Synoptic topics
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What is an indicator?
Igloo writes
When carrying out a titration, the intention is to determine the minimum volume of solution (contained in the burette) necessary to react completely with the relevant compound present in solution in the flask. In the titration process we are determining what is known as the “end-point”, and it is necessary to find a material which changes colour when this stage has been reached. This type of material is known as an indicator.
When an alkali is being titrated with an acid, methyl orange is often used as an indicator. This is because it will show up as yellow in the alkali, but, at the end-point, the colour changes to red (its colour in acid solutions).
When a solution of iodine is being titrated with a solution of sodium thiosulphate, in the presence of starch as indicator, the end-point is marked by a sharp change from blue-black to colourless. This is because starch shows up blue-black in the presence of iodine but is colourless when all of this has been destroyed.
In some cases, one of the reactants is itself coloured, for example a solution of purple potassium manganate(VII). When solutions containing, for example, Fe2+ ions, are being titrated with a solution of potassium manganate(VII), the end-point is marked by the first appearance of a pink-purple colour in the flask, caused by the presence of excess MnO4- ions. This is because at this stage all the Fe2+ ions have been oxidised and the end-point has been reached. This type of titration therefore needs no external indicator, and the potassium manganate(VII) is said to be “self-indicating”.
Risk assessment
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updated: 15 March 2005
