Mass spectrometric investigations of the kinetic stability of chromium and copper complexes with humic substances by isotope-labelling experiments

Isotope-labelling exchange experiments were carried out to investigate the kinetic stability of Cr(III) complexes with humic substances (HS). To compare the results with those of an ion, not expected to form kinetically stable HS complexes with respect to its electron configuration, Cu(II) was investigated under the same conditions. HS solutions of different origin were therefore spiked with ⁵³Cr(III) or ⁶⁵Cu(II) after saturation of HS with chromium and copper of natural isotopic composition. In fractions of metal/HS complexes with different molecular weight, obtained by ultrafiltration and HPLC/ICP-MS using size exclusion chromatography (SEC), respectively, the isotope ratios of chromium and copper were determined by ICP and thermal ionisation mass spectrometry. Distinct differences in the isotopic composition of chromium were found in the permeate of the ultrafiltration compared with the corresponding unseparated solution, which indicates kinetically stable Cr(III)/HS complexes. On the other hand, the copper isotopic composition was identical in the permeate and the unseparated solution, which shows that a total exchange of Cu²⁺ ions took place between free and HS complexed copper ions. The SEC/ ICP-MS experiments also resulted in a different isotopic distribution of chromium in the chromatographically separated complexes whereas the copper complexes, separated by SEC, showed identical isotopic composition. The kinetic stability of Cr(III)/HS complexes could be explained by the d³ electron configuration of Cr³⁺ ions, a fact which is well known from classical Cr(III) complexes, and influences substantially the mobility of this heavy metal in the environment. Received: 7 December 1998 / Revised: 25 March 1999 / Accepted: 27 March 1999     

Synthesis,Characterization and Magnetic Studies of μ‐Oxamido Copper (II) ‐ Chromium (III) Heterodinuclear Complexes

Three new μ‐oxamido‐bridged heterodinuclear copper (II)‐chromium (III) complexes formulated [Cu(Me₂oxpn)Cr‐(L)₂](NO₃)₃, where Me₂oxpn denotes N,N'‐bis(3‐amino‐2, 2‐dimethylpropyl)oxamido dianion and L represents 5‐methyl‐1,10‐phenanthroline (Mephen), 4,7‐diphenyl‐1,10‐phenanthroline (Ph₂phen) or 2,2′‐bipyridine (bpy), have been synthesized and characterized by elemental analyses, IR and electronic spectral studies, magnetic moments of room‐temperature and molar conductivity measurements. It is proposed that these complexes have oxamido‐bridged structures consisting of planar copper (II) and octahedral chromium (III) ions. The variable temperature magnetic susceptibilities (4.2–300 K) of complexes [Cu(Me₂oxpn)Cr(Ph₂phen)₂](NO₃)₃ (1) and [Cu(Me₂oxpn)Cr(Mephen)₂] (NO₃)₃ (2) were further measured and studied, demonstrating the ferromagnetic interaction between the adjacent chromium (III) and copper (II) ions through the oxamido‐bridge in both complexes 1 and 2. Based on the spin Hamiltonian, ? = ‐ 2J?₁ · ?₂, the exchange integrals J were evaluated as + 21.5 an^?1 for 1 and + 22.8 cm^?1 for 2.     

Determination of chromium(III) and total chromium in natural waters using a surface ion-imprinted silica gel as selective adsorbent

A sensitive and selective method has been developed to determine Cr(III) and total Cr in natural water samples by ICP-AES with a Cr(III)-imprinted aminopropyl-functionalised silica gel adsorbent. The Cr(III)-imprinted and non-imprinted adsorbent were prepared by an easy one-step reaction with a surface imprinting technique. Their maximum static adsorption capacities for Cr(III) were 11.12 mg g^?1 and 3.81 mg g^?1, respectively. The relative selectivity factors (α _r) for Cr(III)/Co(II), Cr(III)/Au(III), Cr(III)/Ni(II), Cr(III)/Cu(II), Cr(III)/Zn(II), and Cr(III)/Cr(VI), were 377, 21.4, 15.4, 27.7, 26.4, and 31.9, respectively. Under the optimal conditions, Cr(III) can be absorbed quantitatively, but Cr(VI) was not retained. Total chromium was obtained after reducing Cr(VI) to Cr(III) with hydroxyammonium chloride. The detection limit (3σ) for Cr(III) was 0.11 ng mL^?1. The relative standard deviation was 1.2%. The proposed method has been validated by analysing two certified reference materials and successfully applied to the determination and speciation of chromium in natural water samples with satisfactory results.     

