Theory of Indicator | Theory of Acid Base Indicator

Theory of Indicator

An indicator is used to determine the end-point in titrations. When two solutions are titrated, then the slight excess of one solution is revealed by the colour change in the solution. This stage is called end point. The acid-base indicators show a change of colour at a particular pH or we can say that an indicator has one colour in acid solution and another colour in alkaline solution. The colour change and the pH range of some common indicators are tabulated below-

Indicator	pH Range	Acidic Medium	Neutral Medium	Alkaline Medium
Litmus	4.5 - 7.4	Red	-	Blue
Methyl Orange	3.1 - 4.5	Red	Orange	Yellow
Phenolphthalein	8.0 - 9.5	Colorless	Colorless	Pink

Two main theories are being adopted to explain the behaviour of acid-base indicators.

Ostwald Theory

According to this theory, an indicator is considered as a weak electrolyte which give coloured ion under the influence of strong acid and/or strong base e.g. phenolphthalein (PhH), being a weak acid, ionises into H & Ph ions.

PhH      ⇌       Ph^- + H⁺

Colourless       (Pink)

In acidic solution, H⁺ concentration is high, hence the dissociation of PhH is practically nil due to which the solution remains colourless. In alkaline solution, OH^- concentration is high which reacts with H⁺ to form feebly ionised H₂O shifting the equilibrium towards right i.e. the formation of Ph^- takes place quickly. Hence in alkaline solution, phenolphthalein appears pink.

The unionised methyl orange (MeOH) is yellow. Being a weak base, ion on ionisation. it gives red Me⁺ ion on ionization.

MeOH      ⇌     Me⁺  OH^-

Yellow         (Red)

In presence of alkali which increases OH^- concentration, suppresses the ionisation of MeOH so that solution becomes yellow but when an acid is added in slight excess, the H⁺ ion of acid combines with the OH^- of MeOH to form feebly ionised H₂O. Under this condition, more and more MeOH ionises to form red Me⁺ ions in solution. Hence in acidic medium, methyl orange appears red.

Quinonoid Theory

An acid-base indicator exists in tautomeric forms one of which can exist only in acid medium while another only in alkaline medium. As the pH of such solution changes, the solution shows a change of colour due to intramolecular change from one form to another. The quinonoid form is generally more deep in colour than benzenoid form.

The phenolphthalein is colorless in acidic solution(benzenoid) but shows a red color in alkaline solution(quinoid form)-

Similarly the methyl orange has two tautomeric forms- red form in acidic solution and yellow form in alkaline solution.

Limitations of Acid-Base Indicator

1. Weak acid vs. weak base can not be titrated with either methyl orange or phenolphthalein.
2. It does not actually indicate the acidic or alkaline solution but indicates particular pH range of the solution.
3. Colored solutions can not be titrated.
4. Much care is need near the end point.

Exercise Questions

Theory of Indicator

Theory of Indicator