Lead Orthoplumbate

Lead Orthoplumbate, Red Lead (Minium, Paris Bed, Saturn Cinnabar), Pb₃O₄. - Pliny appears to have been acquainted with this oxide, which he describes as minium, though probably cinnabar and red sulphide of arsenic were confused with it. Dioscorides states that it can be obtained from white lead, and Geber describes its preparation.

Red lead is usually prepared by heating the yellow, powdery form of lead monoxide, known as massicot, to dull redness (400° C.) for twenty-four hours in a reverberatory furnace with a plentiful air- supply, or in barrel-shaped vessels open at the ends to allow of air circulation. The mass is frequently stirred, and samples are removed from time to time, and allowed to cool, to see if the proper scarlet tint has been attained. White lead, however, is frequently the starting- point for the manufacture of red lead. Red lead is also formed when lead monoxide and dioxide are heated together in the right proportion, as well as by heating lead monoxide with nitre. Finely divided lead also is oxidised spontaneously in the air to red lead. On account of its usual manner of preparation red lead is liable to contain the monoxide which has escaped oxidation; this may be removed by repeatedly digesting the powder with lead acetate solution.

Pure red lead may be prepared by heating lead monoxide with potassium nitrate, according to the reaction

3PbO + KNO₃ ⇔ Pb₃O₄ + KNO₂

and extracting unchanged monoxide with lead acetate solution; or by heating the dioxide and potassium nitrate to 470° C.:

3PbO₂ + KNO₃ → Pb₃O₄ + KNO₃ + O₂,

and extracting the resulting mass with potassium hydroxide or lead acetate solution.

Red lead crystallises from fused nitre in small prisms; and it has a density of 8.62 to 9.08. When this oxide is heated it turns violet and then black, but regains its scarlet colour on cooling. This is an enantiotropic change, which has been studied by Le Chatelier, who estimates the transition temperature of the two forms to be 580° C., whilst the melting-point of the compound is 830° C., and its dissociation pressures are:

	445°	500°	555°	636°
Dissociation Pressure, mm. Hg	5	60	183	763

The conditions of formation of red lead from lead monoxide by the absorption of oxygen have been studied by Milbauer, who finds that this absorption which begins at ordinary temperature becomes sufficient to be measured at 240° C.; that at 320°-450° C., whilst the amount of absorption has increased the product has not become red, but is brown, and probably contains the sesquioxide Pb₂O₃; above 450° C., however, a red colour appears and red lead is formed, whilst at 470°-480° C. 100 per cent, of red lead is formed under pressure. Red lead is completely decomposed at 530° C. in a vacuum, but the temperature of complete decomposition is higher when air or oxygen is present. The curve of decomposition has breaks showing the formation of the oxides Pb₂O₃, Pb₃O₄, PbO.

Reinders and Hamburger, however, who have studied the thermal dissociation of red lead, according to the reaction

2Pb₃O₄ ⇔ 6PbO + O₂,

state that the pressure-temperature curve between 445° C. and 607° C. is regular.

Dry chlorine converts red lead into lead dichloride and oxygen. Red lead is practically insoluble in water; dilute acids decompose this oxide forming a salt of bivalent lead, with separation of lead dioxide; but with concentrated acids the unstable plumbic salts are formed. For example, glacial acetic acid produces a mixture of the two acetates, thus:

Pb₃O₄ + 8HC₂H₃O₂ = 2Pb(C₂H₃O₂)₂ + Pb(C₂H₃O₂)₄ + 4HSO.

According to its formula red lead should contain 34.9 per cent, of lead dioxide; the commercial product, however, generally contains about 26 per cent, of the latter oxide; and besides lead monoxide it may contain as impurities oxide of iron, powdered heavy spar, and brick-dust, which should be regarded as adulterants. Various methods are available for the analysis of red lead; these depend upon its oxidising power, which is a measure of the proportion of lead dioxide contained in the specimen. The sample may be heated with hydrochloric acid, and the evolved chlorine passed into potassium iodide, the liberated iodine being titrated with thiosulphate (Bunsen); or the iodine may be liberated directly from potassium iodide without distillation, and similarly titrated. Another method depend upon the oxidation of oxalic acid to carbon dioxide and water by the red lead; excess of oxalic acid is used, and what remains is titrated with permanganate.

On account of its fine colour, red lead is used as a pigment; it is also employed as a cement for steam joints, in the manufacture of secondary batteriesj and for making flint glass.