Chemical Properties of Lead

The key to the chemical behaviour of lead is furnished by the position this element occupies in the electropotential series of the metals. In this series lead occupies a position close to that of tin, and little removed from that of hydrogen, both these metals being slightly more electropositive than hydrogen. It has been found that against a normal hydrogen electrode lead shows a potential of -0.120 volt. Consequently lead possesses very little power of displacing hydrogen from aqueous solutions of acids, and this power is sometimes interfered with by the insolubility of the products of the action. Thus lead dissolves in hydrochloric acid with more difficulty than tin, although lead possesses a greater solution pressure than tin.

A chemical comparison of lead and tin has been carried out by Sackur, who finds that while lead precipitates tin from nitric acid solution, it is precipitated from acetic acid solution by tin. A condition of equilibrium between the two metals, consistent with the law of mass action, is attained in the case of solutions in hydrochloric and sulphuric acids. The conclusion is reached by measurements of the E.M.F. of a voltaic element of the form Pb | Pb(NO₃)₂ | Sn(NO₃)₂ | Sn that the electrolytic solution tension of lead is about 17.2 times as great as that of tin. This great difference, which is partly accounted for by the feeble ionisation of stannous salts, is in accordance with the relative positions of the two elements in the periodic system, and the stronger basic properties of lead oxide as shown by the stabilities of its oxysalts. Since lead is so little capable of displacing hydrogen from dilute acids, its solution by such an acid as acetic acid is to be attributed chiefly to the assistance of atmospheric oxygen in the sense of the reactions:

2Pb + O₂ + 2H₂O = 2Pb(OH)₂
2Pb(OH)₂ + 4HC₂H₃O₂ = 2Pb(C₂H₃O₂)₂ + 2H₂O.

Tin, which is not so oxidisable as lead, is not attacked by acetic acid. The great difference in behaviour between lead and tin towards water containing dissolved air must be attributed to the same cause.

Lead reacts vigorously with fluorine, and with diminishing vigour towards the other halogens in turn. Finely divided lead, produced by heating lead tartrate out of contact with air, is pyrophoric. Ordinary lead in the air becomes coated with a film of the suboxide, Pb₂O, which is also formed as a grey film on the surface of molten lead. By exposure to red heat, however, lead is converted into the monoxide, as in the process of cupellation.

Boiling water is appreciably decomposed by granulated lead, and lead monoxide is reduced by hydrogen; thus the reaction is a reversible one proceeding to an equilibrium at a given temperature. Somewhat diluted nitric acid is the best solvent for lead, and the oxides of nitrogen evolved contain a larger proportion of nitric oxide than in the case of copper. The presence of much sulphuric acid modifies the interaction of nitric acid and lead, causing reduction to nitrous acid; and when dilute nitric acid is electrolysed with lead electrodes in presence of upwards of 40 per cent, of sulphuric acid, about 40 per cent, of the nitric acid is converted into hydroxylamine.

Concentrated sulphuric slowly attacks lead in the cold, and when the acid is heated a vigorous reaction sets in, with evolution of hydrogen, sulphur dioxide, and hydrogen sulphide. Sulphuric acid of density 1-760 and upwards attacks the lead pans in which it is evaporated at a temperature of about 200° C. in sulphuric acid manufacture; and the lead sulphate formed dissolves in the acid, whence it is precipitated on dilution. Pure lead is less affected than the impure metal, but the action varies with the nature of the impurity and the physical condition of the metal.

Composition and Uses of Commercial Lead. - Owing to the purifying treatment to which the metal is submitted, commercial lead attains a high degree of purity. The foreign metals generally present are copper, antimony, iron, zinc, and silver, with less frequently bismuth, and occasionally a minute trace of tin and arsenic. The total metallic impurity, however, rarely exceeds 0.1 per cent., and may fall below 0.01 per cent.

