What is a Universal Indicator?
Universal Indicator is a chemical used to measure the acidity or alkalinity of a substance.
A universal indicator is basically a pH indicator. It is a mixture of several compounds that exhibits several smooth colour changes over a wide range of pH values.
pH is a measurement of the acidity or alkalinity of a substance. Alkaline substances have pH values greater than 7, acidic substances have pH values less that 7 and neutral substances have a pH value of 7.
A universal indicator can be in paper form or in solution form.
Universal Indicator Paper
Universal indicator paper is a strip of coloured paper which changes colour according to the solution being tested. To test pH, the strip can either be placed directly on a surface of a moist substance or few drops of the solution to be tested can be dropped onto the universal indicator using a dropper.
Universal indicator paper is preferably used when the solution to be tested is darker.
Universal Indicator Solution
Universal indicator solution is a solution of:
- Sodium hydroxide
- Methyl red
- Bromothymol blue
- Sodium bisulphite
- Thymol blue
Universal indicator solution is preferable if solution is colourless. This increases the accuracy of indication.
Colours of the Universal Indicator
The colours that indicate the pH of a solution, after adding a universal indicator, are:
- yellow to red indicate an acidic solution
- blue to violet indicate alkali
- green colour indicate a neutral solution.
|< 3||Strong acid||Red|
|3–6||Weak acid||Orange or Yellow|
|> 11||Strong alkali||Indigo or Violet|
However, the colors are usually specific to the formulation of the universal indicator and therefore a commercial preparation supplied together with colour matching charts that explains the expected colors and pH ranges.