Polyprotic Acids: Stepwise Ionization
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A polyprotic acid loses protons through separate equilibria, each with its own Ka, and each successive Ka is dramatically smaller than the one before.
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Phosphoric acid ionizes in three steps, Ka1 near 10⁻³, Ka2 near 10⁻⁸, Ka3 near 10⁻¹³.
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Treating the steps as equally significant ignores how the later ionizations are suppressed by the earlier ones.