When lead is in very minute quantity, an electrolytic method is generally preferable; the lead is precipitated on platinum by using exactly the same apparatus as in Bloxam’s test, described at [p. 566]; the liquid to be tested being placed in the inner cell, the lead film may now be identified, dissolved in nitric acid, and estimated by a colorimetric process. For the estimation of the minute fractions of a grain by a colour method, it is merely necessary to have a very dilute solution of acetate of lead, to add a known volume of SH2 water to the liquid to be tested in a Nessler cylinder, noting the colour, and add to another a known quantity of the standard lead solution and the same quantity of SH2 as was added to the first.
The process has an advantage which is great, viz., that it either detects copper, or proves its absence at the same time; and there are few cases in which the analyst does not look for copper as well as for lead. Lead, if in sufficient quantity, may be most conveniently estimated as oxide, sulphate, or chloride; the chief properties of these substances have been already described.
§ 794. The Detection of Lead in Tartaric Acid, in Lemonade, and Aërated Waters.—To detect lead in tartaric acid a convenient method is to burn it to an ash, digest in a little strong sulphuric acid, and then add either sodic chloride or a drop of HCl; lead, if present, is precipitated as chloride, giving a pearly opalescence. Lemonades often contain minute quantities of iron and copper as well as lead. Neither copper nor iron are precipitated by ammonium sulphide in presence of potassic cyanide. On the other hand, the sulphide of lead is not soluble in the alkaline cyanides. Hence a liquid which, on the addition of potassium cyanide and then ammonium sulphide, becomes dark coloured, or from which a precipitate separates, contains lead.[855]
[855] F. L. Teed, Analyst, xvii. 142-143.
2. COPPER.
§ 795. Copper, Cu = 63·5; specific gravity, from 8·921 to 8·952; fusing-point, 1091° (1996° F.). Copper in analysis occurs either as a film or coating on such metals as platinum, iron, &c., or in a state of fine division; or, finally, as a bead. In thin films, copper has a yellowish or a yellowish-red colour; it dissolves readily in nitric, slowly in hydrochloric acid. If air be excluded, hydrochloric acid fails to dissolve copper, and the same remark applies to ammonia; but, if there be free access of air, ammonia also acts as a slow solvent. Metallic copper in a fine state of division can be fused at a white heat to a bright bluish-green globule, which, on cooling, is covered with black oxide.
§ 796. Cupric Oxide (CuO = 79·5; specific gravity, 6·5, composition in 100 parts, Cu 79·85, O 20·15) is a brownish-black powder, which remains in the absence of reducing gases unaltered at a red heat. It is nearly insoluble in water, but soluble in ClH, NO3H, &c.; it is hygroscopic, and, as every one who has made a combustion knows, is readily reduced by ignition with charcoal in the presence of reducing gases.
§ 797. Cupric Sulphide, CuS = 95·5, produced in the wet way, is a brownish powder so insoluble in water that, according to Fresenius, 950,000 parts of water are required to dissolve one part. It is not quite insoluble in ClH, and dissolves readily in nitric acid with separation of sulphur. By ignition in a stream of H it may be converted into the subsulphide of copper. It must always be washed by SH2 water.