Lead Suboxide, Pb₂O

Berzelius first observed that when lead is heated below its melting-point in the air its surface becomes covered with a grey film, which appears to be the Lead Suboxide, Pb₂O. It was later discovered by Dulong that when lead oxalate is heated to 300° C. out of contact with air it yields the suboxide by the reaction

2PbC₂O₄ Pb₂O + 3CO₂ + CO.

Later observers thought this product was merely a mixture of lead monoxide and metallic lead; but Tanatar has proved that the suboxide is undoubtedly formed by the above reaction, carried out in an atmosphere of carbon dioxide or nitrogen, provided the temperature is kept as low as possible. The product is then a greyish black powder of density 8.342 at 18° C., which is unaltered by dry air or water, but is converted into the monoxide and metallic lead by dilute acids and alkalis. Moreover, the heat of solution of 1 gram-molecule of this substance in acetic acid is 10,048 calories, whilst that of 1 gram-molecule of lead monoxide is 15,500 calories. If, however, the temperature at which lead oxalate is decomposed is too high, a greyish green product results whose molecular heat of solution in acetic acid is 15,500 calories. This product must, therefore, be a mixture of lead monoxide and lead, into which the suboxide is decomposed at high temperature with absorption of 5,452 calories.

The suboxide is also formed by the reduction of the mmioxide by hydrogen at a temperature not exceeding 235° C. Furaier proof that the dark grey powder is not a mixture of lead monoxide and lead is furnished by the fact that mercury dissolves no lead from it, nor sugar solution any lead monoxide. After the powder has been sufficiently heated, however, sugar solution extracts lead monoxide from it, leaving metallic lead behind.