[EQUATION]

DETERMINATION OF GLYCOLIC ACID CONCENTRATION IN A MIXTURE OF SULFURIC & GLYCOLIC ACID[/b]

STEP 1 (A)
20 ml of the mixture sample + 50ml DI water + 50ml sodium fluoride + 20 ml ( .05M) sodium periodate + 2 g KI + 20ml. Then, I titrated with sodium thiosulfate solution until a straw yellow color appeared.  I added 4 ml of starch indicator to the resulting solution and continue to titrate slowly until the blue color disappeared( my endpoint).
STEP 2
I Repeated the same steps as A without sodium periodate.
Then I subtracted (B-A) to get the volume of glycolic acid in the mixture.
It seems this method is not working.

PLEASE WHO HAS A BETTER WAY OF DOING THIS?  PLEASE PLEASE