What is pH, acid, base, pKa? — Biology

Definition 8.7 University Biology — Year 1 · Chapter 8 — Water and Small Biomolecules

Water dissociates slightly, $\mathrm{H_2O} \rightleftharpoons \mathrm{H^+} + \mathrm{OH^-}$ , with $[\mathrm{H^+}][\mathrm{OH^-}] = K_w = 10^{-14}\,\mathrm{mol}^{2}/\mathrm{L}^{2}$ at $25\,{}^{\circ}\mathrm{C}$ . The pH is $-\log_{10}[\mathrm{H^+}]$ : 7 for pure water, lower for acids, higher for bases. An acid $\mathrm{HA}$ releases a proton, $\mathrm{HA} \rightleftharpoons \mathrm{H^+} + \mathrm{A^-}$ , with dissociation constant $K_a = [\mathrm{H^+}][\mathrm{A^-}]/[\mathrm{HA}]$ and p $K_a$ $= -\log_{10}K_a$ ; its conjugate base $\mathrm{A^-}$ accepts one. A strong acid (HCl) is fully dissociated; the acids of the cell (carboxylic acids, phosphates, ammonium) are weak, with p $K_a$ between 2 and 10.

Examples

Example 8.6 (Melting a double helix)

The two strands of DNA are held by two or three hydrogen bonds per base pair plus the stacking (van der Waals) of neighbouring pairs; a single pair would separate at once, but a thousand pairs in a row hold until the temperature reaches about $90\,{}^{\circ}\mathrm{C}$ , and reunite, in exactly the same register, when it is lowered (Chapter 11). Specific, strong in number, reversible.

Example 8.11 (Blood)

Arterial blood holds $24\,\mathrm{mmol}/\mathrm{L}$ of bicarbonate and dissolved $\mathrm{CO_2}$ at $1.2\,\mathrm{mmol}/\mathrm{L}$ (set by the lungs): $\mathrm{pH} = 6.1 + \log(24/1.2) = 6.1 + 1.30 = 7.4$ . Adding $5\,\mathrm{mmol}/\mathrm{L}$ of acid turns $5\,\mathrm{mmol}/\mathrm{L}$ of bicarbonate into $\mathrm{CO_2}$ ; if the lungs blow that $\mathrm{CO_2}$ off and hold the dissolved value, the pH becomes $6.1 + \log(19/1.2) = 7.30$ ; in a closed flask the $\mathrm{CO_2}$ would rise to $6.2\,\mathrm{mmol}/\mathrm{L}$ and the pH fall to $6.1 + \log(19/6.2) = 6.59$ . The lungs make the difference.

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Examples

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