Chromium EDTA oxidation

Although the addition of manganese(u) to the reacting system in conditions of an excess of chromium(vi) over hydrazine causes a retardation of the rate of disappearance of the oxidant, the effect of the divalent cation is to provide a reaction pathway where one half of the hydrazine is oxidized to yield ammonia. A similar observation has been made on the reaction in excess reductant in the absence of Mn . In the presence of edta, however, where the chromium(vi) oxidation of the aminopolycarbo-xylate is very slow, the reaction rate increases— the product now being the substitution-inert Cr" -edta complex ion. Experimentally, half the chro-mium(vi) is converted to this product and this is considered to derive from a dimeric chromium(vi) species involving both edta and hydrazine as ligands. In the reaction with hydroxylamine, the stoicheiometry is dependent on the reagent in excess, i.e. in the presence of excess oxidant... 

As it was not known what kind of organic matter acts as the major ligand for chromium in seawater, Nakayama et al. [38] used ethylene diaminetetra-acetic acid (EDTA) and 8-quinolinol-4-sulfuric acid to examine the collection and decomposition of organic chromium species, because these ligands form quite stable water-soluble complexes with chromium (III), although they are not actually present in seawater. Both of these chromium (III) chelates are stable in seawater at pH 8.1 and are hardly collected with either of the hydrated oxides. The organic chromium species were then decomposed to inorganic... 

Chromium(II) is a very effective and important reducing agent that has played a significant and historical role in the development of redox mechanisms (Chap. 5). It has a facile ability to take part in inner-sphere redox reactions (Prob. 9). The coordinated water of Cr(II) is easily replaced by the potential bridging group of the oxidant, and after intramolecular electron transfer, the Cr(III) carries the bridging group away with it and as it is an inert product, it can be easily identified. There have been many studies of the interaction of Cr(II) with Co(III) complexes (Tables 2.6 and 5.7) and with Cr(III) complexes (Table 5.8). Only a few reductions by Cr(II) are outer-sphere (Table 5.7). By contrast, Cr(edta) Ref. 69 and Cr(bpy)3 are very effective outer-sphere reductants (Table 5.7). 

Chromiain(ii) Complexes.—The oxidation of chromium(ii) in alkaline solution has been studied polarographically and the reaction shown to be irreversible with = — 1.65 V vs. S.C.E. In the presence of nitrilotriacetic acid, salicylate, ethylenediamine, and edta the values were determined as —1.075, —1.33, — 1.38, and —1.48 V, respectively. The production of [Cr(edta)NO] from [Cr (edta)H20] and NO, NOJ, or NO2 suggests that this complex is able to react via an inner-sphere mechanism in its redox reactions. ... 

The chromium(II)-edta system is powerfully reducing (the half-wave potential is -1.48 V at pH 12 vs. SCE) and has been used in the reduction of iron-sulfur clusters.292 No solid complex has been isolated because of its instability to oxidation, but Cru-edta is high-spin in aqueous solution (/ieff = 5.12 BM) and its stability constant has been determined. The edta is believed to be pentadentate with H20 in the sixth position.293... 

Metal salts may be used in the treatment of wool. Flame methods for the determination of aluminium [185], barium, chromium, copper, mercury, strontium, tin, zinc [186] and zirconium [187] in wool have been published. Standard additions to wool cleaned by soaking and washing it with disodium EDTA (800 ml of 0.5 M for 30g wool with soaking for 3 days and double washing) was used as the calibration technique. This compensated for interferences from hydrochloric acid and amino-acids. The samples were equilibrated to a constant humidity for 24 h and then 0.3 g sealed with 5 ml of constant boiling point hydrochloric acid in a glass tube. The tubes were placed in an oven at 110UC for 20 h. The nitrous oxide/acetylene flame was used for the determination of aluminium and zirconium. Sulphate, phosphate, citrate and silicate have been found to interfere in the determination of titanium and zirconium in fire-proofed wool [188], These flame... 

A simple and rapid method for the iodometric determination of microgram amounts of chromium(ni) in organic chelates is based on the oxidation of chromium(III) with periodate at pH 3.2, removal of the umeacted periodate by masking with molybdate and subsequent iodometric determination of the liberated iodate . Iodometric titration was also used for determination of the effective isoascorbate (see 2) concentration in fermentation processes . The content of calcium ascorbate can be determined with high sensitivity by complexometric titration with edta, which is superior to iodometry. The purity of /3 -diketonate complexes of Al, Ga, In and Ni was determined by complexometric titration with edta at pH 5.5-3, with RSD < 0.01 for determining 5-30% metal ion. Good analytical results were obtained by a similar procedure for the metal content of 15 lanthanide organic complexes. ... 

