Calcium tungstate

Structural formula
General
Surname	Calcium tungstate
other names	Calcium tungstate
Molecular formula	CaWO 4
Brief description	whitish, odorless powder
External identifiers / databases
CAS number 7790-75-2 EC number 232-219-4 ECHA InfoCard 100.029.291 PubChem 6101758 Wikidata Q1026374
properties
Molar mass	287.93 g mol −1
Physical state	firmly
density	6.06 g cm −3
Melting point	1620 ° C
solubility	almost insoluble in water
safety instructions
GHS labeling of hazardous substances no GHS pictograms H and P phrases H: no H-phrases P: no P-phrases
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Calcium tungstate is a chemical compound from the group of tungstates .

history

The English and French for tungsten common word tungsten is derived from tung sten (Swedish for "heavy stone") from, which originally calcium tungstate was meant. In 1781 the German-Swedish chemist Carl Wilhelm Scheele recognized a previously unknown salt in this and discovered the element tungsten.

Occurrence

Of course, calcium tungstate occurs in the form of the mineral scheelite or as mixed crystal in powellite and in traces in wolframite and is used in this form for the production of pure tungsten via sodium tungstate .

Extraction and presentation

Calcium tungstate can be produced by a tribochemical reaction of calcium oxide and tungsten (VI) oxide . The representation from sodium tungstate and calcium chloride or calcium nitrate is also possible.

${\displaystyle \mathrm {CaO+WO_{3}\ \longrightarrow {}\ CaWO_{4}} }$

${\displaystyle \mathrm {Ca(NO_{3})_{2}+Na_{2}WO_{4}\ \longrightarrow {}\ CaWO_{4}\downarrow +2\ NaNO_{3}} }$

use

Calcium tungstate is the oldest and still used X-ray fluorescent z. B. for X-ray examinations (also as an intensifying film) in medicine and thin-layer chromatography . This was found out by employees of Thomas E. Edison in March 1896. It is a so-called “pure substance phosphor” because it does not require activation by foreign atoms and it glows blue-violet under X-ray irradiation. Its emission spectrum ranges from 380 nm to around 500 nm, with a maximum at 425 nm, a range in which conventional (unsensitized) X-ray film is particularly sensitive. The mean diameter of the crystalline grains is 4 µm. Today, however, it is largely being replaced by new substances based on rare earth compounds. As an alloy material it is also used to refine steel (tungsten steels) and doped with terbium it also serves as a semiconductor material .

Calcium tungstate is also used in 300 g crystals in the CRESST experiment. An international group of physicists is trying to use these crystals to directly prove the existence of so-called WIMPs , a constituent of dark matter .

literature

• A. Ievinš, V. Grinšteins: For the determination of calcium as tungstate. In: Journal for Analytical Chemistry. 124, 1942, p. 288, doi : 10.1007 / BF01391852 .

Individual evidence

1. ↑ ^{a b c d e} data sheet Calcium tungsten oxide, 98% from AlfaAesar, accessed on December 7, 2019 ( PDF )(JavaScript required) .
2. ↑ Data sheet Calcium tungstate, powder from Sigma-Aldrich , accessed on December 7, 2019 ( PDF ).
3. ^ Rainer Albrecht, Harald Häusler, Rolf Möbius: Representation of calcium and strontium tungstate by tribochemical reaction. In: Journal of Inorganic and General Chemistry. 382, 1971, p. 177, doi : 10.1002 / zaac.19713820209 .
4. ↑ Wolfram (Seilnacht)
5. ↑ Degenhardt 1983
6. ↑ Werner Schlungbaum, Medical Radiation Science , ISBN 3110128500
7. ↑ X-ray course script ( Memento from September 25, 2011 in the Internet Archive ) (PDF; 2.2 MB)
8. ↑ Freyschmidt 1981
9. ↑ Rother, Uwe Jörg: The dose area product with different film-foil combinations and the digital panorama technology. urn : nbn: de: gbv: 18-7246
10. ↑ Entry on Wolfram. In: Römpp Online . Georg Thieme Verlag, accessed on June 13, 2014.
11. ↑ cresst.de: CRESST: The Experiment , accessed on February 26, 2015