step height. Measure the height of the polarographic wave which occurs approximately at a half-wive potential of -0.25 volt. Compare the wave height, which is directly proportional to the concentration of tin with the wave height of standards. RESULTS
The method was applied to the determination of tin in various samples of known tin content (Table 11). Interferences. T h e distillation step separates tin effectively from most elements with which i t is usually associated in minerals, metals, a n d alloys. Only arsenic and antimony of t h e more common elements accompany t h e tin into t h e distillate. While Lingane states (11) that arsenic(V) and antimony(V) are not reduced a t the dropping mercury electrode, his experiments were carried out in chloride solutions while the present investigation involres a mixed chloride-bromide medium. The use of hydroxylamine hydrochloride undoubtedly causes a t least partial reduction of both the antimony and arsenic to the + 3 state. The results in Table I11 indicate that arsenic does not interfere in concentrations u p to 10 times those of tin; antimony affects the polarographic wave to some extent. Although this effect is insignificant for concentration of antimony u p to five times those of tin, larger quantities slightly repress the polarographic wave of the tin. This negative effect of the antimony can be compensated by the addition of equivalent amounts of antimony to the standard.
Table II.
Accuracy
of Proposed Method
Marks NBS 164 NBS 62C XBS 37B NBS 53C NBS 50.4 NBS 170 NBS 152 NBS 55A w.4 44
Material Mn-41 bronze Mn bronze Sheet brass Lead-base bearing metal Cr-W-V steel Open hearth steel Open hearth steel Open hearth steel Ti-alloy Zircaloy 2 Zircaloy 2 Zircaloy 3 Ferroniobium Ferroniobium tantalum Tantalum ore Tungsten ore Tungsten ore a Average of duplicate determinations.
Table 111.
Interference of Arsenic and Antimony
Tin, Mg. Present Found 1.0 1.00 1.02 0.99 5.0 4.99 5.03 1.0 1.00 0.99 0.96 5.0 5.02 4.80 10.0 9 65
Present, Mg. Arsenic Antimony
.. 5
10
20 50
..
.. ..
.. .. ..
...
... ... ...
...
2
5
10 20 50
100
LITERATURE CITED
(1) Allsopp, W. E., Damerell, V. R., ANAL.CHEJI.21, 677. (1949). (2) Am. Soc. Testing Materials, Philadelphia, L‘ASTMhlethode for Chemical ‘Analysis of hfetals,” p. 182, 1956.
(3) (4) (5) (6)
Tin Preeent, yo Tin Founda, 0.61 0.63 0.40 0.39 0.98 0.99 5.12 5.17 0.029 0.025 0.020 0.018 0.033 0.036 0.009 0.007 2.52 2.67 1.32 1.32 1.24 1.24 0.23 0.24 0.19 0.17 1.31 1.32 0.65 0.65 0.18 0.17 0.F5 0.87
70
Ibid., p. 186. Ibid., p. 187. Ibid., p. 339. Baker, I., Miller, M., Gibbs, S., ISD. ESG.CHEM.,ANAL. ED. 16, 269
(1944). (7) Beeghly, H. F., Aiv.4~. CHEJI. 21, 1,513 11949). Clark, R. E. D., A n a l y s t 61, 242 (1936); 62, 661 (1937). Farnsworth, M.,Pekola, J., AKAL. CHEM.26, 735 (1954). Hillebrand, W,F., Lundell, G. E. F., Bright, H. A., Hoffman,. J. I., “Applied Inorganic .4nalysis,” pp. 287, 289, Wiley, Sew York, 1953. Lingane, J. J., IND.ESG. CHEW, AXAL.ED. 15, 583 (1943). Lineane. J. J J Am. Chem. SOC.67, 9T9 (1945): (13) Schemer, J. -4.,BUT.Standards J Research 8 , 309-20 (1931). (14) Stone, I., ISD. ENG.CHEM.,ASAL. ED.13, 791 (1941). RECEIVEDfor reviem- August 21, 1957. Accepted December 26, 1957.
