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Acid base indicators
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Some common acid-base indicators, the pH ranges in which they show colour change and the colours exhibited in acidic and basic medium are given below :
| Indicator | pH range | Colour in acidic medium | Colour in basic medium |
| Methyl orange | 3.1 – 4.5 | Pink | Yellow |
| Methyl red | 4.2 – 6.3 | Red | Yellow |
| Phenolphthalein | 8.0 – 9.8 | Colourless | Pink |
Two theories have been proposed to explain the change of colour of these acid-base indicators. These therories are : (a) Ostwald’s theory (b) Quinonoid theory
The suitable Indicators used in different titrations are :
| Type of titration | Indicator used |
| (A) Strong acid with strong base | Phenolphthalein, methyl orange or methyl red |
| (B) Weak acid with strong base | Phenolphthalein |
| (C) Strong acid with weak base | Methyl orange or Methyl red |
Note : q Phenolphthalein is not suitable if the base is weak, methyl orange is not suitable if the acid is weak.
Universal indicator : It is a mixture of a number of indicators which shows colour changes over different pH ranges.
| pH | 2 | 4 | 6 | 8 | 10 | 12 |
| Colour | Red | Orange | Yellow | Bule | Bule | Purple |
End point and equivalence point : End point is a point at which titration is stopped while equivalence point is the point at which acid and base (or oxidising and reducing agent) have been added in equivalent quantities
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