Lead Chloride, PbCl₂

Lead Chloride, PbCl₂, occurs as the somewhat rare mineral cotunnite, which is of volcanic origin, and has been found in the crater and lava of Vesuvius. It was known to Dioscorides that yellow oxide of lead turns white when placed in warm water with common salt.

Lead chloride may be obtained in various ways. Chlorine slowly attacks lead, forming the chloride; the metal dissolves in dilute hydrochloric acid in presence of air, but in the strong acid, with evolution of hydrogen, to form the chloride; also the oxide, hydroxide, or carbonate may be dissolved in hydrochloric acid; but a more usual way of preparing this salt is to precipitate a moderately concentrated solution of the nitrate or acetate with hydrochloric acid or a soluble chloride. The chloride may then be purified by recrystallisation from hot water.

Lead chloride crystallises anhydrous in white, silky, rhombic needles, having a density of 5.802. Numerous observers have determined the melting-point of this salt, which lies between 485° C. and 512° C. Its boiling-point is 956° C., and its vapour has a density of 9.64 (air = 1) or 138.8 (H = 1) at 1070° C., which corresponds to the molecular formula PbCl₂. In the fused state molecular association exists. The specific heat of the solid salt between ordinary temperature and 100° C. was found by Regnault to be 0.0664, and its molecular heat of formation is 82,800 or 83,900 calories, or, according to Koref and Braune, 85,570 calories.

Lead chloride is sparingly soluble in cold and considerably more soluble in hot water. The solubility of lead chloride in water is as follows:

Temp. ° C	0°	15°	25°	35°	55°	80°	95°
Grams of PbCl2 in 100 grams H2O	0.6728	0.9090	1.0842	1.3244	1.8263	2.6224	3.1654

These data have a twofold bearing in qualitative analysis; for thence it follows that lead is but partially precipitated from solution by dilute hydrochloric acid, but that this substance can be identified by its crystallisation from boiling water, from which it separates in white, shining needles. This salt is less soluble in dilute solutions of hydrochloric acid and other chlorides, but more soluble in concentrated hydrochloric acid solution than in water. Thus its solubility varies in the following manner with the strength of the hydrochloric acid:

Grams HCl per litre Grams PbCl2 per litre	0	1	10	100	200	250	300	400
at 0° C.	5.83	3.6	1.2	1.2	5.2	10.5	17.5	40.0
at 25° C	10.79	7.6	1.8	-	-	-	-	-

In consequence of these facts a concentrated hydrochloric acid solution of lead chloride is precipitated by water, and an aqueous solution by a little hydrochloric acid. The ionic theory affords a satisfactory explanation of these solubilities. First, the addition of a little hydrochloric acid to the saturated aqueous solution increases the concentration of the chloride ions so that the solubility product of lead chloride is exceeded and precipitation takes place; secondly, the presence of much hydrochloric acid promotes the formation of complex ions, so that more lead chloride dissolves. The fact that a concentrated hydrochloric acid solution is not precipitated by hydrogen sulphide is attributed to the lead being present in the form of a complex ion.

The state of ionisation of lead chloride in aqueous solution has been investigated by Noyes, Fernan, and von Ende. Contrary to the opinions of the two former observers, the lafter has concluded that this salt is ionised in two stages, thus:

PbCl₂ ⇔ PbCl^• + Cl'

PbCl^• ⇔ Pb^•• + Cl'

and has estimated the condition of a saturated solution at 25.2° C., which is 0.0388 normal, to be as follows:

Undissociated PbCl₂ - 6.2 per cent.

Pb^•• ions – 50.1 per cent.

PbCl^• ions – 43.7 per cent.

The following results have been obtained by Bottger for the ionic concentration in equivalents per litre of a saturated solution of lead chloride at 19.95° C.: Pb^•• 0.0168; PbCl^• 0.0109; Cl' 0.0444; and undissociated PbCl₂ 0.00127.

Lead chloride solution is hydrolysed to a very small extent; Ley found, by its power of inverting cane-sugar, that a 0.01 normal solution of this salt is hydrolysed to the extent of 0.6 per cent, at 100° C.

Lead chloride may be distilled in an atmosphere of carbon dioxide, but it is reduced by hydrogen below its melting-point, and decomposed by water-vapour at 110° C. and above into a basic chloride and hydrogen chloride gas. The reaction between lead chloride and anhydrous sulphuric acid,

PbCl₂ + H₂S04 ⇔ PbS04 + 2HCl (gas),

has been studied by Schreinermakers, who finds the pressure of hydrogen chloride gas evolved reaches 1 atmosphere at 27.2° C.