Specific adsorption of formate ions at the mercury—aqueous solution interface

Abstract

Adsorption on polarized Hg electrode of formate ions from aqueous solutions of pure HCOONa using differential capacity measurements was studied. Anion specific adsorption was indicated by the cathodic shift of potential of zero charge with increase in bulk-electrolyte activity. Variation of charge due to surface excess of sodium ions (Γ + Z + F) and of charge due to specifically adsorbed formate ions ( q 1), with electrode charge ( q M ) indicated super-equivalent adsorption of formate ions at all positive charges and for all concentrations studied. Esin—Markov coefficient was found to be unreliable criterion of the occurence of specific adsorption. Logarithmic form of constant charge adsorption isotherms were found to be followed. The plots of φ M − 2 vs q 1 at constant q M were not linear and resembled those for F −, BF 4 − and CH 3COO − ions; which is attributed to relatively weak specific adsorption of anions.