Diphosphate

Diphosphate, also called inorganic pyrophosphate in biochemical reactions, contains two phosphorus centres connected through an oxygen bridge. The fully deprotonated ion is ${\mathrm{P}}_2{\mathrm{O}}_7^{4-}$. Other protonation states retain the connected phosphorus–oxygen framework but differ in hydrogen count and electric charge.^[ref]

Counting atoms reveals the bridge. Two separate orthophosphate tetrahedra contain eight oxygens in total; sharing one bridging oxygen gives seven distinct oxygens. This connectivity differs from merely having two independent phosphate ions in one solution.

Guanylate cyclase releases diphosphate while converting GTP to cyclic guanosine monophosphate. A second enzyme, inorganic diphosphatase, hydrolyses diphosphate to two phosphate products.^[ref]

An ideal coupled assay producing $5\mathrm{nmol}$ diphosphate can therefore yield $10\mathrm{nmol}$ phosphate after complete hydrolysis. Forgetting the factor of two would misinterpret a phosphate-based readout. Existing phosphate and other phosphate-releasing reactions require controls if that signal is to be attributed to the intended reaction.

A reaction balance should specify protonation when hydrogen and charge matter. The four-minus formula names one definite ion, not a claim that every solution contains only that ionic form.