blackcool

Please someone help... .

Hi to all great member, This is my first post on this forum after joining a few days ago. Nice meeting all of science people here.... I am really need clarification on how to calculate the purity of Sodium Meta Bisulfite above. I do the titration with Iodine 0.1 N until the color changes to faint black with starch indicator. In water the Na2S2O5 + H2O ---> 2NaHSO3. The purity calculated as: {(VxNx47.5) / W mg)} x 100% = % w/w Na2S2O5 as Na2S2O5. Is the above equation correct?. Any other thought? Google was no help anyway . Thankyou in advance.