Conditional Formation Constants Research Articles

Spectroscopic characterization of chosen Ln(III) polyoxometalate complexes with organic counter cations in solid and in non-aqueous solutions

Polyoxometalate (POM) compositions containing organic counter cations and SiMo 2W 9O 39 8− polyanion have been synthesized using extraction. Several compounds with counter ions containing phosphorus such as: Ph 3P +CPh 3, Ph 3P +Et, Ph 3P +C 16H 33, Ph 4P +, Bu 4P + were obtained and their Eu(III) complexes synthesized. Luminescence characterization of the europium complexes concerning their intensities, quantum yields and luminescence lifetimes of Eu(III) ion both in DMF solutions and solid are discussed. The complexation study has been carried out analyzing absorption spectra of Nd(III) ion in the range of its hypersensitive transition ( 4I 9/2 → 2H 9/2). Results indicated formation of the Ln(POM) 2 sandwich complexes. Computer-assisted target factor analysis has been applied to evaluate the spectra of Nd(III) in DMSO solutions containing varying amounts of the POM. The conditional formation constants with lg β 12 = 8.6 ± 0.5, lg β 12 = 8.2 ± 0.4 and lg β 12 = 9.6 ± 0.7 were obtained for (Ph 3PEt) 5H 3SiMo 2W 9O 39, (Ph 3PC 16H 33) 7HSiMo 2W 9O 39 and (Ph 3PCPh 3) 6H 3SiMo 2W 9O 39, respectively. Studies have shown significant influence of the counter ion structure on the luminescence properties of Eu(III) in the systems studied. The most intensive luminescence and the highest value of quantum yield were observed in the case of the complex (Ph 3PCPh 3) 7H 6Eu(SiMo 2W 9O 36) 2, containing six phenyl groups in the organic counter cation.

Electrochemical and theoretical complexation studies for Zn and Cu with individual polyphenols

Zn and Cu interactions with three selected flavonoids (catechin, quercetin and rutin) have been electrochemically monitored. It has been shown that catechin takes one atom of metal per molecule; quercetin takes two atoms, and rutin is able to take up to three atoms. Not all ligands bind metals equally strong, and weakly bonded metals can be distinguished. Zn shows a sluggish kinetics and, at the same time, the highest conditional formation constants. The method could be applied to a real sample. Theoretical models are proposed for the most favourable compounds.

Effect of polymer nature and hydrodynamic conditions on a process of polymer enhanced ultrafiltration

Technical viability of two polymers, poly(ethyleneimine) and poly(acrylic acid), with different acid-basic behaviour, structure and molecular weight, in a reactive separation process for the recovery of nickel ions across zirconia ultrafiltration tubular membranes, has been studied in a laboratory-scale installation. Suitable experimental conditions (e.g., pH, feed flow rate, polymer and metal concentrations, transmembrane pressure) for the two steps of the process (metal recovery and polymer regeneration) have been optimised and justified with acid–base behaviour of each polymer and measures of polymer–nickel complex formation constants. For this reason, global stability constant of PAA–Ni and conditional complex formation constant of PEI–Ni were measured by potentiometric and spectrophotometric analyses, respectively. Furthermore, the effect of hydrodynamic conditions on two important factors limiting permeate flux (concentration polarization and fouling) have been analysed. The mass transfer coefficient in our system has been studied by means of stagnant layer model and dimensionless analysis.

