← Equilibrium principles in aqueous systems

5 Buffers

Watch: Buffers Opens a YouTube search: pick a video that clicks for you A buffer resists pH change when small amounts of acid or base are added. It’s a mixture of a weak acid and its conjugate base (e.g. CH₃COOH / CH₃COO⁻) in comparable amounts. ✓

Mnemonic: buffers

“A buffer is a couple that argues but never leaves.” You need a weak acid AND its conjugate base together (a conjugate pair) in similar amounts. Add acid → the base partner absorbs it; add base → the acid partner absorbs it. The pH barely moves.

And: “pH = pKa when it’s 50:50.” Equal amounts of HA and A⁻ ⟹ the log term is zero ⟹ pH = pKa. That’s also the half-equivalence point on a titration curve.

How a buffer works

  • Add acid (H⁺): the conjugate base A⁻ mops it up → A⁻ + H⁺ → HA.
  • Add base (OH⁻): the weak acid HA neutralises it → HA + OH⁻ → A⁻ + H₂O.
  • Both HA and A⁻ are present in large reserves, so the ratio [A⁻]/[HA] barely changes ⟹ pH barely changes.
Henderson–Hasselbalch \mathrm{pH}=\mathrm{p}K_a+\log\frac{[\ce{A^-}]}{[\ce{HA}]} When [A⁻] = [HA], pH = pKa. Choose an acid with pKa near the target pH. Worked example Buffer pH A buffer is 0.20 mol L⁻¹ ethanoic acid + 0.30 mol L⁻¹ sodium ethanoate. Ka = 1.8×10⁻⁵. pKa = −log(1.8×10⁻⁵) = 4.74. pH = pKa + log([A⁻]/[HA]) = 4.74 + log(0.30/0.20). pH = 4.74 + log(1.5) = 4.74 + 0.18 = 4.92. ✓ pH = 4.92.

Practise just this