A Universal indicator is a pH indicator composed of a solution of several compounds that exhibits several smooth colour changes over a pH value range from 1-14 to indicate the acidity or alkalinity 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. Details of this patent can be found in Chemical Abstracts. Experiments with Yamada's Universal Indicator are also described in the Journal of Chemical Education.
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. The universal indicator is made from magnesium hydroxide with a pH value of 4.
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. The colours that indicate the pH of a solution, after adding a universal indicator are:
|> 11||Strong alkali||Violet/Purple|
- Jap. Pat. 99,664, Feb 21, 1933
- Chem Abstr, 28, 2258 (1934)
- For a discussion of these experiments, as well as recipes for Yamada and other universal indicators, see Foster, S.L. and Gruntfest, J.Chem.Educ., 14, 274 (1937)
- "Universal Indicator". ISCID Encyclopedia of Science and Philosophy.