[kemistrykid]

Here is the descripition of the experiment:

Oxidations using the iodate ion (IO₃^-) give iodine-containing products in differing ocidation states depending on the conditions. In dilute acid, IO₃^- and I^- react in a 5:1 ratio to give I₂. In the presence of a large excess of Cl^-, (in the form of moderately concentrated HCl), the iodine-containing product is I⁺, in the form of ICl₂, and the reaction ratio differs. Such titrations are know as Andrews' titrations.

Now, I have to work out that reaction ratio (I think its 7:1), and I also have to work out the half-reaction for the tetrathionate/iodate reaction and the overall equation.

It turns out that the results I got for the qualitative results were wrong. Upon adding barium chloride, I should have got a white precipitate, which is a sign that SO₄^2- is present. In the second test, nothing happens, thus showing conclusively that SO₄^2- is present.

I think the half-reaction is something like this:

S₄O₆^2- + 7IO₃^- + 26H⁺ -----> 4SO₄^2- + 13H₂O

I think its balanced...

[Borek]

Well, if you take EBAS and play long enough you may find something like

2S₄O₆^2- + 7IO₃^- + 7Cl^- + 2H⁺ -> 8SO₄^2- + 7ICl + H₂O

but whether it is the reaction you are looking for - I have no idea. And somehow I doubt, as tetrathionate seems to be stable enough to be not oxidized further in analytical solutions, that's why thiosulfate is used in many analytical methods.

[kemistrykid]

Actually I've just found the answer. The reaction is:

2S₄O₆^2- + 7IO₃^- + 14Cl^-+ 2H⁺ ----> 8SO₄^2- + 7 ICl₂^- + H₂O

Very similar to what you suggested. The key thing here is that overall, S is being oxidised to +3, but is reduced to +3 from whatever oxidation state S is in in S₄O₆^2-. :drive1: :ph34r:

[Borek]

Very similar to what you suggested.

IMHO they are identical - ICl/Cl^- and ICl₂^- is analogous to I₂/I^- vs I₃^-.

The key thing here is that overall, S is being oxidised to +3, but is reduced to +3 from whatever oxidation state S is in in S₄O₆^2-.

We have discussed very similar problem in HS forum tonight. There are two different kinds of sulfur with different oxidation numbers in this compound. +3 is average and has nothing to do with reality (not that oxidation numbers have anything to do with reality ).

[kemistrykid]

IMHO they are identical - ICl/Cl^- and ICl₂^- is analogous to I₂/I^- vs I₃^-.We have discussed very similar problem in HS forum tonight. There are two different kinds of sulfur with different oxidation numbers in this compound. +3 is average and has nothing to do with reality (not that oxidation numbers have anything to do with reality ).

lol true!