Chromium III and VI

The cobalt complex is usually formed in a hot acetate-acetic acid medium. After the formation of the cobalt colour, hydrochloric acid or nitric acid is added to decompose the complexes of most of the other heavy metals present. Iron, copper, cerium(IV), chromium(III and VI), nickel, vanadyl vanadium, and copper interfere when present in appreciable quantities. Excess of the reagent minimises the interference of iron(II) iron(III) can be removed by diethyl ether extraction from a hydrochloric acid solution. Most of the interferences can be eliminated by treatment with potassium bromate, followed by the addition of an alkali fluoride. Cobalt may also be isolated by dithizone extraction from a basic medium after copper has been removed (if necessary) from acidic solution. An alumina column may also be used to adsorb the cobalt nitroso-R-chelate anion in the presence of perchloric acid, the other elements are eluted with warm 1M nitric acid, and finally the cobalt complex with 1M sulphuric acid, and the absorbance measured at 500 nm. 

Species may differ by oxidation state for example, manganese(II) and (IV) iron(II) and (III) and chromium(III) and (VI). Oxidation state is influenced by the redox potential. Mobility is affected because oxidation state influences precipitation-dissolution reactions and also toxicity in the case of heavy metals. 

A.K. Minocha, Pankaj Kumar, Jaswinder Singh, L.K. Aggarwal, C.L. Verma, Effect of molybdate (II), chromium (III) and (VI) metal ions on the setting time of ordinary Portland cement. Indian J. Env. Prot., 24, 771-774, 2004. 

Moulin, I. 1999. Lead, Copper, Zinc, Chromium(III) and (VI) Speciation in Cement Hydrated Phases. PhD thesis, Universite Aix-Marseille III, France. 

Popper HH, Grygar E, Ingolic E, et al. 1993. Cytotoxicity of chromium(III) and (VI) compounds. I. In vitro studies using different cell culture systems. Inhal Toxicol 5 345-369. 

For the simultaneous detection of chromium(III) and (VI) after both species have been separated on an ion exchanger, Somerset [153] suggested an oxidation of chro-mium(III) to chromium(VI). The latter can be determined directly via its absorption at 365 nm. Such an oxidation is possible by using potassium peroxodisulfate with a silver catalyst. A complete conversion within a short time is ensured by raising the reaction temperature to about 80 °C. 

Metiers, J.P., Kadara, R.O. and Banks, CJi. (2012) Electroanalytical sensing of chromium(III) and (VI) utilising gold screen printed macro electrodes. Analyst, 137, 896. 

Preparation and chemistry of chromium compounds can be found ia several standard reference books and advanced texts (7,11,12,14). Standard reduction potentials for select chromium species are given ia Table 2 whereas Table 3 is a summary of hydrolysis, complex formation, or other equilibrium constants for oxidation states II, III, and VI. 

Collection of Chromium (III) and Chromium (VI) with Hydrated Iron (III) or Bismuth Oxide... 

Only chromium (III) co-precipitates quantitatively with hydrated iron (III) oxide at the pH of seawater, around 8. In order to collect chromium (VI) directly without pre-treatment, e.g., reduction to chromium (III), hydrated bismuth oxide, which forms an insoluble compound with chromium (VI) was used. Chromium (III) is collected with hydrated bismuth oxide (50 mg per 400 ml seawater). Chromium (VI) in seawater is collected at about pH 4 and chromium (VI) is collected below pH 10. Thus both chromium (III) and chromium (VI) are collected quantitatively at the pH of seawater, i.e., around 8. 

Mullins [37] has described a procedure for determining the concentrations of dissolved chromium species in seawater. Chromium (III) and chromium (VI) separated by co-precipitation with hydrated iron (III) oxide and total chromium are determined separately by conversion to chromium (VI), extraction with ammonium pyrrolidine diethyl dithiocarbamate into methyl isobutyl ketone, and determination by AAS. The detection limit is 40 ng/1 chromium. The dissolved chromium not amenable to separation and direct extraction is calculated by difference. In waters investigated, total concentrations were relatively high (1-5 xg/l), with chromium (VI) the predominant species in all areas sampled with one exception, where organically bound chromium was the major species. 

Parts per billion concentrations of chromium (III) and chromium (VI) in seawater have been determined using high-performance liquid chromatography in conjunction with inductively coupled plasma mass spectrometry [196]. 

Altman C, King EL (1961) The mechanism of the exchange of chromium(III) and chromium(VI) in acidic solution. J Amer Chem Soc 83 2825-2830... 

Sperling, M., S. Xu, and B. Welz. 1992. Determination of chromium(III) and chromium(VI) in water using flow injection on-line preconcentration with selective adsorption on activated alumina and flame atomic absorption spectrometric detection. Anal. Chem. 64 3101-3108. 

Evaluation of the peaks shown in Fig. 4.18 produced with chromium(III) and chromium(VI) standard solutions enables the concentration of these two species in unknown water samples to be deduced. 

The health effects resulting from exposure to chromium(III) and chromium(VI) are fairly well described in the literature. In general, chromium(VI) is more toxic than chromium (III). 

EPA has set the maximum level of chromium(III) and chromium(VI) allowed in drinking water at 100 pg chromium/L. According to EPA, the following levels of chromium(III) and... 

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