Electromotive force measurements of cells without liquid junction, of the type Ag, AgCl, HCl + protein, H2, have been made at 30°C. with the proteins gelatin, edestin, and casein in 0.1 M hydrochloric acid. The data are consistent with the assumptions of a constant combining capacity of each protein for hydrogen ion, no combination with chloride ion, and Failey's principle of a linear variation of the logarithm of the mean activity coefficient of the acid with increasing protein concentration. The combining capacities for hydrogen ion so obtained are 13.4 x 10–4 for edestin, 9.6 x 10–4 for gelatin, and 8.0 x 10–4 for casein, in equivalents of combined H+ per gm. of protein.

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