Lead Dioxide (Lead Peroxide, Puce-coloured Oxide of Lead), PbO₂

Lead Dioxide (Lead Peroxide, Puce-coloured Oxide of Lead), PbO₂, occurs naturally as plattnerite, crystallised in hexagonal prisms, and may be obtained artificially in six-sided tablets. It was discovered by Scheele, who found that red lead is turned brown by chlorine water; Priestley noticed that nitric acid similarly changes red lead, but Proust and Vauquelin were the first to examine this compound systematically.

Lead dioxide may be prepared in many ways. It may be obtained from the monoxide, from lead salts, or from red lead by the action upon them of chlorine or bromine water, preferably in presence of alkali, by fusing litharge with potassium chlorate, or by oxidising it with alkaline permanganate or potassium ferricyanide; also by the action of ozone (Schonbein) hydrogen peroxide, and persulphates on lead salts, as well as by the hydrolysis of plumbic salts. An interesting example of this latter reaction is shown by dissolving red lead in glacial acetic acid and then pouring the solution into water. Red lead is a compound of PbO and PbO₂, and its solution in glacial acetic acid contains lead di- and tetra-acetates, the latter of which is hydrolysed by water, with precipitation of the dioxide. If red lead is digested with dilute nitric acid it does not dissolve as a whole, because plumbic nitrate is not formed. The result is, therefore, the formation of lead dinitrate in solution with separation of the dioxide, thus:

2PbO.PbO₂ + 4HNO₃ = 2Pb(NO₃)₂ + PbO₂ + 2H₂O.

Another important method of obtaining lead peroxide is by electrolytic oxidation, so that it separates at the anode when a suitable lead solution is electrolysed. When electrolysis takes place in alkaline solution the crystalline hydrate H₂PbO₃ is deposited, when in neutral or acid solution the anhydride is obtained, and its separation serves to estimate lead electrolytically. Lead dioxide is also prepared by the electrolysis of a solution of sodium chloride in which the monoxide is suspended. The sodium hydroxide formed by electrolysis dissolves the lead oxide forming plumbite, and this is then oxidised by the hypochlorite present with precipitation of the dioxide.

Colloidal, hydrated, lead dioxide is formed by the hydrolysis of ammonium plumbichloride by much water, thus:

(NH₄)₂PbCl₆ + 4H₂O = Pb(OH)₄ + 4HCl + 2NH₄Cl;

the solution, which is brown, is unstable, and soon becomes colourless on account of the reaction:

Pb(OH)₄ + 4HCl = PbCl₂ + Cl₂ + 4H₂O.

Lead dioxide is prepared commercially by the method of Liebig and Wohler, which consists in fusing 4 parts of litharge with 1 of potassium chlorate and 8 of nitre; and also by a modern process in which lead sulphate is mixed with magnesia, and the mixture is suspended in hot water and treated with chlorine:

PbSO₄ + MgO = PbO + MgSO₄; PbO + MgO + Cl₂ = PbO₂ + MgCl₂.

Lead dioxide, which is called puce-coloured oxide, is perhaps more accurately described as chocolate-coloured. In its natural form it is a brown or grey crystalline powder which has a density of 9.39 to 9.45. The density of the synthetic substance lies between 8.90 and 9.19. Its molecular heat of formation from lead monoxide is 12,100 or 10,100 calories, corresponding to a heat of formation from its elements of 62,400 or 60,400 calories.

Lead dioxide is chiefly employed in the chemical laboratory, and in the arts, as an oxidising agent. When it is heated above 310° C. it loses half its oxygen and is converted into the monoxide; at a lower temperature, however, under the influence of sunlight it yields red lead, Pb₃₀4; but, according to Reinders and Hamburger, red lead is not formed when lead dioxide is heated. When the dioxide is triturated with red phosphorus or sulphide of phosphorus the mixture inflames; on this account it is used in making Swedish matches; when it is rubbed with ordinary phosphorus an explosion takes place. Lead dioxide combines spontaneously with sulphur dioxide gas, becoming red hot and forming lead sulphate; when, however, it is suspended in sulphurous acid solution the following reactions take place:

1. PbO₂ + SO₂ = PbSO₃ + O.
2. SO₂ + O + H₂O = H₂SO₄.
3. PbSO₃ + H₂SO₄ = PbSO₄ + H₂SO₃.

The dioxide likewise combines quantitatively with nitrogen peroxide to form lead nitrate, and also oxidises ammonia with production of the same salt. Hydrochloric acid is oxidised when boiled with lead dioxide, with liberation of chlorine, which may be passed into potassium iodide solution; the dioxide may then be estimated by the titration of the liberated iodine. Probably some lead tetrachloride is formed in this reaction and decomposed again by heat.

Lead dioxide oxidises chromic hydroxide in presence of alkali to chromate, and from the yellow solution lead chromate may be precipitated by acetic acid; it also oxidises a manganous salt in presence of nitric acid to permanganate; both these reactions are employed in qualitative analysis. Phosphorus trichloride when warmed reacts vigorously with lead dioxide, lead phosphate, phosphoryl chloride and lead chloride being formed; and hypophosphorous acid in aqueous solution produces lead phosphate.

Hydrogen peroxide in neutral solution is decomposed catalytically by lead dioxide, but in acid solution a quantitative reaction takes place thus:

PbO₂ + H₂O₂ + 2HNO₃ = Pb(NO₃)₂ + 2H₂O + O₂;

so that lead may be estimated by titration with hydrogen peroxide solution after oxidation to dioxide by bromine in presence of alkali. Oxalic acid in presence of nitric acid is also oxidised to carbon dioxide and water by lead peroxide; and this reaction serves for the volumetric estimation of this latter substance.

Lead dioxide is reduced by hydrogen, the reduction beginning at 189° C. and yielding lead Monoxide, which is not further reduced till the temperature is raised to 211° C. Carbon reduces lead dioxide at 260° C.; carbon monoxide combines with it at 80° C., forming lead carbonate. Lead peroxide, either moist or dry, glows when hydrogen sulphide gas is passed over it, and the gas burns with a characteristic ead flame.

Lead dioxide is soluble in water to the minutest extent; it is distinctly more soluble in mineral acids, presumably with the formation of plumbic salts. Under no conditions does it give rise to hydrogen peroxide; and this may be taken to show that it is not a superoxide like barium dioxide, but a polyoxide, that is a compound of quadrivalent lead, having the constitution O=Pb=O.

Like lead monoxide, the dioxide is an amphoteric oxide, but in accordance with the larger proportion of oxygen it contains its basic properties are very feeble, whilst its acidic properties are more pronounced than are those of the monoxide. The plumbates will be considered here, the plumbic oxy-salts subsequently. According to Bellucci and Parravano, there are threfc types of plumbates - as well as of platinates and stannates - R(OH)₆X₂, R(OX)₄, and RO(OX)₂, derived from the ortho- or meta-acids, thus:

R(OH)₄ + 2XOH = R(OH)₆X₂.
R(OH)₄ + 2XOH = R(OX)₄ + 2H₂O.
RO(OH)₂ + 2XOH = RO(OX)₂ + 2H₂O.