Kitchen chemistry: vinegar

In previous kitchen chemistry posts, we discussed the fermentation processes which produce lactic acid and alcohol. A different fermentation process turns alcohol (at about wine strength) into vinegar (or, more precisely, into acetic acid). However, this process requires oxygen (C₂H₅OH + O₂ → CH₃COOH + H₂O + energy):

This process usually takes place via a bacterial culture called “mother of vinegar” (photo below by “Zinnmann”), and producing gourmet vinegar by fermentation is something that can be done at home (see here, here, here, and here). It is important to remember that (unlike the bacteria producing yoghurt and wine), the “mother of vinegar” culture requires oxygen.

The acetic acid in vinegar is an acid, though not a strong one. Being an acid means that it splits into positively charged hydrogen ions and negatively charged acetate ions. Hydrogen ions are hydrogen atoms without electrons, usually written H⁺ (although in practice they hitch a ride with water molecules). As a reaction, CH₃COOH → CH₃COO⁻ + H⁺:

The presence of hydrogen ions is the sign of an acid, and the stronger the acid, the more hydrogen ions. The strength of an acid is measured by the pH value, where 7 is neutral, 6 is a weak acid, and 1 is a very strong acid (numbers above 7 indicate alkaline substances). Indicator paper (photo below) is often used to determine the pH of a liquid:

However, an extract of red cabbage also does the job (photo below by “Supermartl” – acids with pH 1, 3, and 5 in the tubes on the left; neutral water with pH 7 in the fourth tube; and alkaline solutions on the right):

A common kitchen reaction is to mix vinegar with baking soda (sodium bicarbonate, NaHCO₃), causing the bicarbonate to break up into carbon dioxide gas and water (photo by Kate Ter Haar):

The reaction is H⁺ + NaHCO₃ → Na⁺ + CO₂ + H₂O:

Any other acid (citric acid, for example) would also do the job, of course, since hydrogen ions are doing the work. Sodium ions (sodium atoms with an electron missing, Na⁺) are left over, together with acetate ions from the vinegar, and these form sodium acetate (also the subject of a home experiment). Another common home experiment with vinegar is dissolving the shell of an egg.