Buffer solutions — Chemistry, 14–17
How a solution resists sudden changes in pH. Chemistry, 14–17 years.
A chemical shock absorber
A buffer is a solution that keeps its pH fairly steady when small amounts of acid or base enter it. It contains a weak acid and its matching base, which can take up added H⁺ or release it when needed, rather like a chemical shock absorber.
The problem of unstable pH
Many reactions and living cells only work in a narrow pH range, but even a little acid can shift an unprotected solution sharply. Buffers were developed to control that problem: the weak acid and base share out the extra H⁺ instead of letting its concentration change suddenly.
Adding acid to an acetate buffer
Imagine 1.0 L containing 0.10 mol ethanoic acid and 0.10 mol acetate ions. Add 0.010 mol HCl: acetate reacts with it, leaving 0.090 mol acetate and 0.110 mol acid. Because their amounts are still similar, the pH changes only slightly, rather than falling by one whole pH unit.
A buffer is not unlimited
It is tempting to think that a buffer fixes pH permanently, because its pH barely moves at first. In reality, added acid eventually uses up the acetate or other base, and added alkali uses up the weak acid; after that, the pH can change quickly.
Buffers in living systems
Your blood uses a carbon dioxide–hydrogencarbonate buffer to keep its pH near 7.4, because enzymes and cells are sensitive to pH. Swimming pools, medicines and laboratory reactions also use buffers when a small pH shift would spoil the result.
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