Lead Carbonate, PbCO₃

The natural form of Lead Carbonate, PbCO₃, is cerussite, which occurs in rhombic crystals isomorphous with aragonite, CaCO₃; strontianite, SrCO₃, and witherite, BaCO₃. Lead carbonate is also a constituent of phosgenite, PbCO₃.PbCl₂, and of leadhillite, 2PbCO₃.PbSO₄.Pb(OH)₂. The normal salt may be prepared artificially by precipitating a cold solution of lead acetate or nitrate with ammonium carbonate, or by passing carbon dioxide into a dilute solution of lead acetate. If formed sufficiently slowly lead carbonate separates in the crystalline state. It has been shown by Altmann, however, that a basic carbonate of the composition of white lead 2PbCO₃.Pb(OH)₂ may also be precipitated. Its formation is due to the previous hydrolysis of the lead acetate, which is promoted by dilution and raising the temperature, and especially by carrying out the reaction at 100° C. under a reflux condenser, so that vapourised acetic acid can escape. From lead nitrate solution carbonate is not precipitated by carbon dioxide. Crystallised lead carbonate has a density of 6.47, that of the amorphous salt being 6.43. The molecular heat of formation of the crystallised salt from its elements is 169,800 calories (Thomsen), or 166,700 calories (Berthelot). Lead carbonate is dissociated by heat in the sense of the reaction:

PbCO₃ ⇔ PbO + CO₂.

According to Debray the reaction is not reversible, but Colson has shown that the white oxide formed by the decomposition of lead carbonate, PbCO₃, at as low a temperature as possible absorbs carbon dioxide readily, though the yellow oxide formed by heating the carbonate to 350° C. has not this property, probably because it is polymerised. Thus the reaction is reversible if the disturbance due to polymerisation is eliminated; and a little water-vapour has been found greatly to accelerate the establishment of equilibrium between lead carbonate and its dissociation products. The following are the dissociation pressures of lead carbonate when dry and also when moist:

Temperature ° C.	184	210	233	280	285
Dissociation Pressure, mm. Hg. (dry)	10	32.5	102	548	760
Dissociation Pressure, mm. Hg. (moist)	12	33	104	-	-

Lead carbonate is very slightly soluble in pure water; 1 litre of water dissolves 0.0011 to 0.0017 gram PbCO₃ at 20° C., according to measurements of electric conductivity. There is no hydrolysis below 70° C., but at that temperature the water begins to show an alkaline reaction; and if air free from carbon dioxide is drawn through the liquid so as to remove the liberated carbon dioxide, the carbonate is changed into the basic salt 2PbCO₃.Pb(OH)₂.

Lead carbonate is much more soluble in water containing dissolved carbon dioxide than in pure water, as the following figures show (Pleissner), t = 18° C.:

Mgms. CO2 per litre.	0.0	2.8	5.4	14.4	26.0	43.5	106
Mgms. PbC03 per litre.	1.75	6.0	7.0	8.2	9.9	10.9	15.7

The interaction of lead carbonate and solutions of alkali carbonates has been investigated by Auerbach and Pick.