Hyperbranched bisphosphinoamine ligands: synthesis,characterization and application in ethylene oligomerization

Two hyperbranched bisphosphinoamine (PNP) ligands and chromium complexes were synthesized in good yield with 1.0 generation (1.0 G) hyperbranched macromolecules, chlorodiphenylphosphine (Ph₂PCl) and CrCl₃(THF)₃ as raw materials. The hyperbranched PNP ligands and chromium complexes were characterized by FT-IR, ¹H NMR, ³¹P NMR, UV and ESI-MS. Comparing with the chromium complexes, the hyperbranched PNP ligands, in combination with Cr(III), and activation by methylaluminoxane (MAO) in situ generated species with better catalytic performance for ethylene oligomerization. The effect of solvent, chromium source, ligand/Cr molar ratio, reaction temperature, Al/Cr molar ratio and reaction pressure on the catalytic activity and product selectivity were studied. The results showed that with increase of ligand/Cr molar ratio, reaction temperature and Al/Cr molar ratio, the catalytic activity increased at first and then decreased. However, the catalytic activity continuously increased with increase of reaction pressure. Under the optimized conditions, the catalytic system of hyperbranched PNP/Cr(III)/MAO led to catalytic activity of 2.68 × 10⁵ g/(mol Cr·h) and 37.71% selectivity for C₆ and C₈.     

Speciation of Cr(VI)/Cr(III) by electrothermal atomisation AAS after electrodeposition on a L'vov platform

Summary The pyrolysed graphite L'vov platform of a tube furnace is considered as an electrode for the electrodeposition and speciation of chromium by electrothermal atomisation atomic absorption spectrometry (ETA-AAS). Firstly, a preliminary study of the Cr(VI)/Cr(III) voltammetric behavior at pH 4.70 on a glassy-carbon electrode is carried out. Secondly, the L'vov platform is used as a cathodic macro-electrode for the selective preconcentration of Cr(VI)/Cr(III) on a mercury film. Speciation of Cr(VI)/Cr(III) is carried out on the basis of the electrolysis potential (E_e): at pH 4.70 and E_e=–0.30 V, only Cr(VI) is reduced to Cr(III) and accumulated as Cr(OH)₃ by adsorption on a mercury film; at E_e=–1.80 V both Cr(VI) and Cr(III) are accumulated forming an amalgam with added mercury(II) ions. Once the film has been formed, the platform is transferred to a graphite tube to atomise the element. The reliability of the method was tested for the speciation of chromium in natural waters and it proves to be highly sensitive thanks to the electroanalytical step. In all samples, the Cr(VI) concentration was less than the detection limit (0.15 ng ml^–1), and the concentration of Cr(III) agrees with those of total chromium. The analytical recovery of Cr(VI) added to water samples [3.50 ng ml^–1 of Cr(VI)] was 105±6.2%.     

Filling the Gap in the Metallacrown Family: The 9-MC-3 Chromium Metallacrown

In this work, we report on a long-sought missing complex in the metallacrown family. We synthesized and characterized the novel chromium metallacrown (MC) complex {Cr^III(μ₂-piv)₃[9-MC_{Cr(III)N(shi)}-3](morph)₃} ⋅ MeOH (in which shi³⁻=salicyl hydroxamate, piv=pivalate, and morph=morpholine). The MC with a 9-MC-3 cavity of kinetically inert chromium(III) ions was synthesized by a solvothermal reaction. Magnetization measurements reveal a high spin ground state.     

