Polyprotic Acid Calculator

A polyprotic acid is one that can donate more than one proton (H⁺) per molecule — like sulfuric or phosphoric acid — and calculating its pH requires accounting for each successive ionization step. Enter your acid concentration, select whether it's diprotic or triprotic, and provide the dissociation constants (Ka1, Ka2, and optionally Ka3) into the Polyprotic Acid Calculator to get the solution pH. Secondary outputs include H⁺ concentration and the species distribution fractions (α₀, α₁, α₂) showing what percentage of the acid exists in each ionization state.

M

First acid dissociation constant

Second acid dissociation constant

Third acid dissociation constant (for triprotic acids only)

Results

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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Frequently Asked Questions

What is a polyprotic acid?

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.

How do I calculate the pH of a polyprotic acid?

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.

What are alpha fractions in acid-base chemistry?

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.

What is the relationship between Ka and pKa values?

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.

How accurate is this calculator for very concentrated solutions?

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.

Can I use this calculator for bases?

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.