- 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)
- 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
- 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
- Strong Acid and Weak Base
- Methyl Orange will change sharply at the equivalence point
- Phenolphthalein would not change within the equivalence point
- 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