Lead Chloride Double Salts

The solubility of lead chloride in water is diminished by the presence of small quantities of potassium chloride, and increased when the concentration of the latter salt passes a certain value. The minimum solubility of lead chloride at 25.2° C. is 0.00483 gram-molecule per litre, and is reached when the potassium chloride is of 1.5018 N strength.

This variation in solubility, which is similar to that of lead chloride in hydrochloric acid, is attributed to the same cause, viz. to the mass action of chloride ions at low concentration, and to the formation of complex ions PbCl₄'' at higher concentrations.

The existence of several double chlorides of lead and potassium has been shown by Lorenz and Ruckstuhl, who have examined the freezing-point curve of a fused mixture of the two salts. Thus by a maximum at 430° C. and 33.3 mol. per cent, of potassium chloride, and breaks at 440° C. and 480° C. with 60 mol. and 68 mol. per cent, respectively of potassium chloride, the existence of the following double salts is indicated: 2PbCl₂.KCl, which forms an almost transparent, glassy mass; PbCl₂.2KCl, which occurs in rhombic crystals; and PbCl₂.4KCl, which is said to form reddish white, granular masses. The behaviour of a fused mixture of potassium and lead chlorides on electrolysis shows that a complex anion is present.

The following crystallised double chlorides have been obtained from mixed aqueous solutions of their component salts:

3(PbCl₂.KCl).H₂O; 2PbCl₂.RbCl; PbCl₂.4CsCl; 2PbCl₂.NH₄Cl; PbCl₂.2RbCl.H₂O; PbCl₂. 3TlCl; PbCl₂.2NH₄Cl; 2PbCl₂.CsCl; PbCl₂.2MgCl₂.13H₂O; 3(PbCl₂.NH₄Cl).H₂O; PbCl₂.CsCl; PbCl₂.BaCl₂;

According to Foote and Levy, only one lead ammonium chloride, viz. 2PbCl₂.NH₄Cl, exists. Equilibria in the system water-ammonium chloride-lead chloride have been studied by Bronsted.

Two compounds of lead chloride with ammonium bromide have been described; they are 2PbCl₂.NH₄Br and PbCl₂.2NH₄Br. Lead chloride forms with pyridine the substance PbCl₂.2C₅H₅N, which crystallises in needles, and rapidly loses pyridine in the air.