Colin A. Vincent and J. G. Ward
University of st. Andrews St. Andrews, Fife Scotlond
T h e use of coulometric titration (I, 2) as a means of teltching redox phenomena has much to recommend it. Whereas the principle of electron transfer may be illustrated by, say the comparison of a cell reaction such as Pt(Cl,/CL-iI-, PlPt with the direct reaction taking place on the addition of chlorine water to KI solution, the quantitative nature of the electron transfer may not thus be readily apparent. Again, use of standard redox titrations has its drawbacks in that i t is not always obvious that electrons are involved. Perhaps the simplest method of illustrating the process is to use coulometry, where the total number of electrons involved in a reaction is directly determined. The quantity of electricity, or number of electrons consumed in a reaction is most easily found by using a constant current source (amperostat) and timing the period of the electrolysis. The amperostat acts as an "electron buret" with a constant flow rate. Such a device is described here which costs less than twice the price of the corresponding volumetric instrument. A number of methods have been described for the production of constant electrolysis currents. I n early work, high tension batteries were used with large resistors in series with the cell. While small variations in the relatively low cell resistance had little effect on the current, the series resistance values varied because of heating, the battery voltage decayed with time, and recourse had to be made to manual adjustment of the current. Servomechanical systems have since been devised (e.g., 5, 4) which give currents constant to better than 0.01% over long periods, although their response times are not fast. A number of electronic current regulators using vacuum tubes have also been described (e.g., 5-9) generally involving dc amplifiers Figure 1. (left1 Placement of electrolytic cell in feedback loop.
Figure 2.
(below) Circuit diagram.
A Simple Amperostat for Coulometric Titration
with feedback. A common method in present electrochemical practice is to place the electrolytic cell in the feedback loop of a standard operational amplifier unit (see Fig. 1). I n 1955, Furman, Sayegh, and Adams (10) discussed a constant current circuit using a germanium transistor as control element. The latter had a maximum output of around 5 mA and its very large temperature coefficients meant that it required thermostatting for good control. A transistor-stabilized source has also been described by KuEerovsky, et al. (11). Circuit Description
The circuit of the amperostat is shown in Figure 2. The constant current is controlled by a silicon planar transistor (type 2N696) operating in a common base configuration, taking advantage of the very high output impedance of the collcctor circuit. The current flowing in load R,, (i.e., the electrolytic cell) is determined by the voltage applied to the transistor base, Vp, and the value of RE. The collector, or load, current is given ,where Vgg is the approximately by (VP - VBB)/RB, potential drop at base/emitter junction. For a silicon transistor VBEis about 0.6 V, so that the maximum load current in the above circuit is ((6.2 - 0.6)/270) X lo3 $ 20 mA. The 5 kQ potential divider allows a linear variation of load current down to less than 1 mA. An alternative method of controlling the current is to make Re variable, but this gives a nonlinear current scale. The maximum load resistance which the circuit will accept is determined by the supply voltage. When the voltage drop across the load resistance increases, the transistor supply voltage decreases. If the voltage across the transistor (collector to emitter) falls below about 2 V, the transistor ceases to control the current. The transformer and rectifier circuit provide 25-30 V dc, but if i t is thought desirable, the mains power supply can be removed and the circuit power supplied by a battery a t the points XX. The unit is completely safe electrically and the output terminals may be left open or short-circuited without damaging the transistor. The components may be readily assembled in a 4 X 3 X I-in. diecast box (see Fig. 3). Performance
Within the load limits discussed below, the current remains constant to *0.1% for an hour, once the instrument has attained its working temperature. This was considered acceptable and no attempt was made to introduce temperature compensation, although this may easily be accomplished a t no extra expense. The base/emitter junction potential has a negative temperaVolume 46, Number 9, September 1969
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Figure 4.
Figure 3.
Assembled ompemrtot.
ture coefficient of some 2.5 mV/"C while the zener diode has a positive temperature coefficient of the same order of magnitude. Thus ( V p - VBE),and hence the load current, increases as the temperature of the unit rises. This effect may be almost eliminated by choosing a zener diode with a negative temperature coefficient similar in magnitude to that of the transistor base/emitter junction. This would involve changing the 6.2 V zener diode to a low voltage type since the sign of the temperature coefficient of zener diodes generally changes from positive to negative a t value below about 5.6 V. RE would then have to be decreased in order to pass the same maximum current. The performance of the amperostat a t various current levels is illustrated in Figures 4 and 5. As discussed above, the instrument is limited in supplying current to high impedance loads or cells by the supply voltage, about 25 V in this case. The accuracy should be sufficient for teaching purposes with normal coulometric cells. Electrode Assembly
An inexpensive electrode assembly is shown in Figure 6. The generating electrode is a piece of thin platinum foil, which need be no larger than 1 cm2 for most applications. The counter electrode is a spiral of platinum wire housed in a glass tube terminated by a sintered glass frit ("Pyrex" microfilter) which serves to isolate products of the counter electrode reaction from the main cell solution. The assembly is lowered into a 100-ml beaker containing the solution to be titrated. While very efficient stirring may he effected by means of a magnetic stirrer, manual agitation is adequate for low currents. The unit has been successfully tested in the titration of thiosulfate with electrogenerated iodine ( I @ , acidbase estimations, etc. The former experiment is specially recommended for teaching purposes bccause of the striking visual end-point produced in the presence of starch.
Figure 5. Load limit curves for range of availoble currents.
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Figure 6. arrembly.
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Lilerature Cited (1) LINGANE, J. J., "Electr~analyti~al Chemistry,"Interscience (division of John Wiley & Sons, Inc.), New York, 1958,
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(2) ABRESH,K., AND CLAASSEN, I., "Coulometric Analysis," Chapman and Hall, London, 1965. (31 LINGANE. J. J.. Anal. Chem.. 26. 1021 (1954). i4j DUNN,F: J., MANN, J. B., AND MOSLEY, J. R.,Anal. Chem., 27, 167 (1955). (.5) CARSON, W. N., JR., Anal. Chem., 22, 1565 (1930). (6) DEFORD, D. D., JOHNS, C. J., ANDPITTS, J. W., Anal. Chem., 23, 941 (1951). H. N.. Trans. Farad. Sac. 48. (7) Smno. M.. AND PARTON. ,
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~noMsoN,H. B., BOEHM, M. J., A N D ROGERS,M. T., J. C ~ MEouc., . 32, 463 (1955). D. T., AND COVINGTON, A. K., J. Sci. Inst. 34, HOPKINS, 20 (1957). N. H., SAYEDH, L. J., AND ADAMS,R . N., Anal. FURMAN, Chem., 27, 1423 (1955). M... .AND SISKA. K Y PRIRYL. . , K U ~ E R ~ V S Z.. . M... Chem. Lisly, 59, '604 ' (1965). P. S., A N D MLADENOVIC, S., Anal. Chim. Acta, (12) TUTUNDZIC, 8, 184 (1953).
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61 4
Percentage of rated current posing through load.
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