Isopolymolybdate adsorption as related to inhibition and self-inhibition of electrode processes

•Oxomolybdate adsorption is strongly affected by pH-dependent speciation.•Both electrostatic and specific adsorption interactions are typical for oxomolybdates on mercury.•Oxomolybdate reduction tends to self-inhibition, most pronounced for solutions in which isopolyanions predominate.•Polyoxomolybdate anions inhibit outer-sphere electron transfer, demonstrating barrier properties of oxomolybdate adlayers.

Electrochemical behavior of aqueous isopolymolybdates at various pH (0.1–8.7) is addressed in dropping mercury electrode experiments. Formation of compact adlayers is found to be responsible for decreased current of molybdate reduction (self-inhibition), especially at higher reactant concentrations. Correlation of the observed self-inhibition degree with the decrease of the surface tension is observed in the overall pH range. The difference in adsorption behavior of oxocations, neutral species and oxoanions is illustrated experimentally. Self-inhibition in acidic solutions is accented, as a previously known complication for electroanalysis. This phenomenon is assigned to adsorption of the neutral oxomolybdate species. Inhibition of the outer-sphere electron transfer processes is studied for two model reactions, and the role of pH-affected speciation is found to be the same as for self-inhibition. Similarities with speciation effect in corrosion protection are discussed on the basis of these findings.