Under certain conditions definite colloidal precipitates are formed, particularly in the presence of sulphide of copper or mercury. (2) Sulphuric acid or soluble sulphates produce a precipitate of lead sulphate insoluble in excess of the precipitating salt or sulphuric acid, and only slightly soluble in alkaline solutions. This method is the one generally adopted for gravimetric determination of a lead salt. (3) Potassium chromate produces a precipitate of chromate of lead very little soluble in acid, but soluble in caustic alkali. (4) Potassium iodide produces a yellow lead iodide, soluble on heating, and reprecipitating and crystallizing on cooling. (5) Alkaline chlorides and hydrochloric acid produce needle-like crystals of lead chloride soluble on heating, and reprecipitating on cooling. (6) Potassium nitrate in conjunction with a copper salt (copper acetate) produces a precipitate of a triple copper, lead, and potassium nitrate, crystallizing in characteristic violet-black cubes. This reaction is one made use of in the qualitative determination of small quantities of lead in organic fluids (see [p. 167]).
All the precipitates of lead salts, with the exception of the sulphide, are soluble in fixed alkalies, in ammonium acetate, ammonium tartrate, and ammonium citrate. It is possible to determine the presence of lead in a large volume without evaporating down the whole bulk of fluid. By this means liquid containing lead is treated with sulphide of copper, sulphide of mercury, or baryta-water. Meillère states that he has detected the presence of as small a quantity as 1 milligramme of lead in 1,000 c.c. of water in this manner without evaporating the liquid. Where lead is in organic combination, as is the case in the urine of persons suffering from lead poisoning, it is not decomposed by hydrogen sulphide, and the method is therefore not applicable in such cases, but is useful in water examination.
Electrolytic Reactions.
—Solutions of lead are easily electrolyzed, and give a precipitate of lead at the cathode; simultaneously the peroxide is produced at the anode, and the reaction is acid. In nitric acid solutions Riche pointed out that the whole of the lead is carried to the anode, and this is the reaction made use of in the determination of lead present in the urine (see [p. 172]).
The presence of copper in an electrolyte regulates the precipitation of lead oxide, copper alone being deposited at the cathode, and at the same time the presence of a small quantity of copper promotes the destruction of organic materials.
REFERENCES.
[1] Pliny: lxxxiii., 11, N.c.v.
[2] Stockhusen: De Litharg. Fumo, etc. Goslar, 1656.
[3] Tronchin: De Colica Pictonum. 1758.
[4] John Hunter: Observations of Diseases of the Army in Jamaica. London, 1788.