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Author: Admin | 2025-04-28
Cadmium sulfate is preferably prepared from cadmium metal and acid, the cadmium being dissolved in nitric acid, crystallized several times as a nitrate, precipitated as cadmium sulfate with redistilled sulfuric acid, and purified by repeated recrystallization as the sulfate. Mercurous sulfate is prepared electrolytically in a darkened room. The cadmium amalgam (10% cadmium) is prepared either electrolyticly or by heating the two metals together." The finished cell must be mounted in a suitable enclosure to protect it, and to provide measurement terminals. Bakelite cases thermally lagged with copper (aluminum in later cells) are often used, or fully temperature controlled enclosures for sets of saturated cells. Saturated & Un-saturated Cells Two forms of the Weston Standard Cell are available: saturated and un-saturated. This refers to the concentration of cadmium sulfate in the electrolyte. The saturated type of cell is the most permanent, however it also has a larger temperature coefficient, and is not portable. It is typically used only for very accurate work, and must be temperature controlled. It is this type of cell that was used by the national laboratories to standardize the value of the volt. The un-saturated type of cell is more common, as it is portable. The electrode material and mercurous sulfate are held firmly in place with porous plugs, so the cell won't be damaged if inverted. The temperature coefficient can be ignored in most applications, but is given by: Further variations of both the saturated and un-saturated cell are available in the form of "neutral" or "acid " cells. Acid cells are similar to the neutral of normal cells, with the addition of a small amount of sulfuric acid. The e.m.f. of acid cells is lower than that of normal cells, depending on the concentration of acid. The following formula gives the difference in e.m.f.,
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