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I am doing different titrations to find the amount of vitamin C in fruit juices. I have done a titration using DCPIP but I am looking for a clear method on how to do another titration using NBS. Could you help?
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Igloo writes ...
First you need to make up a solution of NBS in the region of 0.001 mol dm-3. Its precise concentration need not be known.
Next, using a volumetric flask, make up a solution of vitamin C of a known and exact concentration, say 200 mg dm-3, using 5% phosphoric acid as solvent, rather than pure water.
Pipette a known volume of this solution into a conical flask and add about 1-2 cm3 of potassium iodide solution (say of concentration 1 mol dm-3 – but this only needs to be approximate), as well as a few drops of starch indicator. In the meantime place the NBS solution in a burette. Titrate the vitamin C solution with the NBS until the end point is reached, ie colourless to intense blue-black.
Finally, via the use of a pipette, the same volume of the fruit extract can also be titrated with the NBS solution, and the results compared with that of the standard titration.
Since titres are proportional to the concentration of vitamin C it is easy to deduce the concentration of vitamin C in the samples by comparison with the titre obtained for the standard solution (which contains 200 mg dm-3).
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updated: 13 April 2008
