10.2 pH Scale (Indicators and pH Measurement)
10.2.1 Indicators
Indicators are the organic compounds. They have different colours in acidic and alkaline solutions. Litmus is a common indicator. It is red in acidic solutions and blue in alkaline solutions.
Each indicator has a specific colour in acidic medium which changes at a specific pH to another colour in basic medium. For example, phenolphthalein is colourless in strongly acidic solution and red in strongly alkaline solution. It changes colour at a pH of about 9. This means phenolphthalein is colourless in a solution with pH less than 9. If the pH is above 9, phenolphthalein is red as is shown in figure 10.2.
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figure 10.2.
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📊 Data TableScroll ↔
| Indicator | Colour in strongly acidic solution | pH a which colour changes | Colour in strongly alkaline solution |
|---|---|---|---|
| Methyl orange | red | 4 | Yellow |
| Litmus | red | 7 | blue |
| Phenolphthalein | colourless | 9 | red |