Equilibrium study on Cd(II) and Zn(II) chelates of mercapto carboxylic acids

Cadmium(II) and zinc(II) complexes with the chelating agents meso-2,3-dimercapto succinic acid, meso-monomethyl dimercapto succinate, meso-dimethyl dimercapto succinate and the related methylmercapto acetate and mercapto succinic acid were studied potentiometrically in 0.1 M KCl at 25 °C. Models and hypotheses of coordination for the solution complexes are proposed. IR characterisation and stoichiometry of the solid state compounds evaluated by elemental analysis are presented. The obtained formation constants, together with the previously measured ionisation constants of the ligands, were used to compute pCd and pZn values and then to evaluate the effectiveness of the studied chelating agents in binding Cd(II) and Zn(II) in intoxication treatments.     

Evaluation of cadmium–metallothionein stability constants based on voltammetric measurements

A voltammetric study on the binding properties of the commercially available rabbit liver metallothionein (MT) Sigma M 7641 for cadmium ions was performed. The capacity of metallothionein to complex cadmium and the stability constants of Cd–T complex (T denotes the apoprotein or thionein molecule) have been determined from the direct titration of the defined MT concentration with the standard CdCl₂ solution in 0.59 M sodium chloride medium at pH 7.9 and 2.0. At pH 7.9 the formation of the Cd–T complex has been followed measuring the specific anodic signal height of the complex. Stability constants of Cd–T have been evaluated from the experimental data using three different procedures. The calculated apparent stability constants K=(7.6±0.2)·10⁸ dm³ mol⁻¹ of Cd–T complex in 0.59 M NaCl at pH 7.9 and 25°C indicate that under the given experimental conditions all the three procedures are equally applicable.     

Synthesis and Characterization of Functionalized Nanosilica for Zinc Ion Mitigation; Experimental and Computational Investigations

Zinc is an essential trace metal and its concentration above 4ppm reduces the aesthetic value of water. This study explores the possibility of using functionalized nanohybrids as Zn(II) ion scavengers from aqueous solution. Functionalized nanohybrids were synthesized by the attachment of thiosemicarbazide to silica. The material was characterized by TGA, SEM, FTIR, EDX, and BET analysis, which revealed ligand bonding to silica. The functionalized silica was employed as Zn(II) ion extractant in batch experiments and removed about 94.5% of the Zn(II) ions at pH 7, near zero point charge (6.5) in 30 min. Kinetics investigations revealed that zinc adsorption follows an intra particle diffusion mechanism and first-order kinetics (K = 0.1020 min⁻¹). The data were fitted to Freundlich, Dubinin–Radushkevich, and Langmuir models and useful ion exchange parameters were determined. The impact of co-existing ions on Zn(II) ion sequestration was also studied and it was found that the adsorbent can be used for selective removal of zinc with various ions in the matrix. Quantum mechanical investigations revealed that the Zn(II) ion adsorption on ZnBS1 is more favorable, having higher binding energy (BE) (−178.1 kcal/mol) and ∆H (−169.8), and making tridentate complex with the N and S sites of the chelating ligand. The negative ∆G and BE values suggest highly spontaneous Zn(II) adsorption on the modified silica even at low temperatures.     

Mn(II) and Zn(II) interactions with peptide fragments from Parkinson's disease genes

Two peptide sequences from PARK9 Parkinson's disease gene, ProAspGluLysHisGluLeu, (P(1)D(2)E(3)K(4)H(5)E(6)L(7)) (1) and PheCysGlyAspGlyAlaAsnAspCysGly (F(1)C(2)G(3)D(4)G(5)A(6)N(7)D(8)C(9)G(10)) (2) were tested for Mn(II), Zn(II) and Ca(II) binding. The fragments are located from residues 1165 to 1171 and 1184 to 1193 in the PARK9 encoded protein. This protein can protect cells from poisoning of manganese, which is an environmental risk factor for a Parkinson's disease-like syndrome. Mono- and bi-dimensional NMR spectroscopy has been used to understand the details of metal binding sites at different pH values and at different ligand to metal molar ratios. Mn(II) and Zn(II) coordination with peptide (1) involves imidazole N(ε) or N(δ) of His(5) and carboxyl γ-O of Asp(2), Glu(3) and Glu(6) residues. Six donor atoms participate in Mn(II) binding resulting in a distorted octahedral geometry, possibly involving bidentate interaction of carboxyl groups; four donor atoms participate in Zn(II) binding resulting in a tetracoordinate geometry. Mn(II) and Zn(II) coordination involves the two cysteine residues with peptide (2); Mn(II) accepts additional ligand bonds from the carboxyl γ-O of Asp(4) and Asp(8) to complete the coordination sphere; the unoccupied sites may contain solvent molecules. The failure of Ca(II) ions to bind to either peptide (1) or (2) appears to result, under our conditions, from the absence of chelating properties in the chosen fragments.     

