Sodium pyrophosphate, Na₄P₂O₇

The Sodium pyrophosphate is obtained in its anhydrous form by heating disodium hydrogen orthophosphate:

2Na₂HPO₄=Na₄P₂O₇+H₂O.

The product is a white substance, its melting-point being given by Carnelley as 888° C. and by Le Chatelier as 957° C. and also as 970° C. For the density Schroder gives 2.534, and Mohr 2.385. Between 17° and 98° C. Regnault found the specific heat to be 0.2283. The heat of solution is given by Thomsen as 11.85 Cal.

Sodium pyrophosphate crystallizes from water as the monoclinic decahydrate. Its density is given by Playfair and Joule as 1.836, Mohr 1.773, and Dufet 1.824; and its heat of solution by Thomsen as -11.67 Cal. At 20° C. 100 grams of water dissolve 6.2 grams, and at 80° C. 30 grams, yielding a solution of faint alkaline reaction.

Other investigations concern the refractivity; and properties of the solution such as density, vapour-pressure, and electric conductivity.