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)

• 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 pK_a 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/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.

• Is a pH curve an accurate description of reality or an artificial representation? Does science offer a representation of reality?

Utilization:

• Syllabus and cross-curricular links:
• Topic 5.1—thermometric/conductiometric titrations
• Topic 16.2—What are the unusual mathematical features of a pH curve? Students should also be familiar with the use of natural logs when using the Arrhenius expression in topic 16.2

Aims:

• Aim 6: Experiments could include investigation of pH curves, determination of the pKa of a weak acid, preparation and investigation of a buffer solution and the determination of the pK_a of an indicator.
• Aim 7: Data logging, databases, spreadsheets and simulations can all be used. For example, the equivalence point could be determined by using a conductivity probe or a temperature probe.

• Only examples involving the transfer of one proton will be assessed. Important features are:
  • intercept with pH axis
  • equivalence point
  • buffer region
  • points where pK_a = pH or pK_b = pOH.

• For an indicator which is a weak acid:
  • 
  • The colour change can be considered to take place over a range of pK_a⁺_- 1
• For an indicator which is a weak base:
  • 
• 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.