Perniobic Acid, HNbO₄

When niobic acid is treated with hydrogen peroxide it becomes yellow owing to the formation of a perniobic acid, HNbO₄.xH₂O. This compound has been isolated by carefully treating one of the potassium perniobates, K₄Nb₂O₁₁.3H₂O, with sulphuric acid and dialysing the mixture until it was free from both potassium sulphate and excess of sulphuric acid; the gelatinous product was then dried over concentrated sulphuric acid. Perniobic acid, thus prepared, is an amorphous, yellow powder, insoluble in water. At 100° C. it is decomposed with liberation of oxygen. Dilute sulphuric acid is without action on it at ordinary temperatures, but on warming the mixture decomposition ensues with formation of hydrogen peroxide. Concentrated sulphuric acid liberates ozonised oxygen. The acid is much more stable than pervanadic acid, which is consistent with the general rule that in the same group the stability of the per-acid increases with increase in atomic weight of the element. The active oxygen, content agrees with the formula



so that perniobic acid can be looked upon as being derived from hydrogen peroxide by replacement of a hydrogen atom by the NbO^•₂ group:

Nb(OH)₅ + H₂O₂ = NbO₂.O₂H + 3H₂O.

A hydrosol of perniobic acid is obtained by adding 30 per cent, hydrogen peroxide to niobic acid and gently warming the mixture on the water-bath; the cooled product is treated in the cold with more hydrogen peroxide, excess of which is removed by dialysis. The hydrosol has the consistency of glycerine. It is converted into a yellow gel either by standing for a long time or by the action of electrolytes.

Another compound of niobic acid and hydrogen peroxide, of doubtful constitution, has been prepared by treating a solution of niobic acid in concentrated hydrochloric acid with 3 per cent, hydrogen peroxide. The mixture is allowed to stand overnight, and the yellow precipitate, after being washed with water and dried, has the composition Nb(OH)₆ or Nb₂O₅.H₂O₂.5H₂O. On being heated it loses water and oxygen.