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Chromium

DESIGNATIONS

CAS No.: 7440-47-3
Registry name: Chromium
Chemical name: Chromium
Synonyms, Trade names: Chromium
Chemical name (German): Chrom
Chemical name (French): Chrome
Appearance: silver grey, hard, ductile metal

BASIC CHEMICAL AND PHYSICAL DATA

Chemical symbol: Cr
Rel. atomic mass: 51.996 g
Density: 7.19 g/cm3 at 20C
Boiling point: 2672C
Melting point:
1857C
Vapour pressure: 10-6 Pa at 844C
Solvolysis/solubility: soluble in diluted hydrochloric and sulphuric acid

BASIC DATA OF SELECTED COMPOUNDS

 

CAS No: 7789-12-0 1333-82-0
Chemical name: Sodium dichromate (dihydrate) Chromium(VI) oxide
Synonyms, Trade names:   Chromium trioxide, chromic acid anhydride
Chemical name (German): Natriumdichromat (Dihydrat) Chrom(VI)-oxid, Chromtrioxid
Chemical name (French): Dichromate de sodium Oxyde de Chrome (VI)
Appearance: orange to red needles dark red crystals (usually flakes) odourless
Empirical formula: Na2Cr2O7 (2 H2O) CrO3
Rel. molecular mass: 261.98 g (anhydrous)

298.0 g (dihydrate)

99.99 g
Density: 2.35-2.52 g/cm3 2.7 g/cm3
Boiling point: above 400C decomposition not distillable
Melting point: 357C (above 86C conversion to anhydrous salt) 198C (decomposition: brownish red vapours with pungent smell)
Vapour pressure: 0 hPa 0 hPa
Solvolysis/solubility: in water: 73.18 wt% at 20C

in water:

1660g/l at 20C
77.09 wt% at 40C 1990 g/l at 90C
82.04 wt% at 60C ((r) chromic acid)
88.39 wt% at 80C  
in alcohol: insoluble  

ORIGIN AND USE

Usage:
Chromium is used as a catalyst in ammonia synthesis, in the production of chromium steels, stainless steels and chromium alloys and for electroplating. Organic complexes are used as development dyes in colour photography. Inorganic chromium compounds are used as paint pigments. Chromium(VI) salts are widely used in wood preservation and leather tannery.

Origin/derivation:
Chromium virtually only occurs in nature in the form of compounds. The most important chromium ore is chromite. Pure chromium is obtained by reducing chromium(III) oxide with aluminium (thermite method), by way of electrolysis or using chromium iodide.

Production figures:
1985 = 9.935 million t (worldwide)

Toxicity

Humans: 0.5-1 g, oral = lethal, (potassium chromate) acc. MERIAN, 1984
LD 6-8 g, oral (sodium dichromate) acc. KOCH, 1989
Mammals:
Rat LD50 1800 mg/kg, oral (chromium(III)chloride) acc. MERIAN, 1984
Rat LD50 3250 mg/kg, oral (chromium(III)nitrate) acc. MERIAN, 1984
Aquatic organisms:
Freshwater fish LC50 250-400 mg/l (CrVI) acc. MERIAN, 1984
Saltwater fish LC50 170-400 mg/l (CrVI) acc. MERIAN, 1984
Daphnia LC50 0.05 mg/l (CrVI) acc. MERIAN, 1984
Algae LC50 0.032-6.4 mg/l (CrVI) acc. MERIAN, 1984
Brown and rainbow trout 0.20-0.35 mg/l (CrVI) acc. DVGW, 1988
Fish (no spec. differentiation) LD 60-728 mg/l, (sodium dichromate) acc. KOCH, 1989

Characteristic effects:

Humans/mammals: Because of its insolubility, metallic chromium is not toxic in water. The various hexavalent chromium compounds represent the major risk especially due to their genetic effects. Chromium(VI) compounds are active in virtually all test systems designed to determine mutagenic action. There is a strong risk for embryos and foetuses because of the proven placenta passage. The carcinogenic effect of chromium(VI) compounds has been substantiated both in animal experiments and by epidemiological studies on groups of the population subject to workplace exposure. The corresponding latency times are given as between 10 and 27 years. In contrast to chromium(VI) compounds there is no clear-cut evidence of the carcinogenic effect of chromium(III) compounds. Acute poisoning with chromium(VI) compounds becomes apparent for example in the form of damage to the kidneys. Chronic poisoning results in changes in the gastro-intestinal tract as well as in accumulation in the liver, kidneys, thyroid gland and bone marrow. The rate of excretion is slow.

Plants: There are known cases of plant root damage for example caused mainly by chromium(VI). There are major differences in terms of chromium absorption and possible damage both amongst the various types of plant and within the different parts of a plant. The toxic effect of chromium on plants has mainly been described on the basis of batch tests. The findings for oats revealed that the roots remained small and the leaves narrow whilst at the same time exhibiting reddish brown discoloration and small necrotic blotches.

Note: Trivalent chromium is an important trace element for humans and animals in the insulin metabolism.

ENVIRONMENTAL BEHAVIOUR

Water:
The toxicity of soluble chromium compounds in aquatic systems varies depending on the temperature, pH and water hardness as well as on the species of the organism. Chromium(VI) compounds are readily soluble in water, but are readily reduced under natural conditions in the presence of organic, oxidisable material to form less water-soluble, stable chromium(III) compounds.

Soil:
The mobility of chromium in the pedosphere can only be assessed taking into account the adsorptive and reducing capacity of soils and sediments. Once they have become sedimented and established in aquatic sediment, chromium(III)hydroxides exhibit only a minimal tendency to remobilise since there is scarcely any natural oxidation of chromium(III) compounds to form chromium(VI) compounds. Relatively small concentrations of chromium(VI) are sufficient to be toxic with the pH value of the soil playing a major role. The use of fertilisers containing phosphate increases the amount of chromium entering the soil.

