Chemical elements
  Chromium
    Isotopes
    Energy
    Occurrence
    Preparation
    Physical Properties
    Chemical Properties
    Alloys
    Amalgams
    Compounds
      Chromous Fluoride
      Chromic Fluoride
      Chromyl Fluoride
      Chromous Chloride
      Chromic Chloride
      Oxychlorides
      Chromyl Chloride
      Trichromyl Chloride
      Chromium Chlorate
      Chromium Perchlorate
      Chromous Bromide
      Chromic Bromide
      Complex Halogen-halides
      Chromous Iodide
      Chromic Iodide
      Chromium Iodate
      Chromous Oxide
      Chromo-chromic Oxides
      Chromic Oxide
      Chromic Hydroxide
      Barium Chromite
      Cadmium Chromite
      Calcium Chromite
      Cobalt Chromite
      Cupric Chromite
      Cuprous Chromite
      Ferrous Chromite
      Lithium Chromite
      Magnesium Chromite
      Manganese Chromite
      Zinc Chromite
      Chromium Dioxide
      Chromium Trioxide
      Aluminium Chromate
      Ammonium Chromate
      Ammonium Lithium Chromate
      Ammonium Potassium Chromate
      Ammonium Sodium Chromate
      Ammonium Dichromate
      Ammonium Fluochromate
      Barium Chromate
      Barium Dichromate
      Barium Potassium Trichromate
      Beryllium Chromate
      Bismuth Chromate
      Bismuth Potassium Chromates
      Cadmium Chromate
      Cadmium Dichromate
      Cadmium Trichromate
      Caesium Chromate
      Caesium Dichromate
      Calcium Chromate
      Calcium Dichromate
      Cobalt Chromate
      Cobalt Dichromate
      Copper Chromates
      Cupric Dichromate
      Gold Chromates
      Iron Chromates
      Ferric Chromate
      Ferric Ammonium Chromate
      Lead Chromate
      Basic Lead Chromates
      Lead Dichromate
      Lithium Chromate
      Lithium Chlorochromate
      Magnesium Chromates
      Manganese Chromates
      Mercuric Chromate
      Mercuric Dichromate
      Nickel Chromate
      Nickel Dichromate
      Potassium Chromate
      Potassium Dichromate
      Potassium Trichromate
      Potassium Tetrachromate
      Potassium Fluochromate
      Potassium Chlorochromate
      Rubidium Dichromate
      Silver Chromate
      Silver Dichromate
      Sodium Chromate
      Sodium Dichromate
      Sodium Trichromate
      Sodium Chlorochromate
      Stannic Chromate
      Strontium Chromate
      Strontium Dichromate
      Strontium Trichromate
      Thallous Chromate
      Thallic Chromate
      Thallous Dichromate
      Thallous Trichromate
      Thallous Chlorochromate
      Uranyl Chromate
      Zinc Chromate
      Zinc Dichromate
      Zinc Trichromate
      Perchromic Acid
      Chromium Tetroxide Triammine
      Chromous Sulphide
      Chromium Tetrasulphide
      Chromic Sulphide
      Sodium Thiochromite
      Potassium Thiodichromite
      Chromic Sulphite
      Chromous Sulphate
      Chromic Sulphate
      Lithium Chromic Sulphate
      Sodium Chromic Sulphates
      Potassium Chromic Sulphates
      Potassium Chromium Alum
      Ammonium Chromium Alum
      Hydrazine Chromium Alum
      Sulphochromic Acid
      Chromous Selenide
      Chromic Selenide
      Chromic Selenite
      Chromium Nitrides
      Chromium Azide
      Chromic Nitrate
      Chromium Monophosphide
      Chromium Sesquiphosphide
      Chromic Hypophosphite
      Chromous Orthophosphate
      Chromic Orthophosphates
      Chromous Metaphosphate
      Chromic Metaphosphate
      Chromic Pyrophosphate
      Ammonium Chromi-pyrophosphate
      Potassium Chromi-pyrophosphate
      Sodium Chromi-pyrophosphate
      Chromous Thiophosphite
      Chromous Thiopyrophosphite
      Chromous Thiopyrophosphate
      Chromous Arsenide
      Chromium Sesqui-arsenide
      Chromic Arsenite
      Chromic Arsenate
      Chromium Pyroarsenate
      Chromic Thioarsenite
      Chromium Chlorantimonate
      Chromium Orthoantimonichloride
      Tetrachromium Carbide
      Pentachromium Dicarbide
      Tetrachromium Dicarbide
      Chromium Tungsten Carbide
      Chromous Carbonate
      Ammonium Chromous Carbonate
      Potassium Chromous Carbonate
      Sodium Chromous Carbonate
      Chromic Carbonates
      Chromium Thiocarbonate
      Chromous Cyanide
      Chromic Cyanide
      Potassium Chromocyanide
      Hydrogen Chromicyanide
      Ammonium Chromicyanide
      Lithium Chromicyanide
      Sodium Chromicyanide
      Potassium Chromicyanide
      Cobaltous Chromicyanide
      Cupric Chromicyanide
      Lead Chromicyanide
      Manganous Chromicyanide
      Mercury Chromicyanide
      Nickel Chromicyanide
      Silver Chromicyanide
      Zinc Chromicyanide
      Chromous Thiocyanate
      Chromic Thiocyanate
      Chromithiocyanic Acid
      Ammonium Chromithiocyanate
      Sodium Chromithiocyanate
      Potassium Chromithiocyanate
      Barium Chromithiocyanate
      Silver Chromithiocyanate
      Lead Chromithiocyanate
      Chromium Ferrocyanide
      Trichromium Silicide
      Dichromium Silicide
      Trichromium Disilicide
      Chromium Disilicide
      Chromium Aluminium Silicide
      Sodium Chromisilicates
      Chromium Silicofluoride
      Chromium Boride
      Trichromium Diboride
      Chromous Borate
      Chromic Borate
    Detection
    Estimation
    PDB 1huo-9icc

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 + 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 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.


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