Lead (II) chromate
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| General | ||||||||||||||||
| Surname | Lead (II) chromate | |||||||||||||||
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| Molecular formula | PbCrO 4 | |||||||||||||||
| Brief description |
yellow solid |
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| properties | ||||||||||||||||
| Molar mass | 323.18 g mol −1 | |||||||||||||||
| Physical state |
firmly |
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| density |
6.3 g cm −3 |
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| Melting point |
844 ° C |
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| solubility |
almost insoluble in water |
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| safety instructions | ||||||||||||||||
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| Authorization procedure under REACH |
of particular concern : carcinogenic, toxic for reproduction ( CMR ); subject to approval |
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| MAK |
Switzerland: 5 μg m −3 (calculated as chromium) |
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| As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions . | ||||||||||||||||
Lead (II) chromate , PbCrO 4 , (also known colloquially as lead chromate , chrome yellow , Parisian yellow or royal yellow ) is a lead salt of chromic acid . Lead (II) chromate is listed in the Color Index under CI Pigment Yellow 34 .
history
Lead (II) chromate was discovered by Vauquelin in Paris in 1797 and is therefore called Parisian yellow . From 1818 it was produced as a pigment and, due to its extraordinarily high chroma, it soon became a fashionable color ( Neugelb ), especially as a postal yellow .
Since the development of the first yellow azo pigments (CI Pigment Yellow 1 and 3) in 1909 by Hoechst , the poisonous pigment quickly fell out of use in artists' paints. However, these substitute pigments are unsuitable for use in the paint sector, since they have poor hiding power and very low weather resistance.
Lead (II) chromate remained in industrial use in Europe until the 1980s, together with the pigments cadmium sulfide (CI Pigment Yellow 35 and 37) and molybdate red (CI Pigment Red 104), which are also outlawed today . These were among the standard pigments in this color range, but were then outlawed by the European paint industry due to their toxicological properties. Among other things, this explains the strong increase in sales of bismuth vanadate pigments (CI Pigment Yellow 184) and the development of modified organic pigments with higher hiding power. Today, lead (II) chromate has almost disappeared from industrial use in Europe, but is still used in other parts of the world.
It was often used by Vincent van Gogh , for example , who could not afford the expensive cadmium yellow .
Occurrence
In nature, lead chromate occurs as a mineral crocoite with a massive monoclinic crystal structure . For a long time, this ore was used to extract chrome and lead .
Extraction and properties
Lead (II) chromate is an orange-yellow, water-insoluble powder that separates from a solution of a water-soluble lead compound (e.g. lead acetate or lead nitrate ) to which potassium chromate or ammonium chromate has been added.
The representation of lead acetate on sodium dichromate is used industrially. If you use neutral or weakly alkaline chromate solutions instead of acidic, but rather neutral or weakly alkaline chromate solutions to precipitate the lead salt solution, basic lead chromate, chrome red , is formed as the mineral phenicochroite .
In the application as a pigment for paints and emulsion paints lead (II) chromate shows a good hiding power , coverage, high brilliance and chrominance (chroma, and chroma). However, lead (II) chromate slowly converts to other compounds, producing trivalent chromium. This changes the appearance from yellow to matte brown. This means that old paintings (for example by Vincent van Gogh ) become darker and darker over time. This reaction is set in motion primarily by ultraviolet light and thus by solar radiation. Even with the diffuse light in museums, this process cannot be stopped completely. The varnish used also plays an important role , as it contains compounds which contain the elements barium and sulfur and which support the color breakdown.
Chrome yellow tends to deviate and shift in color and can be sensitive to sunlight and sulphurous acid as an air pollutant outdoors . It also reacts with hydrogen sulfide . In relatively dense binders such as oil and synthetic resins, it is more lightfast than when used as a water color .
Lead chromate crystallizes in the monazite structure, i.e. in the monoclinic crystal system in the space group P 2 1 / n (space group no. 14, position 2) with the 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 . An orthorhombic modification (space group Pnma (No. 62) , a = 867 pm, b = 559 pm, c = 713 pm) has also been described.
use
Lead (II) chromate was used as a pigment in artists' paints for a long time because it was cheap, brilliant and extremely fast. Due to its toxicity, however, it has been replaced by other substances such as bismuth vanadate . In Europe, the compound in this area is only important as a pigment for the restoration of historical works of art or buildings.
Unlike most shades of chrome orange , however, chrome yellow is usually not lime-fast.
In industrial application as a pigment in paints and emulsion paints, lead (II) chromate is still used in large quantities, but no longer in Europe. It is also used as a mixed crystal in combination with lead sulfate and / or lead molybdate (e.g. from CI Pigment Red 104). The extraordinary combination of low price, high chroma , good hiding power and high weather resistance (it was therefore often used as a rust protection paint ) means that lead (II) chromate is still used in regions with less security awareness than in Europe. In Europe, the replacement of lead (II) chromate has largely been completed, but at the expense of the price and / or the coloristic properties.