Highly sensitive spectrofluorimetric determination of cysteine by Cu2+-morin complex

p-Toluenesulfonylamide was immobilized on silica gel and on nm-sized silicium dioxide (SiO₂). Their adsorption efficiency toward metal ions was investigated by the batch equilibrium technique. Although silica gel and nm-SiO₂ have the same composition (silicon and oxygen), the difference in their sizes and surface structures results in distinct chemical activity and selectivity. At pH 4, the adsorption capacity of modified silica gel adsorbent was found to be 4.9, 5.0, 33.2, and 12.6 mg g⁻¹ for Cr(III), Cu(II), Pb(II) and Zn(II), respectively. However, the adsorption capacity of nm-SiO₂ adsorbent toward Cr(III) was 26.7 mg g⁻¹ under ultrasonic dispersing. The potential application of p-toluenesulfonylamide-modified silica gel for simultaneous preconcentration of trace chromium, copper, lead and zinc from two standard reference materials and two food samples was performed with satisfactory results. Correspondence: Xijun Chang, Department of Chemistry, Lanzhou University, Lanzhou 730000, P.R. China     

Synthesis and Physicochemical Properties of Some Transition Metal Complexes with 3-Hydroxypyridine

The complexes ML₂ · 2H₂O and CrL₃ · 8H₂O were synthesized by the reaction of Cu(II), Ni(II), Co(II), Cd(II), and Cr(III) nitrates with 3-hydroxypyridine (HL) and identified. The spectroscopic characteristics of neutral, anionic, and cationic forms of the ligand were determined. It was shown that 3-hydroxypyridine enters the complex as an anion to give polymeric chains due to the ligand coordination through the N and O atoms. Complexation of copper, cobalt, and chromium nitrates with 3-hydroxypyridine in ethanol solutions was studied, and the formation constant of the copper complex was calculated. It was found that Cu(II) and Cr(III) complexes are formed in the solid state and in solutions (M : L = 1 : 2 and 1 : 3, respectively; in the case of Cu(II), the M : L ratio is 1 : 3).     

Electroanalytical Detection of Cr(VI) and Cr(III) Ions Using a Novel Microbial Sensor

A microbial sensor, namely carbon paste electrode (CPE) modified with Citrobacter freundii (Cf–CPE) has been developed for the detection of hexavalent (Cr(VI)) and trivalent (Cr(III)) chromium present in aqueous samples using voltammetry, an electroanalytical technique. The biosensor developed, demonstrated about a twofold higher performance as compared to the bare CPE for the chosen ions. Using cyclic voltammetry and by employing the fabricated Cf–CPE, the lowest limit of detection (LLOD) of 1x10⁻⁴ M and 5x10⁻⁴ M for Cr(VI) and Cr(III) ions respectively could be achieved. By adopting the Differential Pulse Cathodic Stripping Voltammetric technique, the LLOD could be further improved to 1x10⁻⁹ M and 1x10⁻⁷ M for Cr(VI) and Cr(III) ions respectively using the biomodified electrodes. The reactions occurring at the electrode surface‐chromium solution interface and the mechanisms of biosorption of chromium species onto the biosensor are discussed. The stability and utility of the developed biosensor for the analysis of Cr(VI) and Cr(III) ions in chromite mine water samples has been evaluated.     

Investigation of deceleration causes in gold electrowinning from aqueous cyanide solutions on porous carbon electrodes

Deceleration of gold electrowinning from model cyanide solutions on porous cathode of graphitized carbon felt was investigated. It was established that calcium ions present in the solution did not negatively affect the electrolysis rate. The main reason of the termination of gold electrowinning on the graphitized cathode was the corrosion of the stainless steel anode resulting in penetration into the cathode chamber of the electrolyzer of CrO₄²⁻ anions further reduced to Cr(III) and probably Cr(II) compounds. X-ray Photoelectron Spectroscopy investigation of the passivated carbon cathode showed that the film formed thereon consisted of Cr(III) compounds containing cyanide and hydroxy ligands. This film covers the active sites on the surface of the porous carbon cathode preventing the deposition thereon of ad-ions of gold(I). Inert aqua and hydroxo complexes of chromium(III) stronger impede the rate of gold(0) deposition on the porous carbon cathode than chromium(III) cyanide compounds. This fact originates presumably from the requirement of lower cathode potentials for the reduction of the former to labile chromium(II) complexes than those necessary for the thermodynamically stable anions [Cr(CN)₆]³⁻.     

