Potassium chromate

Structural formula
	__ K + __ Cr 6+ __ O 2−
Crystal system	orthorhombic
Space group	Pnam (No. 62, position 6)
Lattice parameters	a = 7.663 Å, b = 10.388 Å, c = 5.922 Å
General
Surname	Potassium chromate
other names	Dipotassium chromate; Potassium chromate;
Molecular formula	K 2 CrO 4
Brief description	lemon yellow, crystalline solid
External identifiers / databases
EC number	232-140-5
ECHA InfoCard	100.029.218
PubChem	24597
Wikidata	Q407939
properties
Molar mass	194.19 g mol −1
Physical state	firmly
density	2.73 g cm −3 (18 ° C )
Melting point	985 ° C
boiling point	1000 ° C
solubility	Easily soluble in water (637 g l −1 at 20 ° C); insoluble in ethanol ;
safety instructions
H and P phrases	H: 315-317-319-335-340-350i-410
	P: 201-280-305 + 351 + 338-308 + 313
Authorization procedure under REACH	of particular concern : carcinogenic, mutagenic, toxic for reproduction ( CMR ); subject to approval
MAK	Germany: repealed because it is carcinogenic ; Switzerland: 5 μg m −3 (calculated as chromium);
	As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Potassium chromate is a potassium salt of chromic acid (H ₂ CrO ₄ ) with the empirical formula K ₂ CrO ₄ . It is a non-flammable, lemon yellow, toxic and environmentally hazardous solid. The salt is a strong oxidizing agent and is corrosive to the skin and mucous membranes.

Extraction and presentation

Potassium chromate is produced by the reaction of potassium carbonate and potassium dichromate .

{\ displaystyle {\ ce {K2CO3 + K2Cr2O7 -> 2K2CrO4 + CO2 ^}}}

properties

Potassium chromate

Anhydrous potassium chromate forms lemon-yellow, prism-shaped crystals ( orthorhombic , space group Pnam (space group no.62 , position 6) , lattice parameters a = 7.663 Å , b = 10.388 Å, c = 5.922 Å), which turn into a red above 670 ° C convert hexagonal α-modification. Template: room group / 62.6

The salt is flammable when it comes into contact with flammable substances.

use

Potassium chromate has been almost completely replaced by the cheaper sodium chromate and is only used for very specific applications, such as in analog photography or as an indicator in the titration of sodium chloride according to Mohr ( DAB7 ).

safety instructions

Potassium chromate is poisonous and dangerous for the environment . It causes irritation and damage to the skin and mucous membranes and damages the kidneys, blood and liver. Potassium chromate is also mutagenic and carcinogenic .

Potassium chromate causes poorly healing wounds on injured skin.

proof

The easily soluble chromate anion (CrO ₄²⁻ ) can be precipitated in aqueous solution with metal cations such as Ag ¹⁺ , Ba ²⁺ , Pb ²⁺ and Hg ₂²⁺ as a yellow, red or brown-red precipitate, depending on the cation:

{\ displaystyle {\ ce {Ba ^ {2 +} {} + CrO4 ^ {2 -} -> BaCrO4 v}}}

{\ displaystyle {\ ce {2Ag + + CrO4 ^ {2 -} -> Ag2CrO4 v}}}

It should be noted that in very acidic solutions the chromate-dichromate equilibrium is increasingly shifted towards dichromate.

In addition, the detection can also take place by reducing the chromate anion to Cr ³⁺ , which causes a color change from yellow to green. This reaction is also used for the semi-quantitative determination of ethanol .

Web links

Commons : Potassium Chromate - Collection of pictures, videos and audio files

Individual evidence

↑ ^a ^b John A. McGinnety: Redetermination of the structures of potassium sulphate and potassium chromate: the effect of electrostatic crystal forces upon observed bond lengths . In: Acta Crystallographica . tape 28 , 1972, p. 2845-2852 , doi : 10.1107 / S0567740872007022 .
↑ ^a ^b ^c ^d ^e ^f ^g ^h Entry on potassium chromate in the GESTIS substance database of the IFA , accessed on January 9, 2019(JavaScript required) .
↑ Entry on potassium chromate. In: Römpp Online . Georg Thieme Verlag, accessed on July 15, 2014.
↑ Entry on Potassium 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 14, 2014.
↑ Entry in the register of substances subject to authorization of the European Chemicals Agency , accessed on July 14, 2014.
↑ Swiss Accident Insurance Fund (Suva): Limit values - current MAK and BAT values (search for chromium (VI) compounds ), accessed on October 27, 2015.

[Kristallstruktur-1] John A. McGinnety: Redetermination of the structures of potassium sulphate and potassium chromate: the effect of electrostatic crystal forces upon observed bond lengths . In: Acta Crystallographica . tape 28 , 1972, p. 2845-2852 , doi : 10.1107 / S0567740872007022 .

[GESTIS-2] ↑ ^a ^b ^c ^d ^e ^f ^g ^h Entry on potassium chromate in the GESTIS substance database of the IFA , accessed on January 9, 2019(JavaScript required) .

[3] Entry on potassium chromate. In: Römpp Online . Georg Thieme Verlag, accessed on July 15, 2014.

[CLP_100.029.218-4] Entry on Potassium 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 .

[SVHC-5] Entry in the SVHC list of the European Chemicals Agency , accessed on July 14, 2014.

[Zulassungspflicht-6] Entry in the register of substances subject to authorization of the European Chemicals Agency , accessed on July 14, 2014.

[7] Swiss Accident Insurance Fund (Suva): Limit values - current MAK and BAT values (search for chromium (VI) compounds ), accessed on October 27, 2015.