Chromium

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Other names/abbreviations:

Cr

Element no:	24
Element group:	6 (IUPAC) VI B (American labelling) VI A (European labelling)
Element type:	Transition metals
CAS-Number:	7440-47-3
EINECS-Number:	231-157-5

Chemical properties

Reaction of chromium with acids
Metallic chromium dissolves in dilute hydrochloric acid forming Cr(II) and hydrogen gas, H₂. In aqueous solution, Cr(II) is present as the complex ion [Cr(OH₂)₆]²⁺. Similar results are seen for sulphuric acid but pure samples of chromium may be resistant to attack. Chromium metal is not dissolved by nitric acid, HNO₃ but is passivated instead.

Cr(s) + 2 HCl(aq) Cr²⁺(aq) + 2 Cl^-(aq) + H₂(g)

Cr(VI) is reduced to Cr(III) by ascorbic acid (reductive coefficient = 3.473*10⁶ M^-1*s^-1) [3].


Reaction of chromium with air
Chromium metal does not react with air at room temperature.


Reaction of chromium with ammonia
Cr(III) is precipitated by NH₃ as Cr(OH)₃.

[Cr(H₂O)₆]³⁺(aq) + 3 NH₃(aq) [Cr(OH)₃(H₂O)₃](s) [green] + 3 NH₄⁺(aq)

The precipitate dissolves in excess ammonia.

[Cr(OH)₃(H₂O)₃](s) + 6 NH₃(aq) [Cr(NH₃)₆]³⁺(aq) + 3 H₂O(l) + 3 OH^-(aq)


Reaction of chromium with carbonate
Cr(III) is precipitated by carbonate as Cr(OH)₃. The precipitate is not soluble in excess carbonate.

2 [Cr(H₂O)₆]³⁺(aq) + 3 CO₃^2-(aq) 2 [Cr(OH)₃(H₂O)₃](s) + 3 H₂O(l) + 3 CO₂(g)


Reaction of chromium with halogens
Chromium reacts directly with fluorine, F₂, at 400°C and 200-300 atmospheres to form chromium(VI) fluoride, CrF₆.

Cr(s) + 3 F₂(g) CrF₆(s) [yellow]

Under milder conditions, chromium(V) fluoride, CrF₅, is formed.

2 Cr(s) + 5 F₂(g) 2 CrF₅(s) [red]

Under still milder conditions, chromium metal reacts with the halogens fluorine, F₂, chlorine, Cl₂, bromine, Br₂, and iodine, I₂, to form the corresponding trihalides chromium(III) fluoride, CrF₃, chromium(III) chloride, CrCl₃, chromium(III) bromide, CrBr₃, or chromium(III) iodide, CrI₃.

2 Cr(s) + 3 F₂(g) 2 CrF₃(s) [green]
2 Cr(s) + 3 Cl₂(g) 2 CrCl₃(s) [red-violet]
2 Cr(s) + 3 Br₂(g) 2 CrBr₃(s) [very dark green]
2 Cr(s) + 3 I₂(g) 2 CrI₃(s) [very dark green]


Reaction of chromium with hydroxide ions
Cr(III) is precipitated by hydroxide ions as Cr(OH)₃ The precipitate is amphoteric:

[Cr(H₂O)₆]³⁺ [violet] + OH^- Cr(OH)₃(H₂O)₃(s) [green] + 3 H₂O(l)
Cr(OH)₃(H₂O)₃(s) [green] + 3 OH^- [Cr(OH)₆]^-(aq) [green, octahedral] + H₂O(l)


Reaction of chromium with nucleotides
Cr(VI) is reduced to Cr(III) by nucleotides [3].

	Nucleotide	Reductive coefficient (x 103 M-1·s-1)
	Guanine Cytosine Adenine Thymine Uracil Deoxyribose AMP ADP ATP	0.58 0.51 0.19 0.17 0.08 3.21 0.13 0.47 0.54

Reductive coefficients for nucleotides adapted from [3].


Reaction of chromium with peroxide
Cr(III) is easily oxidized to CrO₄^- by hydrogen peroxide under alkaline conditions:

2 Cr(OH)₄^- + 3 H₂O₂ + 2 OH^- CrO₄^2- [yellow] + 8 H₂O

Cr(VI) dimerizes by acid at concentrations c(Cr(VI)) > 10^-2 M:

CrO₄^2- [yellow] + 2 H⁺ HCrO₄^- Cr₂O₇^2- [orange] + H₂O

CrO₄^2- can be precipitated by several metal ions e.g. Ba. All chromates are soluble in mineral acids and can be precipitated again by sodium acetate.

Under acidic conditions Cr₂O₇^2- reacts with hydrogen peroxide forming a blue instable diperoxochrome(VI)oxide [Cr(O₂)₂O]

Cr₂O₇^2- + 4 H₂O₂ + 2 H⁺ 2 [Cr(O₂)₂O] + 5 H₂O
2 [Cr(O₂)₂O] + 2 H⁺ Cr³⁺ + 3 O₂ + 2 H₂O


Reaction of chromium with phosphate
Cr(III) is precipitated in cold acetic acid by hydrogen phosphate:

Cr³⁺ + H₂PO₄^- CrPO₄(s) [green] + 2 H⁺


Reaction of chromium with saccharides
Cr(VI) is reduced to Cr(III) via soluble and reactive intermediates of Cr(V) by saccharides and derivatives [3].

Monosaccharide

Reductive coefficient (x 103 M-1·s-1)

Hexoses

D-fructose D-galactose D-mannose L-sorbose D-glucose

1.88 1.64 1.29 1.09 0.65

Pentoses

D-ribose D-lyxose D-arabinose D-xylose

6.29 5.2 2.4 0.9

Derivatives

D-glucaric acid D-gluconic acid D-galacturonic acid D-glucoronic acid N-methylglucamine D-glucosamine

10.8 7.8 5.4 3.2 4.6 0.22

Reductive coefficients for saccharides and derivatives adapted from [3].


