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
!.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!