The amount of ascorbic acid (vitamin $\mathrm{C})$ in fruit juice is determined by a titration using a redox reaction. Iodine is the titrant, but because iodine solutions in water are unstable, the iodine is generated by a reaction in which a titrant containing iodate $\left(\mathrm{IO}_{3}^{-}\right)$ ions is added to a solution containing the juice sample, iodide ions However, any iodine formed by reaction (i) is immediately reduced back to iodide by ascorbic acid from the juice sample:
ii. $\mathrm{C}_{6} \mathrm{H}_{8} \mathrm{O}_{6}(a q)+\mathrm{I}_{2}(a q) \rightarrow$
$$
\mathrm{C}_{6} \mathrm{H}_{6} \mathrm{O}_{6}(a q)+2 \mathrm{I}^{-}(a q)+2 \mathrm{H}^{+}(a q)
$$
(I $^{-}$ ), and a few drops of a starch solution that turns dark blue in the presence of iodine. Iodate ions are reduced to iodine
$\left(\mathrm{I}_{2}\right)$ while iodide ions are oxidized to iodine:
i. $\mathrm{IO}_{3}^{-}(a q)+\mathrm{I}^{-}(a q) \rightarrow \mathrm{I}_{2}(a q)$ (not balanced)However, any iodine formed by reaction (i) is immediately reduced back to iodide by ascorbic acid from the juice sample:
ii. $\mathrm{C}_{6} \mathrm{H}_{8} \mathrm{O}_{6}(a q)+\mathrm{I}_{2}(a q) \rightarrow$
$$
\mathrm{C}_{6} \mathrm{H}_{6} \mathrm{O}_{6}(a q)+2 \mathrm{I}^{-}(a q)+2 \mathrm{H}^{+}(a q)
$$
(FIGURE CAN'T COPY)