Apparatus for Automatic Controlled Potential Electrolysis Using an Electronic Coulometer LYNNE L. MERRITT, Jr., ERNEST 1. MARTIN, Jr.l, and RAM DEV BED1 Department o f Chemistry, lndiana University, Bloomington, Ind. An electronic instrumental circuit has been constructed, tested, and used in automatic controlled-potential coulometric electrolyses. It includes a constant-current source to supply current to a large capacitor, an electronic tripper circuit to limit the potential range of the working electrode, and a signal generator-scaler circuit for measuring the total time during which the capacitor i s charged. The capacitor serves as the current source for the electrolysis cell. By measuring the time that the constant current is applied to the capacitor and thus the total number of coulombs used for the elec-
trolysis, the concentration of electroactive species can b e calculated from Faraday’s law. Oxidation of iodide ion to iodine, reduction of dichromate ion to chromic ion, and deposition of silver and copper have been successfully carried out. Possible sources of error are discussed and several methods of improving the instrumental system are proposed.
A
of instruments for carrying out electrolyses a t controlled working-electrode potentials are reviewed by Lingane ( 6 ) and Delahay ( 2 ) . These SUMBER
instruments hare bcen combined with chemical coulometers or mechanical or electrical current-time integrators (1, 6 , 8) to measure the number of coulombs required for the electrolysis. However, chemical coulometers are inconvenient to use and most of the mechanical and electrical integrators developed up to the present time leave much t o be desired in precision and range of currents oyer which they operate. The current integrator described by ILIeites ( 8 ) is precise and accurate t o 1 Present address, Shell Chemical Co., Houston, Tex.
VOL. 30, NO. 4, APRIL 1958
487
10.1% for currents varying about 70fold for any one input resistor. The range of currents can be changed by changing the input resistor. An allelectronic coulometer has recently been described by Booman (1). It is rather difficult to compare the precision of Booman's apparatus with the one described here, because of different methods of expressing the results, but the two devices seem to be roughly comparable, judging by standard deviations for silver depositions-O.04- to 0.05-mg. for a single observation, 14 to 19 determinations, on the instrument described, compared to Booman's value of about 0.0401, for about 0.06 to 61 coulombs, six determinations a t each value. The complexities are also comparable, as the device described herein requires 35 tube envelopes and a relay, while Booman's contains 24 tube envelopes, three vibrators, and six selenium rectifiers. Five tubes can easily be eliminated from the coulometer described, by employing two power supplies rather than three. Another approach has been to carry out the electrolysis a t constant current and to measure the time required for completion of the reaction. This method is generally used for secondary processes, in which a reagent is generated in or added to the solution, or for anodic stripping of plated deposits ( 3 ) . Again, these methods are not as convenient as a primary process in which the number of coulombs required to carry out the electrolysis is measured directly. The apparatus described in this paper is capable of performing electrolytic oxidations or reductions a t controlled working-electrode potentials with automatic registration of the number of coulombs required. EXPERIMENTAL
Instrument. A block diagram of the instrument is shown in Figure 1. The large capacitor serves as the current source for the electrolysis cell. When the potential of the working electrode measured against a reference electrode falls below the limiting value, the tripper circuit activates the relay in such a way that the constant current source is connected to the capacitor and the signal generator is connected to the scaler. When the potential of the working electrode equals the preset limiting value, the tripper throws the relay, so that the constant current passes through the bypass resistor and the signal generator is disconnected from the scaler. Thus the number of coulombs passing through the cell can be determined by multiplying the time shown on the scaler by the current furnished by the constant current source. The latter can be measured accurately by the series resistor, R1, and the potentiometer. The first constant current source em488
ANALYTICAL CHEMISTRY
I i qnol qenerot or
'co"r
I
b I
D.C.
I
Figure 1 . Schematic diagram of coulometer and potentiostat
I
ployed was the magnet supply circuit reported by Elmore and Sands (4, modified by replacing the lB4-P tube with a 657 sharp-cutoff pentode. A 400-ohm, 200-watt power resistor was substituted for the oil-filled Manganin resistance box with some consequent loss in stability of the current. The output was taken from the cathode circuit of the 807 tube and grounded a t the negative terminal of the output. This change kept the cathode of the cell a t ground potential and prevented random cathode-reference electrode potential variations. The performance of the constantcurrent supply was checked by measuring the voltage drop across a General Radio Co. precision decade resistance box with a Leeds & Northrup student potentiometer. A 50-ohm, 10-watt power resistor was connected to the output terminals as a substitute for the electrolysis cell. After a warmup period of 1 hour, the current output was measured a t 10-minute intervals for several hours and for six values of the current. The coarse current control was maintained a t a fixed position and the current varied by changing the bias voltage on the 6J7 control grid from -4.5 to -22.5 volts. The maximum current deviation in this range was 0.02%. In second instrument the constantcurrent, supply described by Ehlers and Sease was used (S), without the potentiometers and resistors used for current ranges below about 6 ma. and with 25watt power resistors instead of the lamps used for the highest current range.
