Properties of anodically formed mercury-barbiturates

Abstract

The anodic formation of a number of mercury 5,5-substituted barbiturates was studied potentio-statically on a stationary mercury drop electrode from solutions of the various barbituric acids. In all cases one monomolecular layer of the salt was formed, having a structure Hg(II) barbiturate (II). In the region of the e.c.m. the neutral barbituric acid molecules are adsorbed; on moving to anodic potentials these molecules are desorbed and the divalent anions of the acids are adsorbed. In some cases very high coverages are reached before film formation. The monomolecular layers behave as dielectrics with ε∼2.5. The salt of rutonal formed at high overpotentials and those of butobarbitone and amylobarbitone have a structure with the heterocyclic ring on the metal surface. The salt of rutonal formed at low overpotentials and that of phenobarbitone have both the heterocyclic and aromatic rings on the metal surface.

The mercury salts of veronal and rutonal are formed by the progressive nucleation and growth of centres, the rate constants being markedly dependent on potential. The reduction of the various salts is more complex; however, the various shapes for the reduction transients can be partly explained by the growth of holes into the original centres of the monolayer phase. The reduction rate is much less sensitive to potential than the corresponding rate for the formation.

The current due to the reduction of oxygen to hydrogen peroxide in the presence of specifically adsorbed barbiturate anions was diminished from the corresponding current in the base solution. On the formation of the veronal salt, a discontinuity in the oxygen reduction current could be observed and in all cases the current through the solid phase was below the accessible limit of measurement.