Chapter 5, Problem 91RQ

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

${\text{IO}}_3^{-}{\text{ + 5I}}^{\text{-}}{\text{+ 6H}}^{\text{+}}\stackrel{}{\to }{\text{3I}}_{\text{2}}{\text{+ 3H}}_{\text{2}}\text{O}$

The iodine is held in solution by adding an excess of ${\text{I}}^{\text{-}}$, which combines with ${\text{I}}_{\text{2}}$ to give ${\text{I}}_{\text{3}}^{\text{-}}$. In the titration, the ${\text{SO}}_3^{2-}$ is converted to ${\text{SO}}_{\text{2}}$ by acidification, and the reaction during the titration is

${\text{SO}}_{\text{2}}{\text{+ I }}_3^{-}{\text{+ 2H}}_{\text{2}}\text{O}\stackrel{}{\to }{\text{ SO}}_4^{2-}{\text{ + 3I}}^{\text{-}}{\text{+ 4H}}^{\text{+}}$

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 g of ${\text{NaIO}}_{\text{3}}$ was dissolved in dilute acid and excess $\text{NaI}$ was added to the solution, which was then diluted to a total volume of 100.0 mL. A 50.0 mL sample of wine was then acidified and titrated with the iodine-containing solution. The volume of iodine solution required was 2.47 mL. (a) What was the molarity of the iodine (actually, I) in the standard solution? (b) How many grams of ${\text{SO}}_{\text{2}}$ were in the wine sample? (c) If the density of the wine was 0.96 g/mL, what was the percentage of ${\text{SO}}_{\text{2}}$ in the wine? (d) Parts per million (ppm) is calculated in a manner similar to percent (which is equivalent to parts per hundred).

$\text{ppm =}\frac{\text{grams of component}}{\text{grams of sample}}{\text{×10}}^{\text{6}}\text{ppm}$

What was the concentration of sulfite in the wine, expressed as parts per million ${\text{SO}}_{\text{2}}$

₂?