Binding of copper(II) to thyrotropin-releasing hormone (TRH) and its analogs

Spectroscopy (UV–Vis, 1H NMR, ESR) and electrochemistry revealed details of the structure of the Cu(II)–TRH (pyroglutamyl–histidyl–prolyl amide) complex. The 1H NMR spectrum of TRH has been assigned. NMR spectra of TRH in the presence of Cu(II) showed that Cu(II) initially binds TRH through the imidazole. TRH analogs, pGlu-His-Pro-OH, pGlu-(1-Me)His-Pro-amide, pGlu-His-(3,4-dehydro)Pro-amide, pGlu-His-OH, pGlu-Glu-Pro-amide, and pGlu-Phe-Pro-amide provided comparison data. The stoichiometry of the major Cu(II)–TRH complex at pH 7.45 and greater is 1:1. The conditional formation constant (in pH 9.84 borate with 12.0 mM tartrate) for the formation of the complex is above 10 5 M −1. The coordination starts from the 1-N of the histidyl imidazole, and then proceeds along the backbone involving the deprotonated pGlu-His amide and the lactam nitrogen of the pGlu residue. The fourth equatorial donor is an oxygen donor from water. Hydroxide begins to replace the water before the pH reaches 11. Minority species with stoichiometry of Cu–(TRH) x ( x = 2–4) probably exist at pH lower than 8.0. In non-buffered aqueous solutions, TRH acts as a monodentate ligand and forms a Cu(II)–(TRH) 4 complex through imidazole nitrogens. All the His-containing analogs behave like TRH in terms of the above properties.

Microspectrophotometric determination of thorium(IV) and uranium(VI) with methylthymol blue

Attempts have been made to improve the sensitivity of complexation reactions of Th I V and U V I with methylthymol blue (MTB) in presence of cationic surfactant cetyldimethylethylammonium bromide (CDEAB). The marked bathochromic shift in λ m a x , considerable increase in sensitivity and extreme stability of these ternary complexes in presence of CDEAB were found to be useful in microdetermination of metal ions under study. The composition and values of conditional formation constants have been evaluated in presence of CDEAB and various analytical parameters, viz. conformity to Beer's law, photometric ranges, Sandell's sensitivity and molar absorptivity have been studied.

Towards Targeted MRI: New MRI Contrast Agents for Sialic Acid Detection

The detection of sialic acid in living systems is of importance for the diagnosis of several types of malignancy. We have designed and synthesized two new lanthanide ion ligands (L1 and L2) that are capable of molecular recognition of sialic acid residues. The basic structure of these ligands consists of a DTPA-bisamide (DTPA, diethylenetriamine pentaacetic acid) whose amide moieties each bear both a boronic function for interaction with the diol groups in the side chain of sialic acid, and a functional group that is positively charged at physiologic pH values and is designed to interact with the carboxylate anion of sialic acid. The relaxometric properties of the Gd3+ complexes of these two ligands were evaluated. The relaxivity of the GdL1 complex has a significant second-sphere contribution at pH values above the pKa of its phenylboronic acid moiety. The interaction of the Gd3+ complexes of L1 and L2 with each of several saccharides was investigated by means of a competitive fluorescent assay. The results show that both complexes recognize sialic acid with good selectivity in the presence of other sugars. The adduct formed by GdL2 with sialic acid has the higher conditional formation constant (50.43+/-4.61 M(-1) at pH 7.4). The ability of such complexes to recognize sialic acid was confirmed by the results of a study on the interaction of corresponding radiolabeled complexes (153SmL1 and 153SmL2) with C6 glioma rat cells. 153SmL2 in particular is retained on the cell surface in significant amounts.

Direct Microspectrophotometric Determination of Bismuth in Silver with Semi-Xylenol Orange

75–300 ng of bismuth in a sample mass of 20 mg of silver can be determined spectrophotometrically in a nitric acid medium (0.18 mol L−1) with Semi-Xylenol Orange as chromogenic agent. Its selectivity is realized by using less toxic reagents in very small amounts to mask some of the interfering impurity cationic species while the matrix effect is negligible. The conditional formation constant of the colored chelate formed was found to be lg K′f=3.1. It exhibits a maximum at 540 nm with a molar absorptivity of 4.2×104 L mo1−1 cm−1. The linear regression equation for this determination is A=0.022+0.0018 C, where C stands for the concentration of Bi (ng per 0.5 mL) with a correlation coefficient γ of 0.9873. The RSD at the level of 200 ng of Bi (n=10) was found to be 5%.

