In room temperature solid di-iodine pentoxide($I_2O_5$) reacts with carbon monoside to form carbon dioxide & Iodine. This can be used to measure the amount of carbon monoxide in an air sample. $5dm^3$ air sample is passed through a tube containing solid $I_2O_5$. $\bbox[yellow]{Discharged \space I_2\space is \space sent\space into\space a\space container\space which\space has\space excess\space aqueous\space KI\space solution.} $ This solution is titrated with $0.005mol/dm^3$$Na_2S_2O_3$ solution( Starch indicator was used). If$10cm^3 of Na_2S_2O_3 $ was used, find the concentration of carbon monoxide in ppm. (density of air sample =$1.4\times10^{-3}gcm^{-3}$

Question: Why have they used KI solution when collecting $I_2$?

My View: Is it because $I_2$ is readily dissolving in KI ?

P.S. I already did the calculation. Just interested in the following phenomenon. Thanks

  • $\begingroup$ Hi Emil, note we have mhchem for chemical equations. If you could use that, that would be great! (Use \ce{formulae} inside a MathJax expression) $\endgroup$ – JavaScriptCoder May 22 '18 at 18:23

Your guess is right. Pure water does not dissolve iodine very well, but a solution of $\ce{KI}$ does, thanks to the formation of $\ce{I3-}$ and other polyiodide anions. And of course we would prefer to have our iodine in a dissolved form when we are going to titrate it.

When it comes to the starch reaction or redox reaction with thiosulfate, $\ce{I3-}$ is as good as $\ce{I2}$.

So it goes.


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