Chromium nitrite reduction

The dry gas has been made by direct reduction of a solid mixture of nitrite and nitrate with chromium(III) oxide (3KNO2 4- KNO3 4-Cr203 —4NO 4- 2K2Cr04) but is now more conveniently obtained from a cylinder. 

All observed interferences, except those due to antimony, gold(III) and nitrite, were masked by the pumped addition of 0.02MEDTA (the pH adjusted to 3 with dilute hydrochloric acid) to the sample prior to the reduction step. In the presence of a 5000-fold excess of iron(III), however, depression of the dimethylarsenic signal cannot be completely restored by EDTA (Table 5). Extraction of the sample solution (at pH 2.0) with a 0.005 M solution of dithizone (diphenylthiocarbazone) in dichloromethane overcomes interferences due to silver(I), gold(III), chromium(VI), molybdenum(VI) and tin(II). 

Phosphate is mostly determined photometrically through the reaction with ammonium heptamolyb-date in acidic medium in the presence of potassium antimonyl tartrate as a catalyst. 12-Molybdophos-phoric acid is formed that upon reduction with, for example, ascorbic acid has a blue color. The lower limit of detection is lOpgl. Chromium(VI), nitrite, and high concentrations of chloride and iron interfere. The lower recoveries caused by iron are eliminated by addition of hydrogensulfite. 

The valence states of most inorganic compounds can be changed by either cathodic reduction or anodic oxidation. Thus in principle appropriate species can be either recycled electrochemically or converted to less toxic and environmentally acceptable forms. Sulphur and nitrogen based compounds are thus typically encountered. For example the reduction of nitrates and nitrites and the oxidation of sulphur compounds, such as sulphite, thiosulphate and dithionite, can be carried out quite effectively. There are several established technologies for cyanide and chromium species. 

FIGURE 1,6 Example of manifolds developed for flow injection speciation analysis, (a) Manifold of the reversed FIA system for speciation of chromium with pH measurements and spectrophotometric detection, q—peristaltic pumps, V—injection valves, L— reaction coil, M.E.—pH glass electrode. (Adapted from Ruz, J. et al. 1986. J. Autom. Ghent. 8 70-74.) (b) Manifold of branched FIA system for simultaneous biamperomet-ric determination of nitrate and nitrite. (Cd)Cu—reductive column with copperized cadmium, L—mixing coils. (Adapted from Trojanowicz, M., W. Matuszewski, and B. Szostek. 1992. Anal. Chim. Acta 261 391-398.) (c) Example signal recordings obtained in the FIA system shown in (b) 1-6—standard solutions, A-I—natural water samples. Concentration in standard solutions 1—0.075 2—0.050 3—0.025 mM nitrate and 4—7.5 5—5.0 6—2.5 pM nitrite. 

Oxidation-reduction reactions can cause substantial errors in analysis. For example, soluble iron(II) and manganese(II) are oxidized to insoluble iron(III) and manganese(IV) compounds when an anaerobic water sample is exposed to atmospheric oxygen. Microbial activity can decrease phenol or biological ojg gen demand (BOD) values, change the nitrate-nitrite-ammonia balance, or alter the relative proportions of sulfate and sulfide. Iodide and cyanide frequently are oxidized. Chromium(VI) in solution may be reduced to insoluble chromium(III). Sodium, silicate, and boron are leached from glass container walls. 

Extensive studies of the reductions of nitro substituents in the cobalt(II) cage complex, [Co(sar)] ", have been reported. These groups do not mediate electron transfer in the reactions of the metal center. The complexes [Ru(Hedta)(NO )] and [Fe(Hedta)(NO )] are catalysts in the electrochemical reduction of NOf, and the mechanism of the reaction has been investigated. Whether the product is mainly NjO or NH/ can be determined by appropriate choice of catalyst and pH. The chromium(I) complex, [Cr(N0)(H20)5] ", is resistant to oxidation. Reactions with the powerful oxidants lO and BrO are thought to be inner-sphere, with attack on the nitrosyl nitrogen atom to give nitrite and chromium(III) as products in a net two-electron reaction. ... 

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