ELECTROREDUCTION OF NIOBIUM(V) IN HYDROCHLORIC ACID SOLUTIONS AT MERCURY ELECTRODES .2. CONTROLLED-POTENTIAL COULOMETRY AND STIRRED-POOL CHRONOAMPEROMETRY

The electroreduction of niobium(V) in concentrated solutions of hydrochloric acid was studied by stirred-pool controlled-potential chronoamperometry and coulometry. In dilute solutions, niobium(V) is monomeric and is reduced stepwise to niobium(IV) and niobium(II), which react to form niobium(III). In more concentrated (10 mF) solutions, niobium(V) is dimeric, and is initially reduced to [Nb(V)Nb(IV)], which can exist in three hydrolytic forms. Only one of these forms is further reduced to [Nb(IV)]2. The latter may dissociate or disproportionate to form [Nb(V)Nb(III)], which can be reduced to [Nb(IV)Nb(III)] and, at more negative potentials, to Nb(III) and Nb(II). At very negative potentials, [Nb(IV)2 is reduced to [Nb(IV)Nb(II)], which decomposes rapidly to form two Nb(III) ions. Niobium(II) can be oxidized to a mixture of niobium(III) and (V), while niobium(III) is oxidized entirely to niobium(V). The dimeric species are probably joined by two oxygen bridges and an intermetallic bond. The latter permits the rapid disproportion of [Nb(IV)]2, and [Nb(IV)Nb(II)], and may stabilize [Nb(V)Nb(IV)], [Nb(V)Nb(III)] and [Nb(IV)Nb(III)] against dissociation. © 1968.