[Mr_C]

For my titration I am going to find out the sulfur dioxide levels in different brands of apricots. I only have this one little problem...

Question word for word:-
Write the half-equations and the net equation for the reaction between sulfur dioxide and iodine in solution.

I think it's...

SO₂ + I₂ => I₂O₂ + S ??

but seeing it says 'half-equations' shouldn't there be some electron movement involving redox methods?

Also, I'd like to verify if this is correct; Iodine Solution @ 0.01gm/mL = 0.0788M

[mike]

SO₂ + I₂ + 2H₂O -> H₂SO₄ + 2HI

I₂ is reduced

SO₂ (S) is oxidised

does this help?

[Borek]

Also, I'd like to verify if this is correct; Iodine Solution @ 0.01gm/mL = 0.0788M

I₂, not I.

[Mr_C]

Thanks for your replies, scooby snacks for all!

mike: hmm....I guess I'll have to take your word for it, just a little curious on how I would work with the products

Borek: ahh, yeah my bad

[mike]

I thought maybe you could add a known amount of I2 and then after it has reacted you could titrate the reamianing I2 with starch and determine the amount of SO2 from that?

[Mr_C]

This is what I plan on doing:
Having a known amount of mushed up dried apricots mixed with water in a beaker [sulfur dioxide level unknown]
The known "standard" iodine @ 0.01M [owing to limitations] is to be dropped in until the end-point reaches
The starch solution is to be inserted as an indicator to the beaker [does it matter what percent it is? it's just an indicator right....]
then from there I would know how much iodine is needed and then i can use that equation mike stated [with appropriate conversions of course] to see how much sulfur dioxide is in the fruit...

[Mr_C]

SO₂ + I₂ + 2H₂O -> H₂SO₄ + 2HI

I₂ is reduced

SO₂ (S) is oxidised

does this help?

I₂ + 2e^- -> 2I^-
SO₂ -> SO₄^2- + 2e^-

would that be it?

Also, the biggest problem I've encountered when titrating it, is that I think it's an equilibrium reaction, when I add the iodine and mix it, it goes to dark blue/black then back to the original colour; but if i had 50mL of iodine and leave it overnight it returns to the original colour; I'm asking when do i know I've reached the endpoint (using starch as indicator)

I'm considering your approach mike but a little unsure how titrating iodine with starch would work? What would I look for?

[Borek]

I₂ + 2e^- -> 2I^-
SO₂ -> SO₄^2- + 2e^-

would that be it?

General idea looks OK.

Also, the biggest problem I've encountered when titrating it, is that I think it's an equilibrium reaction, when I add the iodine and mix it, it goes to dark blue/black then back to the original colour; but if i had 50mL of iodine and leave it overnight it returns to the original colour; I'm asking when do i know I've reached the endpoint (using starch as indicator)

Every reaction has some equilibrium. In case of redox reactions equilibrium is almost always so far on the left or right side, that you don't have to worry about. This case is no different.

Iodine color alone can be used for determination of the end point, but it is not clearly visible. However, free iodine adsorbes on the starch and such a mixture has very clearly visible blue color, thus determination of end point is much easier.

You have to add starch at the moment when there is only small amount of iodine present in the solution. Otherwise iodine can bond to starch so strong that your titration result will have systematic error.

Generally speaking iodine/starch endpoints are not my favorites, as they are not as clearly visible as others.

What kind of determination will you do? Will you titrate just with iodine, or will you add iodine excess to sample to titrate it later with thiosulfate?

[Mr_C]

I've added just starch and iodine alone, and it's turned very dark blue/black (closer to black than blue) - and some sort of precipitate formed...

Maybe I'm using too much starch, the text book which I referred to (although contains many errors) suggests about 5mLs. So what I should do is pour a tiny amount of iodine into the solution and then the starch then titrate it?

The problem is, when I add the iodine, it first go black....then I would stir it, go purple, then stir more, then it goes back to original orange colour. That first purple is that the endpoint?

What kind of determination will you do? Will you titrate just with iodine, or will you add iodine excess to sample to titrate it later with thiosulfate?

Are you saying that once I have my 20mL apricot juice ready to titrate, I should add excess iodine then titrate with a sulfate? Is this a different method to confirm my results?

[Borek]

You have not answered my question, so it is difficult to help. Please quote recipe as closely as possible.

In general - depending on the type of titration starch should be added near the endpoint - or can be added at the beginning.

In iodometry you may either titrate using iodine solution or adding known excess of iodine and than titrating this excess away with thiosulfate.

In the first case concentration of iodine during titration is low (added iodine reacts immediately with titrated substance), so the risk of starch/iodine precipitation is low.

In the second case amount of iodine at the beginning of the titration is huge, so adding starch you will more then surely precipitate some of the iodine. Thus starch should be added near the endpoint, when the amount of iodine is low.

[wala01]

Hey,

I know this was 5 years ago, but i'm curious to know how much of the iodine was for the titration (how much was needed to reach the end point)? I'm doing this experiment too and i really need help ><

THANKS if anyone replies!!