Chapter 2 The nutrients in food
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The vitamin C calculation (Food Science chapter 2) to find quantity doesn't say why phosphoric acid is used and what a blank titre is, and why it is included? Please explain. Thank you.
Igloo writes
Vitamin C (ascorbic acid) readily decomposes under alkaline conditions, since, in the first stage of the degradation, the vitamin C is neutralized by the alkali. This is why it needs to be “preserved” under conditions of low pH (hence the function of the phosphoric acid) in the preparation of the vitamin C extract, prior to titration with the 2,6-dichlorophenolindophenol dye solution.
In order to show that phosphoric acid itself doesn’t react with the dye, a “blank” titration is carried out with the aqueous phosphoric acid alone. This is a type of control experiment. In practice, the titre obtained during this procedure is tiny, perhaps zero cm3, i.e. the “pink” colour obtained during the titration is achieved when the first drop of dye has been added. It is obvious therefore that this (small) titre is subtracted from the titres obtained with the vegetable extracts, before any other calculations are carried out.
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updated: 26 August 2004
