Standardization of Na2S2O3?

Question

Hi. for std'zation of Na2S2O3, KIO3 primary std was used. but i just dont get it why KI is added. and also H2SO4. please explain why? and also specifically, in the experiment we added the acid before the KI. why? and the effect if KI is added first? (in other websites i read that some added KI first...its so confusing)

I think i get what youre saying, but i still dont get about KI. when i said KI, i meant KI, i didnt mean it as a shortcut to writing KIO3. but thanks anyways :)

Answer 1

Because that Na2S2O3 + KIO3 --> Na2S4O6 + I2 is redox reaction. Namely, it is an oxidation-reduction reaction. As you see, this is not a balanced equation and the full reaction is:
10 Na2S2O3 + 2 KIO3 + 12 H(+) --> 5 Na2S4O6 + I2 + 6H2O + 10 Na(+) + 2 K(+)
The reaction will only take place in acidic environment. Since KIO3 is quite neutral, but Na2S2O3 is basic (pH > 7). To make sure the reaction would take place, H2SO4 is added.
Now, as far as the reaction goes, it does not matter which one is added first. However, if you want to do some quantitative analysis, you may want to see when the reaction ends. You do not like to keep adding Na2S2O3 and/ or KIO3 with extra amount of reactants in the solution without seeing any reaction taking place. In this case H2SO4 is to be added first.

Answer 2

Standardization Of Na2s2o3

Answer 3

My Question is why starch as a indicator use in standardization of sodium thiosulphate at the end?why it is not use at the start....