Disodium dihydrogen pyrophosphate, Na₂H₂P₂O₇

Disodium dihydrogen pyrophosphate, Na₂H₂P₂O₇, is formed by heating sodium dihydrogen phosphate at about 200° C.:

2NaH₂PO₄ = Na₂H₂P₂O₇+H₂O.

Above 300° C. it is converted into sodium metaphosphate.

In addition to the anhydrous salt, a tetrahydrate, and also a hexahydrate of density 1.848, are known. The aqueous solution has an acidic reaction, and reacts with sodium hydroxide to form the normal pyrophosphate.