Glycerine.—Five grammes of soap are dissolved in water, decomposed with dilute sulphuric acid, and the clear fatty acids filtered and washed. The filtrate is neutralised with barium carbonate, evaporated to 50 c.c., and the glycerol estimated by the bichromate method detailed under Crude Glycerine.
Starch or gum may be detected by dissolving the soap in alcohol, filtering, and examining the residue on the filter paper. Starch is readily recognised by the blue coloration it gives with a solution of iodine in potassium iodide.
Sugars are tested for by means of Fehlings' solution, in the liquor separated from the fatty acids, after first boiling with dilute acid to invert any cane sugar.
Mercury will be revealed by a black precipitate produced when sulphuretted hydrogen is added to the liquor separated from the fatty acids, and may be estimated by filtering off this precipitate on a tared Gooch's crucible, which is then dried and weighed.
Borax or borates are tested for in the residue insoluble in alcohol. This is dissolved in water, rendered faintly acid with dilute hydrochloric acid, and a strip of turmeric paper immersed for a few minutes in the liquid. This is then dried in the water-oven, when if any boric acid compound is present, a bright reddish-pink stain is produced on the paper, which is turned blue on moistening with dilute alkali.
The amount of the boric acid radicle may be determined by incinerating 5-10 grammes of soap, extracting with hot dilute acid, filtering, neutralising this solution to methyl orange, and boiling to expel carbon dioxide. After cooling, sufficient pure neutralised glycerine is added to form one-third of the total volume, and the liquid titrated with N/2 caustic soda solution, using phenol-phthalein as indicator. Each c.c. of N/2 NaOH solution corresponds to 0.031 gramme crystallised boric acid, H3BO3 or 0.0477 gramme crystallised borax, Na2B4O7·10H2O.
Lyes.
The amounts of caustic alkali (if any), carbonated alkali, and salt present are determined in the manner already described under Alkali and Alkali Salts. The glycerol content is ascertained by taking 2.5 grammes, adding lead subacetate solution, and filtering without increasing the bulk more than is absolutely necessary; the solution is concentrated to about 25 c.c., and the oxidation with bichromate and sulphuric acid conducted as described in the examination of Crude Glycerine. The solution, after oxidation, is made up to 250 c.c., and titrated against standard ferrous ammonium sulphate solution, the formula for the calculation being:—
| Per cent. of glycerol | = | (0.25 - (2.5/n)) × 40 |
where n equals the number of c.c. of oxidised lyes required to oxidise the ferrous ammonium sulphate solution.