Removal of Cu(II), Cd(II) and Cr(III) ions from aqueous solution by dam silt

The removal of heavy metals, such as Cu(II), Cd(II) and Cr(III) from aqueous solution was studied using Chorfa silt material (Mascara, Algeria). The main constituents of silt sediment are quartz, calcite and mixture of clays. The experimental data were described using Freundlich, Langmuir, Dubinin–Radushkevich (D–R) and Langmuir–Freundlich models. The adsorbed amounts of chromium and copper ions were very high (95% and 94% of the total concentration of the metal ions), whereas cadmium ion was adsorbed in smaller (55%) amounts. The Langmuir–Freundlich isotherm model was the best to describe the experimental data. The maximum sorption capacity was found to be 26.30, 11.76 and 0.35 mg/g for Cr³⁺, Cu²⁺ and Cd²⁺, respectively. The results of mean sorption energy, E (kJ/mol) calculated from D–R equation, confirmed that the adsorption of copper, chromium and cadmium on silt is physical in nature.     

Speciation of chromium in waste water using ion chromatography inductively coupled plasma mass spectrometry

Ion chromatography (IC) coupled with inductively coupled plasma mass spectrometry (ICP-MS) was systematically investigated for determining the speciation of chromium in environmental samples. Firstly, the stability of complexes formed by Cr(III) with various aminopolycarboxylic acids was studied by electrospray ionization mass spectrometry (ESI-MS). The results showed that [Cr(EDTA)]⁻ was stable in solution. Secondly, various mobile phases were examined to separate Cl⁻ from chromium species by IC to avoid Cl⁻ interference. The separation of [Cr(EDTA)]⁻ and Cr(VI) was achieved on a new anion-exchange column (G3154A/102) using a mobile phase containing 20 mM NH₄NO₃ and 10 mM NH₄H₂PO₄ at pH 7.0 without Cl⁻ interference. Detection limits for chromium species were below 0.2 μg/L with a direct injection of sample and without prior removal of interferences from the matrix. Finally, the proposed method was used for the determination of chromium species in contaminated waters.     

Electrode processes and their kinetic parameters in the reduction and oxidation of Cr(III) and Cr(II) in molten CsCl

The reduction of chromium ions in the CsCl-CrCl₃ melt in wide concentration and temperature ranges is reported. It is assumed that the melt contains polynuclear chromium complexes. The reduction of Cr(III) to Cr(II) is a reversible one-electron process. The electroreduction of divalent chromium to chromium metal is an irreversible process involving dinuclear and mononuclear chromium complex ions. The electron transfer rate constant has been calculated for various temperatures and chromium chloride concentrations in the salt melt. The activation energy of electron transfer and its dependence on the CrCl₃ concentration in the electrolyte have been determined for the Cr(III) ↔ Cr(II) process.     

Comparative study of Cr(III) adsorption by carbon nanotubes and active carbons

The adsorption of trivalent chromium ions from aqueous solutions on the surface of carbon materials, namely, multiwall carbon nanotubes (NTs) and two samples of active carbon, is studied depending on pH and adsorbate concentration in the system. Isotherms of Cr(III) adsorption by the aforementioned materials are obtained. It is shown that chromium ions are predominantly bound by surface carboxyl groups. The adsorption of chromium ions reduces the electrokinetic potential of NTs and, at chromium concentrations C _Cr(III) > 10^–5 M, leads to the reversal of the surface charge. The adsorption value decreases in the series NT > Merck carbon > Norit carbon, in contrast to an increase in the adsorbate affinity to the adsorbent in this series, as determined from the slope of the initial section of the Langmuir isotherms. Small amounts of chromium ions sorbed at low concentrations in solution (C _Cr(III) ≤ 10^–5 M) are comparable with the concentration of hydrogen ions displaced from the surface, thus making it possible to suppose the existence of an ionexchange adsorption mechanism. As the concentration of Cr(III) increases, the equivalent displacement of H⁺ is violated, thereby indicating the development of other adsorption mechanisms (complexation).     

