Enter your Acid Concentration, Number of Ionizable Protons, and up to three dissociation constants (Ka1, Ka2, Ka3) into this Polyprotic Acid Calculator to find the solution pH, H⁺ Concentration, and the fraction of each species present — α₀ (Fully Protonated), α₁, and α₂ — at equilibrium.
pH
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H⁺ Concentration
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α₀ (Fully Protonated)
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α₁ (First Deprotonation)
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α₂ (Second Deprotonation)
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A polyprotic acid is an acid that can donate more than one proton (H⁺) per molecule. Examples include sulfuric acid (H₂SO₄) which is diprotic, and phosphoric acid (H₃PO₄) which is triprotic.
For polyprotic acids, you need to consider multiple equilibria with different Ka values. The calculator uses alpha fractions to determine the concentration of each species and calculates pH based on the H⁺ concentration from all dissociation steps.
Alpha fractions (α) represent the fraction of total acid in each protonation state. α₀ is fully protonated, α₁ has lost one proton, α₂ has lost two protons, and so on. The sum of all alpha fractions equals 1.
pKa is the negative logarithm of Ka (pKa = -log Ka). Larger Ka values mean stronger acids and lower pKa values. For polyprotic acids, Ka1 > Ka2 > Ka3, so pKa1 < pKa2 < pKa3.
This calculator assumes ideal behavior and may be less accurate for very concentrated solutions (>1 M) where activity coefficients become significant. For most educational and laboratory purposes, it provides good approximations.
This calculator is specifically designed for polyprotic acids. For polyprotic bases, you would need to use Kb values and consider the basic dissociation equilibria instead.