Effect of Nonmagnetic Substitution on the Magnetic Properties and Charge-Transfer Phase Transition of an Iron Mixed-Valence Complex, (n-C(3)H(7))(4)N[Fe(II)Fe(III)(dto)(3)] (dto = C(2)O(2)S(2))

The iron mixed-valence complex (n-C(3)H(7))(4)N[Fe(II)Fe(III)(dto)(3)] exhibits a novel type of phase transition called charge-transfer phase transition (CTPT), where the thermally induced electron transfer between Fe(II) and Fe(III) occurs reversibly at ～120 K, in addition to the ferromagnetic phase transition at T(C) = 7 K. To investigate the mechanism of the CTPT, we have synthesized a series of magnetically diluted complexes (n-C(3)H(7))(4)N[Fe(II)(1-x)Zn(II)(x)Fe(III)(dto)(3)] (dto = C(2)O(2)S(2); x = 0-1), and carried out magnetic susceptibility and dielectric constant measurements and (57)Fe M?ssbauer spectroscopy. With increasing Zn(II) concentration (x), the CTPT is gradually suppressed and disappears at x ≈ 0.13. On the other hand, the ferromagnetic transition temperature (T(C)) is initially enhanced from 7 K to 12 K between x = 0.00 and 0.05, despite the nonmagnetic nature of Zn(II) ions, and then it decreases monotonically from 12 K to 3 K with increasing Zn(II) concentration. This anomalous dependence of T(C) on Zn(II) concentration is related to a change in the spin configuration of the ferromagnetic state caused by the partial suppression of the CTPT.     

Metal ion complexing properties of dipyridoacridine, a highly preorganized tridentate homologue of 1,10-phenanthroline

DPA (dipyrido[4,3-b;5,6-b]acridine) may be considered as a tridentate homologue of phen (1,10-phenanthroline). In this paper some of the metal ion complexing properties of DPA in aqueous solution are reported. Using UV-visible spectroscopy to follow the intense π-π* transitions of DPA as a function of pH gave protonation constants at ionic strength (μ) = 0 and 25 °C of pK(1) = 4.57(3) and pK(2) = 2.90(3). Titration of 10(-5) M solutions of DPA with a variety of metal ions gave log K(1) values as follows: Zn(II), 7.9(1); Cd(II), 8.1(1); Pb(II), 8.3(1); La(III), 5.23(7); Gd(III), 5.7(1); Ca(II), 3.68; all at 25 °C and μ = 0. Log K(1) values at μ = 0.1 were obtained for Mg(II), 0.7(1); Sr(II), 2.20(1); Ba(II), 1.5(1). The log K(1) values show that the high level of preorganization of DPA leads to complexes 3 log units more stable than the corresponding terpyridyl complexes for large metal ions such as La(III) or Ca(II), but that for small metal ions such as Mg(II) and Zn(II) such stabilization is minimal. Molecular mechanics calculations (MM) are used to show that the best-fit M-N length for coordination with DPA is 2.60 ?, accounting for the high stability of Ca(II) or La(III) complexes of DPA, which are found to have close to this M-N bond length in their phen complexes.     