Synthesis and Magnetism of Ferromagnetically Coupled [Cu 3 II CrIII] Heterotetranuclear Complexes Bridged by Oxalate Groups

Three novel -oxalato-bridged Cu ₃ ^II Cr^III-type heterotetranuclear complexes described by the overall formula [Cu₃Cr(ox)₃L₃](ClO₄)₃, where ox represents the oxalato dianions, L stands for diaminoethane (en), 1,3-diaminopropane (pn), and 1,2-diaminopropane (ap) respectively, have been synthesized and characterized by elemental analyses, molar conductivity and magnetic moment (room-temperature) measurements, i.r., e.s.r. and electronic spectral studies. It is proposed that these complexes have oxalato-bridged structures consisting of three copper(II) ions and a chromium(III) ion, in which the chromium(III) ion has an octahedral environment and the three copper(II) ions have square-planar environments. Variable temperature magnetic susceptibility (4.2–300 K) measurements and studies of complexes [Cu₃Cr(ox)₃(en)₃](ClO₄)₃ (1) and [Cu₃Cr(ox)₃(pn)₃](ClO₄)₃ (2) revealed the occurrence of an intramolecular ferromagnetic interaction between the copper(II) and chromium(III) ions through the oxalato-bridge within each molecule. The magnetic data have been used also to deduce the indicated -oxalato-bridges [Cu ₃ ^II Cr^III] heterotetranuclear structure. On the basis of the spin Hamiltonian operator, , the magnetic analyses were carried out for the two Cu^II—Cr^III heterotetranuclear complexes and the spin-coupling constants (J) were evaluated as +6.36 cm^–1 for (1) and +7.02 cm^–1 for (2). The results indicate that the bridging oxalato entity should be able to transmit ferromagnetic interactions in the strict orthogonality [Cu ₃ ^II Cr^III] system.     

Reduction of Cr(VI) Assisted by Sol-Gel Generated Electron-Hole Centers

We report an in-situ harvesting technique of electron-hole (e⁻-h⁺) carriers (e.g., the defect electrons in the O^{2 −} matrix and the self-trapped holes, Si–O⁻–Si) generated during sol-gel processing. In the absence of redox species, the e⁻-h⁺ centers created during room temperature sol-gel polycondensation steps are quickly annihilated and deactivated. However, when Cr(VI) ions are pre-dispersed in sol-gel solutions, the ejected electrons can be effectively harvested for the reduction of Cr(VI) to Cr(III) ions which are encapsulated in the silica gel matrix. The Cr(VI) ions, the possible intermediate oxidation states of chromium ions such as Cr(V) and/or Cr(IV), and the stable Cr(III)-hole complexes in the sol-gel matrix are investigated using uv-visible spectroscopy, electron paramagnetic resonance spectroscopy, and cyclic voltammetry. The chemical stability of Cr(VI) and Cr(III) in sol-gel networks is compared to that in aqueous solutions. The results indicate that the utilization of e⁻-h⁺ carriers generated in the sol-gel can be an effective and selective means for investigating the redox process of Cr(VI) and encapsulating the stable Cr(III) ions in the confined sol-gel environments.     

The synthesis,spectral and magnetic properties of the complexes of chromium with chrysenequinone and chrysenequinonemonoxime

Reactions of the chromium hexacarbonyl with chrysenequinone and chrysenequinonemonoxime gave the tris(chrysenesemiquinone)chronium(III), Cr(ChrySQ)₃, and tris(chrysenesemiquinonemonoxime)chromium(III), Cr(ChrySQM)₃, complexes. From the stretching frequencies of the carbonyl groups in the IR spectra of the complexes, it was concluded that the ligands bonded to the metal in the semiquinone form. Also, the electronic absorption spectra of the two complexes showed strong UV bands due to metal-to-ligand charge transfer. The magnetic susceptibility determination for the Cr(ChrySQ)₃ complex at 300 K gave a value of 3.261 × 10⁻⁶ e.m.u.g⁻¹ with an effective magnetic moment (μ_eff) of 2.55 μ_B. Investigation of Cr(ChrySQ)₃ and Cr(ChrySQM)₃ by EPR spectroscopy at room temperature showed isotropic g values of 1.997 and 1.991 for the two complexes, respectively. The isotropic g values are greater than those observed previously for various osemiquinone chromium complexes. Chrysenequinonemonoxime reacted with Cr(NO₃)₃ to give the bis(chrysenequinonemonoximato) (chrysenequinonemonoxime)chromium(III)nitrate. Spectroscopic studies of the complex suggested that the chrysenequinonemonoxime ligand is attached to the metal in both nitroso and oxime forms. Examination of the chromium complexes by cyclic voltammetry showed some reversible or quasireversible redox reactions due to tautomeric interconversions of the semiquinone—catechol couples through electron transfer.     

