STEP 1 (A)
20 ml of the mixture sample + 50ml DI water + 50ml sodium fluoride + 2 -3 drops phenolphthalein indicator, + 20 ml ( .05M) sodium periodate + 2 g KI + 20ml (20%v/v) HCl. 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).
I Repeated the same steps as A without sodium periodate. ( I expect to have a higher value than A, but i got thesame endpoint.