Vinegar is a solution of acetic acid, which is a weak acid, and can be neutralized by adding a strong base, such as sodium hydroxide. In such a neutralization reaction, the pH of the resulting solution is greater than 7, unlike in strong acid and strong base reactions where the pH is close to 7. Acetic acid cannot be completely neutralized by a weak base, such as ammonia.
The titration method in chemistry is used to determine the equivalence point or point of neutralization in an acid-base reaction. To know when enough base has been added to completely react with the acid, an indicator, phenolphthalein, is added to the acid before titration. Phenolphthalein is colorless in this acidic solution but turns pink when the acid is neutralized at the equivalence point. Sometimes a pH meter is used to indicate when the pH value of the neutralized acid-base is reached.
This method is used to determine the acetic acid concentration of a sample vinegar and to determine whether the vinegar complies with the federal requirement of 4 g of acetic acid per 100 ml of vinegar. By using a known volume of the vinegar and titrating it with a known concentration and volume of a base, the unknown concentration of the acetic acid can be computed.