URGENT!!Analysis of vitamin C tablet

I have to determine content of vitamin C which contains ascorbic acid.
Here are the steps:
(A. Standardize the given thiosulphate by iodate solution)
1.Prepare 250cm3 of iodine solution by KIO, KI, and with some H2SO4.
2.Pipette 25cm3 of iodine solution and titrate with Na2S2O3.
3.Add starch solution when the reaction mixture turns to pale yellow. Record the burette reading when the end-point reaches.

4.Dissolve vitamin C in 150cm2 0.5M H2SO4.Prepare 250cm3 of this solution.
5.Pipette 25cm3 of standrad KIO3 solution into the flask containing vitamin C and KI.
6.Titrate it with standard S2O3 solution, add atarch solution when the reaction mixture turns to pale yellow. Record the results.

The questions I want to ask are:
What are the possible errors in doing this experiment?
What assumptions should be made?
How to improve the accuracy of the results?
Thanks very much for your help! :)

One thing to remember is that ascorbic acid is unstable. It is sensitive to oxygen and also undergoes anaerobic hydrolysis. So don't prepare a sample and let it sit on your desk for any longer than absolutely necessary. Blanketing with nitrogen might be a good idea too. If you are still having problems/questions the best people to talk to would be at www.chromadex.com.