Hence to arrive at the alkalinity we multiply the number of cubic centimeters, read on the burette, by the factor opposite the terms in which we desire to express the alkalinity, divide the weight in grams thus obtained by the original weight taken, and multiply the result by 100, which gives the percentage of alkali in the proper terms. For example, say, we took the 0.505 grams of caustic potash as explained above and required 8.7 cubic centimeter normal acid to neutralize the solution, then
.4886 ----- × 100 = 96.73% KOH in sample. .505
Caustic potash often contains some caustic soda, and while it is possible to express the results in terms of KOH, regardless of any trouble that may be caused by this mixture in soap making, an error is introduced in the results, not all the alkali being caustic potash. In such cases it is advisable to consult a book on analysis as the analysis is far more complicated than those given we will not consider it. The presence of carbonates, as already stated, also causes an error. To overcome this the alkali is titrated in absolute alcohol, filtering off the insoluble carbonate. The soluble portion is caustic hydrate and may be titrated as such. The carbonate remaining on the filter paper is dissolved in water and titrated as carbonate.
To obtain a sample of a cake of soap for analysis is a rather difficult matter as the moisture content of the outer and inner layer varies considerably. To overcome this difficulty a borer or sampler may be run right through the cake of soap, or slices may be cut from various parts of the cake, or the cake may be cut and run through a meat chopper several times and mixed. A sufficient amount of a homogeneous sample obtained by any of these methods is preserved for the entire analysis by keeping the soap in a securely stoppered bottle.
The more important determinations of soap are moisture, free alkali, or fatty acid, combined alkali and total fatty matter. Besides these it is often necessary to determine insoluble matter, glycerine, unsaponifiable matter, rosin and sugar.
The analysis of soap for moisture, at its best, is most unsatisfactory, for by heating it is impossible to drive off all the water, and on the other hand volatile oils driven off by heat are a part of the loss represented as moisture.