Chromium

Chemical reactions

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 CO32−(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+(aq) [violet] + OH−(aq) Cr(OH)3(H2O)3(s) [green] + 3 H2O(l)
Cr(OH)3(H2O)3(s) [green] + 3 OH−(aq) [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−(aq) + 3 H2O2(aq) + 2 OH−(aq) CrO42−(aq) [yellow] + 8 H2O(l)

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

CrO42−(aq) [yellow] + 2 H+(aq) HCrO4−(aq) Cr2O72−(aq) [orange] + H2O(l)

CrO42− 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 Cr2O72− reacts with hydrogen peroxide forming a blue instable diperoxochrome(VI)oxide [Cr(O2)2O]

Cr2O72−(aq) + 4 H2O2(aq) + 2 H+(aq) 2 [Cr(O2)2O](aq) + 5 H2O(l)
2 [Cr(O2)2O](aq) + 2 H+(aq) Cr3+(aq) + 3 O2(g) + 2 H2O(l)


Reaction of chromium with phosphate


Cr(III) is precipitated in cold acetic acid by hydrogen phosphate:

Cr3+(aq) + H2PO4−(aq) CrPO4(s) [green] + 2 H+(aq)


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

Cr2O72−(aq) + 2 H+(aq) + 3 SO2(aq) Cr3+(aq) + SO42−(aq) + H2O(l)

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

Cr2O72−(aq) + Zn(s) + 14 H+(aq) 2 Cr2+(aq) + 7 H2O(l)


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