Ni-EDTA complexes

Ni(CN)4 is greater than that for the Ni-EDTA complex. In fact, the equilibrium constant for the reaction in which EDTA displaces the masking agent... 

The total number of bonds from the ligand to the central metal ion is known as the coordination number. In the Ni -EDTA complex, the coordination number is six. 

FIG. 5, Suggested solution structure and racemization mechanism of the Ni-EDTA complex in aqueous solution. (306)... 

Bryce et al. (1994) demonstrated that in the ternary Ni-EDTA-ferrihydrite system (initial Ni/EDTA molar ratio of 1) the fraction of nickel sorbed was dependent on the component addition sequence (Figure 5.9a), but the fraction of sorbed EDTA was not. EDTA sorption could always be described in terms of the binary EDTA-ferrihydrite system, with the fraction sorbed decreasing sharply above pH 6 (Figure 5.9Zj). When Ni and EDTA were preequilibrated at pH 7, which favored the formation of 1 1 Ni-EDTA complex, the fraction of Ni and EDTA sorbed onto ferrihydrite after 2, 24, and 48 hours was near 100% at pH < 6 but decreased sharply to approximately 0% at pH > 7 (Figure 5.9c). In this system, equilibrium was achieved within 2 hours and the fraction of Ni sorbed at a given pH was equivalent to the fraction of EDTA sorbed, suggesting... 

Many complexing agents are hardly degradable and therefore can be observed in environmental waters. Since the complexing agents such as ethylenediamino tetra-acetic acid (EDTA), nitrilo triace tic acid or aminophosphonic acids are extremely polar compounds, they were determined preferentially by ESl-LC-MS. IC interfaced by ESI to the MS allowed the determination of EDTA in pg L quantities without any pre-concentration [397]. Metal complexes of EDTA were very stable and could be observed after ESI-LC-MS as [M-i-metal] ions in positive ionisation mode [398]. ESI CE-MS was applied to separate and to quantify the stabile Ni-EDTA complexes [399]. 

EDTA forms colored complexes with a variety of metal ions that may serve as the basis for a quantitative spectrophotometric method of analysis. The molar absorptivities of the EDTA complexes of Cu +, Co +, and Ni + at three wavelengths are summarized in the following table (all values of e are in cm )... 

This colour change can be observed with the ions of Mg, Mn, Zn, Cd, Hg, Pb, Cu, Al, Fe, Ti, Co, Ni, and the Pt metals. To maintain the pH constant (ca 10) a buffer mixture is added, and most of the above metals must be kept in solution with the aid of a weak complexing reagent such as ammonia or tartrate. The cations of Cu, Co, Ni, Al, Fe(III), Ti(IV), and certain of the Pt metals form such stable indicator complexes that the dyestuff can no longer be liberated by adding EDTA direct titration of these ions using solochrome black as indicator is therefore impracticable, and the metallic ions are said to block the indicator. However, with Cu, Co, Ni, and Al a back-titration can be carried out, for the rate of reaction of their EDTA complexes with the indicator is extremely slow and it is possible to titrate the excess of EDTA with standard zinc or magnesium ion solution. 

In the case of a solution such as electroless Ni-P, Ni2+ is usually complexed by citrate, and the stability constants are ca. 104 and 2 x 108 (overall value) for the ML and ML2 complexes [67], Thus, pM will change relatively slowly with pH. On the other hand, the stability constant for the Pd-EDTA complex system (ML type only) is reported to be 1024 [67], i.e. Pd2+ is strongly complexed by EDTA. The Pd2+ pM value changes drastically, in a practical electroless deposition sense, over a rather narrow pH range. Consequently, in the case of an electroless Pd solution with EDTA as complexant, the solution may go from a condition of near spontaneous plating out to one where deposition virtually ceases. 

Manganese(II) can be titrated directly to Mn(ni) using hexacyanoferrate(III) as the oxidant. Alternatively, Mn(III), prepared by oxidation of the Mn(II)-EDTA complex with lead dioxide, can be determined by titration with standard iron(II) sulfate. 

Ni/Al, Co/Al, Cu/Al, Rh/Mg/Al and Rh/Ni/Mg/Al catalysts obtained from LDH precursors have been investigated in CO2-reforming of CH4 [93,94]. In addition, Tsyganok et al. also found that Mg/Al LDHs intercalated with an [Ni(EDTA)] chelate complex could be used as a precursor to an efficient catalyst for the above reaction [95]. 

