Editing Pyrophosphate (section)

==Acidity==
Pyrophosphoric acid is a tetraprotic acid, with four distinct [[pKa|p''K''<sub>a</sub>]]'s:<ref>{{cite journal |doi=10.1039/d1qi00209k|title=Induced fit activity-based sensing: A mechanistic study of pyrophosphate detection with a "flexible" Fe-salen complex |year=2021 |last1=Yadav |first1=Prerna |last2=Blacque |first2=Olivier |last3=Roodt |first3=Andreas |last4=Zelder |first4=Felix |journal=Inorganic Chemistry Frontiers |volume=8 |issue=19 |pages=4313–4323 |pmid=34603734 |pmc=8477187 }}</ref>
:{{chem2|H4P2O7 ⇌ [H3P2O7]− + H+}}, p''K''<sub>a1</sub> = 0.85
:{{chem2|[H3P2O7]− ⇌ [H2P2O7](2−) + H+}}, p''K''<sub>a2</sub> = 1.96 
:{{chem2|[H2P2O7](2−) ⇌ [HP2O7](3−) + H+}}, p''K''<sub>a3</sub> = 6.60
:{{chem2|[HP2O7](3−) ⇌ [P2O7](4−) + H+}}, p''K''<sub>a4</sub> = 9.41 
The pKa's occur in two distinct ranges because deprotonations occur on separate phosphate groups. For comparison with the p''K''<sub>a</sub>s for [[phosphoric acid]] are 2.14, 7.20, and 12.37.

At physiological [[pH]]s, pyrophosphate exists as a mixture of doubly and singly protonated forms.