Iodine Titration with Free SO2 in wine
Reaction goes along the lines $\ce{3 I2 -&gt; 2 I3- -&gt; 6 I-}$. The beginning of titration is $\ce{I2}$. The end of titration is $\ce{I-}$. $\ce{I3-}$ forms as an intermediate, but it can be ignored in titration.