Indicators (Edexcel IGCSE Chemistry)
Revision Note
Author
StewartExpertise
Chemistry Lead
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
Two Colour Indicators Table
- 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 an indicator paper and comes in red and blue versions, for dipping into solutions or testing gases
The pH Scale
- The pH scale goes from 0 – 14 (extremely acidic substances can have values of below 0)
- 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
- The higher the pH then the more alkaline the solution is
- A solution of pH 7 is described as being neutral
The pH scale showing acidity, neutrality and alkalinity
Universal Indicator
- 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
Exam Tip
A common error is to suggest using universal indicator as a suitable indicator for an acid-base titration. This is incorrect as a sharp colour change is required to identify the end-point, which cannot be achieved with universal indicator.
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