Thermochemical Study of Cerussite

Abstract

By using thermobalance to dissociate cerussite into lead monoxide in the air, it is shown that the following intermediate basic carbonates were produced : PbCO₃→2PbO·PbCO₃→11PbO·PbCO₃→19PbO₃·PbCO₃→PbO., As the procedure was undertaken in CO₂ gas, however, the following salts were produced by dissociation : PbCO₃→PbO·PbCO₃(or 3PbO·2PbCO₃)→2PbO·PbCO₃→PbO., This fact was quite true with the differential thermal analysis., By using the filament manometer, dissociation pressures of cerussite were measured., From the results thus obtained, the relations between the dissociation pressure and the temperature were expressed by the following equations, and the heats of dissociation (Q) and the dissociation temperatures (t) were calculated as follows., (I) log p=15., 52-7200/T, Q=-32930 cal/mol, t=296., 5°C (II) log p=13;69-7152/T, Q=-32710 cal/mol, t=388., 5°C (III) log p=12., 36-6636/T, Q=-30350 cal/mol, t=426., 9°C (IV) log p=10., 50-5500/T, Q=-25160 cal/mol, t=448., 7°C In these equations, it will probably be said that (I) corresponds to the dissociation of PbCO₃, (II) to that of 2PbO·PbCO₃, (III) to that of 11PbO·PbCO₃, and (IV) to that of 19PbO·PbCO₃., The same experiments were taken with artificial lead carbonate., The process of dissociation is shdwn as : PbCO₃→PbO·PbCO₃→17PbO·PbCO₃→PbO in the air, and shown as : PbCO₃→PbO·PbCO₃(or PbO·3PbCO₃)→PbO·PbCO₃→2PbO·PbCO3→PbO in the CO₂ gas., In cnclusion, the following two facts are remarkable., (1) With the intermediate basic salts, there are remarkable differences in the chemical composition and existing temperatures between the natural and artificial lead carbonate., (2) Even in the case of the same specimen, the intermediate basic salts differ somewhat in their chemical composition and their existing temperature according to the atmnospheric conditions whether the dissociation is taken in the air or in CO₂ gas.,