4.5.7 Titration Curves

• During a titration a pH meter can be used and a pH curve plotted
• A pH curve is a graph showing how the pH of a solution changes as the acid (or base) is added
• The result is characteristically shaped graph which can yield useful information about how the particular acid and alkali react together with stoichiometric information

The features of a pH curve

• All pH curves show an s-shape curve and the midpoint of the inflection is called the equivalence or stochiometric point
• From the curves you can
  • Determine the pH of the acid by looking where the curve starts on the y-axis
  • Find the pH at the equivalence point
  • Find volume of base at the equivalence point
  • Obtain the range of pH at the vertical section of the curve

Four types of acid-base titrations

• There are four combinations of acids and alkalis that you should know about:
  • strong acid + strong base
  • weak acid + strong base
  • strong acid + weak base
  • weak acid + weak base

pH curves for the four types of acid-base titrations

• Without titles for the graph you can easily recognise which combination is shown by looking at the starting and ending pH and deducing whether the acid and alkali are strong or weak
• Sometimes you may see pH titration curves which show pH plotted against volume of acid added
• This produces the mirror image graph from which you can get all the same information

Comparing different versions of pH titration curves

• In this example, strong sodium hydroxide, NaOH (aq), is being added to weak ethanoic acid, CH₃COOH (aq)

NaOH (aq) + CH₃COOH (aq) → CH₃COONa (aq) + H₂O (l)

• The pH on the intercept on the y axis starts at roughly 3 due to the relative strength of the ethanoic acid
• The initial rise in pH is steep as the neutralisation of the weak acid by the strong base is rapid
• Ethanoate ions (conjugate base to ethanoic acid) are formed which then creates a Buffer
  • A buffer consists of a weak acid and its conjugate base or a weak base and its conjugate acid

• At this point, the buffer formed will resist changes in pH so the pH rises gradually as shown in the buffer region
• The half equivalence point is the stage of the titration at which exactly half the amount of weak acid has been neutralised
  • [CH₃COOH (aq)] = [CH₃COO^- (aq)]
  • At this point, it is important to note that the pK_a of the acid is equal to the pH
    • pK_a = pH at half equivalence 
• The equivalence point in a weak acid - strong base titration is above 7

Weak acid - strong base pH curve