Determination of the intrinsic affinities of multiple site-specific Mg(2+) ions coordinated to domain 6 of a group II intron ribozyme

Group II introns are large metallo-ribozymes that use divalent metal ions in folding and catalysis. The 3'-terminal domain 6 (D6) contains a conserved adenosine whose 2'-OH group acts as the nucleophile in the first splicing step. In the hierarchy of folding, D6 binds last into the active site. In order to investigate and understand the folding process to the catalytically active intron structure, it is important to know the individual binding affinities of Mg2+ ions to D6. We recently studied the solution structure of a 27 nucleotide long D6 (D6-27) from the mitochondrial yeast group II intron Sc.ai5gamma, also identifying five Mg2+ binding sites including the one at the 5'-terminal phosphate residues. Mg2+ coordination to the 5'-terminal di- and triphosphate groups is strongest (e.g., log KA,TP = 4.55 +/- 0.10) and is evaluated here in detail for the first time. The other four binding sites within D6-27 are filled simultaneously (e.g., log KA,BR = 2.38 +/- 0.06) and thus compete for the free Mg2+ ions in solution, having a distinct influence on the individual affinities of the various sites. For the first time, we take this competition into account to obtain the intrinsic binding constants, describing a method that is generally applicable. Our data illustrates that any RNA molecule undergoing tertiary contacts to a second RNA molecule first needs to be loaded evenly and specifically with metal ions to compensate for the repulsion between the negatively charged RNA molecules.     

Synthesis and application of modified poly (styrene‐alt‐maleic anhydride) networks as a nano chelating resin for uptake of heavy metal ions

A chelating resin based on modified poly (styrene‐alt‐maleic anhydride) with 3‐aminobenzoic acid was synthesized. This modified resin was further reacted by 1,2‐diaminoethane or 1,3‐diaminopropane in the presence of ultrasonic irradiation to prepare tridimensional chelating resin for the removal of heavy metal ions from aqueous solutions. The adsorption behavior of Fe(II), Cu(II), Zn(II) and Pb(II) ions was investigated by synthesized chelating resins in various pH. Among the synthesized resins, CSMA‐AB1 and CSMA‐AB2 demonstrated a high affinity for the selected metal ions compared to SMA‐AB, and the order of removal percentage changes as follow: Fe(II) > Cu(II) > Zn(II) > Pb(II). The adsorption of all metal ions in acidic medium was moderate, and it was favored at the pH value of 6 and 7. Also, the prepared resins were examined for removal of metal ions from industrial wastewater and were shown to have a very efficient adsorption in the case of Cu(II), Fe(II) and Pb(II); however, the adsorption of Zn(II) was lower than others. The resin was characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, X‐ray diffraction analysis and thermogravimetric analysis/derivative thermogravimetry. Copyright © 2012 John Wiley & Sons, Ltd.     

Reaction of metal-binding ligands with the zinc proteome: zinc sensors and N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine

The commonly used Zn(2+) sensors 6-methoxy-8-p-toluenesulfonamidoquinoline (TSQ) and Zinquin have been shown to image zinc proteins as a result of the formation of sensor-zinc-protein ternary adducts not Zn(TSQ)(2) or Zn(Zinquin)(2) complexes. The powerful, cell-permeant chelating agent N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) is also used in conjunction with these and other Zn(2+) sensors to validate that the observed fluorescence enhancement seen with the sensors depends on intracellular interaction with Zn(2+). We demonstrated that the kinetics of the reaction of TPEN with cells pretreated with TSQ or Zinquin was not consistent with its reaction with Zn(TSQ)(2) or Zn(Zinquin)(2). Instead, TPEN and other chelating agents extract between 25 and 35% of the Zn(2+) bound to the proteome, including zinc(2+) from zinc metallothionein, and thereby quench some, but not all, of the sensor-zinc-protein fluorescence. Another mechanism in which TPEN exchanges with TSQ or Zinquin to form TPEN-zinc-protein adducts found support in the reactions of TPEN with Zinquin-zinc-alcohol dehydrogenase. TPEN also removed one of the two Zn(2+) ions per monomer from zinc-alcohol dehydrogenase and zinc-alkaline phosphatase, consistent with its ligand substitution reactivity with the zinc proteome.     

