Zinc Pyrophosphate, ZnP₂O₇

The anhydrous salt of Zinc Pyrophosphate, ZnP₂O₇, of density 3.7538-3.7574 at 23° C., remains when ammonium zinc orthophosphate, NH₄.Zn.PO₄, is ignited. Under gradual heating in an open crucible this conversion is quantitative and can be used for the estimation of zinc. The pure substance is perfectly white, and any greyness indicates partial reduction. Ammonia, which is evolved during the ignition, begins to reduce it at 500° C.

The hydrated salt, 2Zn₂P₂O₇.3H₂O, is precipitated from a solution of zinc sulphate by sodium pyrophosphate as a white, amorphous mass. It can be obtained as a white crystalline powder from its solution in sulphurous acid. It melts in the blowpipe flame to a clear, transparent condition. If it is heated in a stream of hydrogen, metallic zinc and phosphoric acid sublime, some phosphine is evolved, and a white residue, containing zinc and phosphoric oxide, remains.

Both the crystalline and amorphous varieties dissolve in aqueous acids, alkalies, or ammonia, and give a syrupy mass with alcohol.

An insoluble pentaliydrate, Zn₂P₂O₇.5H₂O, has been prepared by dissolving a zinc salt in a solution of an alkaline pyrophosphate.

When zinc pyrophosphate is heated with water an acid phosphate passes into solution and the tribasic phosphate is precipitated. The compound 3Zn₂P₂O₇.4NH₄.9H₂O has been prepared. Anhydrous crystalline zinc metaphosphate, Zn(PO₃)₂, is obtained by treating zinc oxide with excess of phosphoric acid, heating to redness, and cooling slowly. It is soluble only in hot sulphuric acid.

The tetrahydrate is precipitated in transparent crystals from a solution of zinc chloride by ammonium metaphosphate. It is insoluble in water, but is decomposed on boiling.

Bette prepared a hydrated ammoniate of zinc metaphosphate.

The salts NaZnPO₄, Na₂O.ZnO.P₂O₅, 2Na₂O.ZnO.P₂O₅, Na₂O.2ZnO.3P₂O₅, KZnPO₄, K₂O.ZnO.P₂O₅, have been prepared.

NaZnPO₄ and KZnPO₄ can be precipitated from solutions of zinc salts by sodium or potassium phosphate. The original amorphous precipitates crystallise on standing. The water of crystallisation can be expelled by heating without decomposing the anhydrous salts, but since both salts are hydrolysable they are not suitable for estimating zinc.

A number of other salts of the general formula wNa₂O.uZnO.yP₂O₅.zH₂O, and some of the general formula wK₂O.uZnO.yP₂O₅.zH₂O, have been reported.

The following have been described: 2ZnO.P₂O₅.3NH₄.8H₂O, 2ZnO.P₂O₅.2NH₄.4H₂O, 6ZnO.3P₂O₅.4NH₄.4H₂O, and 6ZnO.3P₂O₅.4NH₄.4H₂O.