The Chemistry of Lead.
Physical Properties.
—Lead belongs to the group of heavy metals, and occupies a position between bismuth and thorium in the list of the atomic weights, the atomic weight being 206·4, and density 11·85. It is blue-grey in colour, and its softness and facility to form a mark upon paper are well known. Lead melts at a temperature of 325° C., and at this temperature a certain (if negligible) amount of volatilization takes place, which vapour becomes reprecipitated in the form of an oxide. Use is made of the volatility of the metal at the higher temperatures, 550° C. and upwards, in the oxidation of lead from a mixture of lead, silver, and gold; the oxide of lead, or litharge, is partially collected and absorbed by the crucible, but the greater part is mainly removed from the surface of the liquid metal as it is formed, while the richer metal is left in the crucible.
Chemically speaking lead is a tetrad, and forms a number of organic derivatives, especially through the intervention of a particular oxide, minium. Lead forms metallic alkalies and alkaline earths, resembling silver in this direction, and also metallic compounds with zinc and copper; in this point it is very similar to silver. Small quantities of lead present in other metals—as, for instance, a small trace in gold—alter its physical qualities to a great extent; whilst the addition of minute traces of other metals to lead—as, for instance, antimony—cause it to become hard, a fact made use of in the manufacture of shot.
A number of oxides of the metal are known: two varieties of protoxides (massicot and litharge), protoxide hydrate, and bioxide. Sulphide, or galena, represents the chief form in which lead is found in Nature, and from which the actual metal is produced by metallurgical processes.
The salts of lead may be divided as follows:
1. The carbonates or hydrated carbonates employed in a large number of industrial and other processes, which are the cause of much lead poisoning.
2. The acetates, both normal and basic, which are particularly concerned in the production of white lead—at any rate in the process of converting metallic lead into the hydrated carbonate through the medium of acetic acid and steam.
3. Chromate of lead, which is used as a pigment, and also in dyeing yarns, etc.
4. The nitrates and chlorides; the chloride particularly is used as an oxidizing agent (plumbing, soldering, tinning of metals).
5. The silicates, silico-borates, silico-fluoborates, which constitute the many varieties of glass and crystals used in optical instruments, and the various glazes and enamel colours used in the potteries.
There are a large number of other derivatives, but these are not of special interest to the subject in hand.
The Action of Water upon Lead.
—The action of water on lead was known even to the ancients, Pliny and Galen having written on the subject. At times, and under certain conditions, as much as 20 milligrammes per litre have been found, as in the Bacup epidemic, and 14 milligrammes per litre in the epidemic at Claremont. Bisserie[6] in 1900 made an exhaustive inquiry into the action of water upon lead; he gives the following conclusions:
1. Water and saline solutions attack lead more or less readily when it is in combination with another metal, such as solder, copper, bronze, iron, or nickel, the result being a hydrated oxide.
2. The maximum effect is produced with water slightly acid and with solutions of chlorides or nitrates. With these it is not necessary to have other metals present, and if the water is thoroughly aerated the pure metal is attacked.
3. Bicarbonates and carbonic acid exercise by themselves an action on wet lead, but the carbonate of lead formed in the process adheres firmly to the surface of the metal, and prevents any further action.
4. Sulphates act in the same way, but in less degree.
5. This protective action is much diminished when the water is even slightly charged with nitrates or organic material. Pouchet has pointed out that lead branch-pipes fixed to iron water-pipes, thus producing an “iron-lead couple,” set up definite electro-chemical changes, and tend to increase the rate at which solution of lead in the pipe water takes place.
Houston[7], in an extensive and very full report on the effect of water upon lead, especially undertaken for the purpose of inquiry into the contamination of supplies of drinking water by means of lead, distinguishes two species of action—namely, plumbo-solvency, which is brought about by the acidity of the water in contact with lead; and a second kind of action, erosion, determined to some extent by the dissolved air in the water. He came to the conclusion that the plumbo-solvency and erosive action of water on metallic lead differed considerably, and that the protective layer or plumbo-protective substance did not always protect lead pipes from the solvent action of water.
Chemical Characters of Lead Salts.
—A short summary of the chemistry of lead salts may not be out of place.
A soluble salt of lead, such as the acetate or nitrate, is precipitated by (1) hydrogen sulphide or alkaline sulphide as a brown or black precipitate, which is insoluble in ammonium sulphide. In dilute solutions this sulphide is, however, appreciably soluble in mineral acids, and may introduce errors in analysis, especially as the solubility is distinctly increased by the presence of certain earthy salts. The sulphide produced through the action of alkaline sulphide on a soluble salt of lead is less soluble than is the corresponding acid sulphide. Soluble salts of lead are at once precipitated by albumin or peptone; the resulting precipitate has no stable composition.
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.