Reducing sugar and Vitamin C

you should know that vitamin C is prone to oxidation, especially under strong heat and presence of transition metals.

first, titrate (or determine) for the total amount of reducing agents in sample by DCPIP solution.

then, to another batch of sample, add a few drops of metal solution (like iron(II) sulfate, manganese(II) sulfate etc.), followed by heating in water bath for at least 15 mins.
addition of metal solution is not a must, nut be sure to heat the sample long enough to destroy the vitamin C.

then, titrate the amount of reducing agent in heated sample -- which is supposed to be NOT containing any vitamin C.
amount of vitamin C is obtained by subtracting the two results.