Lead Sulphate, PbSO₄

Lead Sulphate, PbSO₄, is found naturally as the mineral anglesite or lead vitriol, which often occurs in large transparent crystals, isomorphous with those of celestine and heavy spar. The salt may be prepared artificially by precipitating a lead salt solution with sulphuric acid or a soluble sulphate. Thus obtained it is a white, microcrystalline powder. It may be obtained in a more distinctly crystalline form by causing it to be produced slowly; thus if the end of a platinum wire, covered with fused lead chloride, is allowed to dip into a layer of water which has been poured upon the surface of a saturated solution of potassium sulphate, crystals of lead sulphate are gradually formed. Lead sulphate is also formed by the interaction of lead dioxide and sulphur dioxide.

For commercial purposes lead sulphate is prepared as follows: Granulated lead is dissolved in acetic acid in steam-heated vats. The liquor is then poured off into a large wooden tank, and lead sulphate precipitated by the addition of sulphuric acid. After standing, the clear liquid, consisting of acetic acid, is pumped back into the vats to act upon a further supply of lead, whilst the precipitated lead sulphate is washed and dried.

Natural lead sulphate has a density of 6.30 to 6.39; the density of the synthetic salt is 6.17. The melting-point of the pure salt appears to be above 1100° C., but is difficult to determine on account of the loss of sulphur trioxide at this high temperature. There is, however, a transition point at 850° C. The molecular heat of formation of lead sulphate from its elements is 216,200 calories; the following are the heats of formation in other ways:

Pb + O₂ + SO₂ = PbSO₄ + 145,100 calories.
PbO + H₂SO₄aq. = PbSO₄(ppd.) + aq. + 23,400 calories.

Lead sulphate is not quite insoluble in water; at atmospheric temperature 1 part of the salt dissolves in about 22,000 parts of water. Dibbits found that 1 litre of water dissolves 0.038 gram PbSO₄; but Sehnal found 0.0824 gram. The electric conductivities of aqueous solutions of lead sulphate at different temperatures (k_t) have been measured by Kohlrausch, and the corresponding solubilities deduced therefrom.

Temperature ° C.	0.37	3.48	16.98	18.00	33.23
kt×106	16.73	19.57	31.82	32.4	48.5
Millimols. per litre	0.110	0.117	0.134	0.134	0.144

Applying the same principle, but reckoning the salt to be 82 per cent, ionised, Bottger has calculated the solubility of lead sulphate in water at 19.95° C. to be 4.21×10^-2 gram per litre, a result which agrees closely with that of Kohlrausch. It has been pointed out, however, by Pleissner, that not only does the question of degree of ionisation enter into the calculation of solubility from conductivity data, but also that of hydrolysis. These authors have estimated the state of an aqueous solution of lead sulphate at 18° C. to be as follows:

[PbSO₄] = 0.027 millimols. per litre
[SO₄''] = 0.099 millimols. per litre
[Pb^••] = 0.062 millimols. per litre
[Pb(OH)^•] = 0.037 millimols. per litre
[H^•] = 0.037 millimols. per litre
Solubility product: [Pb^••]×[SO₄''] = 61×10^-10.

The solubility of lead sulphate is considerably less in dilute sulphuric acid than in water, but beyond a certain strength of acid the solubility again increases owing to the formation of complex ions. This is shown by the following figures:

Percentage H2SO4	0	1	29	64	86	99
Grams PbS04 per litre	0.046	0.027	0.012	0.046	0.197	0.72

The solubilities of lead sulphite in hydrochloric acid of different concentrations at atmospheric temperature are as follow:

Grams HCl per 100 grams solution.	10.6	16.3	22.0	27.5	31.6
Grams PbSO4 per 100 grams solvent	0.14	0.35	0.95	2.11	2.86

and in nitric acid, also at atmospheric temperature:

Grams HNO3 per 100 grams solution.	11.6	17.5	34.0	60.0
Grams PbS04 per 100 grams solvent	0.33	0.59	0.78	1.01

Since ammonium acetate solution is used as a solvent for lead sulphate in qualitative analysis, the extent of its solvent action is of practical importance.

The following values have been obtained:

Temperature 25° C.
Grams NH4C2H3O2 per litre	7.98	15.96	31.92
Grams PbSO4 per litre	0.636	1.38	3.02

Temperature 100° C.
Grams NH4C2H3O2 per litre.	280	320	370	450
Grams PbSO4 per litre	71.2	98.8	105.8	111.0

It has been shown by Fox that with solutions of ammonium acetate up to a concentration of 3N the solid phase consists of lead sulphate, but with more concentrated solutions this phase consisted of crystals of (NH₄)₂Pb(SO₄)₂.

When lead sulphate dissolves in sodium acetate solution the solid phase consists of PbSO₄, but when it dissolves in potassium acetate there is double decomposition with the formation of lead acetate and potassium sulphate, the latter uniting with the lead sulphate to form the complex sulphate K₂Pb(SO₄)₂ as the solid phase.

Numerous other salts also increase the solubility of lead sulphate in water, prominent among which are ammonium nitrate, citrate, and tartrate.

Ethyl alcohol diminishes the solubility of lead sulphate, which is practically insoluble in pure alcohol; hence alcohol is added in order completely to separate lead as sulphate in qualitative and quantitative analysis.

Lead sulphate reacts with sodium hydrogen carbonate according to the equation:

PbSO₄ + 2NaHCO₃ ⇔ PbCO₃ + Na₂SO₄ + CO₂ + H₂O;

whence it follows that carbon dioxide under pressure transforms lead carbonate suspended in sodium sulphate solution into lead sulphate.