Lead Acetate, Pb(C₂H₃O₂)₂

Lead Acetate (Sugar of Lead), Pb(C₂H₃O₂)₂.3H₂O, has been known for four or five hundred years. It is prepared by dissolving lead oxide in acetic acid, and is formed slowly by the action of acetic acid and air upon lead. The trihydrate forms monoclinic crystals, isomorphous with those of barium and zinc acetates; it has a density of 2.50. The hydrated salt easily loses water when gently heated, melts at 75° C., and at 100° C. loses acetic acid as well as water, giving rise to a basic salt. The following are the aqueous vapour pressures of this salt at different temperatures:

Temperature ° C.	12.5	15.8	18.3	20.9	21.3	30.1
Vapour Pressure, mm. Hg	2.92	4.29	5.32	6.81	7.14	14.91

The anhydrous salt has a density of 3.251, melts at 280° C., and when strongly heated evolves first acetic acid vapour, then carbon dioxide and acetone, and finally leaves a residue of pyrophoric lead.

If paper is soaked in lead acetate solution and dried, it smoulders like tinder when ignited. This may be due to the formation and combustion of pyrophoric lead.

The molecular heat of formation of the anhydrous salt is 232,600 calories.

Lead acetate is readily soluble in water, and much more soluble in hot than in cold water; 100 grams of water dissolve 50 grams of the salt at 25° C., 100 grams at 40° C., and 200 grams at 100° C.

The state of lead acetate in aqueous solution has been the subject of considerable investigation, as well as the cause of the solubility of lead sulphate in ammonium and sodium acetate solutions.

So far as ionisation of lead acetate takes place, it probably occurs in two stages, as with other salts of lead and of other bivalent metals, thus:

Pb(C₂H₃O₂)₂ ⇔ Pb(C₂H₃O₂)^• + (C₂H₃O₂)'
Pb(C₂H₃O₂)^• ⇔ Pb^•• + (C₂H₃O₂)'.

Noyes and Whitcomb have found that lead acetate solution is, however, much less ionised than the acetates of barium, manganese, nickel, cobalt, zinc, and cadmium under equivalent, conditions; and that, assuming dissociation into three ions, its degree of ionisation is only about 22 per cent, in decinormal solution.

This small degree of ionisation appears to be connected with the spontaneous formation of complex anions, which occurs also with other acetates, and to a less extent with chlorides, sulphates, and nitrates.

The addition of an alkali acetate to lead acetate solution, by increasing the concentration of the acetate ions, increases the concentration of these complex anions; and this fact, as will appear, affords an explanation of the solubility of lead sulphate in acetate solutions. The solubilities of lead sulphate in ammonium acetate solutions of different strengths at 25° C. have been estimated by Noyes and Whitcomb.

These same observers conclude that the lead is present in the acetate solution almost entirely in the non-ionised state, non-ionised lead acetate being formed by metathesis, thus:

PbSO₄ + 2NH₄^• + 2C₂H₃O₂' ⇔ Pb(C₂H₃O₂)₂ + 2NH₄^• + SO₄''.

The solubility of lead sulphate in concentrated solutions of sodium and potassium acetates has been examined by Fox, who concludes that metathesis takes place, according to the above equation; but that complex acetates are formed, such as PbK₂(C₂H₃O₂)₄, which probably would not exist in dilute solution; and that this salt, reacting with the potassium'sulphate in solution, would produce the sparingly soluble salt PbK₂(SO₄)₂, which has been observed to crystallise.

Hydrated lead acetate dissolves in alcohol; 100 grams of alcohol dissolve 3.3 grams of the salt at 25° C. and 100 grams at 100° C.; 100 grams of glycerol dissolve 20 grams of lead acetate at 15° C.

Numerous basic acetates have been described, but probably only two are definite compounds.

The salt Pb(C₂H₃O₂)OH, or Pb(C₂H₃O₂)₂.Pb(OH)₂, is formed when lead oxide is treated with excess of lead acetate solution, by the partial decomposition of the neutral acetate solution with ammonia or alkalis, or by dissolving the salt Pb(C₂H₃O₂)₂.2Pb(OH)₂ in a solution of the normal acetate.

The salt Pb(C₂H₃O₂)₂.2Pb(OH)₂ is formed by treating lead acetate solution with excess of lead oxide, or by pouring a solution of lead acetate into excess of ammonia.

According to Sakabi, the only solid compounds that can exist in contact with lead acetate solutions at 25° C. are: Pb(C₂H₃O₂)₂.3H₂O, Pb(C₂H₃O₂)₂.Pb(OH)₂, and Pb(C₂H₃O₂)₂.2Pb(OH)₂.