Acid or base analytes, or ones that can be transformed into such by chemical treatment, can be determined through a neutralisation reaction using a strong acid or strong base solution.
Strong acid – strong base titration: H3O+ + OH- → 2 H2O
Weak acid – strong base titration: HA + OH- → H2O + A-
Strong acid – weak base titration: H3O+ + B → H2O + BH+
The standard solutions used are strong acids or bases as these react with analytes more completely than weaker solutions and so give a clearer end point.
Indicators are organic substances, either naturally-occurring or synthesised, which change colour according to pH. They are weak acids or bases whose non-dissociated form is a different colour from the conjugate base or acid form.
From the equations in the previous slide, we can conclude that:
The pH at the point at which [HInd] = [Ind-] is called the turning point. The pH turning point needs to coincide as closely as possible with the pH at the equivalence point of a titration. This is the only way to see the change in colour at the right moment, i.e. when the equivalents of a titrand are the same as those of the titrant.
This is why the indicator has to be chosen carefully according to its theoretical equivalence point i.e. where the sharp upturn is seen on the titration curve.
Phenolphthalein is one of the most commonly-used indicators in neutralisation titration. This indicator is said to be monochrome because the colour of the non-dissociated form is not visible to the human eye. This is why the colour change in this case is more correctly referred to as the moment when the colour appears, which happens at a pH of 8.3 with a pale pink colour in solution.
13. Mohr method
14. Vohlard method