What is a Universal Indicator?

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

A roll of universal indicator paper. Image Source

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

Colours of universal indicator solution. Image Source

Universal indicator solution is a solution of:

• Water
• 1-Propanol
• Phenolphthalein
• 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.

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.