The diphenylcarbazide method is almost specific for chromium(Vl). Interferences result only from Fe, V, Mo, Cu, and Hg(II) present at much higher concentrations than the chromium. Iron(lll) can be masked by phosphoric acid or EDTA. Iron(III) can also be separated as Fe(OH>3, after chromium has been oxidized to Cr(VI), or by extraction. Vanadium can be separated from Cr(VI) by extraction as its oxinate at pH -4. Molybdenum is masked with oxalic acid, and Hg(II) is converted into the chloride complex. 

Chemicals used were of standard analytical grade. Fe(lll) was analysed by complexomentric (EDTA) titration [5] with NH4SCN indicator at pH 3. Fe(Il) was estimated by deducting Fe(III) from total Fe found after oxidation of a mix of Fe(IlI) and Fe(II) solution. Chromium was estimated spectrophotometrically at mO 373 as chromate with molar extinction co-efficient of 4815 in IM NaOH solution. Sulphate was analysed by a Nephelometer against a calibrated curve of standard BaS04 suspension. 

The reaction of superoxotitanium(IV) with a number of substrates has been monitored by stopped-flow techniques/ In 1 M perchloric acid, the oxidation of iodide and bromide proceeded with second-order ratde constants of 1.1 x 10 M s and 2M s respectively. It is proposed that the reduction of superoxotitanium(IV) proceeds by a one-electron mechanism. Based on proton dependences, the species TiO " is more reactive than the protonated form Ti02(0H)2. The chromium chelate, bis(2-ethyl-2-hydroxybutyrato)oxochro-mate(V), is reduced by iodide, generating a Cr(IV) intermediate. The reaction is considered to proceed through formation of an iodine atom (T) for which both Cr(V) and Cr(IV) compete. In aqueous solution, [Co(EDTA)] forms a tight ion pair with I . Upon irradiation of this ion pair at 313 nm, reduction of [Co(EDTA)] to [Co(EDTA)] occurs with oxidation of 1 to IJ. The results may be interpreted on the basis of a mechanism in which [Co(EDTA)] and V are the primary photoproducts where the latter subsequently disproportionate to I3 and 1 . The kinetics and mechanism of the oxidation of 1 by a number of tetraaza macrocyclic complexes of Ni(III) have been reported. Variations in rate constants and reaction pathways are attributable to structural differences in the macrocyclic ligands. Of interest is the fact that with some of the Ni(III) complexes, spectrophotometric evidence has been obtained for an inner-sphere process with characterization of the transient [Ni(III) L(I)] intermediates. Iodide has also been used as a reductant for a nickel(III) complex of R-2-methyl-1,4,7-triazacylononane. In contrast to the square-planar macrocycles, the octahedral... 

The kinetics of the oxidation of [Cr(OH2)6] by Ce(IV) and IO4 as well as a series of aminocarboxylate [(edta, 2-hydroxy-ethylene diaminetriace-tate (toh), nta ] chromium(III) complexes by 10 have been reported. In all cases, the product is Cr(VI) [Eqs. (14) and (15)] and two of the studies describe the effect of mixed solvents(Table 6.9). An inner-sphere electron transfer process is proposed for reaction (14). 

Chromium oxide (Cr203). Up to 0.1% colorimetrically with diphenylcarbizide at 540 nm. Above 0.1% but as a minor constituent, colorimetrically with EDTA at 550 nm. As a major constituent by oxidation to dichromate by peroxodisulfuric acid using a silver nitrate catalyst, destruction of permanganate with HCl and titration against ferrous ammonium sulfate using diphenylamine-4-sulfonate indicator. 

Hydrogen peroxide catalyses the reaction between [Cr(H20)6] and edta and the Cr i-edta complex accelerates the decomposition of the substrate. In the absence of edta, the analysis of the kinetic trace suggests that Cr v and/or Cr are catalysts for the decomposition reaction in acidic media. The rate of oxygen evolution decreases markedly in the presence of edta, suggesting that the complexes are less active catalysts for the decomposition process. Any catalytic edta-containing species are considered as having chromium in an oxidation state greater than +3. A detailed reaction scheme for the redox process may be simplified to include the reactions (ox=oxidation, sub=substitution) ... 

With oxides containing higher chromium contents, the Lomi process proceeds very slowly since vanadous picolinate is not able to reduce Cr(III). If the chromium concentration in the oxide exceeds 15%, the dissolution rates will become unacceptably low. Therefore, in such applications, the Cr(III) must first be oxidized to Cr(VI) by an appropriate preoxidation step (AP and NP steps, see below). In order to save this time-consuming additional step, attempts have been made to replace V(II) by another reducing cation. Only a combination of Cr(II) with nitrilo triacetic acid or with EDTA shows a faster reaction, but it suffers from insufficient thermal stability thus, it cannot be used in the decontamination of systems and circuits. 