a
This circuit is much more compact than the magnet supply but, because of lower potentials, it is somewhat more sensitive to large variations in load resistance. Another possibility, which was not investigated, is the circuit described by Gerhardt, Lawrence, and Parsons (6), based on an earlier circuit of Reilley, Adams, and Furman (9). These latter circuits employ dry cells, which are not necessary in the circuit of Ehlers and Sease. The tripper circuit described by Ehlers and Sease (3) was used with a few modifications. The input potential must be negative with respect to the chassis ground and the tripper activates the relay when the highest potential becomes more negative than the preset value. By use of a four-deck switch one input electrode is connected to
either the top or bottom end of the two 10-kilo-ohm potentiometers, the input terminals are exchanged between reference and working electrodes, and the meter is placed between either end of the 10-kilo-ohm potentiometers and sliding contact. This makes it possible to select the proper connections for the following four situations merely by turning the switch to one of four positions. 1. Reduction reaction with reference electrode more negative than working electrode at the final limited potential. 2. Reduction reaction with reference electrode more positive than working electrode at the final limited potential. 3. Oxidation reaction with reference electrode more negative than working electrode at the final limited potential. 4. Oxidation reaction with reference electrode more positive than working electrode at the final limited potential.
The timing and scaling circuit employs a Time Products Corp. Jlodel 2001-2L tuning fork-controlled oscillator as the primary pulse source, followed by a pulse-shaping circuit, Berkeley decade scalers, and a register. The reproducibility of this circuit, determined by comparison with WSW signals, is within 0.008%. The complete electronic circuit is shown in Figure 2. The cell for the determinations described is shown in Figure 3. A "fine" sintered-glass disk separated the cathode and anode compartments and practically eliminated diffusion between the two sides of the cell. The larger compartment held approximately 125 ml. and the smaller chamber held about 25 ml. Kitrogen was passed through the cell by means of the side-arm inlet tube during all electrolyses in which the working electrode potential was below about +0.2 volt. The nitrogen removed oxygen from the solution. The nitrogen gas was purified by passing the tank gas through a 0 . 5 U chromous chloride plus 0.5X hydrochloric acid solution, then through 1 M sodium hydroxide, 0.1M mercuric chloride, and, finally, distilled water. A platinum gauze electrode having an estimated surface area of 66 sq. cm. was used in all determinations. This electrode was held in the large compartment by a tightly fitting rubber stopper. Both reversible and irreversible auxiliary electrodes were used to complete the cell circuit. A stationary silver anode consisting of a closely wound helix of pure silver wire (diameter 2.5 mm., total area 21 sq. cm.) was used in a solution containing 0.8M sodium chloride to give a reversible electrode. The silver chloride coating which formed on
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a n
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3 0
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8
53 L % rl 3
P W
0
ii; 5.
8 P f
4
P ?
