Titration Curves - Interpretation
- 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
Choosing an Indicator
- Not all indicators are suitable for every type of titration
- Indicators behave like weak acids and form an equilibrium between two coloured species:
HA(colour 1) H+ + A- (colour 2)
- The equilibrium constant, Ka, for indicators, is known as KIn and is normally expressed in the logarithmic form, pKIn
- The working pH range of an indicator is a range of pH on either side of the pKIn value
- For example, the pKIn of methyl orange is 3.7
- Below pH 3.7 the dominant colour is colour 1 (red for methyl orange) and above it is colour 2 (yellow for methyl orange)
- The range of change for a weak acid-strong base titration is around pH 8-10
- Using methyl orange for this titration won't work because the range of change is far too high
- You would see the indicator changing to yellow before you reach the end-point
- In this case, you need to use an indicator with a high pH range such as phenolphthalein
Examiner Tip
The word base and alkali are being used interchangeably here, but you should know an alkali is a soluble base.Since we are dealing with titrations here, the bases are always in solution so they are also alkalis.