Lead Monoxide, PbO

Lead Monoxide (Litharge, Massicot), PbO has been known from antiquity, since it is formed as a scum on the surface of molten lead. If this scum is continuously removed, so that a fresh surface of the molten metal is oxidised, a yellowish grey powder may be collected, which is a mixture of the oxide and the finely divided metal. If this powder is further roasted, it is converted into a yellowish red form of lead oxide known as massicot. When the temperature at which the oxidation of the molten metal is so high that the oxide fuses, the solidified product, which is scaly, is called litharge. This form of the oxide is obtained in the process of cupellation, by which silver is won from argentiferous lead; hence the name.

The same oxide is produced when lead-vapour burns in the air; formed in this way it has been called flores plumbi. Lead monoxide occurs crystallised in rhombic octahedra in a mineral found near Vera Cruz; similar crystals are formed when litharge is allowed to cool slowly; they also occur sometimes as a deposit in lead furnaces.

According to the conditions of its formation lead oxide varies in colour from lemon-yellow to red or yellowish brown. Thus when red lead has been heated in a hard glass tube for the preparation of oxygen, the residue of monoxide is lemon-yellow in colour; and when lead hydroxide is boiled with a 10 per cent, sodium hydroxide solution, a light yellow modification of the oxide is produced, whilst with a very concentrated solution a red modification results.

The different forms of lead monoxide have been investigated by Ruer, who finds that the pure oxide of a brownish yellow colour becomes light yellow when heated to about 620° C. and allowed to cool, again becomes brownish yellow on prolonged rubbing with considerable pressure in a mortar, but returns to the yellow colour when the heating and cooling are repeated. Ruer has further found that

Geuther's red oxide can be changed into the yellow form by prolonged heating at 700° C.; and he suggests that the yellow form is stable above 620° C., but unstable below it, and gradually changes into the more stable brownish yellow form, this change being accelerated by friction and pressure. The yellow form of the oxide is considerably more soluble in water than the red form. It is uncertain whether or not the brownish yellow oxide is identical with Geuther's red oxide; the former is more soluble in water than the latter, but this may be due to admixture of the yellow oxide.

The density of the yellow oxide is 9.50 to 9.52 or 9.28 to 9.36, that of the lighter red oxide being 9.28 or 8.74 to 9.126. The melting-point of the oxide lies between 835° C. and 906° C.; its molecular heat of formation is 50,300 calories or 50,800 calories.

Lead monoxide is easily reduced to metal by various reducing agents, such as hydrogen, carbon monoxide, potassium cyanide, carbon, and carbides. The reduction of the oxide by reaction with the sulphide has already been noticed under the metallurgy of lead. According to Glaser, PbO is reduced by hydrogen to Pb₂O at 211° C., and to metal above 235° C.; whilst a temperature above 300° C. is necessary for reduction of the oxide to metal by carbon monoxide. Chemically, lead monoxide is an amphoteric oxide; that is to say it dissolves in acids as a basic oxide, and in alkalis as an acidic oxide. This oxide reacts with ammonium chloride, producing a basic chloride and ammonia, according to the reaction:

PbO + NH₄Cl ⇔ Pb(OH)Cl + NH₃;

the pressure of the ammonia evolved is a function of the temperature and reaches one atmosphere at about 42° C. When boiled with sodium chloride solution lead oxide produces a basic lead chloride and caustic soda; this reaction is the foundation of Scheele's process of alkali manufacture. This oxide will dissolve in a solution of sucrose, especially in presence of alkali, and may be separated from some other oxides, e.g. arsenious oxide, by this means.

Litharge is used in the arts in the manufacture of flint glass, and for glazing earthenware; also for making red lead, white lead, and various salts of lead, and as a drier for painters' oils, such as linseed oil, the setting of which, being due to oxidation, is accelerated by litharge and certain other metallic oxides.

Commercial litharge may contain as impurities iron and copper oxides, as well as carbon dioxide and water absorbed from the air.