Lead Iodide, PbI₂

Lead Iodide, PbI₂, may be obtained by dissolving lead in hydriodic acid, or by precipitating a lead salt by means of a soluble iodide. When obtained in the latter way it is a bright yellow precipitate, which dissolves in much hot water and separates again on cooling in golden yellow six-sided plates, The density of lead iodide is 6.16; when the dry salt is heated it becomes reddish yellow, then bright red and brownish black, and finally melts to a reddish brown liquid at about 380° C. Various estimations of the melting-point are as follow: Ramsay and Eumorfopoulus, 373° C.; Ehrhardt, 375° C.; Carnelley, between 374° C. and 387° C., with a mean value of 383° C. It boils between 861° C. and 954° C., and may be sublimed unchanged in an atmosphere of carbon dioxide.

The molecular heat of formation of lead iodide from its elements is 39,800 calories; or, according to Koref and Braune, 41,850 calories.

Dry lead iodide is stable in the air; but the moist salt, under the influence of sunlight, yields lead carbonate and dioxide together with free iodine. When the salt is heated in the air it loses iodine and forms an oxyiodide.

Solubility curves of lead chloride, bromide, and iodide in water.

The solubility of lead iodide in water is as follows:

Temp. ° C	0	15	25	35	45	55	65	80	100
Grams PbI2 in 100 grams H2O	0.0442	0.0613	0.0764	0.1042	0.1453	0.1755	0.2183	0.3023	0.436

According to von Ende, the solubility at 25° C. is 0.00158 gram-molecule per litre. This salt is thus much less soluble in water than the bromide or chloride. The solubilities of the three salts are shown in Fig.

The solubilities of lead chloride, bromide, and iodide at 20° C. have been determined by Bottger by comparing the electric conductivities of their saturated solutions with the conductivity of pure water. The following results were obtained: PbCl₂ 9.61×10^-1, PbBr₂ 8.34×10^-1, PbI₂ 0.47×10^-1 grams per litre. Moreover, it was concluded that lead iodide is dissociated only into Pb^•• and 2I' ions, no PbI^•• ions being formed; the ion-concentration of this salt was 2.53×10^-3 equivalents per litre, and the solubility product 8.10×10^-9.

An aqueous solution of lead iodide is colourless the yellow colour being that of the solid salt, and not of Pb^•• or I' ions. This salt is more soluble in alkali iodide and acetate solutions than in pure water, owing to the formation of complex ions. Indeed, lead iodide may be dissolved in concentrated potassium iodide solution and reprecipitated by dilution.