Lead(II) chromate

• Chromate
• Chrome yellow
• Paris Yellow
• King Yellow
• Leipzig Yellow
• Lemon

Orange solid

Fixed

6.3 g · cm -3

844 ° C

Almost insoluble in water

Risk

Template: Infobox chemical / molecular formula search is not possible

Lead (II) chromate, PbCrO4, ( colloquially known as chromate, chrome yellow, yellow, or royal yellow Paris ) is a lead salt of chromic acid. Lead (II) chromate is in the Colour Index under C. I. Pigment Yellow 34 listed.

History

Lead (II) chromate in 1797 discovered by Vauquelin in Paris, and therefore is called Parisian yellow. From 1818 onwards, it is manufactured as a pigment and is due to its extremely high chrominance soon a fashionable color ( Neugelb ), especially as a post yellow.

Since the development of the first yellow azo pigments (CI Pigment Yellow 1 and 3) in 1909 by Hoechst was the toxic pigment in artists' colors range quickly into disuse. For use in the coatings sector, however, this replacement pigments are unsuitable because they have a low opacity and a very low resistance to weathering.

In industrial application, lead (II) chromate held in Europe until the 1980s, together with the also now outlawed pigments cadmium sulfide ( CI Pigment Yellow 35 and 37) and molybdate (CI Pigment Red 104). These were among the standard pigments in this color range, but were outlawed due to the toxicological properties of the European paint industry. This established, inter alia, a strong increase in revenue bismuth vanadate pigment (CI Pigment Yellow 184 ) and the development of modified organic pigments with higher hiding power. Today, lead (II ) chromate virtually disappeared from the industrial use in Europe, but is used in other continents on.

Often used, for example, Vincent van Gogh, the expensive Cadmium Yellow could not afford.

Occurrence

In nature, lead chromate is used as mineral Krokoit, with massive monoclinic crystal structure before. This ore served for a long time for chromium and lead.

Extraction and properties

Lead (II) chromate is an orange-yellow, water-insoluble powder, which consists of a solution of a water-soluble lead compound (eg, lead acetate or lead nitrate ), which was treated with potassium or ammonium chromate, separates.

Industrially, the representation is used from lead acetate on sodium dichromate. Used to precipitate the lead salt solution is not acidic, but neutral or slightly alkaline chromate solution, the result is basic lead chromate, chrome red, as mineral Phönikochroit.

When used as a pigment for paints and emulsion paints, lead (II) chromate shows good coverage and high weather resistance and high chrominance ( chroma, and chroma). But lead (II) chromate is changing but slowly to other compounds to where trivalent chromium is formed. Thus the appearance of yellow to dull brown changes. As a result, the old paintings (for example, by Vincent van Gogh) get darker over time. This reaction is initiated mainly by ultraviolet light and thus by solar radiation in motion. Even in the diffuse light in museums, this process can not be completely stopped. An important role is also played by the varnish used, since there connections are included which contain the elements barium and sulfur and support the color decay.

Chromate crystallizes in the monazite structure, ie in the monoclinic crystal system, space group P21 / n with lattice parameters a = 712.7 pm, b = 743.8 pm, c = 679.9 pm, and β = 102.43 °. In the unit cell there are four formula units. Also, an orthorhombic modification ( a = 867 pm, b = 559 pm, c = 713 pm ) has been described.

Use

Lead (II) chromate has long been used as a pigment in artists' colors range, as it was equally low as brilliant and highly authentic. Due to its toxicity, but it was replaced by other substances such as bismuth vanadate. In Europe, the compound has in this area only meaning as a pigment for restoration of historic art or buildings.

In industrial applications as a pigment in paints and emulsion paints, lead (II ) chromate used still in great quantities, but not in Europe. It is used here also as a solid solution in combination with lead sulphate and / or lead molybdate (such as CI Pigment Red 104). The extraordinary combination of low price, high chroma, good hiding power, high weather fastness (it was therefore often used as antirust paint ) indicates that lead (II) chromate is less high safety awareness than in Europe continue to be used in regions. In Europe, the replacement of lead (II ) chromate Although largely complete, but this at the expense of price and / or the color properties.

Other inorganic pigments such as iron oxide yellow (CI Pigment Yellow 42 ) are indeed cheap, but much duller in color. High-quality inorganic pigments such as bismuth vanadate show a pure hue and good weather resistance, but are significantly more expensive than lead (II ) chromate. Organic pigments such as Brilliant Yellow ( for example, CI Pigment Yellow 74, 151) show very pure shades, but are also more expensive and have even optimized variants in a much poorer opacity. Regarding the weather fastness there are dependent on the chemical structure types with higher and niederigerem level. However, all have an inferior level as the inorganic lead (II) chromate.

Trivia

• The color name chrome yellow is the former name of the today known as Daffodil Yellow hue RAL 1007 in RAL color system.
• The above described reaction with potassium chromate is used, inter alia as proof of lead.