Indicators (Edexcel IGCSE Chemistry)

Two Colour Indicators

Spec point — describe the use of litmus, phenolphthalein and methyl orange to distinguish between acidic and alkaline solutions

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 indicator paper and comes in red and blue versions, for dipping into solutions or testing gases

Two Colour Indicators Table

The pH Scale

Spec point — understand how to use the pH scale, from 0–14, can be used to classify solutions as strongly acidic (0–3), weakly acidic (4–6), neutral (7), weakly alkaline (8–10) and strongly alkaline (11–14)

• 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

Spec point — describe the use of universal indicator to measure the approximate pH value of an aqueous solution

• 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 Hint 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.