25 z OXIDATION AND REDUCTION ELEMENTS, chap.
This element suffers from the disadvantage that the con- centration of the zinc chloride may change by evaporation, and from the fact that when current is drawn from it the concentration of the salt may alter on account of zinc dissolving.
To avoid these disturbing factors, a zinc salt easy to prepare pure and in the crystalline form is used for making up the solution, and a layer of this salt is placed over the zinc.
The most suitable salt which has so far been used is the sulphate, which is employed in the normal Clark cell (7), already referred to (p. 124). The electromotive force of this is —
1 normal Clark = 1-433[1 - 00084(^ - 15)] volts.
In the Weston element {8) the zinc is replaced by the closely related metal, cadmium. This cell consists of mercury, mercuric sulphate paste, saturated cadmium sul- phate solution, and cadmium amalgam covered with cadmium sulphate crystals. The cadmium amalgam ia made up of six parts of mercury and one part of cadmium. The electro- motive force of the element is —
1 normal Weston = 1019[1 + 0'00004(^ - 20)] volts.
This element has the great advantage of possessing a very small temperature coefiScient, so that it is unnecessary to exactly determine the temperature when it is used (it is suflBcient to state that the experiment was carried out at the ordinary room temperature). For the composition of the cell, see p. 124.
The elements mentioned, containing difficultly soluble mercury salts, cannot withstand very appreciable current strengths, for such cause the deposition of the small quantity of mercury ion, and it requires a considerable time before a sufficient amount of salt dissolves to re-establish the neces- sary mercury ion concentration. Of the normal elements the calomel cell can stand the greatest cuiTent strength, and this
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