The total alkali is usually expressed in the case of hard soaps as Na2O, and in soft soaps as K2O.
Free caustic alkali is estimated by dissolving 2 grammes of the soap, in neutral pure alcohol, with gentle heat, filtering, well washing the filter with hot neutral spirit, and titrating the filtrate with N/10 acid, to phenol-phthalein.
Number of c.c. required × 0.0031 × 50 = per cent. free alkali Na2O, as caustic.
Free Carbonated Alkali.—The residue on the filter paper from the above determination is washed with hot water, and the aqueous filtrate titrated with N/10 acid, using methyl orange as indicator. The result is generally expressed in terms of Na2O.
Number of c.c. required × 0.0031 × 50 = per cent. free alkali Na2O, as carbonate.
Free Alkali.—Some analysts determine the alkalinity to phenol-phthalein of the alcoholic soap solution without filtering, and express it as free alkali (caustic, carbonates, or any salt having an alkaline reaction).
Combined Alkali.—The difference between total alkali and free alkali (caustic and carbonate together) represents the alkali combined with fatty acids. This figure may also be directly determined by titrating, with N/2 acid, the alcoholic solution of soap after the free caustic estimation, using lacmoid as indicator.
The potash and soda in soaps may be separated by the method described for the estimation of potassium in Pearl ash (page 126).
The potassium platino-chloride (K2PtCl6) is calculated to potassium chloride (KCl) by using the factor 0.3052, and this figure deducted from the amount of mixed chlorides found, gives the amount of sodium chloride (NaCl), from which the sodium oxide (Na2O) is obtained by multiplying by 0.52991.
The potassium chloride (KCl) is converted into terms of potassium oxide (K2O) by the use of the factor 0.63087.