Sodium benzoate in carbonated beverages

Question

Sodium benzoate is a common food preservative and can be commonly found in the ingredient list of carbonated beverages. Carbonated beverages also contain phosphoric acid and are highly acidic. Explain why using the term 'sodium benzoate' is not a completely accurate representation of what is in the beverage.

Answer 1

This is the only way to place this combination of compounds, some salts, some acids (most carbonated beverages have a pH around 2) without resorting to details about the actual system which is known as a "buffer". The presence of carbonic acid and its conjugate base benzoate is a system that prevents drastic (10-fold) changes in pH. The solution exchanges protons in a system that is fairly constant. However, one cannot put both benzoic acid and sodium benzoate on the label as that is not what the ingredients that actually went into making the carbonated beverage were.

 

What is most disturbing is not the nominal inaccuracy of stating "sodium benzoate" on the label. The most distressing thing is that benzoate itself in a low pH environment decomposes into carbon dioxide and benzene. Benzene is a known Class 1 carcinogen and prolonged exposure at ppm levels can lead to cancer of the blood (leukemia). 

Answer 2

Sodium benzoate is the sodium salt of benzoic acid.  In an acidic environment, the sodium salt would not be present in any significant quantity because the benzoic acid would be protonated and would exist mainly as the free acid, not the salt.

benzoate + H⁺ ==> benzoic acid