Nickel-EDTA complex

Nickel may be determined in the presence of a large excess of iron(III) in weakly acidic solution by adding EDTA and triethanolamine the intense brown precipitate dissolves upon the addition of aqueous sodium hydroxide to yield a colourless solution. The iron(III) is present as the triethanolamine complex and only the nickel is complexed by the EDTA. The excess of EDTA is back-titrated with standard calcium chloride solution in the presence of thymolphthalexone indicator. The colour change is from colourless or very pale blue to an intense blue. The nickel-EDTA complex has a faint blue colour the solution should contain less than 35 mg of nickel per 100 mL. 

Exercise 1 Calculate the conditional constant of the nickel-EDTA complex in a solution buffered with a mixture of ammonia (O.lmol/L) and ammonium ions (0.1 mol/L). The measured pH is 9.24 (pK value of ammonium ion). The stability constants are pi = = 10 P3NH3 iq6.77 7.96 ... 

Let s again consider the example of the nickel-EDTA complex in a solution buffered with the couple NH4VNH3 (Exercise 1). In the case in which conditional constants are not used, the mass balance equations are... 

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. 

Show that CN is an appropriate masking agent for Nb+ in a method in which nickel s complexation with EDTA is an interference. 

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. 

Ion chromatography has been applied to the determination of cobalt, nickel, copper, zinc and cadmium as their EDTA complexes using anion separation and suppressor columns and 0.03pm sodium bicarbonate0.03gm sodium carbonate [28] eluant and a conductiometric detector. 

Ferro-Garcia et al. [187] noted that EDTA complexation (see the section on nickel above) may not have the beneficial effect of eliminating the electrostatic repulsion between a positively charged surface and Cu"" (as well as Zn " and Cd- ) cations. This is illustrated in Table 5 and was rationalized by postulating that, in contrast to the smaller inorganic complexes, the Cu-EDTA complex may be excluded from a large fraction of pores in the olive-stone-derived microporous carbon used (with 60% of pores less than 7.5 nm in diameter). 

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... 

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. 

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) ,... 

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 first step in the reaction of the cydta complex of cobalt(ii) with cyanide is reversible formation of a 1 1 seven-co-ordinate adduct. Formation of this adduct follows a second-order rate law and has the extraordinarily low activation energy of 0-8 kcalmol". Further reaction to [Co(CN)5] " is slow. The rate law for reaction of the 1 1 adduct with cyanide is first-order in adduct, second-order in cyanide. Thus the overall reaction is third-order in cyanide, like the related reactions of the edta complexes of cobalt(n) and of nickel(n). The mono-ida and -mida complexes of nickel(ii) react with cyanide by rapid reversible addition of two cyanides the rate-determining step en route to [Ni(CN)4] is reaction with a third cyanide. The overall reaction is thus again third-order in cyanide concentration. [Ni(ida)2] and [Ni(mida)2] react slowly with cyanide by parallel dissociative and associative paths the Ni(ida) and [Ni(ida)(CN)] so formed (or their mida analogues) then react rapidly with further cyanide to give [Ni(CN)4]. Reaction of [Ni(trien)] + with cyanide is fifth-order overall first-order in complex, fourth-order in total cyanide concentration. ... 

The effect of added ligands on this type of reaction has been studied for the replacement of edta complexed to nickel(n) by Eriochrome Black T. 

The effects of added ions on rates, arising from competitive formation of intermediate mixed complexes, have been described for acetate on the reaction of the edta complex of manganese(ii) with cobalt(ii), for acetate and phthalate on cobalt(ii)-nta plus nickel(ii), and for copper(ii) or excess lead(ii) on cadmium(ii)-cydta plus lead(n). ... 

Recent kinetic studies of metal ion exchanges include those of europium(iii) with its cydta complex and of various lanthanides(iii) with their respective dtpa complexes. Kinetic studies of metal ion replacement include reactions of nta, edda, heedta, egta, cydta, and dtpa complexes of zinc(ii) with copper(ii), of edta nedta, cydta, and dtpa complexes of lead(ii) with cobalt(ii), of the edta complex of nickel(ii) with indium(iii), and of ttha complexes of cadmium(ii) with lanthanides(in). There are again several systems mainly concerned with 3 + and 4+ ions, for example those involving edta and dtpa complexes of lanthanides. In systems containing ions of high charge, it seems to be easier to demonstrate the existence of dinuclear intermediates of the type invoked in associative (cf. above) pathways. 

The precipitate forms slowly In neutral or dilute acid solutions (up to 0.1 N hydrochloric or 0.3 N nitric acid). The precipitate forms rapidly from an acid solution made ammonlacal. Tlie advantage of the homogeneous precipitation la the production of a coarse, crystalline, easily filtered precipitate and reduction of occluded contaminants (F2). Separations using thloacetamlde can be made more specific by the addition of EDTA (F3). Some cations such as cadmium, cobalt, iron and nickel fall to precipitate from ammonlacal solution due to formation of EDTA complexes. The precipitation of lead is delayed. In the presence... 

The most sensitive - and perhaps for the radiochemist, the most useful volumetric procedure is complexlmetrlc titration utilizing the lead EDTA complex. A number of Indicators have been used for the direct EDTA titration (W5) The most popular of these are Erlochrome Black T (Cl) (W5)j Eriochrome Red B and X-ylenol orange (W5)- The direct titration with the sodium salt of EDTA is carried out in a pH 10 buffer solution (P5)(p6). Iron, the alkaline earths and the earths interfere but bismuth, aluminum and antimony do not. Cyanide can be used to mask cobalt, nickel, copper, zinc, cadmium, mercury and platinum (Cl),... 

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