Inorganic phosphate (${\mathrm{P}}_i$)

Inorganic phosphate, abbreviated ${\mathrm{P}}_i$, denotes the free phosphate pool in biochemical chemical equations, including ATP hydrolysis. Its constituent chemical species depend on pH and protonation; ${\mathrm{P}}_i$ does not specify a single electric charge or proton count. See the IUBMB recommendations on biochemical reactant pools.