Lead Hydroxides

Two forms of lead hydroxide are known: 2PbO.H₂O or Pb₂O(OH)₂, and 3PbO.H₂O or Pb₃O₂(OH)₂; the ortho-hydroxide, Pb(OH)₂, appears to be unknown in the solid state.

Pb₂O(OH)₂ is formed by the action of water containing dissolved oxygen on metallic lead; by precipitating lead acetate or other lead salt solution by alkali hydroxide or ammonia; and also by allowing a solution of lead oxide in potassium hydroxide to stand in the air, when the absorbed carbon dioxide displaces hydrated lead oxide from combination with alkali, so that Pb₂O(OH)₂ separates in quadratic crystals.

Pb₃O₂(OH)₂ is prepared from the corresponding basic acetate, Pb₃O₂(C₂H₃O₂)₂, by mixing its saturated aqueous solution with ammonia, and allowing the solution to stand at 25°-30° C., with careful exclusion of air. The hydroxide then separates in shining octahedra. A product of the same composition is obtained by precipitating a lead salt solution with caustic alkali, and drying the precipitate at 100° C., and also by precipitating a lead salt with baryta water at atmospheric temperature.

Lead hydroxide is distinctly soluble in water, to which it imparts an alkaline reaction. A saturated solution of Pb₃O₂(OH)₂ at 18° C. contains, according to Pleissner, 100.5 m.g. PbO per litre. Ruer has found, by the conductivity method, that the solubilities in water at 22° C. of the various forms of lead oxide, with the exception of Geuther's red oxide which is less soluble, lie between 1.00×10^-4 and 1.12×10^-4 gram-equivalents per litre; whilst Herz has calculated the solubility lead hydroxide at 25° C. to be 0.93×10^-4. At 130° C. lead hydroxide loses all its water, being converted into the oxide.

It has already been stated that lead monoxide is an amphoteric oxide; thus the hydroxide behaves both as a weak base and a weak acid.

It is possible that in a solution of lead hydroxide in water, which is alkaline in reaction and therefore contains hydroxyl ions, ionisation has proceeded in two stages, thus:

1. Pb(OH)₂ ⇔ Pb(OH)^• + OH'
2. Pb(OH)^• ⇔ Pb^•• + OH'.

From the work of Berl and Austerweil, however, it appears that only the first reaction is to be considered as taking place, and that ionisation has proceeded to the extent of about 28 per cent. The same authors have determined the solubility of lead oxide in sodium hydroxide solutions of different concentrations and temperatures, and conclude that when the alkali solution is below normal strength the sodium salt HPbOONa of the monobasic acid HPbOOH is produced in solution, but that with more concentrated alkali solutions the sodium salt Pb(ONa)₂ of the dibasic acid Pb(OH)₂ is formed to some extent. Thus, towards weak alkalis lead hydroxide behaves as if it were ionised thus:

Pb(OH)₂ ⇔ PbO₂H' + H^•;

and the dissociation constant of this acid at 18° C. is found to be 1.1×10^-12, which shows lead hydroxide to be a weaker acid than phenol.

Hantzsch, however, finds no evidence that hydroxides of the type M(OH)₂ can behave as dibasic acids, and regards their salts as always derived from acids of the formic acid type HMOOH. Moreover, he has shown that the hydroxides of lead, tin, and germanium are feeble acids whose strengths increase in the order in which the metals are named.

Lead monoxide dissolves in lime and baryta water, forming yellow solutions, and also in strong solutions of calcium and strontium chloride. Lead hydroxide combines with polyhydric alcohols and hydroxy-carboxylic acids to form complex compounds in solution in which the lead is very little ionised. Alkali plumbite solution is used as a mordant in cotton dyeing.