[wzhu]

Hi, this is copied from another board, because I feel posting it here could be more helpful. Sorry if I am against the rules by doing this...

I am calibrating ozone generator with iodometric method. A constant ozone output is expected under constant condition (same frequency variable of ozone generator, constant air supply, and temperature). However, my result decreased with time-1.8 mg/min for a 5 min ozonation, 1.3 mg/min for a 10 min ozonation, and 1.0 mg/min for a 20 min ozonation. This variation is abnormal, which might be the result of the generator itself, or the titration method.

The procedure I followed is: Ozone/air gas is introduced through 200 mL 2% potassium iodide solution in a vessel. After a specific time (5, 10, or 20 min), switch of ozone generator, and turn off air gas cylinder. Until bubbles stop coming out from sparger, transfer post-treatment solution to a 500 mL conical beaker, and add 10 mL 2 M sulfuric acid solution. Sit the solution in room temperature for 5 min to release generated iodine from iodate. Titrate with 0.05 M sodium thiosulfate solution. As yellow color almost disappears, add 2 mL 1% starch indicator, and the solution turns blue. Continue titration till the blue color disappears.

I found that after addition of sulfuric acid before titration, the color of the solution turned much deeper, which might indicate formation of iodine. Is this normal or not? Since I found reference talking about releasing iodine from iodate, but should the amount be so much (according to the color change, iodine existence increased about twice as much)?

Besides, according to the data I got, assume that
measurement result * time= ozone dose (D) * time + constant (C), then
1.8 * 5 = D * 5 + C
1.3 * 10 = D * 10 + C
1.0 * 20 = D * 20 + C
C = 5, and D = 0.8.
Which means a constant variation in titration irrelative to time exists. Could this be the formation of iodine from addition of sulfuric acid?

I have been trying to figure out the problem. I would really appreciate it if anyone could offer me some suggestion!

Reply #1:
1. Blind trial - have you done titration of the sample that was not ozonated?

2. Where does iodate come from? Is it ozone + iodide -> iodate? I thought it is much simpler ozone + iodide -> iodine.

Reply #2:
Yes, I have tried to add sulfuric acid directly into the untreated KI solution, sitting for 5 min, and didn't observe color change (seems to be that either ambient oxygen or the sulfuric acid caused significant oxidation), so didn't do titration-I believed the blind trial could serve as an explanation, but it just turned out to be opposite to what I expected...

The reference on iodate formation was in 1979, suggesting iodate formation from either iodide ion or iodine, and its decomposition in acidic condition.

BTW, what is the primary reason to add sulfuric acid? I thought it was because of sodium thiosulfate as titrant. However, the obvious color change of post-treatment solution seems to be a problem, and I just don't know how to explain it.   

Reply #3:
If product of ozone reaction with iodides is iodate, sulfuric acid is necessary for the reaction

IO3- + 5I- + 6H+ -> 3I2 + 3H2O

From what I remember this reaction is specific enough to be used for iodometric determination of small amounts of acids. That could nicely explain why you observe color change after adding acid - and it suggests it is not a problem, but expected chemistry of the system.

That still doesn't explain problems with ozone generator.

Reply #4:
Hey, thanks so much for your comments! But could it be that so much iodine can be released (the color got much deeper), or to say, so much iodate was formed?

I was expecting the obvious color change to be the reason for the constant variation, which could explain the decrease of dosage with time. However, the constant variation itself is just a guess...

Before trying to sit the buffered post-treatment solution for 5 min, results were even not reproducible. I selected 5 min according to a Stantdard Technical Procedure. However, some other documents didn't mention about this at all, which, otherwise, suggested ozone-requirement-free water as solvent. I don't believe water as solvent is contributive to this significant decrease of measurement.

However, I may have to try ozone-requirement-free water if I cannot find other possbile reasons. I am trying to be not too hasty to move before having enough suggestions. Anyone has any thoughts is more than welcome to share~

[marquis]

I believe this reaction takes acidic conditions to proceed.  If the solution is not acidic (or not acidic enought), you don't get the color change from the starch addition.  That might explain the color change with the addition of the sulfuric acid.

After a while, the starch solution goes bad.  Then the color change is weak, if at all.  This could also explain the problem.  If the starch solution is not fresh, you might try making a new batch.

It appears the problem is based on the titration.  If the chamber uses UV bulbs to generate the ozone, you might consider changing them.  See if that makes any difference.Those ozone chambers are notorious for going through the UV bulbs. 

The new ozone chambers use uv spectral detection to determine the ozone concentration.  If you get a chance, switch to those instruments.  They are much more reliable.  When we made the switch, the amount of time spent with instrument calibration dropped dramatically.

[Figatell]

Are you sure Iodide (I^-) is in excess? Because unless Iodine could be volatilized. Then an excess of Iodide ensures which captures liberated iodine to form triiodide ions, I3−.

[Borek]

This was cross posted and not locked earlier:

http://www.chemicalforums.com/index.php?topic=36980