# Topic 18: Acids and bases - 18.3 pH curves

Nature of science: Increased power of instrumentation and advances in available techniques-development in pH meter technology has allowed for more reliable and ready measurement of pH. (3.7)

### Understandings

Essential idea: pH curves can be investigated experimentally but are mathematically determined by the dissociation constants of the acid and base. An indicator with an appropriate end point can be used to determine the equivalence point of the reaction.

The characteristics of the pH curves produced by the different combinations of strong and weak acids 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.

The relationship between the pH range of an acid-base indicator, which is a weak acid, and its pKa value.

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.

### Applications and skills

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.

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/basewith 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

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.

For an indicator which is a weak acid:- HIn(aq) H+(aq) + In-(aq) Colour A Colour B - The colour change can be considered to take place over a range of pKa ± 1.

For an indicator which is a weak base:- BOH(aq) B+(aq) + OH-(aq) Colour A Colour B

Examples of indicators are listed in the data booklet in section 22.

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.