Potassium chromate

Potassium chromate

Potassium chromate sample on a watchglass.
Names
IUPAC name Potassium chromate
Other names Chromic acid, dipotassium salt Dipotassium monochromate Dipotassium chromate
Identifiers
Jmol-3D images	Image
SMILES [O-][Cr](=O)(=O)[O-].[K+].[K+]
Properties
Chemical formula	K2CrO4
Molar mass	194.19 g/mol
Appearance	Yellow solid
Odor	Odorless
Density	2.732 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	56.3 g/100 mL (0 °C) 60.0 g/100 mL (10 °C) 63.7 g/100 mL (20 °C) 67.8 g/100 mL (40 °C) 70.1 g/100 mL (60 °C) 79.2 g/100 mL (100 °C)
Solubility	Soluble in ethylene glycol, trifluoroacetic acid Slightly soluble in acetic anhydride, thionyl chloride Insoluble in alcohols, alkanes, liq. sulfur dioxide
Vapor pressure	~0
Thermochemistry
Std enthalpy of formation (ΔfHo298)	-1,383.27 kJ/kmol
Hazards
Safety data sheet	ScienceLab
Related compounds
Related compounds	Potassium dichromate Potassium ferrate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Potassium chromate (K₂CrO₄) is the potassium salt of the chromate anion. It is a yellow solid, soluble in water. As a Cr(VI) compound, it is a strong carcinogen and an oxidizer.

Properties

Chemical

The addition of an acid to an aqueous solution of potassium chromate will yield potassium dichromate.

2 K₂CrO₄ + H₂SO₄ → K₂Cr₂O₇ + K₂SO₄ + H₂O

Other acids, such as boric acid, hydrochloric acid, nitric acid, glacial acetic acid, oxalic acid, phosphoric acid can also be used. Carbon dioxide also works.

Physical

Potassium chromate is an odorless yellow solid, soluble in water, trifluoroacetic acid, liq. hydrogen cyanide, phosphoryl chloride. It is insoluble in acetone, anhydrous ammonia, aniline, benzonitrile, liq. chlorine, ethyl acetate, methyl acetate, molten antimony(III) chloride, vanadyl chloride (both cold and hot).

It has an average density of 2.7320 g/cm³.

Availability

Potassium chromate is sold by various chemical suppliers. It can also be found on eBay.

Preparation

Potassium chromate can be made by adding potassium hydroxide and conc. hydrogen peroxide or bleach to a suspension of chromium(III) hydroxide.

Cr(OH)₃ + 2 KOH + H₂O₂ → K₂CrO₄ + H₂O + 3/2 O₂

2 Cr(OH)₃ + 6 KClO → 2 K₂CrO₄ + 4 KCl + 2 Cl₂

The chromium hydroxide can be made by dissolving stainless steel in hydrochloric acid, followed by precipitation with sodium carbonate.

If you're using common household bleach, you will end up with sodium chromate. To convert it into potassium chromate, you will need to add a potassium salt, such as potassium carbonate, filter the resulting solution and then recrystallize it.

Chromium(IV) oxide can also replace chromium(III) hydroxide.

It can also be made by adding potassium hydroxide to potassium dichromate in solution.

Heating potassium dichromate will result in potassium chromate, chromium(III) oxide and oxygen.

4 K₂Cr₂O₇ → 4 K₂CrO₄ + 2 Cr₂O₃ + 3 O₂

Heating a mixture of potassium nitrate and chromium(III) oxide with traces of water will also give potassium chromate. This reaction however does not work with indurated chromium oxide (like the OTC chromium oxide pigment).

Projects

• Make potassium dichromate
• Make chromic acid
• Make chromium trioxide
• Make potassium peroxochromate

Handling

Safety

Potassium chromate, like all hexavalent chromium compounds, is highly toxic and carcinogenic on ingestion or inhalation. Handling it without gloves can cause dermatitis, and can also be absorbed through the skin in small amounts, usually if wet. Aqueous solutions are notorious for staining most materials.

Always wear gloves and goggles when handling it, and a dust mask or respirator when handling it as a powder to avoid inhalation of it, which could be fatal.

Storage

Potassium chromate should be stored in closed bottles, with a visible label.

Disposal

Potassium chromate can be safely reduced to the less harmful Cr(III) oxide with a reducing agent, such as ascorbic acid, or various sulfites/metabisulfites/tiosulfates.

References

Relevant Sciencemadness threads

• making potassium chromate from dichromate