Define Titrimetric Method - 2, 6 Dichlorophenol Indophenol Method?
The titrimetric method is a redox titration method which depends on the reduction of the blue dye 2,6 dichlorophenolindophenol to a colourless leuco compound by ascorbic acid in solution or in extracts made out of foodstuff. Ascorbic acid, as you have already studied, is a strong reducing agent because of which it reduces the dye 2,6 dichlorophenolindophenol and itself gets converted to dehydro ascorbic acid. After equivalence point is reached the next drop of dye gives a pink colour to the solution indicating the end point. The dye in this titration is coloured in the oxidized form and colourless in the reduced form. Also the dye is pink in acidic solution and blue in alkaline solution.
The reaction involved in this method is highlighted in Figure
Figure: Conversion of ascorbic acid to dehydro ascorbic acid
In acidic solution the pink dye is reduced to a colourless substance in presence of ascorbic acid. The solution remains colourless as more dye is added until all the ascorbic acid has reacted. As soon as the next drop of dye solution is added the solution becomes light pink due to excess dye indicating that the end point of the titration has reached. The solution of ascorbic acid or its extracts are prepared in metaphosphoric acid or glacial acetic acid or a mixture of the two. The acid prevents the oxidation of ascorbic acid due to oxygen in the atmosphere which is faster in presence of metal ions like Fe3+ and Cu2+. Metaphosphoric acid can also form complexes with metal ions which reduces the catalytic oxidation of ascorbic acid. In foods, metaphosphoric acid releases the ascorbic acid from proteins and inactivates ascorbic acid oxidase.