Food Science
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I am titrating ascorbic acid using iodine, and want then to do a back titration. Can you explain how I can achieve this and whether I would have to decolorise in between the titrations? If decolorisation is needed is the best method using charcoal?
Igloo writes ...
No decolourisation should be carried out. You need the colour to detect the endpoint during the back titration.
Procedure
1 Make up a solution of iodine of appropriate concentration.
2 Determine its accurate concentration with a standard solution of sodium thiosulphate, using starch as indicator when the end point is approaching (when the solution is coloured pale yellow).
3 Add a known volume (in an excess) of the iodine solution to a known volume of your ascorbic acid solution.
4 Leave to stand to ensure that the redox reaction between iodine and ascorbic acid is complete.
5 The solution should still be coloured brown or dark orange.
6 Carry out a back titration with the sodium thiosulphate solution, using starch as indicator when the end point is approaching (when the solution is coloured pale yellow).
I’ll leave you to figure how the calculation is carried out. You’ll need to work out balanced equations for the two redox processes involved.
Always carry out a risk assessment and check with your teacher before starting any practical work.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
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updated: 24 January 2007
