Something about SO2

[dttom]

Will SO2(aq) be changed SO4 2- ion

[sO2+H2O<->H2SO3, (SO3 2- + H2O->SO4 2- +2e- + 2H+,I2+2e- ->2I-)]

and turn iodine solution from brown to colourless?

[budullewraagh]

SO2 can form H2SO3, but won't form H2SO4. if you use hydrogen peroxide in the solution, the oxidation to sulfate will occur.

[woelen]

SO2 indeed reacts with ater and iodine, itself being converted to H2SO4, the iodine being converted to iodide.

 

A very remarkable reaction occurs, if excess SO2 is used. The excess SO2 reacts with I(-), forming deep yellow adducts [i.nSO2](-).

 

You can observe these very special complexes by adding potassium iodide to an acidified solution of sodium sulfite or sodium bisulfite.

 

Think of this. It is really remarkable, because most people think that iodide and sulfite do not react.

[budullewraagh]

ah yes, iodine will oxidize the sulfite. it had appeared that he was asking if water alone would convert SO2 to sulfate

[dttom]

SO2 can form H2SO3, but won't form H2SO4. if you use hydrogen peroxide in the solution, the oxidation to sulfate will occur.

Then whether my half-ionic equation is correct or not?

SO3 2- + H2O-> SO4 2- + 2H+ + 2e-

SO2 indeed reacts with ater and iodine' date=' itself being converted to H2SO4, the iodine being converted to iodide.

[/quote']

You mean sulphite ion will be oxidized to sulphate ion, and iodine will be reduced to iodide? If yes, is the mixture of KI, I- and SO4 2-(react I2 in KI with SO2) colourless?

[YT2095]

Then whether my half-ionic equation is correct or not?

SO3 2- + H2O-> SO4 2- + 2H+ + 2e-

 

that will work

it`s very exothermic though, and doesn`t work all that well in Practice.

[dttom]

How about the second question? Is the mixture react in such way and become colourless?

[woelen]

Then whether my half-ionic equation is correct or not?

SO3 2- + H2O-> SO4 2- + 2H+ + 2e-

 

You mean sulphite ion will be oxidized to sulphate ion' date=' and iodine will be reduced to iodide? If yes, is the mixture of KI, I- and SO4 2-(react I2 in KI with SO2) colourless?[/quote']

Only, when there is an exact neutralization of iodine with SO2 (or acidified sulfite) the resulting liquid will be colorless.

 

If too little SO2 is used, then part of the I2 will remain unoxidized. This will form a brown solution with the already formed I(-) ions and at low concentration you get yellow/brown I3(-), triiodide ions.

 

If too much SO2 is used, then the liquid becomes yellow again. The bright yellow adduct [i.nSO2](-) is formed, with n being 1, 2, 3, or 4.

 

The first reaction and color of triiodide are known very well, but the latter NEVER is mentioned in any schoolbook, not even in higher level chemistry books. It is really remarkable that such common reagents result in formation of such a badly-known compound. Numerous people must have seen the yellow adduct, but nobody apparently cared about it. The issue is resolved last year with the help of a very kind person on Usenet, sci.chem. For the more interested, search sci.chem and look at "sulfite iodide".

 

So, if there is someone out there, who has sulfite and iodide (e.g. Na2SO3 and KI), dissolve the sulfite in some dilute acid, do the same with the iodide and mix. The result is quite unexpected.

[woelen]

that will work

it`s very exothermic though' date=' and doesn`t work all that well in Practice.[/quote']

Well, it is fairly exothermic, so I agree with that one. The second one I do not agree. It works great in practice. The reaction is really smooth, fast and quantitative. I've done this reaction many times with different oxidizers. In fact, (acidified) sulfite by far is my favorite aqueous reductor and I suggest you to get some of this too. It is cheap, does not raise any eyebrows when ordered online, can be shipped and imported without any hazmat or other trouble and is a really versatile reductor. Almost too nice to be true .

[YT2095]

lol, naah, I was just looking at the SO3 + water side of it directly, if it`s done with mediator ions (carriers) then fine, but directly, it isn`t so good and is Very exothermic