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 several commercially available universal pH indicators, most are a variation of a formula patented by Yamada in 1933. 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.
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:
|< 3||Strong Acid||Red|
|> 11||Strong Base||Violet/Purple|
The colours from yellow to red indicate a acidic solution,colours light blue to dark blue indicate bases and green colour tht a solution is neutral.
Wide range pH test papers with distinct colours for each pH from 1 to 14 are also available. Colour matching charts are supplied with the specific test strips purchased.
- 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.