Potassium chromate

Not to be confused with Potassium dichromate.

Potassium chromate is the inorganic compound with the formula K₂CrO₄. This yellow solid is the potassium salt of the chromate anion. It is a common laboratory chemical, whereas sodium chromate is important industrially.

Potassium chromate

Names
IUPAC name Potassium chromate
Other names Potassium dichromate, Chromic acid, (K2CrO4), dipotassium salt
Identifiers
CAS Number	7789-00-6 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:75249 N
ChemSpider	22999 N
ECHA InfoCard	100.029.218
EC Number	232-140-5
PubChem CID	24597
RTECS number	GB2940000
UNII	5P0R38CN2X Y
UN number	3077
CompTox Dashboard (EPA)	DTXSID8064858
InChI InChI=1S/Cr.2K.4O/q;2*+1;;;2*-1 N Key: XMXNVYPJWBTAHN-UHFFFAOYSA-N N InChI=K2CrO4 Key: XMXNVYPJWBTAHN-QALQIXLOAU
SMILES [O-][Cr](=O)(=O)[O-].[K+].[K+]
Properties
Chemical formula	K 2CrO 4
Molar mass	194.189 g·mol−1
Appearance	Yellow powder
Odor	odorless
Density	2.7320 g/cm3
Melting point	968 °C (1,774 °F; 1,241 K)
Boiling point	1,000 °C (1,830 °F; 1,270 K)
Solubility in water	63.7 g/100 mL (20 °C) 75.1 g/100 mL (80 °C) 79.2 g/100 mL (100 °C)
Solubility	insoluble in alcohol
Magnetic susceptibility (χ)	−3.9·10−6 cm3/mol
Refractive index (nD)	1.74
Structure
Crystal structure	rhombic
Hazards
GHS labelling:
Pictograms
Hazard statements	H315, H317, H319, H335, H340, H350i, H410
NFPA 704 (fire diamond)	2 0 0 OX
Safety data sheet (SDS)	Fisher Scientific
Related compounds
Other anions	Potassium dichromate Potassium molybdate Potassium tungstate
Other cations	Sodium chromate Calcium chromate Barium chromate
Related chromates	Potassium hypochromate Potassium perchromate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Structure

Two crystalline forms are known, both being very similar to the corresponding potassium sulfate. Orthorhombic β-K₂CrO₄ is the common form, but it converts to an α-form above 66 °C. These structures are complex, although the chromate ion adopts the typical tetrahedral geometry.

• 
  Structure of β-K₂CrO₄.
• 
  Coordination sphere of one of two types of K⁺ site.
• 
  The environment about the tetrahedral CrO₄²⁻ center in β-K₂CrO₄.

Production and reactions

It is prepared by treating potassium dichromate with potassium hydroxide:

${\ce {K2Cr2O7(aq) + 2KOH -> 2K2CrO4 + H2O}}$

Or, the fusion of potassium hydroxide and chromium trioxide:

${\ce {2KOH + CrO3 -> K2CrO4 + H2O}}$

In solution, the behavior of potassium and sodium dichromates are very similar. When treated with lead(II) nitrate, it gives an orange-yellow precipitate, lead(II) chromate.

Applications

Unlike the less expensive sodium salt, potassium salt is mainly used for laboratory work in situations where an anhydrous salt is required. It is as an oxidizing agent in organic synthesis. It is used in qualitative inorganic analysis, e.g. as a colorimetric test for silver ion. It is also used as an indicator in precipitation titrations with silver nitrate and sodium chloride (they can be used as standard as well as titrant for each other) as potassium chromate turns red in the presence of excess of silver ions.

Occurrence

Tarapacaite is the natural, mineral form of potassium chromate. It occurs very rarely and until now is known from only few localities on Atacama Desert.

Safety

As with other Cr(VI) compounds, potassium chromate is carcinogenic. The compound is also corrosive and exposure may produce severe eye damage or blindness. Human exposure further encompasses impaired fertility, heritable genetic damage and harm to unborn children.