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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 20°C
Boiling point: 2672°C
Melting point: 1857°C
Vapour pressure: 10-6 Pa at 844°C
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 400°C decomposition | not distillable | ||
| Melting point: | 357°C (above 86°C conversion to anhydrous salt) | 198°C (decomposition: brownish red vapours with pungent smell) | ||
| Vapour pressure: | 0 hPa | 0 hPa | ||
| Solvolysis/solubility: | in water: | 73.18 wt% at 20°C | in water: |
1660g/l at 20°C |
| 77.09 wt% at 40°C | 1990 g/l at 90°C | |||
| 82.04 wt% at 60°C | ((r) chromic acid) | |||
| 88.39 wt% at 80°C | ||||
| 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 µg/l | acc. KOCH, 1989 | ||
| Drinkw | WHO | G |
50 µg/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 µg/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.