The absolute hydrogen overvoltage values of titanium in hydrofluoric acid increase, or the cathodic potentials, , decrease (become more negative) with addition of alkali fluorides to the acid. The , decreases with increasing concentration of the salt in the sequence , which is also the sequence of decreasing cationic radius. For example, at 50 ma/cm2 , decreases from −0.8 v in pure O.1N hydrofluoric acid to −1.68 v in the same acid if it is 1M with respect to sodium fluoride. If the titanium cathode dissolves in the acid, the ‐current density relationship is linear; if the dissolution ceases (at higher concentrations of the added fluoride), Tafel's relationship is fulfilled. In between there is a mixed straight line and quasi logarithmic relationship, the latter occurring at higher current densities. Organic colloid solutions of agar‐agar or arabic gum appreciably decrease ; methylene blue solutions increase it slightly.