Conductivity study with caffeinate anion - Caffeic acid and its sodium and potassium salts

Precise electrical conductivities of dilute aqueous solutions of caffeic acid, sodium caffeinate, potassium caffeinate were determined in the temperature range from 278.15 K to 313.15 K (values for caffeic acid and potassium caffeinate solutions are reported for the first time in this paper). The experimental data of caffeic acid salts were analyzed by using the Kohlrausch and Onsager conductivity equation, yielding the limiting conductances of caffeinate anion, their diffusion coefficients at infinite dilution, and the Stokes radii. The equilibrium constants of the first dissociation step of caffeic acid were evaluated by applying the Quint-Viallard conductivity equations for 1:1 type electrolytes, and the Debye-Hückel expression for activity coefficients. Temperature dependence of the equilibrium constant permitted to obtain the standard thermodynamic functions of the dissociation process. The good agreement between experimental and calculated conductances of caffeic acid indicates the reliability of applied molecular model.