Indicators (Edexcel IGCSE Chemistry: Double Science)

What are two colour indicators?

• Two colours indicators are used to distinguish between acids and alkalis
• Many plants contain substances that can act as indicators and the most common one is litmus which is extracted from lichens
• Synthetic indicators are organic compounds that are sensitive to changes in acidity and appear different colours in acids and alkalis
• Phenolphthalein and methyl orange are synthetic indicators frequently used in acid-alkali titrations
• Synthetic indicators are used to show the endpoint in titrations as they have a very sharp change of colour when an acid has been neutralised by an alkali and vice-versa
• Litmus is not suitable for titrations as the colour change is not sharp and it goes through a purple transition colour in neutral solutions making it difficult to determine an endpoint
• Litmus is very useful as an indicator paper and comes in red and blue versions, for dipping into solutions or testing gases

Two Colour Indicators Table

• The pH scale goes from 0 – 14 
• All acids have pH values of below 7, all alkalis have pH values of above 7
• The lower the pH then the more acidic the solution is
  • pH 0-3 = strong acid
    • Extremely acidic substances can have values of below 1
  • pH 4-6 = weak acid
• The higher the pH then the more alkaline the solution is
  • pH 8-10 = weak alkali
  • pH 11-14 = strong alkali
• A solution of pH 7 is described as being neutral

• Universal indicator is a wide range indicator and can give only an approximate value for pH
• It is made of a mixture of different plant indicators which operate across a broad pH range and is useful for estimating the pH of an unknown solution
• A few drops are added to the solution and the colour is matched with a colour chart which indicates the pH which matches with specific colours
• Universal indicator colours vary slightly between manufacturer so colour charts are usually provided for a specific indicator formulation

pH scale with the Universal Indicator colours used to determine the pH of a solution