Pertantalic Acid, HTaO₄.xH₂O

Pertantalic Acid, HTaO₄.xH₂O, is the analogue of perniobic acid and pervanadic acid. It differs from them in its greater stability. Pervanadic acid is a yellow substance which is rapidly decomposed on being warmed; pertantalic acid is a white solid which can be heated for some time at 100° C. without undergoing decomposition. (This behaviour is remarkable in a per-acid.) Pertantalic acid is prepared similarly to perniobic acid, using dilute sulphuric acid and the potassium perniobate, K₃TaO₈.½H₂O. Analysis shows that it contains one atom of active oxygen for each tantalum atom, and the following constitutional formula has been assigned to it:



It is not attacked by dilute sulphuric acid in the cold, but on warming the mixture hydrogen peroxide is formed. Pervanadic acid, on the other hand, is decomposed by cold dilute sulphuric acid. Perniobie acid occupies an intermediate position in the order of stability.

Pertantalates are prepared by the action of excess of hydrogen peroxide on solutions of the 4:3 alkali tantalates; addition of alcohol precipitates them as white, crystalline compounds which yield hydrogen peroxide with warm dilute sulphuric acid and ozonised oxygen with concentrated sulphuric acid. On being boiled with water they evolve oxygen. Their composition usually corresponds to the formula R^•₃TaO₄.xH₂O; they contain four atoms of active oxygen for each tantalum atom, and the following constitutional formula has been assigned to them:



The following are known in this series:

Na₃TaO₈.H₂O A white, amorphous powder.
Na₃TaO₈.14H₂O Pale yellow crystals.
K₃TaO₈.½H₂O A white, crystalline mass.
Rb₃TaO₈ A white, crystalline mass.
Cs₃TaO₈ A white, crystalline mass.

also the following double salts: KCaTaO₈.4½H₂O; KMgTaO₈.7H₂O; NaCaTaO₈.4½H₂O; NaMgTaO₈.8H₂O; RbMgTaO₈.9H₂O.

In addition to the above, a sodium pertantalate of the composition Na₂Ta₂O₉.13H₂O has been prepared by treating an aqueous extract of the residue which is obtained when a solution of 4:3 sodium tantalate is evaporated to dryness with a few drops of hydrogen peroxide and then adding a little alcohol. A white, amorphous powder of the composition indicated is thrown down. It yields hydrogen peroxide with dilute sulphuric acid. The ratio Na₂O:Ta₂O₅:active oxygen is 1:1:3, and when the salt is treated with freshly precipitated aluminium hydroxide, two-thirds of the active oxygen is retained by the precipitate, the other third passing into the filtrate. These figures agree with the constitutional formula



This sodium pertantalate, therefore, appears to be a double salt, NaTaO₄.NaO.TaO₄.13H₂O.