Bismuth vanadate
- Bismutvanadiumoxid
- Bismutgelb
- Yellow vanadium
- C. I. Pigment Yellow 184
Fixed
6.25 g · cm -3
Insoluble in water
Attention
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Bismuth vanadate, rare Bismutvanadiumoxid, Bismutgelb or yellow vanadium is an inorganic, very greenish yellow pigment from the group of substances vanadates. The chemical formula is BiVO4. The Colour Index under C. I. Pigment Yellow 184 listed compound is one of the main substitutes for the formerly common, now classified as toxic, inorganic yellow pigments PbCrO4 chromate and cadmium sulfide CdS. Commercial products often contain molybdenum or tungsten in place of vanadium in the crystal lattice. Besides the classic greenish variant now also reddish variants are offered.
History
In contrast to most of the inorganic pigments bismuth vanadate has to have only a relatively short history. The pigment group was offered commercially until 1985. Although the synthesis was first documented in 1924, began serious attempts of the use as a pigment only in the 70s. Meanwhile, the global annual demand is 1200 tons.
Occurrence
Bismuth vanadate occurs naturally in the minerals pucherite, Clinobisvanit and Dreyerit. However, these are for the industrial production of the pigment is not important.
Production and representation
Production as a solid-state reaction
Bismuth ( III ) oxide and vanadium oxide react in a solid-state reaction to bismuth vanadate.
Production from precipitation process
Bismuth vanadate can also be synthesized by precipitation from a solution of bismuth nitrate and sodium metavanadate.
The color depends strongly on temperature, pH and concentration in the precipitation. In industrially manufactured products after-treatment is often applied, which should further increase alkaline or weather resistance.
Properties
Commercial bismuth vanadate types have a density of about 6.5 g/cm3 and a refractive index of 2.45.
The suitability as a pigment derived from the excellent hiding power, the inorganic pigment unusually pure and bright color, high color strength and excellent weather resistance. At a pH of about 12, however, the pigment is not alkali stable. Only two of the four possible crystal modifications show the brilliant yellow. These four different main modifications also differ in their crystal structure. The occurring in nature as the mineral pucherite bismuth vanadate has an orthorhombic structure with space group PNCA and a0 = 5.33 Å, b 0 = 5.05 Å and c0 = 12.00 Å, and thus differs essentially from the synthetic base type in a monoclinic distorted variant of the calcium tungstate lattice ( scheelite - type) with space group I2 / a crystallized. To increase the stability of the synthetic Bismutvanadatgitters but pure tetragonal modification of the space group I4 1 / a is aimed at. This tetragonal bismuth vanadate with scheelite structure is obtained in the presence of the above-mentioned foreign ions such as molybdenum and tungsten, alone or in combination with alkaline earth metal ions. According to H. Wienand and W. Ostertag has the bismuth vanadate in the presence of sodium tetragonal structure with lattice constants a0 = 5.147 Å and c0 = 11.722 Å. In addition, there is yet another, to be distinguished from the scheelite - type tetragonal bismuth vanadate modification that has the appropriate zircon crystal structure with space group I 4 1/amd, but which is not suitable because of their much paler yellow as pigment.
Since bismuth vanadate is not toxic, it was introduced as a replacement for the outlawed toxic and in the (European) coatings industry yellow pigments chromate and cadmium sulfide.
Use
In particular, the hue of the chromate and cadmium sulfide is very similar to and much the same as that of pure nickel titanate (CI Pigment Yellow 53), makes the pigment to a frequently preferred choice for all types of coating applications.
When used in automotive coatings, industrial coatings, powder coatings and emulsion paints bismuth vanadate is a standard choice for pigments. Due to the high weather resistance, the pigment is also suitable for outdoor use. The only exception to this emulsion paints for use on the facade, since the event of defective pre-treatment of the substrate can ( for example, coverage through a primer ) Damage caused by alkalis which emerge about from incompletely cured concrete arise.