Lead was employed in olden times for making cisterns and coffins, and as a roofing material for important buildings, such as churches. At the present day it is likewise employed for the purposes included in the art of the plumber, in the form of sheets and in pipes for conveying water and gas. It is also used, on account of its power of resisting the action of acids, for the manufacture of chemical plant, such as sulphuric acid chambers. It has been found, however, that for this purpose ordinary lead is better than the purest metal. Further, lead is used in accumulators, for making solders, pewter, type-metal, etc., and when alloyed with arsenic, added in the form of white arsenic or arsenical dross, for the manufacture of shot. The arsenic increases the fluidity of the molten metal, as well as the tendency of its drops to become spherical as they pass through the air. Shot is made by dropping the molten metal through colanders pierced with holes, down a tower or well into water. The size of the shot depends not only on the size of the holes in the colander, but on the initial temperature of the metal, and the height through which the drops are made to fall. The shot is sorted by sieving, and by rolling down an inclined metal plane, the imperfectly shaped shot thus remaining on the plane. Finally, the shot is polished with plumbago.

Ancient lead objects kept in museums sometimes crumble to a fine powder, which consists essentially of lead carbonate, but also always contains chlorides. It was shown by Matignon that a piece of lead which had been dipped in a 30 per cent, solution of sea salt, and then dried so as to become coated with salt crystals, underwent progressive destruction over a period of three years; but that a similar piece of lead which had not been so treated showed no destruction. Consequently it is believed that the following cycle of reactions takes place:

2NaCl + Pb + O + CO₂ = Na₂CO₃ + PbCl₂ = PbCO₃ + 2NaCl.

Lead forms two well-defined classes of compounds derived respectively from the bivalent ion Pb^••, and the quadrivalent ion Pb^••••. In addition to these compounds there is the suboxide Pb₂O, which appears to be a representative of univalent lead. There is evidence, moreover, of the existence of univalent lead ions in solution; for it has been shown by Denham and Allmand that a solution of lead acetate, heated with lead, forms a subacetate with the absorption of heat, thus:

Pb^•• + Pb → 2Pb^•

and that a piece of lead in a solution of lead acetate which is maintained at two different temperatures forms a thermo-cell, the current flowing from the hot solution to the cold, and causing the deposition of traces of spongy lead at the cold end of the column of lead. The anomalous behaviour of the hydrogen electrode in solutions of lead salts is attributable to the reduction of the bivalent lead ion to a univalent ion, just as the discrepancy observed in copper and silver voltameters is due to the formation of subsalts of these two metals.

Lead, like tin, forms no hydride. It resembles the other metals of Subgroup IV B in forming volatile alkyls in which the metal is quadrivalent. The formation of these compounds by the interaction of zinc alkyls and lead dichloride with the separation of metallic lead emphasises the non-existence of lead dialkyls.

It was early observed that the presence of lead in wine could be detected by the addition of sulphuric acid; and Zeller in 1707 suggested an extract of orpiment and lime-water, which would contain sulphide, as a test for lead salts, which it turned black. The introduction of hydrogen sulphide as an analytical reagent was first made in connection with lead salts; for Fourcroy and Hahnemann in 1787 proposed to use water acidified with hydrochloric acid and saturated with hydrogen sulphide as a test for lead.

Detection in the Dry Way

Lead salts impart a bluish grey tint to the Bunsen flame, which does not, however, show a clearly defined spectrum. The spark spectrum of lead contains characteristic lines in the orange, green, and violet. Owing to the easy reducibility of lead oxide, lead salts yield a bead of the metal when heated with sodium carbonate on charcoal before the blowpipe. The metal is identified by its malleability, by the fact that it marks paper, and by dissolving it in nitric acid and applying to the solution one or other of the characteristic wet tests.

Detection by Reactions in Solution

From moderately concentrated solutions lead is precipitated as chloride by hydrochloric acid, and this reaction is employed to separate lead, together with silver, and mercurous mercury in qualitative analysis.

