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Universal Indicator

A universal indicator is prepared by mixing a number of common indicators together so that the mixture obtained can pass through a series of colour changes over a much wider pH range. For example, one such mixture which may show various colours at different pH is as given in Table 25.5

Table 25.5. Colours of Universal Indicator at Different pH Values

pH                                           Colour

3.0                                           Red

5.0                                           Orange red

5.5                                           Orange

6.0                                           Orange-yellow

7.0–7.5                                    Greenish-yellow

8.0                                           Green

9.5                                           Blue

10.0                                         Violet

Such mixtures are commonly known as universal indicators. Universal indicators are available commercially ‘ as solutions and as test papers. A pH paper is a strip of paper which is prepared by dipping the strip in the solutions of different indicators and then drying them.

pH paper can be used to find the approximate pH of any solution. The pH paper is dipped in a given sample of the solution, the colour developed in the paper is compared with the colour chart and approximate pH of the solution can be predicted.