Trivalent chromium Cr

Trivalent chromium (Cr+3) is an essential trace element in humans and some species of laboratory animals, but the database is incomplete for other groups of organisms... 

Chromium occurs in three basic forms metallic chromium (Cr(0)), trivalent chromium (Cr(III)), and hexavalent chromium (Cr(VI)). Hexavalent chromium can exist as chromium hexavalent ion and as part of a number of compounds including calcium chromate, chromic acid, chromium trioxide, lead chromate, strontium chromate, potassium dichromate, and zinc chromate. 

The toxicity of chromium is profoundly related to its oxidation state, whether the metal is in the +3 or +6 oxidation state. Chromium is a naturally occurring metal that is widely used for industrial purposes including plating, leather tanning, as a dye and as a wood preservative. Trivalent chromium (Cr ) is an essential trace nutrient required for proper glucose metabolism and other biological functions. 

For passivation, the passivation current density ipjs must be applied either by means of an anodic current (polarisation) or in the reaction vith an oxidant at passivation potential Upas. In the active and passive range, trivalent chromium (Cr ) is dissolved. Above the transpassive breakthrough potential Ua, i.e. after the transition to the transpassive range, the current density, and with it the rate of corrosion, rises once again, since at this high oxidation potential chromium then dissolves in hexa-valent form (Cr ) as chromate. 

The high corrosion resistance offered by chromate films is attributed to the presence of both hexavalent and trivalent chromium in the coating. Analyses of coatings by wet chemical methods and with surface-sensitive techniques have shown that both hexavalent chromium, Cr or CifVl), and trivalent chromium, Cr or CifllO. are present in the films. The trivalent chromium is believed to be present as an insoluble hydrated oxide, whereas the hexavalent chromium imparts a self-healing character to the film during oxidative (corrosive) attack by species such as chloride... 

Cross-linked xanthan gums have also been used to reduce the permeabiUty of thief 2ones. Trivalent chromium is the preferred cross-linker (54). Cross-linker effectiveness is less at high salinity. However, Cr(III) has been used ia the field at salinities as great as 166,000 ppm total dissolved soHds (55). 

Ghromium(III) Compounds. Chromium (ITT) is the most stable and most important oxidation state of the element. The E° values (Table 2) show that both the oxidation of Cr(II) to Cr(III) and the reduction of Cr(VI) to Cr(III) are favored in acidic aqueous solutions. The preparation of trivalent chromium compounds from either state presents few difficulties and does not require special conditions. In basic solutions, the oxidation of Cr(II) to Cr(III) is still favored. However, the oxidation of Cr(III) to Cr(VI) by oxidants such as peroxides and hypohaUtes occurs with ease. The preparation of Cr(III) from Cr(VI) ia basic solutions requires the use of powerful reducing agents such as hydra2ine, hydrosulfite, and borohydrides, but Fe(II), thiosulfate, and sugars can be employed in acid solution. Cr(III) compounds having identical counterions but very different chemical and physical properties can be produced by controlling the conditions of synthesis. 

Water-Soluble Trivalent Chromium Compounds. Most water-soluble Cr(III) compounds are produced from the reduction of sodium dichromate or chromic acid solutions. This route is less expensive than dissolving pure chromium metal, it uses high quaHty raw materials that are readily available, and there is more processing fiexibiHty. Finished products from this manufacturing method are marketed as crystals, powders, and Hquid concentrates. 

The primary routes of entry for animal exposure to chromium compounds are inhalation, ingestion, and, for hexavalent compounds, skin penetration. This last route is more important in industrial exposures. Most hexavalent chromium compounds are readily absorbed, are more soluble than trivalent chromium in the pH range 5 to 7, and react with cell membranes. Although hexavalent compounds are more toxic than those of Cr(III), an overexposure to compounds of either oxidation state may lead to inflammation and irritation of the eyes, skin, and the mucous membranes associated with the respiratory and gastrointestinal tracts. Skin ulcers and perforations of nasal septa have been observed in some industrial workers after prolonged exposure to certain hexavalent chromium compounds (108—110), ie, to chromic acid mist or sodium and potassium dichromate. 

Metal Finishing and Corrosion Control. The exceptional corrosion protection provided by electroplated chromium and the protective film created by applying chromium surface conversion techniques to many active metals, has made chromium compounds valuable to the metal finishing industry. Cr(VI) compounds have dominated the formulas employed for electroplating (qv) and surface conversion, but the use of Cr(III) compounds is growing in both areas because of the health and safety problems associated with hexavalent chromium and the low toxicity of trivalent chromium (see... 

The compositions of the conversion baths are proprietary and vary greatly. They may contain either hexavalent or trivalent chromium (179,180), but baths containing both Cr(III) and Cr(VI) are rare. The mechanism of film formation for hexavalent baths has been studied (181,182), and it appears that the strength of the acid and its identity, as well as time and temperature, influences the film s thickness and its final properties, eg, color. The newly prepared film is a very soft, easily damaged gel, but when allowed to age, the film slowly hardens, assumes a hydrophobic character and becomes resistant to abrasion. The film s stmcture can be described as a cross-linked Cr(III) polymer, that uses anion species to link chromium centers. These anions may be hydroxide, chromate, fluoride, and/or others, depending on the composition of the bath (183). 

Chromium plating from hexavalent baths is carried out with insoluble lead-lead peroxide anodes, since chromium anodes would be insoluble (passive). There are three main anode reactions oxidation of water, reoxidation of Cr ions (or more probably complex polychromate compounds) produced at the cathode and gradual thickening of the PbOj film. The anode current density must balance the reduction and reoxidation of trivalent chromium so that the concentration reaches a steady state. From time to time the PbOj film is removed as it increases electrical resistance. 

In groundwater, hexavalent chromium tends to be mobile due to the lack of solubility constraints and the low adsorption of CH6 anion species by metal oxides in neutral to alkaline waters (Calder 1988). Above pH 8.5, no CH6 adsorption occurs in groundwater Cr adsorption increases with decreasing pH. Trivalent chromium species tend to be relatively immobile in most groundwaters because of the precipitation of low-solubility Cr 3 compounds above pH 4 and high adsorption of the Cr+3 ion by soil clay below pH 4 (Calder 1988). 

Among warm-blooded organisms, hexavalent chromium was fatal to dogs in 3 months at 100 mg/kg in their food and killed most mammalian experimental animals at injected doses of 1 to 5 mg Cr/kg body weight, but it had no measurable effect on chickens at dietary levels of 100 mg/kg over a 32-day period. Trivalent chromium compounds were generally less toxic than hexavalent chromium compounds, but significant differences may occur in uptake of anionic and cationic CL3 species, and this difference may affect survival. 

Trivalent chromium was less effective than Cr+6 in reducing fecundity of Daphnia magna 44 pg Cr+VL vs. 10 pg Cr+6/L (USEPA 1980). Annelid worms (Tubifex sp.) accumulated about 1 mg total chromium/kg whole body during exposure for 2 weeks in sediments containing 175 mg ( T+3/kg, suggesting that benthic invertebrates have only a limited ability to accumulate chromium from sediments or clays (Neff et al. 1978). 

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