EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 复习笔记:2.5.1 Indicators

EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 复习笔记:2.5.1 Indicators

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

 

2.6.1-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

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

 

 

Universal-indicator-and-the-pH-scale2

 

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.

转载自savemyexams

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