● The characteristics of the pH curves produced by the different combinations of strong and weak
acid and bases.
● An acid–base indicator is a weak acid or a weak base where the components of the conjugate
acid–base pair have different colours.
Guidance
● For an indicator which is a weak acid:
HIn(aq) s H+(aq) + In–(aq)
● For an indicator which is a weak base:
BOH(aq) s B+(aq) + OH–(aq)
● Examples of indicators are listed in the data booklet in section 22.
● The relationship between the pH range of an acid–base indicator, which is a weak acid, and its pKa
value.
Guidance
The colour change can be considered to take place over a range of pKa ± 1.
● The buffer region on the pH curve represents the region where small additions of acid or base
result in little or no change in pH.
● The composition and action of a buffer solution.

● The general shapes of graphs of pH against volume for titrations involving strong and weak acids and bases with an explanation of their important features.
Guidance
● Only examples involving the transfer of one proton will be assessed. Important features are:
intercept with pH axis equivalence point buffer region points where pKa = pH or pKb = pOH
● Selection of an appropriate indicator for a titration, given the equivalence point of the titration and the end-point of the indicator.
● While the nature of the acid–base buffer always remains the same, buffer solutions can be
prepared by either mixing a weak acid/base with a solution of a salt containing its conjugate, or by partial neutralization of a weak acid/base with a strong acid/base.
● Prediction of the relative pH of aqueous salt solutions formed by the different combinations of
strong and weak acid and base.
Guidance
● Salts formed from the four possible combinations of strong and weak acids and bases should be considered. Calculations are not required.
● The acidity of hydrated transition metal ions is covered in topic 13. The treatment of other hydrated metal ions is not required.