Theoretical investigation on sulfur‐containing chelating resin‐divalent metal complexes

The complex structures and interactions of sulfur‐containing chelating resin poly[4‐vinylbenzyl‐(2‐hydroxyethyl)]sulfide (PVBS), poly[4‐vinylbenzyl‐(2‐hydroxyethyl)]sulfoxide (PVBSO), and poly[4‐vinylbenzyl‐(2‐hydroxyethyl)]sulfone (PVBSO₂) with divalent metal chlorides (Cu(II), Ni(II), Zn(II), Cd(II), and Pd(II)) were investigated theoretically. Results indicate that PVBS tends to coordinate with metal ions by sulfur and oxygen atoms forming five‐membered ring chelating complexes; while PVBSO and PVBSO₂ prefer to interact with metal ions by the oxygen atom of the sulfoxide or sulfone and hydroxyl group to form six‐membered ring chelating compounds. Theoretical calculations reveal that sulfur atoms of PVBS are the main contributor when coordinate with metal ions, while oxygen atoms also take part in the coordination with Cu(II), Zn(II), and Cd(II). As for PVBSO, the oxygen atoms of sulfoxide group play a key role in the coordination, but sulfur and hydroxyl oxygen also participate in the coordination. Similarly, sulfone group oxygen atoms of PVBSO₂ dominate the coordination of Ni(II), Cu(II), and Pd(II), while the affinities of Zn(II) and Cd(II) are mainly attributed to the hydroxyl oxygen atoms. The computational results are in good agreement with the XPS analysis. Combined the theoretical and experimental results, further understanding of the structural information on the complexes was achieved and the adsorption mechanism was confirmed. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Unusual metal ion selectivities of the highly preorganized tetradentrate ligand 1,10-phenanthroline-2,9-dicarboxamide: a thermodynamic and fluorescence study

Some metal ion complexing properties of the ligand PDAM (1,10-phenanthroline-2,9-dicarboxamide) in aqueous solution are reported. Using UV-visible spectroscopy to follow the intense π-π* transitions of PDAM as a function of metal ion concentration, log K(1) values in 0.1 M NaClO(4) and at 25 °C are, for Cu(II), 3.56(5); Ni(II), 3.06(5); Zn(II), 3.77(5); Co(II), 3.8(1); Mg(II), 0.1(1); Ca(II), 1.94(4); and Ba(II), 0.7(1). For more strongly bound metal ions, competition reactions between PDAM and EDTA (ethylenedinitrilo-tetraacetic acid) or tetren (1,4,7,10,13-pentaazatridecane), monitored following the UV spectrum of PDAM, gave the following log K(1) values in 0.1 M NaClO(4) and at 25 °C: Cd(II), 7.1(1); Pb(II), 5.82(5); In(III), 9.4(1); and Bi(III), 9.4(1). The very low log K(1)(PDAM) values for small metal ions such as Cu(II) or Zn(II) are unprecedented for a phen-based ligand (phen = 1,10-phenanthroline), which is rationalized in terms of the low basicity of the N donors of the ligand (pK(a) = 0.6) and the fact that PDAM has a best-fit size corresponding to large metal ions of ionic radius ~1.0 ?. Large metal ions with ionic radius ≥1.0 ? show large increases in log K(1) relative to their phen complexes, which in turn produces unparalleled selectivities, such as a 3.5 log units greater log K(1)(PDAM) for Cd(II) than for Cu(II). PDAM shows strong fluorescence in aqueous solution, suggesting that its carboxamide groups do not produce a fluorescence-quenching photon-induced electron transfer (PET) effect. Only Ca(II) produces a weak CHEF (chelation enhanced fluorescence) effect with PDAM, while all other metal ions tested produce a decrease in fluorescence, a CHEQ (chelation enhanced quenching effect). The production of the CHEQ effect is rationalized in terms of the idea that coordination of metal ions to PDAM stabilizes a canonical form of the carboxamide groups that promotes a PET effect.     

