Other determinations include those of specific gravity, alkalinity, proportion of salts and chloride, and tests for metals, arsenic, sulphur compounds, sugar, and fatty acids.
Specific gravity is determined at 15° C., and may be taken in specific gravity bottle, or with a Westphal balance or hydrometer It usually ranges from 1.3 to 1.31.
Alkalinity, which is usually sodium carbonate, and may be somewhat considerable if the soap has been grained with caustic alkali, is determined after dilution with water by titrating with N/2 acid, using methyl orange as indicator.
Salts.—These may be determined by gently incinerating 5-6 grammes of the glycerine, extracting the carbonaceous mass with distilled water, filtering, and evaporating the filtrate on the water bath. The dried residue represents the salts in the weight taken.
Chloride of sodium (common salt) may be estimated by dissolving the total salts in water, adding potassium chromate, and titrating with N/10 silver nitrate solution.
Copper, lead, iron, magnesium, and calcium may also be tested for in the salts, by ordinary reactions.
Arsenic is best tested for by the Gutzeit method. About 5 c.c. is placed in a test-tube, a few fragments of granulated zinc free from arsenic, and 10 c.c. dilute hydrochloric acid added, and the mouth of the tube covered with a small filter paper, moistened three successive times with an alcoholic solution of mercury bichloride and dried. After thirty minutes the filter paper is examined, when a yellow stain will be observed if arsenic is present.
Sulphates.—These may be precipitated with barium chloride in acid solution, in the usual way, dried, ignited, and weighed.
Sulphites give with barium chloride a precipitate soluble in hydrochloric acid. If the precipitate is well washed with hot water, and a few drops of iodine solution together with starch paste added, the presence of sulphites is proved by the gradual disappearance of the blue starch-iodine compound first formed.
Thiosulphates are detected by precipitating any sulphite and sulphate with barium chloride, filtering, acidifying, and adding a few drops of potassium permanganate solution, when in the presence of a mere trace of thiosulphate, the solution becomes cloudy.