Reaction of chromium with sulfide
Cr(III) is not precipitated by sulfide in 0.4 M hydrochloric acid. Sodium sulfide precipitates Cr(III) as the hydroxide. The precipitate is dissolved by excess sulfide.

Cr(VI) is reduced to Cr(III) by H₂S


Reaction of chromium with thiols
Cr(VI) is reduced to Cr(III) by thiols [3].

	Thiols	Reductive coefficient (x 103 M-1·s-1)
	L-Cys-OEt Glutathione (GSH) Dithiotreitol (DTT) L-Cys Mercapto-propane diol(MP) Mercapto-ethanol (ME)	730 450 184 168 100 50

Reductive coefficients for thiols adapted from [3].


Reaction of chromium with water
Chromium metal does not react with water at room temperature.


Redox reactions of chromium
Cr(VI) is reduced to Cr(III) by reduction agents like SO₂ og KI

Cr₂O₇^2- + 2 H⁺ + 3 SO₂ Cr³⁺ + SO₄^2- + H₂O

Cr(VI) is reduced to Cr(II) by strong reduction agents like Zn

Cr₂O₇^2- + Zn + 14 H⁺ 2 Cr²⁺ + 7 H₂O


Solubility Water: Insoluble (20 °C) [6]

Quantitative analysis

Method 3500-Cr C+D Colorimetric Methods [1]. Sample water (or soil digested in water) is reacted with diphenylcarbazide in an acidic solution to produce an intensely red-violet colored solution. Hexavalent chromium is measured against a range of standards using light absorption at 540 nm.

All chromium
Method limit of detection in water = 0.005 mg/L
Method limit of detection in soil = 1.00 mg/kg

Hexavalent
Method limit of detection in water = 0.0005 mg/L
Method limit of detection in soil = 0.50 mg/kg

Safety

Symbol: -
R-phrases: -
S-phrases: -

Physical properties

Appearance
	Physical state @ 20°C:	Solid [6]
	Color:	Silver gray-metalic [6]
	Odor:	Odourless [6]

Bulk properties
	Molecular weight (g/mol):	51.9961(6) [1]
	Melting point (°C):	1900 [1]
	Boiling point (°C):	2690 [1]
	Density (g/cm3):	7.14 [1]
	Molar volume (cm3):	7.29 (20 °C) [4]
	Velocity of sound (m/s):	5940
	Elastic properties: Young's modulus (GPa): Rigidity modulus (GPa): Bulk modulus (GPa): Poisson ratio:	190-279 [4] 115 [4] 160 [4] 0.21 [4]
	Hardness: Mineral hardness: Brinell hardness (MN/m2): Vicker hardness (MN/m2):	8.5 1120 1060

Thermodynamic properties
	ΔH°atomization (KJ/mol):	397
	ΔfH°gas (KJ/mol):	397±3 (Monoatomic) [1]
	ΔH°fusion (KJ/mol):	21±2 [1]
	ΔH°vaporization (KJ/mol):	342±6 [1]
	Cp (solid) (J/K·mol):	22.9817 (20 °C) [4] 25.3539 (100 °C) [4] 30.1199 (400 °C) [4] 33.5803 (700 °C) [4]
	Coeff. of linear thermal expansion (106 K-1):	6.5 (20 °C) [4] 6.6 (100 °C) [4] 8.4 (400 °C) [4] 9.4 (700 °C) [4]

Crystal structure
	Crystal type:	Body centered cubic [4]

Electronic properties
	Electron configuration:	1s2-2s2-2p6-3s2-3p6-3d5-4s1
	Atomic radius (Å)	1.85
	Ionic radius (Å):	Cr(VI): 0.44 (6-coordinate) [1] Cr(V): 0.49 (6-coordinate) [1] Cr(IV): 0.55 (6-coordinate) [1] Cr(III): 0.615 (6-coordinate) [1] Cr(II): 0.73 (6-coordinate, low-spin) [1] Cr(II): 0.80 (6-coordinate, high-spin) [1]

Conductivity
	e°(V):	-0.74
	Electrical resisitvity (μΩ·cm):	13.20 (20 °C) [4] 12.90 (25 °C) [4] 18 (152 °C) [4] 31 (407 °C) [4] 47 (652 °C) [4]
	Thermal (W/m·K):	84 (197 °C) [4] 74 (447 °C) [4] 69 (727 °C) [4] 63 (927 °C) [4]
	Electrical (Ω·m):	774 (20 °C)

References

1: Standard Methods for the Analysis of Water and Wastewater, APHA, 1992, 18th edition

2: Greenwood,N.N., Earnshaw,A. Chemistry of the elements
2nd edition (1997) Edited by Greenwood,N.N., Earnshaw,A. pp. 1-1340, Butterworth-Heinemann. Oxford. Great Britain

3: Kaiwar,S.P., Rao,C.P. In vitro reduction of Cr(VI) by low molecular weight biomimetic components: a comparative study using UV-Vis spectroscopy
Chem. Biol. Interact. (1995) 95 89-96

4: Buch,A. Pure Metals Properties. A Scientific-Technical Handbook
1st edition (1999) Edited by Buch,A. pp. 1-306, ASM International and Freund Publishing House Ltd. Ohio. USA

5: CRC Handbook of Chemistry and Physics
75th edition (1994) Edited by Lide,D.R. , CRC Press Inc. Boca Raton, USA

6: Merck. ChemDAT The Merck Chemical Database Ver. 1.1.5

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© Michael Pilgaard
Created: October 3, 2006
Last update: October 8, 2008