Even complex rate laws may be easily constructed by examining the dependence of F xch on the concentrations of the various species in solution. The rate of exchange of Ni between Ni + and Ni(edta) obeys the rate law... 

A kinetic-photometric method for the determination of EDTA, zinc, and bismuth by interchange reactions of azomethine groups, was reported by Rios and Valcarcel [26], The method involves the in situ formation of the Ni(II) complex with 6-methylpicolinaldehyde thiosemicarbazone when Ni(II), 6-methylpicolinaldazine, and thiosemcarbazide are mixed at pH 4.5. The reaction was monitored spectrophotometrically at 396 nm. 

EDTA was determined indirectly by its interference effect, since the Ni(II) complex with EDTA is more stable than the Ni(II) comples of 6-methylpicolinaldehyde thiosemicarbazone. Since the addition of EDTA decreases both the initial reaction rate and the final absorbance, it can be determined kinetically within the range 5 to 25 pM or photometrically in the range of 5 to 35 pM. 

An indirect AAS method was described for EDTA in the antibiotic streptomycin [42]. The method involved formation of the Ni(II) complex with EDTA, release of complexed Ni(II) by pH adjustment, and subsequent determination of Ni(II) by AAS. EDTA was also determined through its masking effect on the extraction of copper oxinate into methylisobutyl ketone at pH 6.5. The decrease in AAS signal of Cu(II) was linearly proportional to the EDTA concentration [43]. 

NMR and CD methods. (251) From the spectral data it is concluded that these complexes stereospecifically adopt the A-s-cis form(s) in solution [48]. The solution structures of a variety of EDTA-type complexes have been deduced from variable temperature studies. (306) Racemization of Ni(EDTA) and Ni(l,3-DDTA) is rapid at 80°C but is slow for the other complexes studied. The kinetic data indicate predominant 5-coordination by EDTA and 1,3-PDTA. The suggested racemization mechanism (Fig. 5) involves a 7-coordinate intermediate. 

Suppose we have two solutions of Ni + and of Fe " " in water, each 0.001 M in metal ion and 0.1 M in EDTA. If the pH of the solutions is adjusted to 12, will either or both of the metal ions be precipitated as their hydroxides or stay in solution as the EDTA complexes [At pH 12, H4(EDTA) can be regarded as completely dissociated to EDTA .] The relevant data are as follows ... 

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)... 

Nickel (Ni, at. mass 58.71) usually occurs in the II oxidation state, but some complexes contain nickel in higher oxidation states (111 and IV). Nickel(II) sulphide is precipitated at pH 4. Nickel(II) hydroxide (precipitated at pH 7) dissolves in ammonia owing to formation of ammine complexes, but is insoluble in excess of NaOH. Nickel(ll) also forms stable cyanide, oxalate, and EDTA complexes. 

Silver may be separated from Ce, Zr, Th, Be, and Fe(III), on strongly acidic cation-exchangers, by converting these metals into anionic complexes, or separated from Cu, U, Al, and Zn by selective elution with nitric acid [14]. After retention of Pb, Ag, and Hg on Dowex 50, lead is eluted first with 0.25 M ammonium acetate, then silver with 0.5 M ammonia solution. Silver has been separated on a cation-exchanger from Hg, Co, Ni, and Zn on the basis of the differing stabilities of their EDTA complexes at pH 4.6 [15]. Silver retained in a column with a macroporous cation-exchange resin bed has been eluted with 2 M HNO3 or 0.5 M HBr in aqueous acetone solution [16]. 

Mixed Donors. The 4-methylpyridine adduct of bis-[l-(2-thienyl)-4,4,4-trifluoro-butane-l,3-dionato]nickel(ii) is shown to have weaker co-ordination than the nonadduct by X-ray studies. The crystal structure of bis-(8-amino-2-methylquinoline) nitratonickel(ii) nitrate shows that the nitrato ligand is bidentate (66). A kinetic study has been made of nickel(ii) murexide complex formation in DMS0-MeN02- Nickel(ii) complexes of some optically active ethylenediamine-NN -diacetic acid-type polyaminocarboxylic acids have been prepared and solid-state spectra and t.g.a. recorded. N.m.r. temperature dependence for racemization of Ni(edta) ,... 

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