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g
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Il
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I
I
I
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VOL. 30, NO. 4, APRIL 1958
489
this anode was removed periodically by reduction a5 the cathode with a platinum anode in dilute sulfuric acid electrolyte. A flat sheet of perforated platinum, having an area of approximately 8.0 sq. cm., was used as an irreversible electrode. The reference electrode was a saturated calomel cell. The calomel electrode n a s connected to the cell by an inverted U-tube-salt bridge filled with 1 . O J 1 sodium nitrate in 3% agar solution for the silver determinations or 30% potassium chloride in 3% agar solution for the other determinations. The salt bridge was placed as close as possible to the working electrode. The solution in the large compartment was stirred by a motor-driven stirrer. The maximum output potential t o the cell is limited to 24 volts by the working voltage of the large capacitors. The constant current source is capable of a constant output u p t o about 150 volts. The maximum current is limited by the output of the constant current source to about 90 ma. Operation. The constant current source, t h e tripper circuit, t h e signal generator, and t h e scaler are turned on and alloned to warm u p for 0.5 hour, so t h a t their operation becomes stable. T h e current is switched through t h e bypass resistor during this period. The current is measured accurately by determining the potential drop across the series resistor, R1,with the potentiometer. R1 is so chosen that the potential drop across it !Till be about l volt. The proper setting of the tripper circuit can be determined experimentally or by calculation, if the voltmeter in the tripper circuit is known t o be accurate. I n the latter case, the potential corresponding to the desired residual concentration or to that for start of deposition of the next less noble element is calculated from the Kernst equations and the maximum expected concentration. To determine the optimum setting experimentally, the supporting electrolyte and the electroactive species are introduced into the cell and the potentiometer settings necessary just to close the tripper circuit are noted. The potentiometers are then reset so that the meter reading is a t least 0.25 volt more negative (for a reduction reaction) for a 1-electron reaction or 0.12 volt more negative for a 2-electron reaction. Care is taken to prevent undesired reactions within the limits of the potential range of the working electrode. The cell is prepared for the actual determination by placing the anolyte and anode in the small compartment (for a reduction reaction) and the supporting electrolyte and cathode in the large conipartinent. The salt bridge connected to the reference electrode is introduced, the stirring motor and the nitrogen gas, if needed, are turned on, and current is passed through the supporting electrolyte for a few seconds to remove any impurities. The timer is reset to zero, the sample is introduced into the supporting electrolyte in the large compartment, and the timer cir490
0
ANALYTICAL CHEMISTRY
B I
Figure 3. A. E. F.
M. N. S. T.
Electrolysis cell
Platinum gauze electrode Platinum sheet electrode Fine sintered-glass disk Coarse sintered-glass disk Inlet for nitrogen Salt bridge Bearing for stirrer
cuit and current-source-to-cell circuit
are turned on again. The electrolysis is allowed to proceed until the tripper circuit closes the relay for only a few milliseconds out of every minute. The scaler is read, the electrolyte is removed from the cell, and the cell is ready for another determination. Another sample may be added directly to the electrolyte, if the volume is not too great and the sample is not to be used for any other purpose. The weight of substance reacting is calculated by means of Equation 1.
where
weight, grams current,amperes time,seconds F = value of the faraday (96,494 coulombs) Af = molecular weight n = number of electrons involved in the reaction per molecule z t
= = =
Reagents. All chemicals were reagent grade. A standard solution of silver nitrate mas prepared by dissolving a rreighed amount of t h e dried salt in n a t e r and diluting t o the mark in a volumetric flask. I t Tvas stored in bronn bottles. -4standard cupric sulfate solution was prepared in a similar manner from lveighed amounts of cupric sulfate pentahydrate. The iodide standards were prepared from weighed amounts of dried potassium iodide and used on the day of preparation. Standard solutions of potassium dichromate were prepared from weighed samples of Bureau of Standards potassium dichromate. Aliquot portions of these standard solutions were added to the supporting electrolyte in the large compartment of the electrolysis cell. Silver Determinations. Coulometric depositions of silver were carried out using a platinum cathode and a reversible silvex-silver chloride anode for from 38.78 t o 0.194 mg. of silver ion. The catholyte contained 0.8X sodium nitrate, 0.05J1 perchloric acid, and the standard silver solution. The anolyte contained 0.851 sodium chloride and