The reduction of ferricytochrome c by hydrated electrons and by several free radicals has been studied by pulse radiolysis. The reduction of oxidized cytochrome c by [Fe(edta)] - follows first-order kinetics for both protein and reductant, with a rate constant of 2.57 x 10 1 mol" s" at pH 7 and activation enthalpy and entropy of 6.0 kcal mol" and —18 cal K" mol", respectively. These values are comparable to those for outer-sphere cytochrome c reductions and redox reactions involving simple iron complexes, and are compatible with outer-sphere attack of [Fe(edta)] " at the exposed haem edge, although the possibility of adjacent attack through the haem pocket is not ruled out. The rate data at pH 9 are consistent with [Fe(edta)] " reduction of two slowly interconverting forms of the protein, native kt = 2.05 X10 1 mol" S" ) and high-pH kt = 2.67 x 10 1 mol" s" ) isomers. A possible route for the transfer of the electron from Cr + to ferricytochrome c has been suggested as a result of the chemical analysis of the chromium(m) product. The reduction by Cr + of the native protein and of ferricytochrome c carboxy-methylated at the haem-linked methionine (residue 80) has been studied kinetically. At pH 6.5 the former process is simple and corresponds to a second-order rate constant of 1.21 x 10 1 mol" s". The latter, however, is complex - two chromium-... 

Reduction of [Co(EDTA)] by Cr " shows a reactivity pattern corresponding to other inner-sphere reactions of this oxidant and its derivatives. The product is a carboxylate-bound EDTA complex of chromium(III). 

ESR and visible spectroscopic evidence has been found for chromium(V) intermediates during the oxidation of [Cr(L)(OH] (L = hydroxyethylenediaminetriacetate or edta) and [Cr(LL)2(OH)2] (LL = ox, mal) with H202. Oxidation of [Cr(Hedta)(OH2)] with IO4 to Cr04 has also been studied/ The photochemical or thermal formation of chromium(V) complexes with crown ethers is reported from reactions of Cr207 in nonaqueous solvents in the presence of crowns. " ... 

It is well known that paracetamol undergoes redox reaction with dichromate in presence of higher H SO amount (6.0 mol dm ) to form chromium (III) as the reaction product. This reaction is slow, but is sharply increased by the addition of trace amounts of Mn (II) and EDTA. Therefore, in order to take full advantage of the role of Mn (II) and EDTA, the reaction conditions (HCIO, concentration, and temperature) and reagent concentrations (dichromate, paracetamol, Mn (II), and EDTA) must be optimized. Oxidation of paracetamol by dichromate has been studied kinetically as a function of [PCM], [Cr(VI)j, [Mn(II)j, [EDTA], [bpy], [HCIOJ, and [SDS]. The results are compiled in Tables 1-2 and Figures 1-5. 

In presence of Mn (II), the reaction proceeds through the formation of a ter-molecular complex between Cr(VI), paracetomal, and Mn (II) (Scheme 2) [19] because the direct oxidation of Mn (II) by chromium (VI) is thermodynamically unfavorable [20]. The positive catalytic effect ofMn (II) is due to a one-step three-electron reduction of chromium (VI), which is in conformity the reduction of Cr(VI) —> Cr (III) without passing through formation of Cr(IV) as an intermediate. Table 1 shows the effect of EDTA and bpy on the reaction rate. It was found that whereas the reduction of paracetamol by chromium (VI) is slow, reduction in presence of EDTA/bpy at a similar concentration is fairly fast. It should be emphasized here that the complexing agents (EDTA and bpy) themselves are resistant to... 

Although a number of spectrophotometric methods are available for the determination of paracetamol, these are generally associated with some or the other demerits. The use of chromium (VI) for the determination of paracetamol has been suggested but the reaction requires a high concentration of H SO and very high temperature for the complete consumption of chromium(VI). The results from this study show that oxidation of paracetamol by Cr (VI) is enhanced in presence of complexing events (Mn(II), EDTA, and bpy) and surfactant. Of these, Mn (II) is the most effective as only 5-10 minutes are required for the completion of the reaction. This is very significant for any industrial use to avoid or minimize the use of h her acid concentrations. The present method is simple, accurate, rapid, economical, and precise. 

Binary systems synthesized consisted of Cu/Fe, Ni/Fe, Cu/Al and Ni/Al and Cu/Cr for 4-10 wt percent Cu or Ni in the calcined mixed oxide. Anionic complexing agents acetic, citric and oxalic acids and EDTA were used in molar ratios of 1 1 with the initial copper or nickel. Two stage precipitations were used starting with an initial formation of aluminum, chromium or ferric hydroxide by addition of NaOH to an aqueous solution of A1 nitrate, Cr nitrate or Fe chloride. In the second stage aqueous solutions of Cu sulfate or Ni nitrate were mixed with the initial precipitate with or without the presence of a 1 1 mole ratio of selected anionic complexing agents to complete the precipitation. A second mode of coprecipitation used was to preadsorb oxalic acid on the initially precipitated AI, Cr or Fe hydroxide. 

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