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.(eg., phenolphthalein and methyl orange).
Those substances which change to specific colours in different pH value ranges of the medium are called as pH indicators. pH indicators are used as acid – base indicators also.
SELECTION OF pH INDICATORS:
Every pH indicators 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.
SOLUTIONS
INDICATORS pH RANGE. ACIDIC BASIC
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 neutralised 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 obtained is called titration curve. The titration curve is useful in the choice of a suitable indicators in an acid – base titrations. 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 indicators for any titration depends on the nature of the acid and base involved and the working range of the indicators.
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 indicators which changes its colour within this range may be used. as a suitable indicator in the titration of strong acid against strong base, phenolphthalein can be used as indicator 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 it’s working ranges from 8.3 – 10. Methyl orange cannot be used as an indicator. Since it has a working ranges 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 with in this pH range.
- Titration of weak acid against weak base 😦 example, Acetic acid Vs ammonium hydroxide)
In this type of titration there is no sharp colour change in the pH value at the end point. Therefore, in this titration of a weak acid against a weak base none of the indicators shown in the table are quite satisfactory.
by:
Pragadeeswari.D,
B.Tech. Chemical engineering 2nd year.
Chemnation 
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