During the saponification process, why does using potassium hydroxide produce a liquid soap?

Answer 1

The statement about melting points of K salts of fatty acids being lower than that of the Na salts is true but usually does not come into play. Both salts will generally be solids at room temperature if they are anhydrous.

The K salts are preferred in making concentrated liquid soap because they are more water soluble than the Na salts. Thus it is easier to make the soap liquid and homogeneous if KOH is used. There are companies which still use NaOH for this application anyway.

Answer 2

The trigycerides (esters formed from glycerol and three molecules of long-chain carboxylic acids) in the fats/oils react with the alkali to form glycerol (pentane-1,2,3-triol) and the sodium or potassium salts of the carboxylic acids. These salts are the soap. Sodium hydroxide is used if a hard soap is required; potassium hydroxide is used if a soft soap is needed.

Answer 3

The potassium salts of the fatty acids used to make soap have a lower melting point than the corresponding sodium salts.