Lead Chromate, PbCrO₄

Normal Lead Chromate, PbCrO₄, occurs naturally as crocoite in Siberia, Brazil, and other parts; the mineral crystallises in hyacinth-red, translucent, monoclinic prisms, and has a density of about 6.0. It was prepared artificially by Bourgeois by heating precipitated lead chromate with dilute nitric acid in sealed tubes at 130° C. The salt is prepared artificially as a bright yellow precipitate by mixing solutions of a lead salt and potassium chromate or dichromate. Probably the precipitate is at first amorphous, but it gradually becomes crystalline. Crystals of lead chromate are also obtained by exposing to the air a solution of the precipitated chromate in caustic soda, by fusing together lead chloride and potassium chromate, by allowing solutions of lead acetate and potassium chromate to mix slowly by diffusion, and by electrolysing a dilute solution of chromic acid with lead electrodes.

Precipitated lead chromate tends to change colour during filtration, becoming orange. This is due to the formation of the red basic salt, Pb₂CrO₂ or PbCrO₄.PbO, by hydrolysis, which is the more likely to take place when the salt is prepared from basic lead acetate.

Lead chromate is valued as a pigment under the names of chrome yellow, Paris yellow, and Leipzig yellow, and consequently it is important to realise the best conditions for its preparation. It has been shown that the rapid mixing of cold, concentrated solutions of the reacting salts under conditions which prevent the formation of basic salt, produces a crystalline chrome yellow of clear colour and good covering power. Commercial chrome yellow frequently contains lead sulphate, and a pigment known as Cologne yellow is prepared by heating lead sulphate with a solution of potassium dichromate. In some cases potassium chromate is treated with sulphuric acid, or the dichromate is mixed with a sulphate, such as Glauber salt, alum, etc., and the mixture employed to precipitate a suitable quantity of sugar of lead solution. It is recognised that the admixture of lead sulphate to the extent of about 10 per cent, produces a lighter tint; and this is due not only to a physical cause, but to the fact that the presence of a soluble lead salt reduces or prevents hydrolysis, which by producing the basic chromate would cause darkening of colour.

The solubility of lead chromate in water is exceedingly small; indeed, the chromate is probably the least soluble salt of lead. It is practically insoluble also in dilute acetic acid; consequently chromate ions in presence of acetic acid constitute a very delicate test for lead ions in solution. The solubility of lead chromate in water at 25° C. is about 10^-4 grams per litre according to Kohlrausch, but von Hevesy and Paneth obtained the value 1.2×10^-5 grams per litre by employing radium-D as a radioactive indicator, that is, by mixing with the lead salt radium-D, which is chemically indistinguishable from lead, and estimating the radioactivity of the solution, since the original ratio between the two salts is maintained in solution.

Lead chromate readily dissolves in dilute hydrochloric and nitric acids. The solubility of this salt in dilute hydrochloric acid, and the equilibrium between chromate and dichromate in solution, have been studied. It is found that the solubility in dilute solutions is nearly proportional to the hydrogen ion concentration, but in more concentrated solutions to the square of this value; this is attributed to the formation of dichromate ions thus:

2HCrO₄' ⇔ Cr₂O₇ + H₂O,

which results in more lead chromate being dissolved.

The following equilibrium constants have been calculated:

[H^•].[CrO₄'']/[HCrO₄'] = 3.7×10^-7; [H^•]².[CrO₄'']²/[Cr₂O₇''] = 3.4×10^-13;

[H^•].[Cr₂O₇']/[HCr₂O₇] = 1.0×10^-3; [HCrO₄']²/[Cr₂O₇''] = 2.5.

Lead chromate also dissolves in sodium hydroxide solution, being decomposed into a mixture of sodium chromate and plumbite. A solution of sodium carbonate reacts with lead chromate thus:

2PbCrO₄ + 2Na₂CO₃ + H₂O ⇔ PbO.PbCrO₄ + 2NaHCO₃ + Na₂CrO₄;

in presence of excess of sodium hydrogen carbonate, however, the following reaction takes place:

PbCrO₄ + 2NaHCO₃ ⇔ PbCO₃ + Na₂CrO₄ + H₂O + CO₂.

Boiling with potassium nitrate solution causes lead chromate to pass into the colloidal state.

Besides its use as a pigment, lead chromate is employed for dyeing or printing on calico. The process, which is a mechanical one, is carried out by passing the fabric first through a solution of lead salt and then through a solution of chromate. The material may, however, be agitated with precipitated lead chromate suspended in water, alcohol, or benzene. When the two latter media are employed the colour is not so deep as when water is used.

Lead chromate fuses when strongly heated, forming a brown liquid, which on cooling solidifies to a light brown crystalline mass. At a high temperature the salt evolves oxygen, and, on account of its oxidising property, it is employed in organic analysis to aid the combustion of substances containing the elements chlorine and sulphur, which are retained by the lead. A mixture of asbestos and lead chromate may be used for absorption of sulphur compounds.