Niobium Sulphates

Electrolytic reduction of solutions of niobic acid in sulphuric acid furnishes solutions which contain the niobium either as Nb₂O₃ or Nb3O₇, according to the experimental conditions, but no sulphates of niobium have been isolated from these solutions. Reddish- brown crystals of an ammonium niobium sulphate, which have the probable composition (NH₄)₂SO₄.Nb₂(SO₄)₃.6H₂O, have, however, been prepared by electrolytic reduction, as well as an acid ammonium niobium sulphate, (NH₄)₂SO₄.Nb₂(SO₄)₃.H₂SO₄.6H₂O. The latter is a brown powder which is somewhat stable in a dry atmosphere. It dissolves in water to give a brown solution which oxidises rapidly, becoming successively blue, green, and finally colourless; addition of ammonium hydroxide or hydrochloric acid produces a blue coloration immediately.

Evaporation of a solution of niobium oxysulphide in sulphuric acid and heating to 340° C. yields a compound of the composition Nb₂O₅.SO₃, which can be alternatively written NbO₂.SO₄.NbO₂, i.e. as hyponiobic sulphate. At 420° C. the product has the composition 2Nb₂O₅.SO₃.