Universal indicator

This article is about the chemical pH indicator. For the musical artists, see Universal Indicator (music).
A roll of universal indicator paper
Colours of universal indicator

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 to 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.[1] Details of this patent can be found in Chemical Abstracts.[2] 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
< 3 Strong acid Red
3–6 Weak acid Orange or yellow
7 Neutral Green
8–11 Weak base Blue
> 11 Strong base Violet or purple

The colours from yellow to red indicate an acidic solution, colours light blue to dark blue indicate bases and green colour indicates that a solution is neutral.

Universal indicator components
Indicator Low pH colour Transition pH range High pH colour
Thymol blue (first transition) Red 1.2  2.8 Yellow
Methyl red Red 4.4  6.2 Yellow
Bromothymol blue Yellow 6.0  7.6 Blue
Thymol blue (second transition) Yellow 8.0  9.6 Blue
Phenolphthalein Colourless   8.3  10.0 Fuchsia

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.

Types of Universal Indicator

Colour Change of Universal Indicator after immersing into Basic NaOH solution.
Universal Indicator Test Kit
Colour Change of Universal Indicator after immersing into a Neutral solution

A universal indicator is collectively a mixture of indicators which show a colour change in a solution,which interprets how acidic or basic a solution is. A universal indicator can be in paper form or present in a form of a solution.[5]

See also

References

  1. Jap. Pat. 99,664, Feb 21, 1933
  2. Chem Abstr, 28, 2258 (1934)
  3. For a discussion of these experiments, as well as recipes for Yamada's and other universal indicators, see Foster, L. S.; Gruntfest, I. J. (1937). "Demonstration experiments using universal indicators". Journal of Chemical Education 14 (6): 274. doi:10.1021/ed014p274.
  4. "Universal Indicator". ISCID Encyclopedia of Science and Philosophy.
  5. Walker, Denise (2007). Acids and alkalis (1 ed.). London: Evans. p. 13. ISBN 0-237-53002-3. Retrieved 4 June 2015.
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