Basic Lead Chlorides

Various basic lead chlorides exist. Owing to the tendency of lead to form basic salts, basic chlorides are more common in nature than the normal salt. Thus the mineral matlockite is PbCl₂.PbO, and is formed artificially by igniting the normal chloride in the air until no more fumes are evolved. The mineral mendipite PbCl₂.2PbO is also well known. When lead chloride is fused with lead oxide a yellow product results, which is used as a pigment under the name Cassel yellow; it has approximately the composition PbCl₂.7PbO, but is not a single substance. Various complex basic. chlorides are formed when chlorine and air act on litharge at high temperature; but they are probably mixtures. The basic lead chlorides have been investigated by Ruer, who has traced the freezing-point curve of fused mixtures of lead oxide and lead chloride, and concludes that only the following three anhydrous basic chlorides exist:

1. PbCl₂.PbO, which occurs naturally as matlockite. This compound crystallises in long, thin needles, and cannot be melted without decomposition, but decomposes at 524° C., yielding the second basic chloride.
2. PbCl₂.2PbO, which exists naturally as mendipite, forms long, shining needles, that melt, undecomposed, at 693° C.
3. PbCl₂.4PbO has not been found in nature, but crystallises in yellow plates, and yields a bright yellow powder. It melts, unchanged, at 711° C.

Several hydrated basic lead chlorides, or lead hydroxychlorides, exist, among them being the mineral laurionite PbCl₂.PbO.H₂O or Pb(OH)Cl. An artificial form of this substance was at one time prepared by precipitating lead chloride solution with lime water. The process was patented by Pattinson in 1849, and the product used as a substitute for white lead. This compound, as well as PbCl₂.3PbO.H₂O, appears also to be formed when lead chloride is shaken with water and the hydrated oxide Pb₃O₂(OH)₂; PbCl₂.3PbO is also supposed to be produced in Scheele's process for manufacturing caustic soda by the action of litharge on sodium chloride solution, thus:

2NaCl + 4PbO + H₂O ⇔ 2NaOH + PbCl₂.3PbO,

but according to Berl and Austerweil the compound PbCl₂.4PbO.2H₂O is produced when a normal solution of sodium chloride is employed, thus:

2NaCl + 5PbO + H₂O ⇔ 2NaOH + PbCl₂.4PbO.

The hydroxychloride formed in Scheele's process turns yellow when heated till it is anhydrous. In 1787 Turner took out a patent for preparing caustic soda by Scheele's reaction, and the ignited basic chloride was employed as a pigment under the name of Turner's yellow, or patent yellow.

Stromholm has prepared the salt 2PbCl₂.6PbO.H₂O, and finds that it crystallises in yellow needles; he has also obtained the salt PbCl₂.6PbO.2H₂O, crystallised in needles which on heating change in colour from grey to yellow.