Complexation of metal ions, including alkali-earth and lanthanide(III) ions, in aqueous solution by the ligand 2,2',6',2'-terpyridyl

Some metal-ion-complexing properties of the ligand 2,2',6',2'-terpyridyl (terpy) in aqueous solution are determined by following the π-π* transitions of 2 × 10(-5) M terpy by UV-visible spectroscopy. It is found that terpy forms precipitates when present as the neutral ligand above pH ～5, in the presence of electrolytes such as NaClO(4) or NaCl added to control the ionic strength, as evidenced by large light-scattering peaks. The protonation constants of terpy are thus determined at the ionic strength (μ) = 0 to avoid precipitation and found to be 4.32(3) and 3.27(3). The log K(1) values were determined for terpy with alkali-earth metal ions Mg(II), Ca(II), Sr(II), and Ba(II) and Ln(III) (Ln = lanthanide) ions La(III), Gd(III), and Lu(III) by titration of 2 × 10(-5) M free terpy at pH >5.0 with solutions of the metal ion. Log K(1)(terpy) was determined for Zn(II), Cd(II), and Pb(II) by following the competition between the metal ions and protons as a function of the pH. Complex formation for all of these metal ions was accompanied by marked sharpening of the broad π-π* transitions of free terpy, which was attributed to complex formation affecting ligand vibrations, which in the free ligand are coupled to the π-π* transitions and thus broaden them. It is shown that log K(1)(terpy) for a wide variety of metal ions correlates well with log K(1)(NH(3)) values for the metal ions. The latter include both experimental log K(1)(NH(3)) values and log K(1)(NH(3)) values predicted previously by density functional theory calculation. The structure of [Ni(terpy)(2)][Ni(CN)(4)]·CH(3)CH(2)OH·H(2)O (1) is reported as follows: triclinic, P1, a = 8.644(3) ?, b = 9.840(3) ?, c = 20.162(6) ?, α = 97.355(5)°, β = 97.100(5)°, γ = 98.606(5)°, V = 1663.8(9) ?(3), Z = 4, and final R = 0.0319. The two Ni-N bonds to the central N donors of the terpy ligands in 1 average 1.990(2) ?, while the four peripheral Ni-N bonds average 2.107(10) ?. This difference in the M-N bond length for terpy complexes is typical of the complexes of smaller metal ions, while for larger metal ions, the difference is reversed. The significance of the metal-ion size dependence of the selectivity of polypyridyl ligands, and the greater rigidity of ligands based on aromatic groups such as pyridyl groups, is discussed.     

Dynamic adsorption and desorption of heavy metal ions on poly(acrylaminophosphonic-carboxyl-hydrazide) chelating fiber

The dynamic adsorption and desorption properties, including the effect of pH value and flow rate on the adsorption, eluent acidity and volume, eluting velocity and re-use, of Cu(II), Pb(II), Zn(II), Cd(II), Mn(II), Ni(II), Co(II) and Hg(II) ions on the column loaded with poly(acrylaminophosphonic-carboxyl-hydrazide) chelating fiber were investigated. The recovery of Mn(II), Co(II), Cd(II), Ni(II) and Zn(II) ions in the presence of Na, K, Ca and Mg ions was examined. The preconcentration of trace amounts of Mn(II), Co(II), Cd(II), Ni(II) and Zn(II) ions from model solution samples was carried out with satisfactory results. The amount of the metal ions detected after preconcentration and recovery by this technique was basically in agreement with the added amount. The method is rapid, precise and simple. Received: 15 October 1997 / Revised: 17 March 1998 / Accepted: 20 March 1998     

Dynamic adsorption and desorption of heavy metal ions on poly(acrylaminophosphonic-carboxyl-hydrazide) chelating fiber

The dynamic adsorption and desorption properties, including the effect of pH value and flow rate on the adsorption, eluent acidity and volume, eluting velocity and re-use, of Cu(II), Pb(II), Zn(II), Cd(II), Mn(II), Ni(II), Co(II) and Hg(II) ions on the column loaded with poly(acrylaminophosphonic-carboxyl-hydrazide) chelating fiber were investigated. The recovery of Mn(II), Co(II), Cd(II), Ni(II) and Zn(II) ions in the presence of Na, K, Ca and Mg ions was examined. The preconcentration of trace amounts of Mn(II), Co(II), Cd(II), Ni(II) and Zn(II) ions from model solution samples was carried out with satisfactory results. The amount of the metal ions detected after preconcentration and recovery by this technique was basically in agreement with the added amount. The method is rapid, precise and simple. Received: 15 October 1997 / Revised: 17 March 1998 / Accepted: 20 March 1998     

