Cr-EDTA complex

For instance, Cr(III) ions may coexist with Co(II) and Cu(II) ions in a complex sample. The latter two ions may produce CL emission under similar conditions as for Cr(III). Fortunately, the formation rate of the Cr-EDTA complex is relatively slower, making possible the selective determination of this metal ion in waste water [8], urine, blood, and hair [9], Owing to the small number of CL reagents explored in recent years, the elements covered by CL techniques are still rather limited. Up to now only a few elements have been found to produce direct CL emission when reacted with CL reagents. Most of the publications so far involve indirect methods for the detection of elements. 

Ciesla P, Karocki A, Stasicka Z. Photoredox behaviour of the Cr-EDTA complex and its environmental aspects. J Photochem Photobiol A Chem 2004 162 537 14. 

Added edta has an accelerative effect on the reaction with local rate maxima at edta Cr ratios of 1 2 and 1 1. The proposed mechanism involves stable Cr -edta complexes and ternary complex formation with hydrazine ... 

The purple [Cr (edta)] complex was shown to be a reaction product. 

The effect of edta on the reaction was reinvestigated in another paper. Though the above experimental results were verified, the reaction with added edta was found to be pH-dependent and the local rate maxima resulted from a combination of a monotonic edta acceleration and a rate decrease due to change in pH. When the pH was maintained constant, no maxima were observed. Spectroscopic evidence for Cr -edta complexes was attributed to experimental error in the measurements. 

Nazmutdinov, R.R., TsirUna, G.A., Kharkats, Y.I. et al (1998) Activation energy of electron transfer between a metal electrode and reagents of nonspherical form and complicated charge distribution. Cr(EDTA) complexes. J. Phys. Chem. B, 102, 677-686. 

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... 

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. 

Rhodium forms an EDTA complex isomorphous with the corresponding ones of Ru, Fe, Ga and Cr. In Rh(EDTAH)(H20) one carboxylate is proton-ated and thus the acid is pentadentate, the water molecule completing the octahedron (Figure 2.43). 

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. ... 

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... 

The kinetics of reactions of M0O4- leading to the binuclear complexes Co(NH3)5OMo03 and Cr(edta)OMo03- have been studied around pH = 7.55 Interestingly, the reaction mechanisms for the formation of these two species differ. The Co complex is formed by equation (4)... 

General anion binding site [Gd(dipic)3], [Cr(CN)J"-, [Cr(oxalate)3], and EDTA complexes of above cations Cytochrome-c ... 

Back-titration is useful for the determination of cations that form stable EDTA complexes and for which a satisfactory indicator is not available. The method is also useful for cations such as Cr(III) and Co(III) that react only slowly with EDTA. A measured excess of standard EDTA solution is added to the analyte solution. After the reaction is judged complete, the excess EDTA is back-titrated with a standard magnesium or zinc ion solution to an Eriochrome Black T or Calmagite end point. For this procedure to be successful, it is necessary that the magnesium or zinc ions form an EDTA complex that is less stable than the corresponding analyte complex. 

Chromium is usually determined by the diphenylcarbazide method. This method is particularly useful for determining traces of chromium. Larger amounts of chromium can be determined either by the chromate method, or by the method based on the Cr(IlI)-EDTA complex. 

Fig. 8 The effect of the addition of the cationic surfactant tetradecyltrimethylammonium bromide (TTAB) in the separation buffer for the separation of ion complexes with the chelating agent EDTA. TTAB, 0.5 mM, was added to the separation buffer for the separation of N03 (1), Cu-EDTA, Pb-EDTA, EDTA (2), Cr-EDTA (3), and Fe-EDTA (4). (For details of experimental conditions, see Ref. 341.)...

Silica gel H H20-glycol monomethyl ether-methylethyl ketone-acetone-NH4OH (40 20 20 20 0.15) Natural colour 20% ammonium peroxy disulphate solution EDTA complexes of Co, Cu, Ni, Mn, Cr, Fe... 

Such phenomena have been reported for electrode reactions of Cr(EDTA) and Fe(CN)g " in which the viscosity of the solvent (water or dimethyl sulfoxide) is varied by the addition of dextrose or sucrose [64—66]. Solvent friction has also been invoked in reductions of thiophenecarboxylatopenlaamminecobalt(HI) complexes [67] and of metallocenes [68, 69] at Hg electrodes in organic solvents. Of particular interest in the present context are studies by Murray et al. [70, 71] of the reduction kinetics of Co(bpy)3 in a variety of organic solvents, in which kei was found to be proportional to 1/rj, to 1/tl, and to the mean diffusion coefficient D of the reactants over a huge range (11 orders of magnitude) of kei-... 

An analogous comparison of the reactivity of anions and anionic complexes of Table VI reveals several interesting facts. In this table there are 40 compounds which react with e m at a rate which is more than 70% of the calculated diffusion controlled rate (11 of these compounds have been measured in the present study for the first time). However, only 18 compounds have a values between 0.7 and 1.5 and we consider them to agree reasonably well with the calculated ones. In some cases, the high a values may be because of the fact that they were calculated from the experimental rate constants without correcting for salt effects. These include the following compounds Cr(EDTA)", (6.9) Cr(OX) v3", (6.10) Co(CN)o3", (6.20) Co(CN)5CF, (6.21) Co(CN)5N023", (6.22) Co(N02)63", (6.23) and Co(EDTA)", (6.24). 

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