., 32, 31R (1960). (4) Gutmiller, F. W. (ed.), “Silicon Con\ - - - - , -
trolled Rectifier Manual,” General Electric Co., Auburn, N. Y . 1961. (5) Lingane, J. J., “Electroanalytical Chemistry,” Interscience, New York, 1958. (6) Wadsworth, N. J., Analyst 8 5 , 673 (1960).
RECEIVEDfor review July 10, 1963. Accepted September 30, 1963. Presented at the South Carolina Academy of Sciences Meeting, Columbia, April 1963, and at the Southeastern Regional ACS Meeting, Charlotte, Kovember 1963. From the thesis submitted by Joe B. Davis to the Graduate School of Clemson College in partial fulfillment of the requirements for the degree of master of science, January 1963. Work supported in part by National Defense Education Act Fellowship.
Controlled Potential Coulometry of Metals in Fused Lithium Chloride-Potassium Chloride Eutectic ROY D. CATON, Jr,,I and HARRY FREUND Department o f Chemistry, Oregon State University, Corvallis, Ore.
b The controlled pcltential coulometric determination of platinum, copper, chromium, and valadium in fused lithium chloride-potcrssium chloride eutectic was studied. Platinum(l1) was determined with a relative error of 0.67% by plating out the metal, and copper(1) with a relative error of o.45Y0 both by deposition of the metal and b y stripping from graphite or platinum electrodes. Chromium was determined with a relative error of 0.38y0by redLCtion of chromium (111) to chromium(ll:l, and vanadium with a relative error of 1.0% b y oxidation of vanadium(ll1). Using a coulometric procedure, the standard electrode potentials of the vanadium (Ill)-vanadium( II) and chromium(111)chromium(l1) systems were determined in solutions ranging from 0.016M to 0.04M. Polarogram of niobium solutions made by anodizing the metal in the melt exhiktited two waves. Coulometric and polarographic data indicated that these solutions contained niobium(1V) and niobium(ll1) in a concentration ratio of 3 to 1.
A
procedures capable of direct app1ic:ttion to molten systems a t high tenperature are of increasing importance to present-day technology. The existence of species often stable only in the high temperature solvent make in situ methods of analysis mandatory. Thus far, the analytical techniques employed are extensions of those used in aqueoLis solutions-potentiornetry, polarography, chronopotentiometry, and couloinetry. These electroanalytical methods and their relaNALYTICAL
1 Present address, Department of Chemistry, University of New Mexico, Albuquerque, N. hl.
Figure 1.
Fused salt cell assembly
tive accuracies have been reviewed (17). Secondary coulometric titrations with electrolytically generated Fe(III), using potentiometric and amperometric end point detection, have been applied to the determination of V(I1) and Cr(I1) in the LiCl-KCl eutectic by Laitinen and Bhatia (6). The use of controlled potential coulometry as a reliable means for the determination of metals in the LiC1-KC1 eutectic was recently described by Van Norman (17). In the latter instance zinc and cadmium were determined by anodic stripping from a bismuth pool electrode and nickel was determined by stripping the predeposited metal from a platinum gauze electrode. The analyses mere performed Uranium with an accuracy of il%. in bismuth-uranium alloys was determined by stripping in a like manner to give results accurate to within + ~ 2 7 ~ . The present investigation describes the controlled potential coulometry or redox couples of type Mm+ M”+
+
( m - n)e-, which have not yet been described elsewhere. Studies of two such systems, Cr(III)/Cr(II) and V(III)/ V(I1) in the LiCl-KCl eutectic, form a portion of the work described here. The standard electrode potentials of these two systems, previously determined by Laitinen and coworkers (8, I O ) , were also evaluated here using a controlled potential coulometric procedure described by Stromatt, Peekema, and Scott (16). Cozzi and Vivarelli (9, 3) reduced concentrated hydrochloric acid solutions of niobium (V) chloride a t a mercury pool electrode to obtain chloro complexes in which the metal exhibited valence states of 4, 3, and possibly 2. Similar species might be expected in the fused LiC1-KCl eutectic in which the chloride concentration is 29.7hf a t 450” C. The electrochemical behavior of niobium in the melt was studied to determine the nature of the stable species in solution and their possible analytical determination by means of polarography and controlled potential coulometry. EXPERIMENTAL
Apparatus. CELL. The cell assembly used is shown in Figure 1. The container, which measured 67 X 260 mm., was constructed of Vycor glass, but the cover and all other components were made of borosilicate glass. The solvent, kept under an argon atmosphere a t all times during the experiments, was divided into separate portions using 10-mm.-diameter fritted compartments as described elsewhere (7, 9). Electrodes were held firmly in place with sleeves of rubber tubing, and access tubes not being used were kept closed with glass plugs. The side arm to the cell was connected t o a drying tube filled with anhydrous magnesium perVOL 36, NO. 1, JANUARY 1964
* 15
