3.1.17 (HL)—Buffer pH and composition

$pH ≈ pKa + log10([A−]/[HA])$

Buffer pH depends on pKa and the conjugate-to-parent ratio. Dilution changes both concentrations by the same factor, so the ratio and pH remain approximately constant.

When a small amount of acid is added, A⁻ removes it to form HA; added base is removed by HA to form A⁻. Dilution leaves their ratio nearly unchanged but reduces buffer capacity, so resistance to a large addition is not unchanged.

$basic buffer: pOH ≈ pKb + log₁₀([BH⁺]/[B]); then pH = pKw − pOH$

Worked buffer example: an ethanoate buffer contains $0.100\,\mathrm{mol\,dm^{-3}}$ $\ce{CH3COOH}$ and $0.200\,\mathrm{mol\,dm^{-3}}$ $\ce{CH3COO^-}$ , with $\mathrm{p}K_a=4.76$ . Substitution gives $\mathrm{pH}=\mathrm{p}K_a+\log_{10}([\ce{A^-}]/[\ce{HA}])=4.76+\log_{10}(0.200/0.100)=5.06$ . The pH is above $\mathrm{p}K_a$ because the conjugate base is more concentrated than the acid.

Representative question

[Maximum number: 3]

This $1.00 \mathrm{~mol} \mathrm{dm}^{-3}$ solution of nitrous acid was used to prepare a buffer with pH 3.00 .

Calculate the concentration of the conjugate base of nitrous acid required to make this buffer. The $\mathrm{pK}_{\mathrm{a}}$ of nitrous acid is 3.25.

Concentration of conjugate base:

（a）

（ii）

$K_{a}<10^{-3.25}>=5.62 \times 10^{-4}$

« $\left[\mathrm{H}^{+}\right]=10^{-3.00}$ » =0.001 «mol dm ${ }^{-3}$ »
$\text { « }\left[\mathrm{A}^{-}\right]=\left(5.62 \times 10^{-4} \times 1.00\right) / 0.001 »=0.562 \text { «mol } \mathrm{dm}^{-3} \text { » }$

Alternative solution：
$\mathrm{pH}=\mathrm{p} K_{a}+\log _{10}[\text { salt }] /[\text { acid }]$ $\text { « } \log _{10}[\text { salt }] /[\text { acid }] \text { 》 }=-0.25$

OR
«[salt]/[acid]» = 0.562、

《 $\left[\mathrm{A}^{-}\right]=0.562 / 1.00 »=0.562$ «mol $\mathrm{dm}^{-3}$ » ↓

Award［3］for correct final answer．

Retrieve the route: track proton transfer and conjugates, calculate pH and Kw, distinguish strength, balance neutralization, read titration curves, use Ka/Kb and hydrolysis, select indicators, and explain and calculate buffer behaviour.

Check donor versus acceptor, one-proton differences, logarithm direction, ion comparison, strength versus concentration, equivalence versus endpoint, pKa landmarks, conjugate equations and dilution ratios.

3.1.17 (HL)—Buffer pH and composition

Buffer pH and Composition

Calculating Buffer Composition

Assessment in practice

Proton Transfer Reactions Summary

Question 1