3rd. The ammon. sulphide solution of the antimony and tin. Evaporate to dryness, dissolve in hot strong hydrochloric acid, dilute, divide into two equal portions: in one throw down both metals by a rod of zinc: in the other throw down only Sb by a slip of tin. Wash off both precipitates, dry and weigh. The first is antimony and tin together, the second is antimony alone (Gay Lussac). The difference is the tin.
Usually only some of these metals will be present. Tin has derived more importance lately since Hehner has proved its almost constant presence in canned provisions.
As to the delicacy of the precipitation of antimony by zinc or galvanism, Mohr (Toxicologie, 1876) states that a solution containing ·00005 gramme of Sb in one cub. centimetre gives a distinct reaction in fifteen minutes. Such a solution gives with H2S only a colouration, and after a long time a faint precipitate. 1/30000 part gives with zinc a clear reaction in one half-hour: with H2 S, only a colour, no precipitate, in twelve hours. 1/40000, doubtful: 1/50000, imperceptible with zinc: of course, nothing with H2 S. The reaction is only decisive if other metals are excluded.
From the solution of Sb Cl3, or tartar emetic in H Cl, gallic acid throws down Sb, and not As or tin. The precipitate after washing and drying contains 40·85 per cent. of Sb. (Chem. News, XXIV. 207, 251.)
To sum up, the decisive characters of antimony are:—
1. An orange red precip. by H2 S in slightly acid solutions.
2. The insolubility of this precip. in ammon. carbonate.
3. Its solubility in ammonium sulphide.
4. Its solubility in hot H Cl, with evolution of H2 S, and formation of a solution of Sb Cl3, which is precipitated by water and cleared up again by tartaric acid.