Complexation of N-tris[(1,2-dicarboxyethoxy)ethyl]amine with Ca(II), Mn(II), Cu(II) and Zn(II) in aqueous solution

In a search for environmentally friendly metal chelating ligands for industrial applications, the protonation and complex formation equilibria of N-tris[(1,2-dicarboxyethoxy)ethyl]amine (TCA6) with Ca(II), Mn(II), Cu(II) and Zn(II) ions in aqueous 0.1?M NaCl solution were studied at 25°C by potentiometric titration. A model for complexation and stability constants of the complexes were determined. With all of the metals, complex formation was dominated by ML^4?. Comparison of TCA6 and six other chelating agents showed TCA6 to be suitable for applications where strong calcium binding is essential.     

Metal-ion-complexing properties of 2-(pyrid-2'-yl)-1,10-phenanthroline, a more preorganized analogue of terpyridyl. A crystallographic, fluorescence, and thermodynamic study

Some metal-ion-complexing properties of the ligand 2-(pyrid-2'-yl)-1,10-phenanthroline (MPP) are reported. MPP is of interest in that it is a more preorganized version of 2,2';6,2'-terpyridine (tpy). Protonation constants (pK(1) = 4.60; pK(2) = 3.35) for MPP were determined by monitoring the intense π-π* transitions of 2 × 10(-5) M solutions of the ligand as a function of the pH at an ionic strength of 0 and 25 °C. Formation constants (log K(1)) at an ionic strength of 0 and 25 °C were obtained by monitoring the π-π* transitions of MPP titrated with solutions of the metal ion, or 1:1 solutions of MPP and the metal ion were titrated with acid. Large metal ions such as Ca(II) or La(III) showed increases of log K(1) of about 1.5 log units compared to that of tpy. Small metal ions such as Zn(II) and Ni(II) showed little increase in log K(1) for MPP compared to the tpy complexes, which is attributed to the presence of five-membered chelate rings in the MPP complexes, which favor large metal ions. The structure of [Cd(MPP)(H(2)O)(NO(3))(2)] (1) is reported: monoclinic, P2(1)/c, a = 7.4940(13) ?, b = 12.165(2) ?, c = 20.557(4) ?, β = 96.271(7)°, V = 1864.67(9) ?(3), Z = 4, and final R = 0.0786. The Cd in 1 is seven-coordinate, comprising the three donor atoms of MPP, a coordinated water, a monodentate, and a bidentate NO(3)(-). Cd(II) is a fairly large metal ion, with r(+) = 0.96 ?, slightly too small for coordination with MPP. The effect of this size matching in terms of the structure is discussed. Fluorescence spectra of 2 × 10(-7) M MPP in aqueous solution are reported. The nonprotonated MPP ligand fluoresces only weakly, which is attributed to a photoinduced-electron-transfer effect. The chelation-enhanced-fluorescence (CHEF) effect induced by some metal ions is presented, and the trend of the CHEF effect, which is Ca(II) > Zn(II) > Cd(II) ~ La(III) > Hg(II), is discussed in terms of factors that control the CHEF effect, such as the heavy-atom effect.     

N2S2Ni metallodithiolate complexes as ligands: structural and aqueous solution quantitative studies of the ability of metal ions to form M-S-Ni bridges to mercapto groups coordinated to nickel(II). implications for acetyl coenzyme A synthase

