22 2 ELECTROMOTIVE FORCE. chap.
case 71 = 1, since the electrolyte HCl consists of monovalent ions) is passed through the element in the direction of the arrow, ix, from the dilute to the concentrated solution, the following changes take place : —
(1) A gram-equivalent of mercury combines with an equivalent of chlorine from the calomel, and forms
HgCl;
(2) The gram-equivalent of mercury thus set free from the calomel combines with an equivalent of chlorine from the OOl-normal HCl solution. This latter solution, therefore, loses a gram-ion of chlorine ;
(3) At the contact surface of concentrated and dilute solution the same process takes place as in the above example ;
(4) A gram-equivalent of hydrogen from the 01-normal JBLCl solution combines with an equivalent of chlorine from the adjacent calomel solution, and forms a gram-molecule of hydrochloric acid. The concentration of the latter solution is thereby increased ; and
(5) The gram-equivalent of mercury, which becomes free on account of process (4), is deposited at the mercury electrode.
The result of these processes may be summed up as follows : —
By processes (1) and (5) the left side loses and the right side (in the diagram) gains a gram-ion of mercury. The old equilibrium can be re-established by allowing this quantity of mercury to flow back from right to left, and no work is required for this. This part of the whole process cannot, therefore, cause any electromotive force. By processes (2) and (4) a gram-ion of chlorine is brought from the dilute to the concentrated solution. This process is therefore the same, but in the opposite direction, as when unpolarisable electrodes of the first order are used, and consequently the same electro- motive force occurs, but with the other sign. By process (3) we obtain the same electromotive force at the surface of contact as in the former example. The total electromotive force is therefore given by —
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