Page:Text-book of Electrochemistry.djvu/139

 ��ELECTROLYTES.

��CHAP. VIII.

��ZnSo,

��^solution

��ZnS€^

���Fia. 28.

��tliis by a porous cell, a solution of zinc sulphate (or dilute sulphuric acid, which soon forms zinc sulphate) containing a zinc pole. The electromotive force of this combination varies between 11 and 1*18 volts, according to the concentrations of the solutions. A more suitable "normal" or "standard" element, and that generally used, is the Clark cell. This is constructed as follows : A mix- ture of 90 per cent, mercury and 10 per cent, zinc (which,

in an electromotive respect, acts like pure zinc) is put into the limb J of a vessel AB of the form shown in Fig. 28. This amalgam, which is easily fused, is allowed to solidify round the platinum wire p. Pure mercury is poured into the limb B over the platinum wire jpi. A paste, made by rubbing together crys- ■f 7H2O), solid mercurous sulphate, mercury, and a concen- trated solution of zinc sulphate, is poured on to the mercury to the depth of 1 cm., and this, as well as the zinc amalgam in Ay is covered to a depth of at least 1 cm. with crystals of zinc sulphate. The vessel is now filled with a saturated solution of zinc sulphate, leaving only a* small air bubble to allow for the expansion by heat of the solution; the vessel is then closed by a cork, P, through which passes the thermometer 7. The electromotive force of this cell at 15° has been accurately determined to be 1*438 volts. Between 10° and 25° the E.M.F. decreases by 00012 volt for a rise of temperature of 1°. As the temperature coefficient of this cell is com- paratively large, the Weston element (see Chap. XV.) has recently been used to a considerable extent as standard, and this seems to be quite justifiable, since the latest investiga- tions prove that when the composition is correctly chosen (12 to 13 per cent, cadmium amalgam) the E.M.F. is very constant.

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