2.5 Indicators
Indicators are dyes that change colour under varying conditions of acidity. Although not as accurate as instruments such as pH meters in determining acidity, indicators can be used to give less precise measure of acidity. You should recall in our introduction to acids and bases that we mentioned that litmus is red in acids and blue in bases. Litmus is an indictor that changes colour from red to blue in the pH range of 5.5 to 8.0. Other indicators and their colours are listed in the table of Acid - Base Indicators which you should print out and keep available.
Indicators are coloured compounds that exist in both acidic and basic forms. A general formula may be used for indictors - HIn - and a reaction written:
HIn |
↔ |
H+ + In- |
acidic form |
basic form |
|
colour 1 |
colour 2 |
Indicators may be in solution form or paper form. pH paper is prepared by treating the paper with the indictor solution. When the paper is then dipped into the solution you are testing, it will change colour depending on the acidity of the solution.
Here are some questions to try. A short acid-base indicator table is given here:
| Indicator | pH range | Colour change |
| methyl orange | 3.2 - 4.4 | red to yellow |
| litmus | 5.8 - 8.0 | red to blue |
| phenolphthalein | 8.2 - 10.0 | colourless to pink |
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1. |
A given solution turns methyl orange yellow, litmus blue, and phenolphthalein red. What is the approximate pH of the solution? | ||||||||||||||||
Solution: Methyl orange in yellow when pH is above 4.4 Therefore the solution would have to have a pH above 10.0 |
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2. |
What color would methyl orange, litmus, and phenolphthalein turn when testing: a. vinegar (pH = 3) |
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Solution:
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