THE POLAROGRAPHIC DIFFUSION CURRENT AND THE HEIGHT OF THE MERCURY RESERVOIR D. SPENCER Sir John Cass College, Aldgate, England
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principle of zero suppression' is well known in polarography, hut the following may he a new and useful application. The resnlts obtained more than justify the extra complications introduced. In the experimental verification of the relation i,rudh,where id is diffusion current and h is height of mercury reservoir, a potential of one volt is applied to a polarographic cell containing an air-free M/1000 cadmium solution in M/10 potassium chloride (E1/a = 0.7 v.), and the variation of id, due to deposition of Cd++, with h studied.l The above method suffers from the disadvantage that the increase of i, only amounts to about 20 per cent of the initial current, so that changes in galvanometer readings are relatively small anti difficult to read accurately. A typical plot obtained by students using this method is shown in Figure 1 (Line I). To increase the accuracy of the experiment the followine nrocedure mas adonted. -
' ~ N G A N B J. , J., AND H. KIRLINOER, Ind. Eng. Chem., A n d Ed., 12,750 (1940). ' Miimen, 0. H., J. CHEM.I~DvI'c.,18,172-7 (1941).
The usual galvanometer shunt mas removed and the circuit shown in Figure 2 arranged so that by adjustment of the potentiometer Rba small current, i', could be scnt throueh the ealvanometer in the owwosite direc. tion to that flowing in the polarizing circuit, and this small current, if,was arranged to be equal and opposite t,o that flowing a t the lowest height of the mercury reservoil-that is, to return the. galvanomet,er spot to zero. As this small opposing current must remain constant, resistance, R3, ~vasconnected to the accumulator t,o act as a load and steady t,he voltage. Owing to removal of the shunt the galvanometer was now much more sensitive, and an increase in height of the reservoir caused a much larger deflection of the galvanometer spot. Consequently, the change of id with h could he determined accurately (Line I1 in Figure 1). The increase in sensitivity of the galvanometer increased the variations in current during drop formation, hut this did not affectthe accuracy since "average" values for id were determined after noting the minimum and maximum readings of the galvanometer a t any one height. A Cambridge galvanometer with a resistance of 450 ohms and a full-scale deflection for 1.5 microamperes was nsed in these experiments.
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di Figurm l
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= Conventional
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Modified mothod.
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