Indicators and the pH Scale

The pH Scale is a numerical scale that measures how acidic or alkaline a solution is. The scale ranges from 0 (very acidic) to 14 (very alkaline).

  • Acidic substances have a pH value lower than 7
  • Alkaline solutions have a PH value greater than 7
  • Neutral solutions have a pH value equal to 7

If you add the right amount of an acid to an alkali, it will form a neutral solution. And the same is in reverse, if you add the right amount of an alkali to an acid solution, it will form a neutral solution.

This means that if we had something from the left side of the pH scale (an acid) and added the right volume of something from the ride side of the pH scale (an acid), we would eventually form a substance in the middle. We would form a neutral solution that has a pH of 7.


An indicator is a substance that changes colour in response to changes in pH. The pH of a solution can be measured using an indicator or a pH meter.

Universal indicator

A universal indicator is an acid-base indicator that can be used to determine the pH of a solution. When added to a solution, it produces a range of colours that can be compared to the pH indicator chart. This provides an accurate measurement of the solution’s pH.

  • On the left side of the scale, between pH 0-6, are acidic solutions. These solutions will range in colour from dark red (strong acid) to yellow (weak acid).
  • In the middle, at pH 7, you get neutral solutions, which is indicated by a colour change to green. This is where you find pure water.
  • On the right side of the scale, between pH 8-14, are alkaline solutions. These solutions will range in colour from dark green (weak alkali) to purple (strong alkali).

When performing acid-base reactions, we can use universal indicators to determine when the reaction has reached a neutral state. If neutralisation has occurred, the solution should change to a green colour.

Litmus Paper

Litmus paper is paper that has been treated with a mixture of dyes. The two types of litmus paper are blue and red.

  • Blue litmus paper turns red when put in an acidic solution
  • Red litmus paper turns blue when put in an alkaline solution

pH meter

A pH meter is a more precise and accurate way to determine the acidity or basicity of a solution compared to using a universal indicator. When placed in a solution, it provides a numerical value that represents the pH of the solution.

For example, if the reading is 2, the solution is acidic. The numerical data provided by the pH meter is more accurate than the colour-based method of using a universal indicator.