The filtrate from the hydric sulphide precipitate is divided into two parts; the one half is used in the search for zinc, the other half is used for chromium.
Search for Zinc.—The liquid is alkalised with ammonia and then ammonium sulphide is added. There will always be a precipitate of a dark colour; the precipitate will contain earthy phosphates, iron and, in some cases, manganese. The liquid with the precipitate is treated with acetic acid to strong acid reaction and allowed to stand for several hours. The portion of the precipitate remaining undissolved is collected on a filter, washed, dried and heated to redness in a porcelain crucible. The residue thus heated is cooled and dissolved in a little dilute sulphuric acid. To the acid solution ammonia is added, and any precipitate formed is treated with acetic acid; should the precipitate not completely dissolve, phosphate of iron is present; this is filtered off, and if SH2 be added to the filtrate, white zinc sulphide will come down (see “[Zinc]”).
Search for Chromium.—The second part of the SH2 filtrate is evaporated to a thin extract, mixed with double its weight of sodic nitrate, dried and cast, little by little, into a red-hot porcelain crucible. When the whole is fully melted, the crucible is removed from the flame, cooled, and the mass dissolved in water and filtered. Any chromium present will now be in solution in the easily recognised form of potassic chromate (see “[Chromium]”).
INVESTIGATION OF THE RESIDUE ([p. 52]) AFTER THE TREATMENT OF THE ORIGINAL SUBSTANCE WITH HYDROCHLORIC ACID AND POTASSIC CHLORATE FOR PRESENCE OF SILVER CHLORIDE, LEAD AND BARIUM SULPHATES.
The residue is dried and intimately mixed with three times its weight of a mixture containing 2 parts of sodic nitrate and 1 part of sodium hydrate, This is added, little by little, into a red-hot porcelain crucible. The melted mass is cooled, dissolved in a little water, a current of CO2 passed through the solution to convert any caustic soda into carbonate, and the solution boiled. The result will be an insoluble portion consisting of carbonates of lead and baryta, and of metallic silver. The mixture is filtered; the insoluble residue on the filter is warmed for some time with dilute nitric acid; the solution of nitrates of silver, lead and barium are concentrated on the water-bath nearly to dryness so as to get rid of any excess of acid, and the nitrates dissolved in water; then the silver is precipitated by hydrochloric acid, the lead by SH2, and the barium by sulphuric acid.
VII.—The Spectroscope as an aid to the Identification of certain Poisons.
§ 33. The spectra of many of the metals, of phosphine, of arsine and of several other inorganic substances are characteristic and easily obtained.
It is, however, from the employment of the micro-spectroscope that the toxicologist is likely to get most assistance.
