Universal indicator


A universal indicator is a pH indicator made of a solution of several compounds that exhibits several smooth colour changes over a wide range pH values 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, 1-Propanol, 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
pH rangeDescriptionColour
< 3Strong acidRed
3–6Weak acidOrange or yellow
7NeutralGreen
8–11Weak alkaliBlue
> 11Strong alkaliViolet or Indigo

The colours from yellow to red indicate an acidic solution, colours blue to violet indicate alkali and green colour indicates that a solution is neutral.
IndicatorLow pH colourTransition pH rangeHigh pH colour
Thymol blue Red1.2 – 2.8Yellow
Methyl orangeRed3.2 – 4.4Yellow
Methyl redRed4.8 – 6.0Yellow
Bromothymol blueYellow6.0 – 7.6Blue
Thymol blue Yellow8.0 – 9.6Blue
PhenolphthaleinColourless8.3 – 10.0Fuchsia

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

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.