Basic Lead Chromates

The basic salt, Pb₂CrO₅, PbCrO₄.PbO, or 2PbO.CrO₃, is formed as a red powder by the action of cold caustic soda solution on the normal chromate. It is also produced by mixing together lead chromate and oxide in presence of water, and is known commercially by various names - chrome red, orange chrome, Derby red, Chinese red - according to its depth of colour. It is converted by acetic acid into the normal salt, and the two salts are in equilibrium in contact with an aqueous solution of 2×10^-5 molecules CrO₃ per litre at 25° C. Mixtures of the two salts produce tints intermediate between yellow and red; such mixtures are known commercially.

The basic salt, Pb₃Cr₂O₉, occurs in dark red hexagonal prisms, of density 5.75, as the mineral phoenicite or phcenicochroite. This compound may be prepared artificially by allowing solutions of lead nitrate and potassium chromate to mix by diffusion, by exposing a solution of lead chromate in caustic potash to the air for several months, or by immersing galena in potassium dichromate solution for six months. This last process, which was carried out by Meunier, probably nearly reproduces the conditions by which the mineral is naturally produced.

Double salts of lead chromate with alkali chromates have been obtained. The potassium salt, K₂CrO₄.PbCrO₄ or K₂Pb(CrO₄)₂, is formed as a yellow amorphous precipitate when 50 c.c. of a saturated solution of potassium chromate is mixed with 10 c.c. of saturated lead acetate solution. The ammonium salt, (NH₄)₂Pb(CrO₄)₂, is prepared similarly, and both salts are decomposed into their components by water.