• The equivalence point is where there is an equal proportion of hydrogen ion added to hydroxide ions
  • The End point is the volume of an alkali or acid which when added together the indicator changes colour

  • Suitable Indicators for particular Titrations require:
    • Colour change must be sharp rather than gradual.
    • The end point of the titration given by the indicator must be the same as the equivalence point, otherwise the titration will yield the incorrect answer
    • The different colours must be distinct (e.g. yellow à Orange is minor change, however colourless to pink is a major change)
  • The indicator must change in the equivalence point (the vertical sections)
  1. Strong acids and Strong Base
    • Methyl Orange changes colour within the equivalence point and therefore is suitable
    • Phenolphthalein changes colour within the equivalence point is also suitable
    • Phenolphthalein is preferred as the colour change is more obvious
  2. Weak Acid and Strong Base
    • Methyl Orange is not suitable as it does not change within the equivalence point of the curve
    • Phenolphthalein changes sharply at 25cm3, well within the equivalence zone, and is therefore suitable
  1. Strong Acid and Weak Base
    • Methyl Orange will change sharply at the equivalence point
    • Phenolphthalein would not change within the equivalence point
  2. Weak Acid and Weak Bases
    • Neither indicator is suitable
    • No indicator would be viable for the equivalence point over the two pH units

The Half-Neutralisation Point

  • The half neutralisation point is the distance half way between zero and the equivalence point
  • At this horizontal stage, additional acid has little effect on the pH
  • Therefore, by adding acid/base up until this point with the confidence that there will be insignificant change lays the basis of buffers
  • The half neutralisation point also allows for the pKa to be determined in weak acids

HA + OH à H2O + A

  • At the Half Neutralisation Point:

[HA] = [A]

Ka = [H+][A]

[HA] 

Ka = [H+]

-log10Ka = -log10[H+]

pKa = pH