Lead remaining in solution after the addition of hydrochloric acid is detected and separated by means of hydrogen sulphide, with which it yields a black precipitate of the sulphide. Copper, mercury, and bismuth also give black or dark brown precipitates with the same reagent, but the lead is separated and identified by the solubility of its sulphide in moderately concentrated nitric acid and the subsequent precipitation of the sulphate by dilute sulphuric acid. The lead sulphate may then be dissolved in ammonium acetate solution, and the lead precipitated as chromate.

The sulphide and chromate tests are both very delicate, and may be employed for the detection of lead in potable water. The sulphide test is generally employed in presence of a little dilute hydrochloric acid, though it is even more delicate in presence of alkali. In either case the absence of other metals which might give brown colorations or black precipitates must be ascertained. The chromate test is carried out in presence of dilute acetic acid. Other and less important tests for lead are as follow - Alkalis precipitate lead hydroxide, soluble in excess of the precipitant, but insoluble in ammonia; alkali carbonate precipitates basic lead carbonate, insoluble in excess; alkali iodide precipitates yellow lead iodide, soluble in hot water from which it crystallises in golden spangles; metallic zinc separates lead in crystals, forming the lead tree.

A reagent for detecting traces of lead and manganese exists in the form of tetramethyldiaminodiphenylmethane, which gives a deep blue colour with lead and manganese dioxides. The substance to be tested is incinerated with sulphuric acid; a few drops of sodium hypochlorite solution are added to the ash, excess of chlorine is eliminated by heating, and the reagent is added. By the blue colour produced lead has been detected in water which has passed through lead pipes, or in an animal body, when other methods have failed.

Estimation of Lead

Lead may be estimated by three series of methods - (i) gravimetric, (ii) volumetric, (iii) electrolytic.

(i) Gravimetric Methods. - (a) Lead is precipitated from solution as sulphate by dilute sulphuric acid, a volume of alcohol equal to twice the volume of the solution being added to secure complete precipitation. The precipitate is then filtered off through a weighed filter, or preferably a Gooch crucible, dried, and weighed.

(b) Lead may also be precipitated as basic carbonate or oxalate, and these are converted by ignition into monoxide, which is weighed.

(c) Lead is occasionally weighed as chloride, chromate, molybdate, or oxalate.

(iii) Volumetric Methods. - (a) The method of Alexander consists in titrating an ammonium acetate solution of lead sulphate with ammonium molybdate solution which has been standardised with a similar lead solution of known strength. The end point is found by a spot reaction with a solution of tannin which gives a yellow colour with molybdate.

(b) Lead may also be estimated by precipitating with excess of standard chromate solution, filtering and washing the precipitate, and estimating the excess of chromate by adding acidified potassium iodide, and titrating the liberated iodine with thiosulphate.

It is possible also to titrate a hot solution of a lead salt by standard chromate, employing the shaking method.

(c) A further method of estimating lead consists in obtaining it as sulphate, dissolving this in ammonium acetate, and oxidising to dioxide by bromine water, the dioxide being then estimated iodometrically.

(d) After lead has been converted into sulphate it may be estimated volumetrically by decomposing this salt with hydrogen sulphide, and titrating the liberated sulphuric acid.

(e) Traces of lead are estimated colorimetrically by means of hydrogen sulphide, the depth of colour produced being matched by means of a standard lead solution. By this method 0.05 mg. of lead per litre can be estimated.

(iii) Electrolytic Methods. - Lead has usually been estimated electro- lytically by separation from nitric acid solution, as lead peroxide at the anode. From a faintly acid, ammoniacal or alkaline solution, however, it can be separated as metal at the cathode. Classen has found that with a current density of 1.5 to 1.7 amperes per square dcm. the separation of lead as lead dioxide is complete in four to five hours at a temperature of 40° to 50° C. The lead dioxide is then washed and dried at 180° C. The special precautions necessary for this estimation have been studied by Vortmann.

The introduction of rotating electrodes has enabled electro-analysis to be carried out very rapidly; and the estimation of lead by this means, as well as the separation of various metals by the use of graded potential, has been successfully accomplished by Sand.