Aqueous Acid–Base Coulometry Revived, with Applications to the Determination of Protonation and Complex Formation Constants: A Proof of Concept

Aqueous acid–base coulometry is applied to the study of protolysis and metal complexation equilibria. The drawbacks of the Pt anode (unwanted oxidation reactions due to the high overpotential for proton generation) have been overcome using a pre-loaded Pd electrode, which effectively replaces water oxidation as the proton source by oxidation of absorbed hydrogen. As this happens just above the reversible potential of the Pd–H electrode (at +0.05 V vs NHE), it permits the work with oxidation-sensitive species. Proton-generating coulometry is complemented by hydroxide generation at a Pt cathode in conjunction with a silver anode. The complementarity of both approaches is demonstrated by the determination of the relevant equilibrium constants of well known systems: the protolysis of acetic acid, p-nitrophenol, ethanolamine, oxine, and 2,2′-dipyridyl and equilibrium constants for the complexation of Cd2+ with ethylenediamine and Ge(IV) with oxine and catechol. A new scheme for GE calibration is presented. Limitations of the methodology are briefly discussed.