Other inorganic pigments such as iron oxide yellow (CI Pigment Yellow 42) are inexpensive, but their color is much more cloudy. High-quality inorganic pigments such as bismuth vanadate show a pure color shade and good weather resistance, but are significantly more expensive than lead (II) chromate. Organic pigments such as brilliant yellow (e.g. BCI Pigment Yellow 74, 151) show very pure color tones, but are also more expensive and even in optimized variants have significantly poorer hiding power. With regard to weather resistance, there are types with a higher and lower level, depending on the chemical structure. However, all of them have a lower level than the inorganic lead (II) chromate.
Trivia
- The color name chrome yellow is the earlier name of the color RAL 1007 in the RAL color system , which is now called daffodil yellow .
- The reaction with potassium chromate described above serves, among other things, as evidence for lead .
Individual evidence
- ↑ a b c d data sheet lead (II) chromate (PDF) from Merck , accessed on December 15, 2010.
- ↑ a b Entry on lead (II) chromate in the GESTIS substance database of the IFA , accessed on February 1, 2016(JavaScript required) .
- ↑ Entry on lead chromate in the Classification and Labeling Inventory of the European Chemicals Agency (ECHA), accessed on February 1, 2016. Manufacturers or distributors can expand the harmonized classification and labeling .
- ↑ Entry in the SVHC list of the European Chemicals Agency , accessed on July 16, 2014.
- ↑ Entry in the register of substances subject to authorization of the European Chemicals Agency , accessed on July 16, 2014.
- ↑ Swiss Accident Insurance Fund (Suva): Limit values - current MAK and BAT values (search for chromium (VI) compounds ), accessed on October 27, 2015.
- ^ A b Kurt Wehlte : Materials and techniques of painting . Otto Maier, Ravensburg 1967. ISBN 3-473-48359-1 (formerly: ISBN 3-473-61157-3 ), pp. 102-103.
- ↑ a b c d G. Buxbaum, G. Pfaff; Industrial inorganic pigments; Wiley-VCH; 2006.
- ↑ a b c W. Herbst, K. Hunger; Industrial organic pigments; 3. Edition; Wiley-VCH; 2004.
- ^ Pigments for industrial paint mixing systems; Clariant; 2004.
- ^ H. Tan, H. Tian, J. Verbeeck, L. Monico, K. Janssens , G. van Tendeloo: Nanoscale Investigation of the Degradation Mechanism of a Historical Chrome Yellow Paint by Quantitative Electron Energy Loss spectroscopy Mapping of Chromium Species . In: Angewandte Chemie . tape 125 , no. 43 , 2013, p. 11570–11573 , doi : 10.1002 / anie.201305753 .
- ↑ Wissenschaft aktuell: From yellow to brown - Why van Gogh's paintings are getting darker and darker , accessed on February 15, 2011.
- ↑ Dr. A. Eibner: Painting materials as the basis of the painting technique. For art students, artists, painters, varnishers, manufacturers and dealers , 1909, Julius Springer Verlag, Berlin
- ^ H. Effenberger, F. Pertlik: "Four monazite type structures: comparison of SrCrO 4 , SrSeO 4 , PbCrO 4 (crocoite), and PbSeO 4 " in Zeitschrift für Kristallographie 1986 , 176 , pp. 75-83. Full text (PDF file; 718 kB)
- ↑ G. Collotti, L. Conti, M. Zocchi: "The structure of the orthorhombic modification of lead chromate PbCrO 4 " in Acta Cryst 1959 , 12 , p. 416. doi : 10.1107 / S0365110X59001220
- ↑ F. Quittner, J. Sapgir, N. Rassudowa: "The rhombic modification of the lead chromate" in the journal for inorganic and general chemistry 1932 , 204 (3), pp. 315-317. doi : 10.1002 / zaac.19322040309
- ^ Description of chrome yellow ( memento of March 8, 2017 in the Internet Archive ) at Kremer Pigments
- ↑ Michael Stahr, Joachim du Puits, Karl-Heinz Pfestorf: Praxiswissen Bausanierung: Recognizing and remedying structural damage , p. 481, edition from 2013 or p. 808 in the 6th edition from 2015, Springer Verlag
- ^ CI Pigment Red 104 in: Chemical Dictionary Online , accessed September 16, 2012.
- ↑ Guide to the use of environmentally friendly substances (PDF file; 82 kB)
literature
- H. Kühn, M. Curran: Chrome yellow and Other Chromate Pigments, in Artists' Pigments. A Handbook of Their History and Characteristics , Vol. 1, L. Feller, Ed., Cambridge University Press, London 1986, p. 187-204.
Web links
- Chrome yellow , ColourLex