Cobalt blue

Blue powder

Fixed

4.5 g · cm -3

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The pigment Thénards Blue ( Dumont Blue, Blue Leithners ) is cobalt aluminate of the formula CoAl2O4. It is accordingly a Mischphasenoxidpigment. The classic version is in the Colour Index under CI Pigment Blue 28 listed, a grünlichere variant with the formula Co ( Al, Cr) 2O4 under CI Pigment Blue 36

The color is also cobalt blue or cobalt blue, cerulean ( from sky blue, " heavenly " from the Latin coelestis ) called. Cobalt aluminate is the basis for the production of cobalt glass.

History

Cobalt blue was used early in Persia for the decoration of pottery, in China it was from the Tang Dynasty ( 618-906 AD) used in ceramic glazes. As a pure substance it was discovered in 1795 in the Vienna Porcelain Manufactory of the chemist and porcelain painter Josef leithner by annealing of aluminum sulfate and cobalt nitrate (II ) ( Co ( NO3 ) 2). The industrial production of the deep blue pigment was initiated by the University of Paris Professor Louis Jacques Thénard.

Chemistry

Cobalt aluminate is a cobalt - aluminum spinel with the following stoichiometric composition:

The production serves as a test reaction to aluminum in a Spinellprobe (melting sample) in inorganic analytical chemistry.

Color theory

Cobalt blue as the color is a neutral blue high color saturation

  • HSV 215 °, 100 %, 67 %
  • RGB255 (0, 71, 171 ) in decimal or hexadecimal 0047AB
  • CMYK100 (100, 58, 0, 33)

Use

  • Pigment powder

Cobalt blue greenish C. I. PB 36

Pigment

The special feature of this pigment is its extremely deep color. The chrominance of about 85% is far outside the gamut of photo technology in color reproduction and printing technology accessible. In print special colors are necessary because the brilliance is outside the CMYK color space of the classical four-color printing. In the RGB color space of CRT monitors and flat panel displays of the usual color is outside the gamut and can not be reproduced. The pigment shows a powder a significantly lighter shade than about Ausmischungen in oil.

In the paint area cobalt blue is used in applications where the level of fastness of copper phthalocyanine is not sufficient. This applies to the weathering of exterior paints. For other areas, the cheaper and at the same time stronger phthalocyanine blue color is used.

Detection reaction

During annealing of aluminum salts with cobalt ( II) nitrate produced the blue pigment and can therefore be used as a test reaction.

Cobalt glass

For cobalt glass the proportion of cobalt aluminate is color critical, you get an intense dark blue glass. Cobalt glass filters in transmission of yellow light. Therefore, it is used in chemical analysis to optically separate the pale violet flame color of potassium salts of the rich yellow color of sodium compounds in flame tests. At great depth, it is also suitable as Augen-/Geräteschutz when viewing the eclipse.

Toxicity

Cobalt oxides themselves are classified as harmful (Xn, R -phrases 22-43-50/53 ). The pigment, however, the Cobalt is built into Spinellmischphase and can not be expanded due to the stable crystal structure. There are also no concerns for glasses, as it is bound to be a stable cobalt silicate.

CAS registry number Cobalt(II) nitrate Spinel group Cobalt(II) oxide Aluminium Nuance IBM
195428
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