Chromium concentration

Operation nd Control. Control of chromium chromate conversion coating baths is accompHshed by controlling chromium concentration and pH. The quaHty of the conversion coating is sensitive to aluminum accumulations in the coating bath as well as to rinse water purity. Sulfate contamination is a particular problem. 

The hydrolysis equilibria for H2Cr04 given in Table 3 are only valid in HNO or HCIO solutions. Other acids yield complexes such as those shown for chloride and bisulfate ions. The exact composition of chromate(VI) anion(s) present in aqueous solution is a function of both pH and hexavalent chromium concentration (68). However, at pH values above 8, virtually all the Cr(VI) is present as the CrO anion. When the pH is between 2 and 6, an equilibrium mixture of HCrO and Ci2 is present when the pH is below 1, the principal species is H2Cr04 (68,69). At very high Cr(VI) concentrations... 

Instrumental Quantitative Analysis. Methods such as x-ray spectroscopy, oaes, and naa do not necessarily require pretreatment of samples to soluble forms. Only reUable and verified standards are needed. Other instmmental methods that can be used to determine a wide range of chromium concentrations are atomic absorption spectroscopy (aas), flame photometry, icap-aes, and direct current plasma—atomic emission spectroscopy (dcp-aes). These methods caimot distinguish the oxidation states of chromium, and speciation at trace levels usually requires a previous wet-chemical separation. However, the instmmental methods are preferred over (3)-diphenylcarbazide for trace chromium concentrations, because of the difficulty of oxidizing very small quantities of Cr(III). 

The chromium concentration as a function of distance from surface is shown, in Fig. 12.20. The 12% chromium boundary (broken line) represents... 

Fig. 12.20 Chromium concentration in relation to distance from surface for a chromised...

Two ions of the transition metal take part in this reaction. However, in the case of supported one-component catalysts the formation of the active bond seems to occur on the interaction of the monomer with isolated ions of the transition metal. That may be illustrated by the data showing that the activity of chromium oxide catalysts decreases linearly with decreasing chromium content (or even increases per chromium ion) to the rather low (0.01%) chromium concentrations on the catalyst surface (62, 69). In... 

Alternatively, hexavalent chromium can be reduced, precipitated, and floated by ferrous sulfide. By applying ferrous sulfide as a flotation aid to a plating waste with an initial hexavalent chromium concentration of 130 mg/L and total chromium concentration of 155 mg/L, an effluent quality of less than 0.05 mg/L of either chromium species can be achieved if a flotation-filtration wastewater treatment system is used.15... 

Cary E.E., Allaway W.H., Olson O.E. Control of chromium concentrations in food plants, I. Absorption and translocation of chromium by plants. J Agric Food Chem 1997 25 300-304. 

Results reported by various workers for chromium concentrations in seawater are listed in Table 2.1. In most of these methods, co-precipitation with... 

Dubovenko et al. [180] used chemiluminescence to determine total chromium in brines. The method is based on the enhancement of the chemiluminescence by chromium in the reaction of 4-(diethylamino) phthalhydrazide with hydrogen peroxide. The detection limit is 0.025 pg/1 of chromium, and the chemiluminescence is directly proportional to chromium concentrations in the range 5 x 10 10 to 10 6 M. 

Chromium concentrations in field collections of selected species of marine, freshwater, and terrestrial plants and animals... 

Table 2.1 (continued) Chromium Concentrations in Abiotic Materials... 

Table 2.3 Chromium Concentrations in Field Collections of Selected Species of Marine, Freshwater, and Terrestrial Plants and Animals (Values shown are in mg Cr/kg [ppm] whole organism or designated body part fresh weight [FW], dry weight [DW], or ash weight [AW] ... 

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