phosphotungstic acid

noun

phos·​pho·​tung·​stic acid -ˌtəŋ-stik- 

: any of several acids obtainable from solutions of phosphoric acid and tungstic acid and used chiefly as precipitants (as for alkaloids and for basic dyes for pigments) and in analytical reagents

especially : a greenish yellow crystalline acid H₃PW₁₂O₄₀·xH₂O used as a reagent and as a biological stain