A Universal indicator is a pH indicator composed of a blend of several compounds that exhibits several smooth colour changes over a pH value range from 1-14 to indicate the acidity or basicity of solutions. Although there are a number of commercially available universal pH indicators, most are a variation of a formula patented by Yamada in 1923.[1] Details of this patent can be found in Chemical Abstracts.[2] Color of the universal indicator is dark pink.
Experiments with Yamada's Universal Indicator are also described in the Journal of Chemical Education.[3]
A universal indicator is typically composed of water, propan-1-ol, phenolphthalein sodium salt, sodium hydroxide, methyl red, bromothymol blue monosodium salt, and thymol blue monosodium salt.[4]
The colours that indicate the pH of a solution, after adding a universal indicator are:
| pH range | Description | Colour |
|---|---|---|
| 0-3 | Strong acid | Red |
| 3-6 | Acid | Orange/Yellow |
| 7 | Neutral | Green |
| 8-11 | Base | Blue |
| 11-14 | Strong Base | Violet/Purple |
There are now also available wide range pH test papers with distinct colours for each pH from 1 to 14. Colour matching charts are supplied with the specific test strips purchased.
A universal indicator is a solution which undergoes several colour changes over a wide range of pH's. The colour is used to "indicate" pH directly. Universal indicators are usually mixtures of several indicators. The easiest universal indicator to prepare is red cabbage juice. Stew red cabbage in water and pour off the water for use as an indicator solution. The pigment in red cabbage juice is anthocyanin, which changes colour from red in acid solution to purplish to green in mildly alkaline solution to yellow in very alkaline solution.
Another universal indicator is used in the classic 'chemical stoplight' demonstration, which changes from green to yellow to red as the pH goes from alkaline to acidic. The indicator is a solution that is 0.05% in each of the following: methyl red, methyl yellow, thymol blue, and bromthymol blue in ethanol.[1]