Electrolysis : Determination Of Conductivity

If an ordinary current from a battery is passed between platinum electrodes in a solution of an electrolyte, and a galvanometer included in the circuit the current strength diminishes as electrolysis proceeds. This is partly due to the accumulation of the products of electrolysis at the electrodes. These form a galvanic cell which tends to send a current in the opposite direction to that driven round the circuit by the battery. This reverse electromotive force, tending to oppose the direct electromotive force of the battery effecting decomposition, is known as the electromotive force of polarisation.

In order to obtain accurate measurements of the conductivity of electrolytes it is necessary to eliminate polarisation. F. W. Kohlrausch (1869) did this by using an alternating current, i.e., a current which flows alternately in one direction and then in the other, with a very small interval of time between the reversals of direction. Such a current is supplied by an induction coil attached to a battery. The ions are driven first in one direction and then in the other by the alternating current, and the amounts deposited on the electrodes are exceedingly small.

Polarisation is still further reduced by depositing platinum black on the electrodes, by electrolysing between them a solution of 1 gm. of chloroplatinic acid and 8 mgm. of lead acetate in 30 c.c. of water, with an accumulator, and reversing the current from time to time. The platinum black exposes a large surface.

Expt. 6. - A convenient type of electrolytic cell, shown on the right in,

Fig: Apparatus for measurement of conductivity

consists of a small bottle, with parallel platinised platinum electrodes. The wires from the electrodes, which are covered with glass inside the cell, pass into glass tubes containing mercury, and wires dip into the mercury to make contact. These wires pass through rubber tubes, so that the cell may be immersed in a tank of water kept at a constant temperature, say 18° or 25°.

Since a galvanometer cannot be used with an alternating current, a telephone is employed. A resistance-box is connected with the cell, telephone, coil, and a metre wire-bridge with a scale and sliding contact. The connections are shown in

Fig: Diagram of conductivity apparatus

The slider is placed near the middle of the bridge, plugs are taken out of the box until the sound in the telephone is appreciably reduced, and the slider is moved until the sound in the telephone is a minimum.

The resistance of the conductivity cell is given by the Wheatstone bridge formula

ohms. Ohm's law has been proved experimentally to apply to electrolytes. The conductance is 1/r, i.e. the current passing in amperes for 1 volt potential difference between the electrodes.

The electrodes of the conductivity cell will not usually be exactly 1 cm² in area, parallel, and 1 cm. apart, so that the conductance, 1/r, is not usually equal to the conductivity k. Since the relation between the two depends only on the construction of the cell, it is possible to determine this once for all by using as electrolyte a solution of known conductivity, viz., a normal solution of potassium chloride (74.555 gm. per litre, weighed in air), for which at 18° k_18° = 0.09822. If the resistance of the cell containing this solution is r, k_18° = C/r = 0.09822, where C is the cell constant. If any other solution is used, and if the resistance is r' ohms, the conductivity is k'_18° = C/r'.

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Electrolysis
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