The nickel(II) complex of an N2S2 ligand, derived from a diazacycle, N,N'-bis(mercaptoethyl)-1,5-diazacycloheptane, (bme-dach)Ni, Ni-1', serves as a metallodithiolate ligand to NiII, CuI, ZnII, Ag, and PbII. The binding ability of the NiN2S2 ligand to the metal ions was established through spectrochemical titrations in aqueous media and compared to classical S-donor ligands. For M = Ni, Zn, Pb, binding constants, log K = ca. 2. were computed for 1:1 Ni-1'/M(solvate) adducts; for Ag+ and Cu+, the 3:2 (Ni-1')3M2 adducts were the first formed products even in water with log beta3,2 values of 26 and >30, respectively. In all cases, the binding ability of Ni-S-R is intermediate between that of a free thiolate and a free thioether. The great specificity for copper over nickel and zinc by N2S2Ni, which serves as a reasonable structural model for the distal nickel of the acetyl CoA synthase active site, relates to biochemical studies of heterogeneity (metal content and type) in various preparations of acetyl CoA synthase enzyme.     

Simultaneous determination of speciation parameters of Cu,Pb, Cd and Zn in model solutions of Suwannee River fulvic acid by pseudopolarography

There is a growing awareness of the importance of quantitative determinations of speciation parameters of the trace metals Cu, Zn, Cd and Pb in aqueous samples containing chemically heterogeneous humic substances, especially when they are present together, interacting with one another and competing for specific binding sites of the humic substances. Such determinations require fundamental knowledge and understanding of these complex interactions, gained through basic laboratory-based studies of well-characterized humic substances in model solutions. Since the chemical heterogeneity of humic substances plays an important role in the thermodynamics (stability) and kinetics (lability) of trace metal competition for humic substances, a metal speciation technique such as pseudopolarography that can reveal the special, distinctive nature of metal complexation is required, and it was therefore used in this study. A comparison of the heterogeneity parameters (Gamma) for Zn(II), Cd(II), Pb(II) and Cu(II) complexes in model solutions of Suwannee River fulvic acid (SRFA) shows that GammaCd>GammaZn>GammaPb>GammaCu, suggesting that SRFA behaves as a relatively homogeneous complexant for Zn(II) and Cd(II), whereas it behaves as a relatively heterogeneous complexant for Pb(II) and an even more heterogeneous complexant for Cu(II) under the experimental conditions used. The order of values of log K* (from the differential equilibrium function, DEF) for the trace metals at pH 5.0 follow the sequence: log K*Cu>log K*Pb>log K*Zn>log K*Cd. These results are in good agreement with the literature values. The results of this work suggest the possibility of simultaneously determining several metals in a sample in a single experiment, and hence in a shorter time than required for multiple experiments.     

A photometric study of the complexation reaction between Alizarin complexan and zinc(II), nickel(II), lead(II), cobalt(II) and copper(II)

The complexation reaction between Alizarin complexan ([3-N,N-di(carboxymethyl)aminomethyl]-1,2-dihydroxyanthraquinone; H(4)L) and zinc(II), nickel(II), lead(II), cobalt(II) and copper(II) has been studied by a spectrophotometric method. All these metal ions form 1:1 complexes with HL; 2:1 metal:ligand complex were found only for Pb(II) and Cu(II). The stability constants are (ionic strength I = 0.1, 20 degrees C): Zn(2+) + HL(3-) right harpoon over left harpoon ZnHL(-) log K +/- 3sigma(log K) = 12.19 +/- 0.09 (I = 0.5) Ni(2+) + HL(3-) right harpoon over left harpoon NiHL(-) log K +/- 3sigma(log K) = 12.23 +/- 0.21 Pb(2+) + HL(3-) right harpoon over left harpoon PbHL(-) log K +/- 3sigma(log K) = 11.69 +/- 0.06 PbHL(-) + Pb(2+) right harpoon over left harpoon Pb(2)L + H(+) log K approximately -0.8 Co(2+) + HL(3-) right harpoon over left harpoon CoHL(-) log K 3sigma(log K) = 12.25 + 0.13 Cu(2+) + HL(3-) right harpoon over left harpoon CuHL(-) log K 3sigma(log K) = 14.75 +/- 0.07 Cu(2+) + CuHL(-) right harpoon over left harpoon Cu(2)L + H(+) log K approximately 3.5 The solubility and stability of both the reagent and the complexes and the closenes of the values of the stability constants make this reagent suitable for the photometric detection of several metal ions in the eluate from an ion-exchange column.