Polyprotic Weak Acid pH Approximation

$$[H^+] \approx \sqrt{K_{a1} \cdot C} \implies \text{pH} \approx 0.5 \cdot (\text{p}K_{a1} - \log_{10}C)$$

$$[A^{2-}] \approx K_{a2}$$

Concept: $$\text{Approximates pH for polyprotic acids } (H_2A) \text{ when } K_{a1} \gg K_{a2}. \text{ Anion concentration } [A^{2-}] \text{ is independent of } C.$$

Notation:

• $$\text{Notation: } K_{a1}, K_{a2} = \text{First and second ionization constants of } H_2A$$
• $$\text{Notation: } C = \text{Initial molar concentration of polyprotic acid}$$
• $$\text{Notation: } [A^{2-}] = \text{Concentration of divalent conjugate anion}$$