0.05J1 perchloric acid. Nineteen determinations gave a standard deviation for a single determination of 0.05 mg. Silver was also determined in an irreversible anode system. The supporting electrolyte in both cell compartments was 0.831 sodium nitrate and 0.0551 perchloric acid. Fourteen determinations of silver ranging from 38.78 doim to 0.194 nig. gave a standard deviation of a single determination of 0.04 nig. Silver was also deposited in the presence of cupric ion using a platinum cathode and silver-silver chloride anode. The amounts taken ranged from 19.44 mg. of silver and 0.665 mg. of copper to 5.83 mg. of silver and 13.29 mg. of copper. Ten determinations gave a standard deviation for a single observation of 0.08 mg. I n all cases the cathode n-as limited to 0.15 volt positive n ith respect to the saturated calomrl electrode. Copper Determinations. Copper n as electrodeposited on a platinum cathode using both the reversible and t h e irreversible anode systems. With a silver-silver chloride anode, an anolyte of 0.821 sodium chloride plus O . O L l 1 perchloric acid, and a catholyte of 0.8JI sodium sulfate plus 0.05JI sulfuric acid, 11 determinations gave a standard deviation of a single observation of 0.07 m a Cupric ion taken ranged - froin 13729 to f.99 mg. \Then a olatinum anode in 0.851 sodium sulfaie plus 0.05X sulfuric acid was used, with a catholyte of the same composition, six determinations ranging from 6.65 to 1.99 mg. gave a standard deviation of a single observation of 0.02 mg. Copper was also deposited in the presence of nickelous ion. The amounts ranged from 13.29 mg. of cupric ion in the presence of 4.32 mg. of nickelous ion to 3.324 mg. of cupric ion plus 13.4 mg. of nickelous ion. The reversible silver-silver chloride anode in 0.8i11 sodium chloride plus 0.05Y perchloric acid and a platinum cathode in 0.8N sodium sulfate plus 0.05X sulfuric acid was used, Ten determinations gave a standard deviation of 0.08 mg. I n all cases the cathode was limited to -0.20 volt with respect to the saturated calomel electrode. Oxidation of Iodide Ion to Iodine. Iodide ion was oxidized t o iodine by using the platinum gauze electiode as t h e anode. The laige anode conipartment contained t h e iodide ion in 1.OM peichloric acid. The perforated platinum sheet m s used as cathode in a solution of 1.0X perchloric acid. The tripper limited the anode to 4-0.83 volt with respect to the saturated calomel electrode. Eighteen determinations gave a standard deviation of 0.18 mg. for a single determination. Iodide ion ranged from 55.64 to 1.67 mg. Standardization of Dichromate Solutions. The norniality of a n y 0-4dizing or reducing agent can be readily determined from the final timer reading, the current, and t h e volume taken. 4 s a n example, dichromate \\-as reduced to chromic ions a t a potential of 0.10 volt positive with respect to the saturated calomel electrode. The ano-
lyte and catholyte were 1.Y in sulfuric acid. Excess sodium iodide (about 1 gram) was added before electrolysis. The large platinum electrode lvas used for the anode and the small platinum electrode for the cathode. B 0.10116N solution (by calculation from weight and volume) gave a n average value of 0.10122.V in four determinations with a standard deviation of 0.00006. Twomilliliter samples were used and the current was 60.00 ma. Determination of Silver a n d Copper in Silver Solder. A carefully analyzed sample of silver solder was obtained through the couitesy of Harvey Diehl, I o n a State College. A sample weighing 0.1167 gram n a s dissolved in 2 to 3 ml. of 6.1- nitric acid with heating. After solution was complete, 10 ml. of ‘70 to i2YG perchloric acid was added, and the solution evaporated to fumes of perchloric acid. The sides of the beaker w r e riiised n-ith distilled v a t e r and the solutioii was again evaporated to fumes of perchloric acid. The solution was diluted to about 50 nil. v i t h water and transferred to the large compartment of the electrolysis cell. Electrolysis of the silver was carried out a t +O.l5 volt with respect to the saturated calomel electrode. using the irreversible platinum anode. After this electrolysis was complete, the potential was reset to -0.20 volt us. S.C.E. The timer n a s reset aiid electrolysis of the copper was carried out. The silver and copper found w r e 50.86 and l5.i4YG, respectively; Diehl’s figures ivere 50.92 and 15.’74%, rrspectivcly.
If the current a t any given instant is proportional to the concentration of electroactive ion remaining in solution, it = act (3) From derivations based on diffusion layer theory (6), the constant, a, would be giveii by Equation 4.
a=- nFD.4
where D
il 6
= = =
diffusion coefficient electrodearea diffusion layer thickness
Substituting Equation 3 in Equation 2 and rearranging, give Equation 5. ACTUAL
ELECTROLYSIS
TIME,
YINUTES
Figure 4. Plot of logarithm of cell current against electrolysis time-oxidation of iodide to iodine A.
Current limited b y output of constant current circuit
B.
