Marsh’s and Reinsch’s tests have been mentioned under arsenic. It, however, will be necessary to add a few observations on their special use for Sb.
A fractional part, say one-fourth, of the suspected matter, after mincing or pounding, is digested with hot distilled water containing 5 per cent. of pure hydrochloric, and a little tartaric, acids, well shaken or stirred in a covered or closed vessel, and after some hours filtered. To a portion of the filtrate is added a little more hydrochloric and a little sulphuric acid (to reduce the higher oxides of As and Sb), and the whole is boiled for ten minutes. A portion of the filtrate is now subjected to
Reinsch’s Test.—First it is absolutely necessary to have pure copper; so pure, in fact, that a quantity, larger than would be used in testing, will not, if totally dissolved up, yield any As or Sb to another piece of copper boiled in the solution.
Dr. Taylor’s mistake in the case of Smethurst, more fully treated of hereafter, was a very natural one. The trace of arsenic in his copper would not have affected the conclusion in ordinary cases: but it would be better not to test at all than to use materials which are not proved beforehand to be free from the poison we are seeking. Pure “electrotype” copper can now be purchased; or it can be made pure by either of the following methods.
(a) “Pure” commercial sulphate of copper is boiled with a slight excess of chlorine water, then treated with dilute ammonia till a slight permanent precipitate forms: after standing twelve hours it is filtered (the precipitate containing iron and arsenic), acidulated with pure sulphuric acid, and subjected to the current from two Daniell cells, the terminals being two plates of hard wax well coated with purified graphite: the coating must communicate with the copperwire from the battery, and the wire must not dip into the solution. The distance between the terminals should be so regulated that the copper may be deposited slowly and in a tough layer on the negative pole: the thin plate so obtained may afterwards be easily detached, hammered or rolled, and cut into suitable pieces: it is absolutely free from arsenic.
(b) Pure crystallized chloride of copper is mixed with pure carbonate of soda in excess, the mixture dried with constant stirring, heated to near redness, powdered, mixed with an equal volume of lamp-black, and introduced into a “plumbago” crucible lined with a paste of purified graphite and oil. The crucible is covered, and gradually heated in a Fletcher’s or Griffin’s gas furnace (not with coal or coke), and finally kept at a very high temperature till the copper is reduced. The fumes contain chlorides of copper, sodium, &c., and are poisonous. The copper, after separation from the slag, may be cast, hammered, or rolled, and is free from As or Sb.
I suggest these processes more for manufacturers than for chemists, but expense and trouble should really be subordinate considerations where life is concerned.
Now for the application. Two flasks containing pure diluted (25 per cent.) hydrochloric acid are placed on a sandbath, and nearly closed by small glass funnels. About a square inch of pure copper, cleaned by sand-paper, is placed in each: to one is added the suspected liquid, to the other an equal bulk of 5 per cent. hydrochloric acid. Both are boiled, with occasional inspection. If the following are present, the copper will be darkened:—
Arsenic.—Stain steel-grey: dried and heated in closed tube it gives easily a sublimate of octahedral crystals of As2 O3. (See Arsenic, ante.)
Antimony.—Stain black, or in small quantity, violet: in the closed tube it gives with difficulty an amorphous white sublimate of Sb2 O3, soluble in H Cl, and then precipitated orange by H2 S.