Chemical elements
    Physical Properties
    Chemical Properties
      Lead Tetramethyl
      Lead Tetraethyl
      Lead Tetraphenyl
      Lead Ethoxide
      Lead Fluoride
      Lead Tetrafluoride
      Hydrofluoplumbic Acid
      Lead Chloride
      Lead Chloride Double Salts
      Basic Lead Chlorides
      Lead Tetrachloride
      Ammonium Plumbichloride
      Lead Chlorite
      Lead Chlorate
      Lead Perchlorate
      Lead Dibromide
      Double Salts of Lead Bromide
      Basic Lead Bromides
      Lead Bromate
      Lead Iodide
      Lead Iodide Complex Salts
      Basic Lead Iodides
      Lead Tetra-iodide
      Lead Iodate
      Lead Periodates
      Lead Suboxide
      Lead Monoxide
      Lead Hydroxides
      Lead Dioxide
      Plumbic Acids
      Hexahydroxyplumbic Acid
      Colloidal Plumbic Acid
      Potassium Plumbate
      Lead Plumbate
      Calcium Orthoplumbate
      Lead Orthoplumbate
      Red Lead
      Metaplumbic Acid
      Calcium Metaplumbate
      Lead Metaplumbate
      Basic Lead Plumbate
      Lead Sulphide
      Lead Sulphohalides
      Lead Polysulphide
      Lead Sulphite
      Lead Sulphates
      Lead Sulphate
      Basic Lead Sulphates
      Lead Hydrogen Sulphate
      Plumbic Sulphate
      Lead Persulphate
      Lead Thiosulphate
      Lead Dithionate
      Lead Selenide
      Lead Selenite
      Lead Selenate
      Lead Telluride
      Lead Tellurite
      Lead Azide
      Lead Azoimide
      Lead Hydrazoate
      Lead Imide
      Lead Hyponitrite
      Lead Nitrites
      Lead Nitrate
      Lead saltpetre
      Basic Lead Nitrates
      Lead Hypophosphite
      Lead Phosphite
      Lead Orthophosphate
      Lead Monohydrogen Phosphate
      Lead Dihydrogen Phosphate
      Lead Pyrophosphate
      Lead Metaphosphate
      Lead Arsenite
      Lead Orthoarsenate
      Lead Hydrogen Arsenate
      Lead Pyroarsenate
      Lead Antimonate
      Lead Carbonate
      White Lead
      Lead Formate
      Lead Acetate
      Sugar of Lead
      Complex Lead Acetates
      Plumbic Acetate
      Lead Tetra-acetate
      Lead Oxalate
      Lead Tartrate
      Lead Silicates
      Lead Borates
      Normal Lead Chromate
      Lead Dichromate
      Basic Lead Chromate
      Lead Molybdate
      Lead Tungstate
      Lead Metatungstate
      Lead Diuranate
      Lead Peruranate
    PDB 1afv-3qjk

Normal Lead Chromate, PbCrO4

Normal Lead Chromate, PbCrO4, occurs naturally as crocoite in Siberia, Hungary, Brazil, and other parts; the mineral crystallises in yellow, 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 Pb2CrO5 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 name of chrome yellow, Paris yellow, or Leipzig yellow; and consequently it is important to realise the best conditions for its preparation. It has been shown by Free 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. It is recognised that the admixture of lead sulphate to the extent of about 10 per cent, produces a lighter shade; 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 gram per litre according to Kohlrausch, but von Hevesy and Paneth obtained the value 1.2×10-5 gram per litre by employing radium-D as a radioactive indicator, i.e. by mixing with the lead salt radium-D, which is 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 by Beck and Stegmtiller, who find that the solubility in dilute solutions is nearly proportional to the hydrogen ion concentration, but in more concentrated solutions to the square of this concentration. This is on account of the formation of dichromate ions, thus:

2HCrO4' ⇔ Cr2O7' + H2O,

which results in more lead chromate being dissolved.

The following equilibrium constants have been calculated:

= 3.7×10-7; = 3.4×10-13;

= 1.0×10-3; = 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 + H2OPbO.PbCrO4 + 2NaHCO3 + Na2CrO4;

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

PbCrO4 + 2NaHCO3PbCO3 + Na2CrO4 + H2O + CO2.

Boiling with potassium nitrate solution causes lead chromate to pass into a 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.

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