Perniobates

Perniobates having the general formula R^•₃NbO₈, where R stands for an alkali metal, can be prepared by the action of excess of hydrogen peroxide on solutions of niobates which also contain alkali hydroxide or alkali carbonate; addition of alcohol precipitates the perniobates as white powders, which are washed with alcohol and ether. The rubidium and caesium salts become dark on being exposed to the air, but otherwise the perniobates are stable in air, and they are not decomposed in cold aqueous solution. Addition of sulphuric acid does not yield a precipitate in the cold, but on boiling, a yellow precipitate, which is probably niobium perhydroxide, is thrown down and oxygen is evolved. The following constitutional formula has been ascribed to the perniobates:



but the evidence for this formula is weak. In addition to this series the only well-defined perniobate is the potassium salt, K₄Nb₂O₁₁.3H₂O.

Potassium Perniobate, K₄Nb₂O₁₁.3H₂O

Potassium Perniobate, K₄Nb₂O₁₁.3H₂O, is prepared by treating the saturated aqueous extract from the fusion of niobium pentoxide and excess of caustic potash with hydrogen peroxide in small quantity, warming gently on the water-bath, and filtering; more hydrogen peroxide is added to the filtrate, which is precipitated with alcohol; the precipitate is treated in aqueous solution with more hydrogen peroxide, caustic potash, and alcohol. It forms a soluble, white powder, which evolves oxygen on being warmed; dilute sulphuric acid produces hydrogen peroxide, and concentrated sulphuric acid produces ozonised oxygen. Analysis shows that the ratio K₂O: Nb₂O₅:active oxygen is 2:1:4, from which result the salt can be looked upon as the potassium salt of pyro-per niobic acid,



When the salt is allowed to remain in solution for some time it slowly precipitates another potassium perniobate, the composition of which closely approximates to the formula KNbO₄.xH₂O.

Potassium Perniobate, K₃NbO₈

Potassium Perniobate, K₃NbO₈, is prepared by the action of caustic potash and 3 per cent, hydrogen peroxide on 4:3 potassium niobate. It yields tetragonal crystals.

Sodium Perniobate, Na₃NbO₈

Sodium Perniobate, Na₃NbO₈, is obtained as a white powder by the action of caustic soda and 3 per cent, hydrogen peroxide on 7:6 sodium niobate.

Rubidium Perniobate, Rb₃NbO₈

Rubidium Perniobate, Rb₃NbO₈, is obtained as a white, crystalline powder by the action of rubidium carbonate and excess of hydrogen peroxide on 4:3 rubidium niobate.

Cesium Perniobate, Cs₃NbO₈

Cesium Perniobate, Cs₃NbO₈, is prepared similarly to the rubidium salt.

The following double perniobates have also been isolated: KCaNbO₈. 4H₂O; NaCaNbO₈.4H₂O; KMgNbO₈.7H₂O; RbMgNbO₈.7½H₂O; NaMgNbO₈.8H₂O; CsMgNbO₈.8H₂O.