3. Of Sugar of Lead.
Sugar of lead is the acetate of this metal. It is sold in the form either of a white heavy powder, or of aggregated masses of long four-sided prismatic crystals. It has a sweetish astringent taste, and a slight acetous odour. It is very soluble.
When in the solid state, it may be known by its solubility in water, and by the effects of heat. It first undergoes the aqueous fusion, then abandons a part of its acid empyreumatized, as may be perceived by the smell, next becomes charred, and finally presents globules of lead reduced by the charcoal of the acid. The best way of effecting its reduction on the small scale is to char it, and then direct on the mass the point of a blowpipe-flame: in an instant globules are developed. It is not easily reduced in a tube; at least I have never been able to succeed in that way.
In the fluid state the acetate of lead, as well as all its soluble salts, may be detected by the following system of reagents,—hydrosulphuric acid, bichromate of potass, hydriodate of potass, and metallic zinc,—which are the best of the numerous reagents yet proposed.
1. Hydrosulphuric acid causes a black precipitate, the sulphuret of lead. This is a test of extreme delicacy; and it acts in whatever state of combination the lead exists, whether fluid or solid.
It is preferable to the hydrosulphate of ammonia as a medico-legal test; for, as Fourcroy observed, the hydrosulphate of ammonia acts on many sound wines as if they contained lead,[1] while hydrosulphuric acid never causes with them a black precipitate, unless they contain either lead or some other metallic impregnation. It must be remembered that many other metallic solutions, such as those of mercury, copper, silver and bismuth, yield a black precipitate with this test.
2. Chromate of potass, both in the state of proto-chromate and bichromate, causes a fine gamboge-yellow precipitate, the chromate of lead. For the characteristic action of this reagent, it is desirable that the suspected liquid be neutral. It forms with solutions of the sulphate of copper a precipitate nearly of the same colour as the chromate of lead.
3. Hydriodate of potass causes also a lively gamboge-yellow precipitate, the iodide of lead. The action of this test is impaired in delicacy by a considerable excess of nitric acid, or acetic acid. These acids cause a yellow coloration with the test, though no lead be present.
4. A rod of zinc held for some time in the solution displaces the lead, taking its place, and throwing down the lead in the form of a crystalline arborescence. This is a very characteristic test; and also one of much delicacy; for I have found a small thread of zinc will very easily detect a twentieth part of a grain of lead dissolved in the form of acetate in 20,000 parts of water. It acts also on the nitrate of lead. Its action is impaired or prevented by an excess of acetic or nitric acid.
- ↑ Mem. de l'Acad. des Sc. 1787, 281, sur les vins lithargyriés
