Dear Duy

as Dr aaron mentioned: the total available iodine equivalent (counting I3– as equivalent to I2)?

but you can determining Iodine by reaction with peroxy disulphate .

Hi may I know what is your solution ? If it's water, you can try isolate iodine from iodide and iodate by partitioning your solution with Diethyl ether.

Iodine has higher solubility in diethyl ether whereas iodide and iodate more soluble in water.

Determination of iodine can be done by iodometric titration, whereas iodide and iodate can be analyzed by ion chromatography.

However the result might not accurate as the iodate tend to oxidize iodide to iodine, even in room temperature, neutral pH solution but in a lower rate. Acids will speed up the process of oxidation.

Removal of iodine from your solution will break the equilibrium as aqueous layer is less saturated with iodine, this might promote oxidation of iodide by iodate.

An additional complication is that iodine and iodide react to form triiodide.

Are you trying to determine the actual concentration of I2 in solution at a given point in time (for instance analyzing the speciation to determine equilibrium constants etc.), or the total available iodine equivalent (counting I3– as equivalent to I2)?

For the first case, a spectroscopic measurement would probably be best (UV-vis might be a good option). For the second case, some sort of titration would probably be the best way to get this answer. (The suggestions from Ooi Yan Jie are a good place to start.)

Also, see the attached link, which is an image of the Pourbaix diagram for iodine. 

http://sti.srs.gov/fulltext/tr9900270/image26.gif

Dear Duy

as Dr aaron mentioned: the total available iodine equivalent (counting I3– as equivalent to I2)?

but you can determining Iodine by reaction with peroxy disulphate .

Firstly, I'm so sorry for not replying timely. And I want to thank you for all of your answers.

Ooi Yan Jie: my sample is a water solution of povidone iodine, iodide and potassium iodate. I need to determine content of available iodine (include free iodine and iodine bonding with povidone), So i'm afraid that partitioning the solution with Diethyl ether is possible to find out only "free iodine" concentration.

Aaron Bloomfield: the aim is to determine available iodine (both free iodine and iodine in complex with povidone). I've done a search about using UV-Vis to quantitative iodine but there's still no progresses. Could you explain more about how UV-Vis can apply in this situation?

Parisa Ziarati: yes the total available iodine equivalent (counting I3– as equivalent to I2). Because peroxydisulfate react with both iodine and iodate. As a result, I find it difficult to titrate iodine by peroxydisulfate in this situation.

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Recently, I've tried to vary pH by adding sodium hydoxide to diminish the reaction between sodium thiosulfate with potassium iodate in the assay process but the result is modest.

I think titration method may be not suitable for this situation.

For UV-Vis you can make solutions with known amounts of I2 and I3– and measure their absorbance at a particular wavelength (this might be useful: https://www.ncbi.nlm.nih.gov/pubmed/15852827 ) Then you can look at the UV-Vis spectrum of the sample (probably diluted by a known amount), and compare to the your spectra from the solutions of known concentrations. In this way you can calculate the concentration of both I2 and I3–