[wyleslie]

Hi.  I am an atmospheric science grad student working in the analytical chemistry field for her thesis work, so forgive me if I ask or say anything that is obvious.

My work involves instruments we use to measure ozone concentrations in the atmosphere.  These instruments are electrochemical galvanic cells, using dilute (1% and 0.5%) KI solutions to react with ozone and give us a reading of ozone concentrations.  I am trying to verify the 1:1 ratio of iodine produced in the solution to ozone reacting in the solution, and this is where my hang-up is occurring.

I am taking small portions of 1% or 0.5% KI solutions, exposing them to ozone and then attempting to titrate for iodine formed in the solution.  I know how much ozone is entering the solution.  The problem is that I am getting between 20-40% less iodine titrated than ozone entering the solution.

Because I am more of a physicist than a chemist, this is a very perplexing problem for me.  I hope I've given enough info about my problem, but I'd be very grateful for any tips or suggestions as to what can be happening to my iodine.  I know iodine is volatile, and that I may be losing some, but how exactly?  Could it be evaporating?  Sublimating?

Thanks for the help

[Borek]

In the presence of excess I^- iodine is in solution in form of the I₃^-, such solutions are considered to be stable enough (ie you shouldn't loose iodine in a few hours span in normal room temperature).

Can you give some more details of the procedure you follow?

[wyleslie]

Sure.
I am exposing 6 mL of the 1% and 0.5% KI solutions to ~200 ppb ozone for 90 minutes.  This amount of ozone is representative of stratospheric ozone levels if brought down to the pressure level of my laboratory.  90 minutes is the approximate time it takes for an ozone balloon carrying our instrument to ascend to 30 km.
The ozone exposure takes place in a glass test tube, and in order to perform the titration, I have to transfer it to a small flask.  I add about 0.5 mL of starch solution, and titrate with 0.0005 M sodium thiosulfate.

These solutions I am using are kept neutral using two pH buffers and another ingredient, KBr; they are standard solutions used in these instruments.  I read about the fact that I₂ can be sequestered as I_3^-, and the recommendation was acidifying the solution prior to titration.  I also read that an additional reaction with activated oxygen can occur, which forces slower reactions (i.e. not all the iodine would be available for titration immediately after ozone exposure.  These authors who mention this suggest a 45 minute wait after ozone exposure to allow these "slow" reactions to catch up.)

Our study initially was to see if these buffers we use have an effect on the electrochemistry of the ozone instruments.  But now, we are stuck at this problem of not getting enough iodine out of the solution.

I hope that helps.

[Borek]

Honestly, can't say I have any idea. Just a few remarks.

I₃^- presence is not a problem, quite the opposite, it helps to keep iodine in soultion. Its creation and dissociation are fast enough to not interfere with thiosulfate titration. I don't think lowering pH will change this equilibrium.

My textbook suggests neutral pH during titration.

Is the difference between expected and found amount of iodine consistent or does it look random? Thiosulfate solutions are not stable, so they have to be standardized now and again. That won't explain random differences.

Are you adding starch at the very beginning of the titration? Try to wait till you are close to the end point.

[wyleslie]

The iodine differences are fairly consistent, but there are some random differences mixed in with the bunch (but that could just be operator error).  I can try re-standardizing the thiosulfate solution I have to see if the molarity has changed much. 

It seems to me that if that is what the problem is, my results would have a consistent pattern (i.e. the amount of iodine titrated would constantly be decreasing, do to a constant change in the molarity of the titrant).  What I mean is, if the initial molarity of the titrant was 0.1 (I dilute to get 0.0005M), it would either decrease in molarity, or it would increase.  But it wouldn't increase, then decrease, than increase again.  Right now, the amount of iodine I am titrating now is the same (nearly) as it was last week, and the week before.
Is my logic right?

As for adding the starch solution later, I would do that (that's what I do in the thiosulfate standardization), but the amounts of iodine I am titrating are so low (I am using a 10 mL buret, and I only add about 0.3 mL of titrant), that I don't think I'd visibly be able to see the color change without the starch solution.

As for the starch solution, does it matter what starch I use?  I've read all sorts of literature/ textbooks, and nothing is very specific. 

Thanks again!

[Borek]

the amounts of iodine I am titrating are so low (I am using a 10 mL buret, and I only add about 0.3 mL of titrant)

Wow, with about 20 drops per milliliter that means intrinsic 16% error. Seems to me like you are past the reasonable limits of the method. That in turn means you can expect many strange effects. Why do you use this particular method?

Logic you presented is the same that made me ask about the differences

I have never heard about important differences in starch used. I was using potato starch, but that's mainly because that was the only type available here.

[wyleslie]

Seems to me like you are past the reasonable limits of the method. That in turn means you can expect many strange effects. Why do you use this particular method?

We aren't chemists.  I am in a physics department, and this was initially thought to be a straight forward problem, but has turned out not to be.  I don't know of any other method of doing this other than using a photometer, which we don't have access to.
Anyway, thanks for your help and input.  I'm going to try re-standardizing, just to make sure.

[Borek]

I am afraid amount of iodine produced is so low you will need some kind of instrumental approach, be it spectroscopy or something else.