a crystalline soluble solid acid formed by the reaction between one molecule of phosphorus pentoxide and two water molecules. Formula: H4P2O7
See also polyphosphoric acid
pyrophosphoric acid in American English
(ˌpaɪrəfɑsˈfɔrɪk)
a viscous liquid acid, H4P2O7, which crystallizes when left standing at ordinary temperatures and is easily converted to orthophosphoric acid upon dilution with water