[765] Journ. f. pract. Chem., xiii. 168.
[766] Journ. Chem. Soc., June 1893, 886.
The material to be tested is boiled gently in a porcelain vessel with dilute hydrochloric acid and a small strip of copper about 1 inch long by 1⁄4 inch broad, till the absence of arsenic or antimony has been ascertained, or a coating has been produced. When the coating is decided, the piece of copper is taken out, washed first with water, then with a little alcohol to get rid of fatty matter, and finally with water. It is then placed in a mixture of dilute caustic potash and peroxide of hydrogen, and allowed to digest in the cold. At the same time a second piece of copper is introduced into the material which has given a deposit on the first piece, the washings of the first piece being added, and the boiling continued.
The treatment of the first piece of copper by caustic potash and peroxide of hydrogen dissolves any antimony or arsenic and restores the copper to its original brightness; when this is accomplished, the second piece of copper is taken out and replaced by the first, and this second piece, if stained, is digested with potash, peroxide of hydrogen, and washed as in the former case. The process is repeated until the slips of copper cease to be stained in the slightest degree—until, in short, the whole arsenic or antimony has been withdrawn.
The alkaline liquid contains the arsenic, as arsenate of potassium; the antimony, if present, as antimonate; and the solution is also contaminated by a little hydrated copper oxide; this latter separates on boiling, and can be filtered off, and the filtrate is boiled down to a small bulk. The liquid is washed into a small distillation-flask with strong hydrochloric acid, ferrous chloride is added, the flask, fitted with a safety tube, is connected with a condenser, and the arsenic distilled into water. To obtain the last traces of arsenic it may be necessary to distil it twice in this way, adding, each time, fresh strong acid and distilling to dryness. The distillate is then tested for arsenic by adding an equal bulk of saturated SH2 water. The sulphide of arsenic may be dealt with as described ([p. 573]).
The residue in the flask is now tested for antimony by saturating with SH2; should antimony be present, the precipitate by SH2 will probably be dark coloured, because of a small quantity of copper. The precipitate is collected, dissolved in dilute caustic soda, boiled, filtered to remove copper sulphide, the filtrate acidified by hydrochloric acid, and sulphuretted hydrogen water added. If antimony was present, this time the precipitate will be of an orange colour, and may be dealt with as described ([p. 589]).
The test experiments with regard to this combined process appear satisfactory.
§ 739. Arsenic in Glycerin.—Arsenic has been frequently found in commercial glycerin, the quantity varying from 0·1 to 1 mgrm. in 100 c.c. The best method to detect the presence of arsenic in glycerin is as follows:—A mixture of 5 c.c. of hydrochloric acid (1 : 7) and 1 grm. of pure zinc is placed in a long test-tube, the mouth of which is covered with a disc of filter-paper previously moistened with one or two drops of mercuric chloride solution, and dried. If arsenic is present, a yellow stain is produced upon the filter-paper within fifteen minutes, and it subsequently becomes darker.[767]