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The Range of an Indicator

The range of an indicator is the pH range over which it changes colour ·

At low pH, a weak acid indicator is mainly present as Hin (in acid form) and the colour of this form predominates. As the pH increases, the intensity of colour A starts decreasing while that of B starts increasing. The observable change from colour A to colour B takes place over a range of pH and this range of pH is known as the range of an indicator. An indicator is most effective if it undergoes a distinct change in colour over a narrow range of pH. For. most of the indicators, the range is within ±1 of the pK1n value. Since indicators have quite different values of pKin, the pH ranges of various indicators also vary widely as given in Table 27 .1.

It may be noted that end point of each indicator is in the centre of the pH range.

Table 27.1. Indicators and their pH Ranges

Litmus contains several substances and changes colour over a broader range from about pH 5 to pH 8.

A universal indicator is a mixture of indicators which gives a gradual change from one colour to the other over a wide range of pH. The approximate pH of the solution can be determined from the colour obtained when a few drops of universal indicator are added to the solution. The colour of the solution is then compared with that given on the standard chart. The corresponding pH is the pH of the solution.

Certain dyes from flowers and vegetables can be extracted with ethanol and used as acid-base indicators. For example, turmeric gives brown colour is alkaline medium. Dye from red cabbage also serves as acid-base indicator. In order to extract dye from some flower, the flower is crushed and the dye extracted with ethanol or warm ethanol.