Standard cells Clark, 295 Weston

Mercurous Sulfate. Mercurous s Ai2LX.e[7783-36-0] Hg2S04, is a colodess-to-shghfly-yellowish compound, sensitive to light and slightly soluble ia water (0.05 g/100 g H2O). It is more soluble ia dilute acids. The compound is prepared by precipitation from acidified mercurous nitrate solution and dilute sulfuric acid. The precipitate is washed with dilute sulfuric acid until nitrate-free. Its most important use is as a component of Clark and Weston types of standard cells. 

A standard cell produces a precise voltage and, before the advent of reliable voltmeters, was needed to calibrate medical and laboratory equipment. It is generally agreed that the first standard cell was the Clark cell (see p. 299), but the most popular was the Weston saturated cadmium cell, patented in 1893. 

Weston s cell was much less temperature sensitive than the previous standard, the Clark cell. We recall how the value of AG changes with temperature according to Equation (4.38). In a similar way, the value of AG(Ceii) for a cell relates to the entropy change A ceii) such that the change of emf with temperature follows... 

The red form of HgO is of increasing importance in electronics in the production of miniature dry cells, and Hg2S04 is employed in both the Weston and Clark standard cells. Hg2Cl2 still finds use as an antiseptic and Hgl2 is used to treat skin diseases. Mercury fulminate, Hg(OCN)2, is an important detonator for explosives.142... 

Standard cell — An electrochemical cell composed of two - half-cells containing electrodes built according to standard (normal) conditions. Frequently the term is also used for electrochemical cells showing a well-defined, reproducible, and stable cell voltage suitable for calibration purposes (- Weston cell, - Clark cell). 

Clark cell A type of cell formerly used as a standard source of e.m.f. It consists of a mercury cathode coated with mercury sulfate, and a zinc anode. The electrolyte is zinc sulfate solution. The e.m.f. produced is 1.4345 volts at 15°C. The Clark cell has been superseded as a standard by the Weston (Trademark) cadmium cell. The cell is named for the English engineer Josiah Latimer Clark (1822-98). 

The Clark cell was at one time widely used as a standard cell, but it has now been rejected, in favour of the Weston cell (q.v.), owing to its high temperature coefficient of e.m.f. 

Normal element (cell) - Galvanic element of exceptional reproducibility of the cell voltage. Since 1908 the -> Weston normal element is used as the international standard because it has a higher stability than the - Clark cell. 

VSE For making electric batteries with zinc sulfate in the standard Clark cell and with cadmium sulfate in the stan dard Weston cell. 

The standards of electromotive force available up to the present time did not come up to the ideal which Mr. Weston had set before him, even the best of these, the Clark cell, being subject to a variation of 0.077 per degree centigrade. 

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