Page:Text-book of Electrochemistry.djvu/147

 lit CONDUCTIVITY OF ELECTROLYTES, chap.

completely immersed. The electrodes consist of platinum plates electrolytically covered with a film of platinum black, so that their surfaces become exceedingly great,' and the

���quantity of ions deposited per unit of surface ia corre- spondingly small, and consequently also the polarisation.

The capacity of the resistance-cell is determined by measuring the resistance, pi, offered by a solution of known

resistance (mi = sr )■ If the resistance of the solution under

examination is p, the specific resistance and the conductivity are found from the relationship —

p : pi = in : mi = Xi '. X.

[The condnctivity of electrolytes may also be measured satisfactorily in the following way, due to Steoud and Henderson {Phil. Mag., 1897, 43, 19). The detrimental effects of polarisation in the electrolytic cell are very lai^ely reduced by inserting a second cell with a very different lei^h of

' KohlrauKck foutid that the surfiice of an electrode covered with platinum black was Beveral tbouaaod times greater than that of the polished electrode. According to Lnmmer and Eurlbaum (5), the electrode Bhould be platinii;ed with a 3 per cent solution of platinic chloride, containing about -^ per cent of lead acetate.

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