Lead Nitrate, Pb(NO₃)₂

Lead Nitrate, Pb(NO₃)₂, or lead saltpetre, has long been known, and is mentioned in the Alchymia of Libavius by the name calx plumb. dulcis. It is obtained by dissolving lead, lead oxide, or lead carbonate in warm, dilute nitric acid, and crystallising the solution; commercially, lead scale or litharge is used for this purpose. The salt separates from aqueous solution in regular octahedra, in combinations of these forms with the cube, or in dodekahedra. Monoclinic crystals of lead nitrate have also been observed. According to Hauer, the form and transparency of the crystals depends upon the rate of cooling and degree of acidity of the solution. It was pointed out by Retgers that crystals separated from an aqueous solution are porcelain-like, but that from a solution acidified by nitric acid they are clear; the explanation being that in the former case they are slightly basic owing to hydrolysis, whilst in the latter case they are pure.

Lead nitrate has a density of 4.531 at 24° C.; its refractive index for sodium light at 20° C. is 1.7820; its molecular heat of formation from its elements is 105,500 calories. Lead nitrate decrepitates when heated, detonates on red-hot charcoal, and deflagrates when rubbed with sulphur. The action of heat upon lead nitrate has been investigated by Baekeland, who finds that when the salt is heated to 35.7° C. in an evacuated and sealed glass tube, brown fumes are evolved, but are completely reabsorbed when the tube is cooled, according to the reaction

Pb(NO₃)₂ ⇔ PbO + O + 2NO₂.

The connection between temperature and dissociation pressure was found to be as follows:

Temperature ° C.	223	230	250	274	296	357	448
Pressure mm. of mercury	6.2	6.9	11.8	32.6	78.4	514.0	1180.0

If, however, the salt is kept at 357° C. its power of evolving gas so as to regenerate the pressure after evacuation is much diminished, until the pressure attainable falls from 514 mm. to 260 mm. The residue is now slightly yellow and is a basic lead nitrate of the composition 3Pb0.2N₂O₅ or 2Pb(NO₃)₂.PbO. After the salt has been left at the same temperature in communication with the vacuum pump for ten days its dissociation pressure falls to zero, whilst its composition becomes 3PbO.N₂O₅ or Pb(NO₃)₂.2PbO. This second basic salt loses all its nitrous fumes at a red heat, leaving a residue of oxide. So it is shown that lead nitrate is decomposed by heat in stages, thus:

3Pb(NO₃)₂ → 2Pb(NO₃)₂.PbO → Pb(NO₃)₂.2PbO → 3PbO.

The introduction of oxygen or nitrogen peroxide into the dissociation tube before the salt is heated, retards dissociation in accordance with the law of mass action. According to Colson, perfectly dried lead nitrate shows no sign of decomposition in a vacuum till 283° C. is reached. On account of its decomposition, the melting-point of lead nitrate is unknown. This salt dissolves readily in water; the following solubility table includes the results of Mulder, Kremers, Michel and Kraft, and Euler:

Temperature ° C.	Grams Pb(NO6)2 per 1000 grams of:
	Water		Solution
	Mulder	Kremers	Michel and Kraft.
0°	36.5	38.8	27.33
10°	44.4	48.3	31.6
17°	50.0	54.0	34.2
20°	52.3	56.5	35.2
25°	56.4	60.6	36.9
30°	60.7	66.0	38.8
40°	69.4	75.0	41.9
50°	78.7	85.0	45.0
60°	88.0	95.0	47.8
80°	107.6	115.0	52.7
100°	127.0	138.8	57.1
17°	Euler. 52.76		Euler. 34.54

The density of a saturated solution at 17° C. is 1.405 (Euler), and at 25.3° C. compared with water at the same temperature 1.458.

The solubility of lead nitrate in water is much diminished by the addition of nitric acid, so that its aqueous solution is precipitated by concentrated nitric acid. The following figures show the influence of nitric acid on this solubility at 25° C.:

Mols. HNO3 per litre	0	2.02	4.64	8.77	14.35
Mols. Pb(N03)2 per litre	1.63	0.536	0.187	0.042	0.0017

The densities of lead nitrate solutions of various strengths are as follow:

Per cent. Pb(N03)2	5	10	15	20	25	30	35
Relative densities at 15° C (water at 4°C. = 1)	1.045	1.095	1.147	1.204	1.268	1.336	-
Relative densities8 at 17.5° C. (water at 17.5° C. = 1)	1.040	1.092	1.144	1.200	1.263	1.333	1.409

The solution of lead nitrate in water is accompanied by lowering of temperature; the heat of solution of 1 molecule Pb(N03)₂ in 400 molecules of water is -7,600 calories, and of 1 molecule in 930-1860 molecules of water -8,200 calories.

From numerous estimations of depressions of freezing-point and conductivities of solutions of lead nitrate of various concentrations it is concluded that this salt is not appreciably hydrolysed at 25° C., but undergoes ionisation in two stages, thus:

1. Pb(NO₃)₂ → Pb(NO₃)^• + NO₃'.
2. Pb(NO₃)' → Pb^•• + NO₃'.

The extent of ionisation at 25.3° C. has been estimated by Lewis, with the following results:

Mol. Pb(NO3)2 per litre	1	0.4	0.25	0.05	0.01
Concentration Pb•• ions	0.0503	0.0408	0.0354	0.0168	0.0054
Percentage ionisation	5	10	14	34	54

Warm solutions of lead nitrate are appreciably hydrolysed. Thus Walker and Aston estimated, by measurement of the inversion of cane sugar, that a N/2 solution of this salt is hydrolysed to 0.15 per cent, at 60° C.; Long, however, found only 0.1 per cent, hydrolysis of a similar solution at 85° C.

The solubilities of lead nitrate in aqueous and absolute ethyl alcohol and in methyl alcohol are as follow:

Solvent	Grams Pb(NO3)2 per 1000 grams solvent at
	4° C.	8° C.	22° C.	40° C.	50° C.
Aqueous ethyl alcohol (density 0.9282)	4.96	5.82	8.77	12.8	14.9
Absolute ethyl alcohol	-	-	0.04 (20.5° C.)	-	-
Absolute methyl alcohol	-	-	1.37	-	-

Lead nitrate has an astringent taste. It is used in calico printing, as a mordant in dyeing, and for the manufacture of chrome yellow.