The conductivity of aqueous solutions of lanthanum ferricyanide

The conductivity of lanthanum ferricyanide in water has been measured at 18, 25 and 30° C. In dilute solutions the salt shows the behaviour of a comparatively weak electrolyte, and when the limiting Debye-Huckel and Onsager equations are applied to thirty measurements at ionic strengths lower than 0·002, a constant value K = 1·82 x 10-4 is found for its dissociation constant at 25° C. The dissociation constants at 18 and 30° C have also been calculated, and hence ∆H, ∆G and ∆S for dissociation. At the higher concentrations the conductivity curve shows abnormalities which find a reasonable explanation in the theory that the degree of dissociation falls to a minimum value of approximately 0.5, and increases again at higher concentrations. The composition of the hydrate stable at room temperature is found to be LaFe(CN)6,5H2O. One molecule of water is readily lost over concentrated sulphuric acid, and this may account for the lower degrees of hydration reported in the literature.