Selection of pH indicators and Titration

pH INDICATORS

An indicator is a substance which indicates the completion of a reaction by sharp colour change at the end point without taking part in the reaction. The substances which are used to indicate the end point in acid-base reactions are called as acid-base indicators. (e.g.,) phenolphthalein and methyl orange.

Those substances which change to specific colours in different pH range values of the medium are called as pH indicators. Incidentally pH indicators are used as acid-base indicators also.

Selection of pH indicators

Every pH indicator changes its colour specifically in a ranging pH which is called as indicator range. For some of the indicators, the indicator ranges are given as below.

Colour of the indicator

Indicator      pH range      Acidic solution       Basic solution

Methyl orange :    3.1 - 4.4      Pink  Yellow

Methyl red :          4.4 - 6.2      Red   Yellow

Phenol red :          6.8 - 8.4      Yellow        Red

Phenolphthalein :           8.3 - 10       Colourless  Pink

When a base is added to a solution of an acid, the H⁺ ions will be slowly neutalised by the OH^- ions of the base. Hence, there is a steady decrease in the H⁺ ion concentration and pH value increases uniformly. At the end point there is a steep rise in the pH value. The pH values can be plotted against the volume of the base added and the curve so obtained is called titration curve. The titration curves are useful in the choice of a suitable indicator in an acid-base titration. A suitable indicator in an acid-base titration is one whose range is well within the sharp rising portion of the titration curve. Thus the choice of a suitable indicator for any titration depends on the nature of the acid and base involved and the working range of the indicator.

1.Titration of a strong acid against a strong base : (Example, HCl vs NaOH)

In this type of titration, the change in the pH value at the end point is roughly from 4 to 10. Therefore any indicator which changes its colour within this range may be used as a suitable indicator in the titration of strong acid against strong base and phenolphthalein can be used as indicators for this type of titrations.

2.Titration of a weak acid against a strong base : (Example, Oxalic acid vs NaOH)

There is a little change in the pH value at the end point in this type of titration. The pH value changes from 6.5 to 10. Thus phenolphthalein is the suitable indicator for this titration as its working range is 8.3 - 10. Methyl orange is not a suitable indicator. Since it has a working range below pH 5.

3.Titration of strong acid against weak base : (Example, HCl vs Na₂CO₃]

When a strong acid like HCl is titrated against a weak base like Na₂CO₃, the pH changes from 3.5 to 7.5 at the end point. The best indicator for this type of titration is methyl orange which changes its colour within this pH range.

4.Titration of weak acid against weak base : (Example, CH₃COOH vs NH₄OH)

In this sypte of titration there is no sharp change in the pH value at the end point. Therefore, in the titration of a weak acid against a weak base none of the indicators shown in the table are quite satisfactory.

There are two theories to explain the function of acid-base indicators.

1. Ostwald's theory

This theory was proposed by Ostwald's in 1891. It is based on Arrhenius theory. According to this theory, the acid-base indicator is either a weak acid or a weak base. They are partially ionised in solution. The ionised and unionised forms have different colours. The indicator exists predominantly in one of the two forms depending on the nature of the medium and hence there is colour change when the nature of the medium changes. Phenolphthalein is a weak acid and it is partially ionised in solutions.

HPh (Unionised form (colourless) < -- -- -- > H⁺ + Ph ^- (ionised form (pink) )

In acidic medium, excess H⁺ ions are present which suppress the dissociation of HpH due to common ion effect. Hence the indicator exists predominantly in unionised form and it is colourless. In alkaline medium, the OH^- ion neutralises H⁺ ion to form water. Consequently the dissociation of HpH is favoured and the indicator is predominantly in the ionised form and it is pink in colour.

Methyl orange is a weak base and its ionisation can be written as

MeOH (Unionised form (yellow)) < -- -- -- > Me⁺ +    OH^- (ionised form (pink))

In the presence of a base excess OH^- ions suppress the dissociation of MeOH due to common ion effect. Hence in basic medium, the indicator is mostly in unionised form which is yellow.

In acidic solution the H⁺ ions combine with OH^- ions to form unionised water. Hence in acidic solution, the indicator is mostly in ionised form and has pink colour.

This theory also explains why phenolphthalein is not a suitable indicator in the titration of a strong acid against a weak base. The reason is the OH^- ions produced by the weak base at the end point is too low to cause the ionisation of phenolphthalein. Hence, the pink colour does not appear exactly at the equivalence point. The pink colour appears only after a sufficient excess of the weak base is added.

For a similar reason, methyl orange is not a suitable indicator in the titration of a strong base against a weak acid. The weak acid does not furnish sufficient H⁺ ions to shift the equilibrium towards the right. A sufficient excess of the weak acid has to be added to get the colour change.

Quinonoid Theory

According to this theory the colour change of an acid-base indicator arises as a result of structural change. It is supposed that an indicator exists as an equilibrium mixture of two tautomeric forms namely, benzenoid and quinonoid forms.

One form exists in acidic solution and the other form in basic solution. At least one of the tautomers is a weak acid or a weak base. The two forms possess two different colours and as the pH of the solution containing the indicator is changed, the solution shows a change of colour. The colour change is due to the fact that one tautomer changes over to the other.

For example, phenolphthalein is tautomeric mixture of the two forms.