Plumbic Sulphate

Plumbic Sulphate, lead disulphate, Pb(SO₄)₂, is formed by the electrolytic oxidation of lead in presence of sulphuric acid. It was prepared by Elbs and Fischer by the electrolysis of sulphuric acid of density 1.7 to 1.8 with a lead anode. The temperature did not exceed 30° C., the current density was 0.02 to 0.06 ampere per sq. cm., and the anode was separated from the cathode by immersion in a porous pot containing a considerable volume of the acid. The anode mud contained 60 to 85 per cent, of plumbic sulphate; a purer product subsequently crystallised from the anode liquid. The crystals of plumbic sulphate were of a faintly greenish yellow tint, but yielded a practically white powder when dried on a porous plate. They could not, however, be completely freed from sulphuric acid.

One hundred c.c. of sulphuric acid dissolve about 0.345 gram of plumbic sulphate at 30° C. This salt is hydrolysed by water into sulphuric acid and lead dioxide; the same effect is produced more slowly by the action of sulphuric acid of density less than 1.65. Hydrolysis, however, takes place in two stages, the basic sulphate PbOSO₄.H₂O being an intermediate product.

Concentrated hydrochloric acid dissolves plumbic sulphate forming a yellow solution containing lead tetrachloride or chloroplumbic acid; glacial acetic acid also dissolves the salt at 40°-50° C., and white needles of the tetra-acetate crystallise from the solution on cooling. Sodium acetate solution likewise dissolves plumbic sulphate, forming sodium plumbi-acetate; double salts, such as K₂Pb(SO₄)₃ and (NH₄)₂Pb(SO₄)₃, are also formed with sulphates of the alkali metals, ammonia, and amines. Cold concentrated sodium hydroxide solution forms sodium plumbate.

Plumbic sulphate resembles, but is more energetic than, lead dioxide as an oxidising agent; ferrous salts are oxidised to ferric, alcohol to aldehyde, and oxalic acid to carbon dioxide by this compound. These oxidations are accompanied by the separation of lead sulphate, without the appearance of the dioxide.