Food chain:
Chromium(III) compounds absorbed with food are relatively harmless whereas chromium(VI) compounds have an extremely toxic effect. Animals and humans normally only absorb small amounts of chromium through inhalation; the majority of substances containing chromium are absorbed with food and drinking water. Resorption in the intestines is largely dependent on the chemical appearance of chromium: up to approx. 20 - 25 % of chromium complexes are absorbed whereas the figure for inorganic chromium is roughly 0.5 % (MERIAN, 1984).

ENVIRONMENTAL STANDARDS

Medium/ acceptor Sector Country/ organ.

Status

Value Cat. Remarks Source
Water: Drinkw D

L

50 g/l     acc. KOCH, 1989
Drinkw WHO

G

50 g/l     acc. KOCH, 1989
Groundw D(HH)

G

0.050 g/m3   Investigation acc. LAU-BW, 1989
Groundw D(HH)

G

0.20 g/m3   Rehabilitation acc. LAU-BW, 1989
Groundw NL

G

1 mg/l   Reference acc. TERRA TECH, 6/94
Groundw NL

L

30 mg/l   Intervention acc. TERRA TECH, 6/94
Waste water D

G

2 mg/l     acc. KOCH, 1989
Soil:   CH

G

75 mg/kg   Soil acc. LAU-BW, 1989
  NL

G

100 mg/kg   Reference acc. TERRA TECH, 6/94
  NL

L

380 mg/kg   Intervention acc. TERRA TECH, 6/94
Sewage sludge D

L

100 mg/kg AD   Soil acc. LAU-BW, 1989
Sewage sludge D

L

1200 mg/kg DM   Sewage sludge acc. LAU-BW, 1989
Sewage sludge CH

L

1000 mg/kg DM   Sewage sludge acc. LAU-BW, 1989
Sewage sludge EC

L

1-3 mg/kg DM   Soil acc. LAU-BW, 1989
Sewage sludge EC

L

20-40 mg/kg DM   Sludge acc. LAU-BW, 1989
Sewage sludge EC

L

1.5 kg/ha   Load in 10 a acc. LAU-BW, 1989
Compost D

G

100 mg/kg AD   Soil acc. LAU-BW, 1989
Compost D

G

2 kg/(ha*a)   Compost acc. LAU-BW, 1989
Compost CH

L

150 mg/kg DM   Compost acc. LAU-BW, 1989
Compost D(HH)

G

300 mg/kg DM   Investigation acc. LAU-BW, 1989
Air: Emiss. D

L

5 mg/m3   mass flow > 25 g/h1) acc. TA Luft, 1986
Emiss D L 1 mg/m3   mass flow > 5 g/h2) acc. TA Luft, 1986
Workp AUS

L

1 mg/m3   Cr and insoluble
Cr compounds
acc. MERIAN, 1984
Workp AUS

L

0.5 mg/m3   Soluble salts acc. MERIAN, 1984
Workp B

L

0.5 mg/m3   Soluble salts acc. MERIAN, 1984
Workp CH

L

1 mg/m3   Cr and insoluble
Cr compounds
acc. MERIAN, 1984
Workp CH

L

0.5 mg/m3   Soluble salts acc. MERIAN, 1984
Workp DDR

L

0.5 mg/m3   Cr and insoluble
Cr compounds
acc. MERIAN, 1984
Workp I

L

0.5 mg/m3   Cr and insoluble
Cr compounds
acc. MERIAN, 1984
Workp NL

L

0.5 mg/m3   Soluble salts acc. MERIAN, 1984
Workp SF

L

0.5 mg/m3   Soluble salts acc. MERIAN, 1984
Workp SF

L

1 mg/m3   Cr and insoluble
Cr compounds
acc. MERIAN, 1984
Workp SU

(L)

1 mg/m3   Chromium oxide acc. KETTNER, 1979
Workp USA

(L)

0.5 mg/m3 TWA Metallic Cr acc. ACGIH, 1979
Workp USA

(L)

0.5 mg/m3 TWA Cr(III) acc. ACGIH, 1979
Workp USA

(L)

0.5 mg/m3 TWA Cr(VI), water soluble acc. ACGIH, 1979
Workp USA

(L)

0.5 mg/m3 TWA Cr(VI), water insoluble acc. ACGIH, 1979
Workp YU

L

1 mg/m3   Cr and insoluble
Cr compounds
acc. MERIAN, 1984

Notes:

1) Cr(VI)-compounds, stated as Cr
2) Cr(VI)-compounds in respirable forms, stated as Cr

Alkali chromates: have a suspected carcinogenic potential

Comparison/reference values

Medium/origin Country Value Source
Atmosphere worldwide 5 pg/m3 acc. KOCH, 1989
Surface/groundwater worldwide 0.5 g/l acc. KOCH, 1989
Fly ash (coal) USA 43-259 mg/kg acc. HOCK, 1988
Plants   0.2-1 mg/kg acc. HOCK, 1988

Assessment/comments

The amounts of chromium found in the hydrosphere, pedosphere, atmosphere and biosphere can primarily be attributed to industrial emissions. The natural emissions into the atmosphere are quoted as being roughly 58,000 t per annum, whereas anthropological emissions are almost up to 100,000 t/a.

As regards environmental behaviour, chromium(III) compounds can be said to have a high degree of stability in contrast to chromium(VI) compounds.

Refuse containing chromium should be critically viewed in particular on account of its behaviour in the geological substrate on waste tips. Chromates are estimated to be stable for up to 50 years in an alkaline milieu and migrate even through tenacious soils to reach the groundwater.

The incineration of sludges containing chromium(III) should therefore be avoided on account of possible chromate formation.


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