The weight, less the amount of stearic acid or beeswax added, multiplied by 20 gives the percentage of fatty acids.
Care must be taken that the cake does not contain enclosed water.
The results of these methods are returned as fatty acids, but are in reality insoluble fatty acids, the soluble fatty acids being generally disregarded. However in soaps made from cocoa-nut and palm-kernel oils (which contain an appreciable quantity of soluble fatty acids) the acid liquor is shaken with ether, and, after evaporation of the ethereal extract, the amount of fatty matter left is added to the result already obtained as above, or the ether method described below may be advantageously employed.
Where the soap under examination contains mineral matter, the separated fatty acids may be dissolved in ether. This is best performed in an elongated, graduated, stoppered tube, the total volume of the ether, after subsidence, carefully read, and an aliquot part taken and evaporated to dryness in a tared flask, which is placed in the oven at 100° C. until the weight is constant.
In a complete analysis, the figure for fatty acids should be converted into terms of fatty anhydrides by multiplying by the factor 0.9875.
In this test the resin acids contained in the soap are returned as fatty acids, but the former can be estimated, as described later, and deducted from the total.
Total Alkali.—The best method is to incinerate 5 grammes of the soap in a platinum dish, dissolve the residue in water, boil and filter, making the volume of filtrate up to 250 c.c., the solution being reserved for the subsequent determination of salt, silicates, and sulphates, as detailed below.
Fifty c.c. of the solution are titrated with N/1 acid, to methyl orange, and the result expressed in terms of Na2O.
Number of c.c. required × 0.031 × 100 = per cent. Na2O.
The total alkali may also be estimated in the filtrate from the determination of fatty acids, if the acid used for decomposing the soap solution has been measured and its strength known, by titrating back the excess of acid with normal soda solution, when the difference will equal the amount of total alkali in the quantity taken.