§ 809. Detection and Estimation of Copper.—Copper may occur either in the routine process of precipitating by SH2, or it may, as is generally the case, be searched for specially. If copper is looked for in a precipitate produced by SH2, it is taken for granted that the precipitate has first been treated successively by carbonate of ammonia, sulphide of sodium, and hydrochloric acid; in other words, arsenic, antimony, and lead have been removed. The moist precipitate is now treated with warm nitric acid, which dissolves out copper sulphide with separation of sulphur; if there is sufficient copper, the fluid shows a blue colour, which of itself is an indication of copper being present. The further tests are—(1) Ammonia gives a deeper blue; (2) ferrocyanide of potash a brown-red colour or precipitate; (3) a few drops mixed with a solution of tartrate of soda, alkalised with sodic hydrate, and boiled with a crystal or two of grape-sugar, gives quickly a red precipitate of oxide of copper; (4) a needle or a clean iron wire, or any simple galvanic combination, immersed in, or acting on, the liquid, soon becomes coated with the very characteristic reddish metallic film. Various other tests might be mentioned, but the above are ample.
Special Examinations for Copper.
(1) In Water and Liquids generally.—The liquid may be concentrated, and the copper separated by electrolysis. A simple method is to place the liquid in a large platinum dish, and insert a piece of zinc, adding a sufficient quantity of ClH to dissolve the zinc entirely; the copper is found as an adherent film on the inner surface of the dish. It is neater, however, and more accurate, to connect the platinum dish with the negative plate of a battery, suspending in the liquid the positive electrode. The modifications of this method are numerous; some chemists use (especially for small quantities of copper) two small platinum electrodes, either of foil or of wire, and on obtaining the film, weigh the electrode, then dissolve the copper off by nitric acid, and re-weigh. Such solid substances as peas are conveniently mashed up into a paste with water and ClH; an aliquot part is carefully weighed and put in a platinum dish, connected, as before described, with a battery; at the end of from twelve to twenty-four hours all the copper is deposited, and the dish with its film dried and weighed. The weight of the clean dish, minus the coppered dish, of course equals the copper. Fat and oils are best thoroughly washed with hot acid water, which will, if properly performed, extract all the copper. By the use of separating funnels and wet filters, the fat or oil can be separated from the watery liquid.
A galvanic test has been proposed, which is certainly very delicate, 1⁄100 of a mgrm. in solution being recognised with facility. A zinc platinum couple is made with two wires; on leaving this in an acid liquid containing a mere trace of copper, after several hours the platinum will be found discoloured. If the discoloration is from copper, on exposing the wire to hydrobromic acid fumes (easily produced from the action of potassic bromide and sulphuric acid) and bromine, the wire will become of a violet colour. This colour is easily recognised by rubbing the wire on a piece of porcelain.[889]
[889] Chem. News, Nov. 30, 1877.
(2) Animal Matters, such as the liver, brain, spinal cord, &c., are best entirely burnt to an ash, and the copper looked for in the latter.[890] The same remark applies to bread and substances consisting almost entirely of starchy matters. Any injurious quantity of copper can, however, be extracted with hydrochloric acid and water; and, although this method of extraction is not quite so accurate, it is quicker.