Coulometric titration of sulfite in wine. Sulfur dioxide is added to many foods as a preservative. In

Question:

Coulometric titration of sulfite in wine. Sulfur dioxide is added to many foods as a preservative. In aqueous solution, the following species are in equilibrium:

Bisulfite reacts with aldehydes in food near neutral pH:

Sulfite is released from the adduct in 2 M NaOH and can be analyzed by its reaction with I^-₃ to give I^- and sulfate. Excess I^-₃ must be present for quantitative reaction.

Here is a coulometric procedure for analysis of total sulfite in white wine. Total sulfite means all species in Reaction A and the adduct in Reaction B. We use white wine so that we can see the color of a starch-iodine end point.

1. Mix 9.00 mL of wine plus 0.8 g NaOH and dilute to 10.00 mL. The NaOH releases sulfite from its organic adducts.

2. Generate I^-₃ at the working electrode (the anode) by passing a known current for a known time through the cell in Figure 16-9. The cell contains 30 mL of 1 M acetate buffer (pH 3.7) plus 0.1 M KI. In the cathode compartment, H₂O is reduced to H₂ + OH^-. The frit retards diffusion of OH^- into the main compartment, where it would react with I^-₃ to give IO^-.

3. Generate I^-₃ at the anode with a current of 10.0 mA for 4.00 min.

4. Inject 2.000 mL of the wine/NaOH solution into the cell, where the sulfite reacts with I^-₃, leaving excess I^-₃.

5. Add 0.500 mL of 0.050 7 M thiosulfate to consume I^-₃ by Reaction 15-19 and leave excess thiosulfate.

6. Add starch indicator to the cell and generate fresh I^-₃ with a constant current of 10.0 mA. A time of 131 s was required to consume excess thiosulfate and reach the starch end point.

(a) In what pH range is each form of sulfurous acid predominant?

(b) Write balanced half-reactions for the anode and cathode.

(c) At pH 3.7, the dominant form of sulfurous acid is HSO^-₃ and the dominant form of sulfuric acid is HSO₄^2-. Write balanced reactions between I^-₃and HSO^-₃ and between I^-₃ and thiosulfate.

(d) Find the concentration of total sulfite in undiluted wine.

Figure 16-9