EDTA complexes, with chromium

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... 

BackTitrations. In the performance of aback titration, a known, but excess quantity of EDTA or other chelon is added, the pH is now properly adjusted, and the excess of the chelon is titrated with a suitable standard metal salt solution. Back titration procedures are especially useful when the metal ion to be determined cannot be kept in solution under the titration conditions or where the reaction of the metal ion with the chelon occurs too slowly to permit a direct titration, as in the titration of chromium(III) with EDTA. Back titration procedures sometimes permit a metal ion to be determined by the use of a metal indicator that is blocked by that ion in a direct titration. Eor example, nickel, cobalt, or aluminum form such stable complexes with Eriochrome Black T that the direct titration would fail. However, if an excess of EDTA is added before the indicator, no blocking occurs in the back titration with a magnesium or zinc salt solution. These metal ion titrants are chosen because they form EDTA complexes of relatively low stability, thereby avoiding the possible titration of EDTA bound by the sample metal ion. 

Chromium (ITT) can be analy2ed to a lower limit of 5 x 10 ° M by luminol—hydrogen peroxide without separating from other metals. Ethylenediaminetetraacetic acid (EDTA) is added to deactivate most interferences. Chromium (ITT) itself is deactivated slowly by complexation with EDTA measurement of the sample after Cr(III) deactivation is complete provides a blank which can be subtracted to eliminate interference from such ions as iron(II), inon(III), and cobalt(II), which are not sufficiently deactivated by EDTA (275). 

Iron (and nickel, if present) can be determined by adding an excess of standard EDTA to the cold solution, and then back-titrating the solution with lead nitrate solution using xylenol orange as indicator provided the solution is kept cold, chromium does not react. The solution from the back-titration is then acidified, excess of standard EDTA solution added and the solution boiled for 15 minutes when the red-violet Cr(III)-EDTA complex is produced. After cooling and buffering to pH 6, the excess EDTA is then titrated with the lead nitrate solution. 

Optical activity of cobalt(III), chromium(III) and rhodium III) complexes with aminopolycarboxy-late, edta-type and related ligands. D. J. Radanovic, Coord. Chem. Rev., 1984, 54, 159-261 (195). 

Seitz, Suydam, and Hercules 186> recently developed on the basis of luminol chemiluminescence a method for chromium-III ion determination which has a detection limit of about 0.025 ppb. The method is specific for free chromium-III ions as chromium-VI compounds have no catalytic effect and other metal ions can be converted to a non-catalytic form by complexing with EDTA, since the chromium-III complex of EDTA, which is in any case not catalytically active, is formed kinetically slowly 186>. To detect extremely small light emissions, and hence very small metal concentrations, a flow system was used which allows the reactants to be mixed directly in front of a multiplier. (For a detailed description of the apparatus, see 186>). 

Chromium(III)-edta complexes are typically prepared1220 by refluxing the ligand with a suitable chromium(III) salt for about half an hour followed by recrystallization. The formation of [Cr,n(Hedta)H20] from acetato, formamato and trifluoroacetato chromium(III) complexes... 

Ethylenediaminetetraacetic acid (EDTA, formula 17.2) forms coloured complexes with cations which have chromophoric properties (e.g., Fe, Cr, Cu, Co, Ni). These complexes, which are not very intensely coloured, form the basis of several less sensitive spectrophotometric methods, such as that for chromium(III)... 

With chromium(III), EDTA forms a violet complex in slightly acidic medium. The complex is formed slowly in the cold, but more rapidly if the solution is heated [28,29]. The sensitivity of the method is not high. The molar absorptivity is 1.4-10 at 540 nm (a = 0.003). The colour intensity diminishes as the pH is reduced. In a hot solution EDTA reduces Cr(VI) to Cr(III). This reaction is catalyzed by traces of Mn(II). 

Coloured ions, and those giving coloured complexes with EDTA, interfere in the determination of chromium as its EDTA complex. Oxalic and citric acids interfere in the colour reaction. 

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 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-... 

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... 

Formation reactions of chromium(iii)-carboxylate complexes often do not follow the normal kinetic pattern, as has been illustrated by the study of the reaction of [Cr(NH3)6(OH2)] + with amino-acids. Here reaction takes place exclusively by a carbon dioxide-catalysed pathway in which ammonia is displaced to give an intermediate carbonato-chelate. The ratedetermining step in the formation of chromium(rii)-edta complexes is a ligand interchange process. The dependence of rates on pH can be explained in terms of the relative reactivities of variously protonated forms of edta and of the [Cr(OH2)e] + and [Cr(OH2)s(OH)] + cations. The chelation step in the conversion of the quadridentate chromium(m)-edta complex to the quinquedentate form in aqueous solution has been studied over the pH range 0—12. From the rate-pH profile (see Figure) it is apparent that... 

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. " ... 

Because of the opposite charges of the two redox chromium species and the possibility to use an eluent without carbon, lEC is often preferred before detection by ICP-MS. Byrdy et have used an anion-exchange column that allows the retention of both Cr(VI) and Cr(III) after transformation of this last form into an anionic complex with EDTA. However, the ammonium sulfate mobile phase did not allow the analysis of Cr" " because this isotope is interfered by Although the presence of chloride ions did not disturb the detection of... 

Other methods reported for the determination of beryllium include UV-visible spectrophotometry [80,81,83], gas chromatography (GC) [82], flame atomic absorption spectrometry (AAS) [84-88] and graphite furnace (GF) AAS [89-96]. The ligand acetylacetone (acac) reacts with beryllium to form a beryllium-acac complex, and has been extensively used as an extracting reagent of beryllium. Indeed, the solvent extraction of beryllium as the acety-lacetonate complex in the presence of EDTA has been used as a pretreatment method prior to atomic absorption spectrometry [85-87]. Less than 1 p,g of beryllium can be separated from milligram levels of iron, aluminium, chromium, zinc, copper, manganese, silver, selenium, and uranium by this method. See also Sect. 5.74.9. 

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... 

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