Broadly defined, as indicator is substance which is used for the visual detection and determination of a specific constituent present in any sample. The visual observation used is primarily that of colour, but observations of fluorescence and turbidity are also used.Indicators under this general definition include all reagents which can be used in colorimetry flurimetry and turbidimetry. It is convenient, therefore, to define on indicator in a more limited way, as substance which is used for the visual detection of the completion of a particular reaction that is for the end point of a titration.
Acid-Base Indicators :- Acid- base indicator are organic substances which have one colour in acid solution while an altogether different colour in alkaline solution.
Theories of Acid-Base Indicators :-
Two important theories have been put forwarded to explain their behaviour:
(I) Ostwald's theory (1891) : According to this theory:
(i) Acid- base indicators are weak organic acids or bases.
(ii) They have different colours in ionised and non-ionised states i.e.
Hln H+ + ln-
(one colour) (different colour)
(iii) The colour of the indicator depends on the relative proportions of the unionised indicator molecules and its ions.
On the basis of above postulates. Ostwald explained the action of phenophthalein, methyl orange, methyl red and other acid-base indicators.
(a) Action of Phenolphthalein:- Phenolphthalein is a weak acid (HPh) and is almost unionised. Its unionised molecules are colourless whilst on ionisation give colourless H ions and pink coloured Ph- ions.
HPh H+ + Ph-
(colourless) (colourless) (pink)
In the presence of acid due to increase in the concentration of common H ions, the dissociation of HPh is suppressed and thus the solution becomes colourless.
On the other hand, the addition of strong bases (like NaOH, KOH), however, the OH- ions produced from them combine with the H ions from the phenonphthalein to form feebly ionised water. The equilbrium (i) is thus disturbed and more of the phenolphthalein ionises to produce Ph- ions. The latter combine with Na ions to form the strongly ionised sodium salt NaPh and hence remains in the ionic state giving pink coloured Ph- ions.
(b) Methyl orange :- It is a weak base and can be represented as MeOH. Its undissociated molecule is yellow while gives red coloured Me ions on dissociation,
MeOH Me+ + OH-
(yellow) (Red) (colourless)
If a base (i.e., OH- ions) is added to the indicator, the OH- ions will suppress the ionisation of the indicator. Hence, the indicator will remain yellow in an alkali. However, if a small excess of acid (say, HCl) is added, the latter will force the equilibrium to the right by removing OH- ions to form H2O. This will result in the formation of red coloured Me ions in the solution.
MeOH Me+ + OH-
(yellow) (Red) (colourless)
HCl Cl- + H+
(Highly ionized) (Unionised)
(II) Modern Quinoid Theory :- Main postulates of this theory are:
(i) The indicators used in acid-alkali titrations are aromatic organic compounds which are equilibrium mixtures of at least two tautomeric forms, ordinarily one form is benzenoid while the other is quinoid.
(ii) The two forms have different colours. The quinoid form is usually deeper in colour than the benzenoid form. Out of these one form exists in acidic solution while other in alkaline solution.
(iii) Change in pH causes the transition of benzenoid form to quinoid form and vice versa and consequently a change in colour.
This theory explains the action of phenolphthalein, methyl orange and other acid-base indicators.
(a) Action of Phenolphthalein:- Phenolphthalein is an acidic indicator undergoing the following transformation:
In alkaline medium, OH- ions combine with H ions produced by the indicator, the equilibrium shifts to the right producing pink colour. In acid medium the dissociation of the organic acid is suppressed, the equilibrium shifts to the left and the solution becomes colourless.
(b) Action of Methyl Orange
Methyl orange is a basic indicator. More correctly it is an amphoteric compound containing both acidic group -SO3H and basic group -N(CH3)2. The quinoid form (red) combines with the OH-ions in alkaline medium favouring the formation of yellow.
(c) Action of Methyl Red
Methyl red is a basic indicator and the colour change takes place according to the following scheme:
Acid-Base Indicators with pH range of colour change
Colour in alkali
pH range of colour change
3.2 - 4.4
3.8 - 5.4
4.8 - 6.0
6 - 7.5
6.3 - 8.4
8.2 - 10
8 - 9.6
9. 4 - 10.6
Choice of Indicators:-
At the equivalence point of acid- base titration there occurs a sudden jump in pH of the solution. An indicator, the pH range of colour change of which falls within this limit, is suitable and is used in that titration. No sudden pH jump of the solution in the titration of weak acid with weak base occurs and so not indicator is suitable for this titration.