Sodium benzoate (E211), also called benzoate of soda, has chemical formula 5CO. It is the sodium salt of benzoic acid and reverts to this form when dissolved in water. It boils at 249 degrees celsius.
As a food additive it is used as a preservative, effectively killing most yeast, bacteria and fungi. Sodium benzoate is effective only in acidic conditions (pH < 3.6) making its use most prevalent in foods such as preserves, salad dressings, carbonated drinks, jams, and fruit juices.
It is found naturally in cranberries, prunes, greengage plums, cinnamon, ripe cloves and apples. Concentration as a preservative is limited by the FDA in the U.S. to 0.1% by weight (because it is poisonous in larger quantities) though organically grown cranberries and prunes can concievably contain levels exceeding this limit.
It is also found in alcohol-based mouthwash.