When fused with caustic soda, hydrogen is liberated and a vanadate is formed.
According to Ditte (Comptes rendus, 101, p. 698) it exists in three forms: a red amorphous soluble form which results when ammonium metavanadate is heated in a closed vessel and the residue oxidized with nitric acid and again heated; a yellow amorphous insoluble form which is obtained when the vanadate is heated in a current of air at 440° C.; and a red crystalline form which is almost insoluble in water.
Metavanadic acid is obtained in the form of yellow scales by boiling copper vanadate with an aqueous solution of sulphur dioxide.
Pyrovanadic acid is deposited as a dark brown unstable powder when an acid vanadate is decomposed by nitric acid.
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