Polyprotic acids of the type H₂M constitute two ionizable protons. As a result, on titration with a base, they exhibit two equivalence points in the titration curve. During titration, the species H₂M, HM⁻, and M²⁻ will be present in the solution at different points. The fractions of H₂M, HM⁻, and M²⁻ present at the various instances of the titration are denoted by α₀, α₁, and α₂, respectively.

A graph with the alpha values is plotted against the volume of base added during titration. Here, a value of 0.7 for the α₀ at the beginning of the titration suggests that 70% of the solution is H₂M, with the remaining 30% as HM⁻. On adding the base, the fraction of H₂M, α₀, decreases to nearly zero at the first equivalence point. Simultaneously, α₁, representing the fraction of HM⁻, increases and approaches unity. As more base is added, the fraction of HM⁻ decreases and reaches zero at the second equivalence point while the fraction of M²⁻, represented by α₂, approaches unity.