The determination is carried out in the following way: Decomposition of the soap by sulfuric acid and extraction of the fatty acida with light petroleum. Oxidation of the free glycerol remaining in the aqueous phase by periodic acid to formic acid and formaldehyde. On reaction with chromotropic acid, the aldehyde formed gives an absorbing compound whose absorbance is propotional to the free glycerol content. Spectrometric measurement of the absorbance at a wavelength about 571 mm.