Pyropervanadates

Pyropervanadic acid has not been isolated, but ammonium and potassium salts derived from it are known. The ammonium salt has the composition (NH₄)₄V₂O₁₁, and is prepared by dissolving ammonium metavanadate, NH₄VO₃, in an aqueous solution of hydrogen peroxide, adding ammonium hydroxide until the solution smells of ammonia, and then precipitating the salt with alcohol. The reaction is thought to take the following course: Ammonium meta- pervanadate, NH₄VO₄, is first formed, and this reacts with water to yield ammonium pyropervanadate, (NH₄)₂H₂V₂O₉, in solution:

+ H₂O =

This substance undergoes further change with ammonium hydroperoxide, NH₄O.OH, which is also formed in solution at the same time, to yield the compound (NH₄)₄V₂O₁₁:

= + H₂O.

Ammonium pyropervanadate crystallises in minute, slender, yellow, rhombic prisms, remaining undecomposed for some time when dry, then slowly decomposing with evolution of oxygen. With concentrated sulphuric acid it yields ozonised oxygen, and with dilute sulphuric acid hydrogen peroxide.

The ammonium and potassium salts of orthopervanadic acid have also been prepared.