Sulfites are used worldwide in the wine industry as antioxidant and
antimicrobial agents. However, sulfites have also been identified as causing
certain allergic reactions suffered by asthmatics, and the FDA mandates that
sulfites be identified on the label if they are present at levels of 10 ppm
(parts per million) or higher. The analysis of sulfites in wine uses the
"Ripper method" in which a standard iodine solution, prepared by the reaction
of iodate and iodide ions, is used to titrate a sample of the wine. The iodine
is formed in the reaction
$$
\mathrm{IO}_{3}^{-}+5 \mathrm{I}^{-}+6 \mathrm{H}^{+} \longrightarrow 3
\mathrm{I}_{2}+3 \mathrm{H}_{2} \mathrm{O}
$$
The iodine is held in solution by adding an excess of \(\mathrm{I}^{-}\), which
combines with \(\mathrm{I}_{2}\) to give \(\mathrm{I}_{3}^{-}\). In the titration,
the \(\mathrm{SO}_{3}^{2-}\) is converted to \(\mathrm{SO}_{2}\) by acidification,
and the reaction during the titration is
$$
\mathrm{SO}_{2}+\mathrm{I}_{3}^{-}+2 \mathrm{H}_{2} \mathrm{O} \longrightarrow
\mathrm{SO}_{4}^{2-}+3 \mathrm{I}^{-}+4 \mathrm{H}^{+}
$$
Starch is added to the wine sample to detect the end point, which is signaled
by the formation of a dark blue color when excess iodine binds to the starch
molecules. In a certain analysis, \(0.0421 \mathrm{~g}\) of \(\mathrm{NaIO}_{3}\)
was dissolved in dilute acid and excess NaI was added to the solution, which
was then diluted to a total volume of \(100.0 \mathrm{~mL}\) A \(50.0
\mathrm{~mL}\) sample of wine was then acidified and titrated with the iodine-
containing solution. The volume of iodine solution required was \(2.47
\mathrm{~mL}\).
(a) What was the molarity of the iodine (actually,
\(\left.\mathrm{I}_{3}^{-}\right)\) in the standard solution? (b) How many grams
of \(\mathrm{SO}_{2}\) were in the wine sample? (c) If the density of the wine
was \(0.96 \mathrm{~g} / \mathrm{mL}\), what was the percentage of
\(\mathrm{SO}_{2}\) in the wine?
(d) Parts per million (ppm) is calculated in a manner similar to percent
(which is equivalent to parts per hundred).
$$
\mathrm{ppm}=\frac{\text { grams of component }}{\text { grams of sample }}
\times 10^{6} \mathrm{ppm}
$$
What was the concentration of sulfite in the wine, expressed as parts per
million \(\mathrm{SO}_{2} ?\)
