This may contain lead, mercury, tin, bismuth and antimony. It is heated for a considerable time with aqua regia, the solution filtered, and the second residue, should one remain, washed with dilute hydrochloric acid. If the second residue is fused with cyanide of potassium, the compounds present are reduced to the metallic state. The liberated metals are treated with nitric acid, which dissolves lead, but leaves tin as insoluble metastannic acid. The nitrate of lead is then filtered from the metastannic acid, and both metals are identified as described in the treatment of residue A.
The solution, obtained by the action of aqua regia on residue I, is treated with sulphuretted hydrogen. The tin and antimony are separated from the lead, mercury and bismuth by treating the precipitate produced with sulphide of ammonium, which dissolves only the sulphides of the first two metals. The solution in sulphide of ammonium is afterwards examined for these metals, as directed under the head of solution IV., the search for arsenic, however, being here omitted.
Upon treating the residue insoluble in sulphide of ammonium with nitric acid, lead, copper and bismuth go into solution, mercury remaining undissolved. The liquid is filtered, and the undissolved mercury submitted to the special examination previously described.
Sulphuric acid is added to the solution and the precipitate of sulphate of lead formed, separated, washed, and examined as directed while treating of residue A.
Finally, the solution separated from the lead is tested for bismuth and copper, as in examination of precipitate III.
SOLUTION II.
The solution is concentrated by heating on a water-bath, a small quantity of carbonate of soda cautiously added to a portion, and notice taken if a precipitate forms. The part taken is then acidulated with a little hydrochloric acid, returned to the principal solution, and sulphuretted hydrogen conducted through the fluid, as in the examination of solution B. In case a precipitate fails to form, all metals are absent; if, on the contrary, a precipitate (c) is produced, it is examined as directed below.
EXAMINATION OF PRECIPITATE c.
If the solution merely became turbid, or the precipitate formed was of a pure white color, it consists probably of sulphur. It is, however, indispensable, even in this case, to collect the precipitate and examine it for arsenic. Provided it is of a pure yellow color, it is treated with ammonia. In case it is entirely dissolved by this treatment, and the addition of carbonate of ammonia failed to produce a precipitate in solution II., it is certain that arsenic, and no other metal, is present. Under these circumstances, the ammoniacal solution is examined as directed in the article on the detection of arsenic. If, on the other hand, the precipitate is not yellow, or being yellow, is but imperfectly soluble in ammonia, and a precipitate was formed by the addition of carbonate of ammonia to solution II., it is necessary to likewise search for tin, antimony, mercury, copper, bismuth and cadmium. In this case, the precipitate is placed in a small flask, allowed to digest for several hours with ammonia and sulphide of ammonium in a moderately warm place, and the solution filtered.
The remaining residue (III.) is washed, labelled, and preserved for subsequent examination; the filtrate (IV.) is treated as directed below.