Sulfur dioxide is a common preservative in wine; it prevents oxidation and bacterial growth. When SO₂ is added to wine, it reacts with water to form an equilibrium system with the bisulfite ion:

In this equilibrium system, SO₂ is called “molecular SO₂”; in its HSO₃^– form, it is called “free SO₂.” Only molecular SO₂ acts as a preservative. The amount of molecular SO₂ in the equilibrium system is highly pH dependent—the lower the pH, the more the equilibrium shifts to the left and the greater the amount of free SO₂. The recommended amount of free SO₂ is 0.8 ppm for white wine and 0.5 ppm for red wine. The table shows the amount of free SO₂ required to obtain the correct amount of molecular SO₂ as a function of pH for both red and white wine. For dilute solutions such as these, 1 ppm = 1 mg/L.

Study the table and and answer the questions.

a. A 225-L barrel of white wine has an initial free SO₂ concentration of 22 ppm and a pH of 3.70. How much SO₂ (in grams) should be added to the barrel to result in the required SO₂ level?

b. A 225-L barrel of red wine has an initial free SO₂ concentration of 11 ppm and a pH of 3.80. How much SO₂ (in grams) should be added to this barrel to result in the required SO₂ level?

c. Gaseous SO₂ is highly toxic and can be difficult to handle, so winemakers often use potassium metabisulfite (K₂S₂O₅), also known as KMBS, as a source of SO₂ in wine. When KMBS is added to wine, the metabisulfite ion (S₂O₅^2-) reacts with water to form the bisulfite ion (HSO₃^–). Write the balanced equation for the reaction that occurs when the metabisulfite ion reacts with water.

d. Determine the percent by mass of SO₂ in KMBS.

e. How much KMBS must a winemaker add to the barrels of wine in problems (a) and (b) to achieve the required amount of molecular SO₂?

Transcribed Image Text:

SO₂(aq) + H₂O(1) H*(aq) + HSO3 (aq)