Indicators (Edexcel IGCSE Chemistry (Modular))
Revision Note
Indicators
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 an indicator paper and comes in red and blue versions, for dipping into solutions or testing gases
Two Colour Indicators Table
Indicator | Colour in acid | Colour in alkali |
---|---|---|
litmus | red | blue |
phenolphthalein | colourless | pink |
methyl orange | red | yellow |
The pH scale
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
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
Examiner 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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