- Sulfur is burned to produce sulfur dioxide, which is used to bleach and preserve foods, especially fruits. It prevents the growth of bacteria and mold. Apples, pears, apricots, peaches, and figs are dried and preserved with SO2. Although packagers try to remove as much of the sulfur compounds as possible, some usually remains. You may test for the presence of residual sulfur compounds by reacting fruit with barium chloride to form a new compound that is not soluble. A substance that is not soluble will form a cloudy precipitate. You will compare fruit preserved with sulfur and without sulfur. This is a two day lab.
MATERIALS
Sulfured dried fruit from grocery store
Unsulfured dried fruit from health food store
Hydrogen peroxide (H2O2) solution, 3%
Barium chloride (BaCl2) solution, 3 M
Funnel with stand
Filter paper, or coffee filter
Four beakers, 250 mL
Stirring rod
Distilled waterPROCEDURE
1. Overnight, soak several slices of each kind of dried fruit in distilled water. Use enough water to cover the slices.
2. Set up the filtering funnel with the filter paper. Poor the liquid through the funnel, and then place the fruit in the funnel to squeeze out as much liquid as possible. Use a stirring rod. Do this step for each of the two samples.
3. Add 25 mL of 3% H2O2 to each filtrate in the beakers you collected them in under the funnel. This will oxidize the sulfur atoms so they can be identified.
4. To separate the sulfur (in the form of sulfate) from the solution, add BaCl2 drop by drop. Be careful with BaCl2. It is toxic. Stop adding barium chloride when it appears no more white precipitate is formed.
5. You may want to filter the barium sulfate precipitate and dry it.
REACTIONS
When sulfur dioxide dissolves in the moisture in the fruit, it reacts to form the sulfite ion (SO32- ):
- SO2 (g) + H2O (l) --------> SO32- (aq) + 2H+ (aq)
The sulfite ion reacts with hydrogen peroxide (H2O2) to form the sulfate ion (SO42-):
- SO32- (aq) + H2O2 (aq) -------> SO42- (aq) + H2O (l)
sulfite sulfateThe sulfate ion reacts with the barium ion in the BaCl2 to produce the precipitate, barium sulfate (BaSO4):
Ba2+ (aq) + SO42- (aq) -------> BaSO4 (s)
white
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- SO2 (g) + H2O (l) --------> SO32- (aq) + 2H+ (aq)
SULFUR DIOXIDE
IN
FOODS
INTRODUCTION