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Wiktionary
polyphosphoric acid

n. (context chemistry English) Any of several ill-defined polymers of orthophosphoric acid; they are viscous, hygroscopic liquids

WordNet
polyphosphoric acid

n. a series of oxyacids of phosphorus

Usage examples of "polyphosphoric acid".

To get an idea of what's happening, have a chemist drop polyphosphoric acid in water.