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 HMdecreases and reaches zero at the second equivalence point while the fraction of M2−, represented by α2, approaches unity.

Tags
Polyprotic AcidsTitrationH2MHM 8722M2 8722Equivalence PointsAlpha ValuesPHFraction Of SpeciesTitration CurveBase AdditionChemical Equilibrium

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3.11 : Composition of Polyprotic Acid Solutions as a Function of pH

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3.1 : Acid–Base Titration: Overview

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3.2 : Titration of a Strong Acid with a Strong Base

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3.3 : Titration of a Weak Acid with a Strong Base

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3.4 : Titration of a Weak Base with a Strong Acid

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3.5 : Titration of a Weak Acid with a Weak Base

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3.6 : Solution Composition During Acid/Base Titrations

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3.7 : Mixtures of Acids

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3.9 : Titration of Polyprotic Base with a Strong Acid

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3.10 : Titration of Polyprotic Acids with a Strong Base

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3.12 : Buffers: Overview

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3.13 : Buffers: Buffer Capacity

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3.14 : Leveling Effect

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