Synthesis and ferromagnetic interaction in oxamido-bridged heterodinuclear copper(II)-chromium (III) complexes

Four μ- oxamido heterodinuclear complexes, [Cu (oxae) Cr (L)₂ ] (NO₃) ₃, where oxae denotes the N, N'bis (2-aminoethyl) oxamido dianion and L represents 1,10-phenanthroline (phen); 5-nitro-1,10-phenanthroline (NO_2-phen); 5-methyl-1, 10-phenanthroline (Me-phen) and 2, 2′-bipyridine (bpy), have been synthesized and characterized by elemental analyses, magnetic moments (at room temperature) and molar conductivity measurements and spectroscopy. It is proposed that these complexes have extended oxamido-bridged structures consisting of a copper (II) ion and a chromium (III) ion, which have a square planar environment and octahedral environment, respectively. The cryomagnetic properties of the [Cu(oxae)Cr(bpy)₂(NO₃)₃(1) and [Cu(oxae)Cr(phen)₂](NO₃)₃(2) complexes have been measured over the range of 4.2–300 K. The leastsquares fit of the experimental data based on the spin Hamiltonian, ? = - 2J?₁·?₂, the exchange integrals (J) were evaluated as +36.9 cm^?1 for 1 and +35.8 cm^?1 for 2. The reds have connived that the spin coupling between the adjacent copper (II) and chromium (III) ions through oxamido-bridge in both 1 and 2 is ferromagnetic.     

Effect of chromium(III) and of other ions on the absorptiometric determination of copper with 2, 2'-biquinolyl

In the determination of small amounts of copper in certain alloys by liquid-liquid extraction of the bis-chelate of copper(I) with 2, 2'-biquinolyl, recovery of copper has been reported to be low when chromium(III) is present. The adverse effect of chromium(III) could be overcome by adding iron(II). It is now shown that the inhibiting effect of chromium is attributable to the formation of a kinetically inert ternary complex of chromium(III), copper(II) and citrate ions containing an equal number of atoms of each of the two metals. Copper can be displaced from this complex by any of the transition cations Mn(II), Fe(II), Co(II), Ni(II) and Zn(II). Zinc is shown to form a ternary complex formally analogous to that of copper. The formation of the ternary complexes has been studied polarographically. The formation of binuclear complexes of various hydroxy-acids is reviewed and a probable structure for the ternary complex is proposed which explains its stability and kinetic inertness. Analogies are drawn between this complex and ternary complexes of UO(2)(VI)-Al(III)-citrate and Cu(II)-Al(III)-tartrate which also cause interference in established analytical procedures.     

The catalytic adsorptive stripping voltammetric determination of chromium with TTHA and nitrate

A sensitive voltammetric method is presented for the determination of trace amounts of total chromium (Cr(III) and Cr(VI)) in natural waters, employing the square wave mode. The method is based on the preconcentration of the Cr(III)-TTHA complex by adsorption at the HMDE at the potential of –1.0 V vs. Ag/AgCl. The adsorbed complex is then reduced producing a response with a peak potential of –1.29 V and the peak height of the Cr(III) reduction is measured. The catalytic action of the nitrate ions on the Cr(III)-TTHA reduction has been elucidated using cyclic voltammetry. The adsorption of chromium complexes at the HMDE was investigated using out-of-phase a.c. voltammetry and the potential range of adsorption was determined.Based on these investigations optimal conditions for the determination of the total chromium concentration in the range 155–2000 ng 1^–1 have been established. The determination limit is 15 ng 1^–1 and the RSD is 3.5% for chromium concentrations 200 ng 1^–1.The usefulness and wide scope of this method for reliable and highly sensitive chromium analysis down to the ultra trace levels existing in various types of natural waters is demonstrated by determinations of the total chromium content in lake, sea and rain water.