[GaeGanzi]

I think that the chemical reaction formula in this case is:
SO2+I2+2H20→H2SO4+2HI

If this is right, KI, which is involved in Lugol's iodine, does not react
with SO2?

Please help me.

[Arkcon]

Yes.  The reaction I'd seen before was that one.  To help you understand it better:  What type of reaction is this?  Can you see what is happening to both reagents?  Does that give you a clue as to why the reaction won't proceed on the KI?  (Note: this is just a clue as to why it won't proceed on the KI, you would have to do more work to be sure.)

[GaeGanzi]

Yes.  The reaction I'd seen before was that one.  To help you understand it better:  What type of reaction is this?  Can you see what is happening to both reagents?  Does that give you a clue as to why the reaction won't proceed on the KI?  (Note: this is just a clue as to why it won't proceed on the KI, you would have to do more work to be sure.)

I know that this is oxidation reduction reaction.
In this case(SO₂+I₂+2H₂0→H₂SO₄+2HI),
for SO₂, SO₂+2H₂O→SO₄^2-+4H⁺+2e^- happens.
For I₂, I₂+2e^-→2I^- happens.

But, the problem here is that depending on the circumstance, SO₂ can work as oxidizing agent.
: SO₂+4H⁺+4e^-→S+2H₂O

Fortunately, Lugol's solution contains KI, which provides iodide ion.
: 2I^-→I₂+2e^-

That's why I think that another possible reaction can be considered:
SO₂+2H₂O+4KI→S+2I₂+4KOH

I am confused.

[Arkcon]

Good work.  You recognize that this is a red-ox reaction.  And you know some additional reactions the KI-I2 system has.  Now, you have to consider if SO2 has sufficient reducing power to affect the KI salt.

[GaeGanzi]

Good work.  You recognize that this is a red-ox reaction.  And you know some additional reactions the KI-I2 system has.  Now, you have to consider if SO2 has sufficient reducing power to affect the KI salt.

I am sure that SO₂ has sufficient reducing power to affect the I₂.

Of course, It is taken for granted that reducing power of SO₂ cannot make KI deoxdized.
However, as i mentioned above, SO₂ is an oxidizing agent in some cases. For example, when SO₂ meets H₂S, this reaction happens: 2H₂S+SO₂→3S+2H₂O
Obviously, SO₂ works as an oxidizing agent.
Therefore, I want to know whether SO₂ has sufficient oxidizing power to affect I^-.

[Arkcon]

As someone has posted in another, similar thread, http://www.chemicalforums.com/index.php?topic=56657.msg204213#msg204213 you have to compare red-ox potentials.  Wikipedia has a handy table:  http://en.wikipedia.org/wiki/Standard_electrode_potential_%28data_page%29

[Borek]

depending on the circumstance

As Arkcon wrote, it all depends on the redox potentials (and sometimes on kinetics). According to tables, standard electrode potential for iodine half reaction is higher than the standard electrode potential for (both) SO₂ reactions. That means iodine will be the oxidizing agent.

Interestingly, standard potentials for I₂/I^- reaction and SO₂/S reaction are very similar (+0.54V vs +0.50V), so correctly selecting concentrations and SO₂ pressure it should be possible to reduce iodides to iodine, but you would need very low pH (around zero) and a relatively high pressure of SO₂ (atmospheric at least). It still doesn't mean reaction would go as expected, but at least in terms of potentials it looks possible.

[nightowl]

That's why I think that another possible reaction can be considered:
SO₂+2H₂O+4KI→S+2I₂+4KOH

I find this highly plausible as I had worked out similar thoughts on this issue over an exercise sheet of reaction diagrams and found the same answer in the solutions I got at the student body's department.
However, it also states that in the reaction of SO₂ (g) with KI, there is a <b>white</b> precipitate forming.
Is there any form of white elemental sulfur? I've looked through dozens of books and the Internet but I don't seem to find any. 

[Arkcon]

I don't have a citation, but yes, I had heard that a finely divided elemental sulfur precipitate can look white.

[Borek]

I can confirm it looks white, at least initially.