noun
: any of four sodium salts of pyrophosphoric acid: as
a. : a crystalline acid salt Na 2 H 2 P 2 O 7 used chiefly as a leavening agent, in acid cleaning compositions, and in electroplating; disodium dihydrogen pyrophosphate — called also sodium acid pyrophosphate
b. : the crystalline normal salt Na 4 P 2 O 7 used chiefly as a builder in soaps and detergents and as a deflocculant (as in treating drilling muds for oil wells) — called also tetrasodium pyrophosphate