C. D.
Tripping begins Experimental curve Extrapolated curve
DISCUSSION
I n ail t.lectrol~-sisconducted with a limited current supply aiid a limited n-orking clectrotle potential, as with the instrument described in this article, electrolysis proceeds at a coiistant rate until the potential of the working electrode reaches the limited value. At this point. tripping begins and the current through the cell gradually decreases Lingaiie (7‘) has observed that the logarithni of the current plotted against time i i a straight line and has Fhonii (6) that this relationship i? to be e~pectetltheoretically. Figure 4 shows such a plot for the oxidation of iodide ion to iodine as carried out with this instrunwilt. The deviation a t very low curwiit> could be due to leakage iii the coildenser or through the solution, tCJ overstepping the limited potential v:ilue by inability of the relay t o operate fast enough, or to failure of the fuiidaniental relationship to hold a t very low currents. The bulk of the deviation is probably due t o the “iiornial residual current” always found i n electrolyses. The condensers n-eie scslccteti for low leakage, which amounted to only 1 volt in a day or more. The error involved is very slight, as the currents are very small. A useful relationship was observedafter tripping begins, current passing
(4)
6
RECORDED
TIME,
MIN.
Figure 5. Plot of cell current against time recorded on register, oxidation of iodide to iodine A.
Current limited b y 6. Tripping begins output of constant C. Experimental curve current circuit D. Extrapolated curve
through the cell plotted against time recorded by the scaler gives a straight line (Figure 5). This relationship can be derived in the follon ing manner. Let io = constant current supplied by thp source it = current passing through the cell a t time t, after tripping begins a t time t o f, = time recorded on the scaler, after tripping begins i.e., time that current io has been applied to cell circuit after tripping begins C, = molar concentration a t start of tripping Ct = molar concentration a t time t
Then, from Faraday’s law, iot,
=
nFV(C0 - C,)
(2)
Equation 5 represents a straight line when 1, is plotted against it. This plot may be useful for determining the final recorded time by extrapolation to zero it. As the Schmitt trigger circuit has a certain hysteresis. about 0.040 volt for the tripper circuit employed in this instrument, the potential of the working electrode must fall by this amount before the tripper circuit reactivates the relay. This hysteresis can be set a t any desired value by varying the amplification of the direct current amplifier (6SLi); however, the 40-mv. range seems suitable for all ordinary separations. At the start of the triggeriiig action, when concentration polarization is significant, the tripper action is rapid. Triggering action gradually decreases until, finally, the relay may be activated once, for only a few milliseconds, every few minutes. Sources of Error. The time required for charging t h e capacitor contributes a slight error t o t h e recorded electrolysis time. At t h e beginning of a n electrolysis, t h e capacitor is uncharged and a t the conclusion of tripping. it is charged. The charging time error may be calculated in terms of weight error for a n individual determination froni the equation Keight error =
clFiV
~
grams
(6)
where M is the molecular weight of the reacted species, AT’ is the difference between the initial aiid final anodecathode potentials, n is the number of electrons consumed per molecule or ion of the reactant, F is the value of the Faraday, and C is the capacitance in farads. The capacitor can be charged, if desired, to the control potential or to the final potential expected, before the electrolyses begins. This is automatic, if the solution is preelectrolyzed before the u n k n o m is added, in order t o remove traces of impurities in the reagents. As the tripping process is concluded, the current source to cell circuit must VOL. 30, NO.
4, APRIL 1958
491
remain closed only long enough to recharge the capacitor until the working electrode reaches the limiting potential set for the electrolysis. However, a small “time lag” prevents the tripper circuit from responding immediately t o any sudden changes in the potential of the rvorking electrode and the current source to cell circuit remains closed for longer periods of time than are now required for charging the capacitor. The response time lag for the relay operation may account for the deviations from linearity at low currents for the curves shown in Figures 4 and 5 . No attempt was made to determine the causes of the errors in these determinations. Hon-ever, to obtain accurate results, the optimum electrolysis conditions given by Lingane (6) should be determined for the reaction of each given species. Advantages, Improvements, and Applications. B y using t h e capacitor
