3.3 - Using indicators
Learn the colour changes of litmus, methyl orange and phenolphthalein, then use observations without claiming more than an indicator can show.
Recognising indicator colours
An acid–base indicator changes colour over a particular pH range. Use only a small amount, or indicator paper, to observe a sample. The name of the indicator matters: the same colour can mean different things with different indicators.
These are the usual colours in acidic and alkaline laboratory test solutions, away from each indicator's transition range:
| Indicator | Acid colour | Alkali colour |
|---|---|---|
| Litmus | Red | Blue |
| Methyl orange | Red | Yellow |
| Phenolphthalein | Colourless | Pink |
For litmus paper, describe the starting and final colours: blue litmus turns red in acid; red litmus turns blue in alkali. Red litmus placed in acid stays red, so using only red paper gives less information than testing with both colours.
With phenolphthalein, colourless means no colour, not white. If acid is added to a pink alkaline solution until it passes through the indicator's transition, the indicator becomes colourless. This observation is different from saying the solution has become cloudy.
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