Indicators and pH

Indicators and pH

Understanding pH and Indicators

pH Scale

  • The pH scale is used to measure the acidity or alkalinity of a solution. It ranges from 0 to 14.
  • A pH of 7 is neutral. This is the pH of pure water at 25°C.
  • A pH less than 7 is acidic. The lower the pH, the stronger the acid.
  • A pH greater than 7 is alkaline (or basic). The higher the pH, the stronger the alkali.

Indicators

  • Indicators are substances that change colour depending on the pH of the solution they are in.
  • Litmus paper is a commonly used indicator. It turns red in acidic solutions and blue in alkaline solutions.
  • Universal indicator is a mixture of indicators which can show different colours at different pH values across the pH scale, allowing for a more precise pH measurement.
  • Some indicators are natural and can be found in plants. For instance, red cabbage is a natural indicator.

Using Indicators

  • Acid-base titrations use indicators to identify the point at which the reaction between an acid and a base (alkali) is complete. This point is known as the endpoint.
  • The selection of an indicator for a particular titration depends on the pH range over which it changes colour and the pH at the end-point of the titration.

Importance of pH

  • Maintaining a certain pH is vital in many settings. For example, human blood has a pH around 7.4. If the pH of blood changes significantly it can be life-threatening.
  • pH is also important in environmental contexts: for example, rainwater with a pH less than 7 is considered acid rain and can damage buildings, harm wildlife and affect water sources.

Practical Work with pH and Indicators

  • Practical work with indicators and pH often involves titrations. In a titration, a known volume of an acidic solution (or a solution of known concentration) is added to an alkaline solution (or vice versa) until the solution is neutralised.
  • Indicators are used to show when neutralisation has occurred. This can be by change in colour.
  • An accurate measurement of pH during a titration can help calculate the concentration of an unknown solution.
  • Remember to use indicators and other chemicals safely in the laboratory, following all appropriate safety precautions.