for storage of a portion of t h e current delivered by t h e constant current source, a very efficient electronic coulometer is obtained. The recorded electrolysis time a n d t h e value of t h e constant current may be used t o calculate t h e number of coulombs of electricity delivered to the electrolysis cell. The coulometer system for the determinations reported was used t o measure the number of coulombs for currents ranging approximately from 35 ma. to as lorn as 5 ha.; or, for a current range of 7000 t o l. The instrumental circuit may be improved, the electrolysis time decreased, and the accuracy of the individual determinations increased by modifying the circuit in several mays. An increase in the value of capacitor, C, decreases the number of tripping cycles per unit time and increases the accuracy of measuring the recorded
time. The charging time error for the capacitor is increased; however, this error may be taken into account by Equation 6. The errors associated with relay operation may be decreased by substituting for the relay an electronic circuit using thyratrons or high vacuum tubes. Such a circuit will be shortly described. It will eliminate all moving parts but will require two to four additional tubes. One main difficulty is finding suitable circuits and tubes to switch the higher currents desired in many determinations. Faster relays could be used in place of the one specified by Ehlers and Sease. A4n increase in the attainable range of output currents would be advantageous in some cases. By using a high current, tripping can be initiated at the beginning of the electrolysis and the total time for electrolysis decreased. For other purposes, such as the determination of small amounts of materials, a smaller current output from the constant current source is advisable. Smaller, high-voltage, lowleakage capacitors can be employed. The time pulses can easily be of a higher frequency, should more precise time measurements be necessary. Frequency standards in the range 200 to 3000 cycles per second are simpler and cheaper than those below this range. Improvements in the design of the apparatus along these lines are being investigated in these laboratories. The automatic controlled-potential instrumental system and the electronic coulometer may be used for either controlled-potential or constant current coulometry. The controlled-potential instrumental circuit may be used for any reactions now carried out by constant controlled-potential electrolysis. I n addition t o the applications de-
scribed, controlled-potential coulometry may be used to determine the number of electrons, n, involved in an electrode reaction. W , i, T,F , and M are measured or known and n may be calculated from Equation 1. New and useful preparative methods, particularly of organic compounds, may be developed by the application of controlled-potential coulometry. By using the appropriate instrumental system and electrolysis conditions, this method may be used in any desired concentration ranges. ACKNOWLEDGMENT
This work was supported by the U. S. Atomic Energy Commission under contract At(ll-1)-120. Their support is gratefully acknorvledged here. LITERATURE CITED
(1) Booman, G. L., AN?LL.CHEM. 29, 213-8 (1957). ( 2 ) Delahay, P., ‘%‘em, Instrumental
Methods in Electrochemistry,” Part
4, Interscience, New York, 1954.
( 3 ) Ehlers. V. B.. Sease. J . W.. ANAL. CHEhf. 26, 513-6 (1954). ’ ( 4 ) Elmore, W. C., Sands, M., “Electronics,” pp. 390-3, McGram-Hill, Sew York, 1949. ( 5 ) Gerhardt, G. E., Lawrence, H. C., Parsons, J. S., ASAL. CHEW 27, 1752-4 (1956). ( 6 ) Lingane, J. J., “Electroanalytical \
I
Chemistrv,” Interscience, New York, 1953. ( 7 ) Lingane, J. J., J . -4m. Chem. SOC.67,
1916 (1945). 18) Meites. L.. -4s.4~. CmM. 27. 116-9 (195h). ’ ( 9 ) Reilley, C. S.,Adams, R. N., Furman, Tu’. H., Ibid., 24, 1044-5 (1952). RECEIVEDfor reviem May 24, 1957. Accepted December 16,1957. Abstracted \
I
from theses presented to the Graduate School of Indiana University by Ernest L. Martin, Jr., and Ram Dev Bedi in psrtial fulfillment of the requirements for the degree of doctor of philosophy. Contribution No. 812 from the Chemistry Laboratories, Indiana University.
Differential Therma I Ana lysis Apparatus for Heating and Cooling Data D. D. WILLIAMS, R. D. BAREFOOT, and R. R. MILLER lnorganic and Nuclear Chemisfry Branch, Chemistry Division,
b A versatile apparatus for differential thermal analysis, based on the concept of Rosenhain, has been developed. A constant reproducible temperature gradient is established in a heavy-walled metal tube 32 inches long. The sample holder, containing the base and differential thermocouples, i s pulled in either direction through this tube at the rate 492
ANALYTICAL CHEMISTRY
U. S.
Naval Research Laboratory, Washingfon, D.
required to produce the desired rates of temperature change. Individual runs require approximately 1 hour, but consecutive runs may b e made immediately. Simple modifications permit controlled atmosphere work.
A
DIFFERENTIAL thermal
analysis (DTA) system based on the thermal gradient produced in a metal
C.
tube has been developed to test salt systems rapidly for phase changes and reaction temperatures. The system, distinguished by its versatility, is based on the fire-clay furnace developed by Rosenhain (4). It was recently modified by Evans, Fromm, and Jaffee ( 2 ) to determine the melting points of low melting alloys. T h e principle of the apparatus was also