Synthesis and spectroscopic characterisation of chosen heteropolyanions and their Ln(III) complexes containing tetrabutylammonium counter ion

Synthesis and spectroscopic characterisation of tetrabutylammonium salts of heteropolyanions (NBu 4HPAS) of Keggin’s (NBu 4) 5H 2PW 11O 39, (NBu 4) 6HPMo 2W 9O 39, (NBu 4) 6H 2SiMo 2W 9O 39, (NBu 4) 4H 3PMo 2W 9O 39, (NBu 4) 6HPMo 2W 9O 39, Dawson’s type (NBu 4) 9HP 2W 17O 61 and Preyssler heteropolyanion (NBu 4) 12[(Eu)P 5W 30O 110] are presented. Extraction procedure has been selected as the best method for synthesis of compositions containing an organic counter anion. Elemental and thermogravimetric analysis, spectrophotometric determination of W and Mo, and FTIR spectra have been applied for analysis of the compositions obtained. The complexation studies were carried out with the use of Nd(III) and Er(III) absorption spectra (in the range of their hypersensitive transitions) and Eu(III) luminescence lifetime measurements. Threshold bootstrap computer-assisted target factor analysis (TB CAT) has been applied to evaluate the Nd(III) and Er(III) spectra in DMF and DMSO solutions and to estimate conditional formation constants for Ln(III)(NBu 4HPAS) 2 sandwich species (Ln(III)Nd(III), Er(III)). Luminescence characterisation of the NBu 4EuHPAS compounds concerning their intensity, quantum yield and luminescence lifetime of Eu(III) ion in non-aqueous solutions and solid are discussed.

Low temperature preparation of strontium barium niobate powders from metal carboxylate gels

Sr x Ba 1− x Nb 2O 6 ( x=0.4, 0.5, 0.6) ceramic powders have been synthesized by using the aqueous organic gel routes. The desired metal cations are chelated in a solution using citric acid and ethylenediaminetertraacetic acid (EDTA) as the chelating agents. The thermal decompostion of the metal carboxylate precursors gels have been studied by TG/DTA and the products derived from calcinations of the gels at different temperatures have been characterized by XRD and TEM. The results reveal that tetragonal tungsten–bronze-type SBN with fine particle size could be achieved at 800 °C. The pH 8 used during the preparation of Sr–Ba–Nb precursor gels is determined by calculation of the conditional formation constants K cf of Sr–EDTA and Ba–EDTA complexes. The influences of the pH and the molar ratio of citric acid:Nb cation on the formation of homogeneous Sr–Ba–Nb precursor gels are also studied.

Threshold bootstrap target factor analysis study of neodymium with pyridine 2,4 dicarboxylic acid N-oxide—an investigation of traceability

Threshold bootstrap computer-assisted target factor analysis (TB CAT) has been applied to the evaluation of UV-Vis spectra of Nd(III) in aqueous solutions containing varying amounts of pyridine 2,4-dicarboxylic acid N-oxide (dipyr NO). Using the total uncertainty budget concept, probability distributions of the conditional formation constant(s) of the respective Nd(III) dipyr NO species have been evaluated. The experimental data are compatible with two different systems: a single Nd(III) dipyr NO species with log β11=2.8±0.11or, alternatively, a two species system with log β11=3.7±0.35and log β12=5.2±0.8. The results seem to favor the single species interpretation. However, a discussion of the influence of spectral correlation indicates that the situation should be considered with caution. The results are compared to a previous TB CAT study on Nd(III) picolinic acid N-oxide interaction illustrating the advantage of arguing in terms of probability density functions. The pyridine N-oxides with substituted carboxylic acids are structurally poorly investigated. Hence, this study calls for more structural information on rare earth pyridine N-oxide derivates.

Spectrophotometric study of complexation equilibria with H-point standard addition and H-point curve isolation methods

The use of H-point curve isolation (HPCIM) and H-point standard addition methods (HPSAM) for spectrophotometric studies of complex formation equilibria are proposed. One step complex formation, two successive stepwise and mononuclear complex formation systems, and competitive complexation systems are studied successfully by the proposed methods. HPCIM is used for extracting the spectrum of complex or sum of complex species and HPSAM is used for calculation of equilibrium concentrations of ligand for each sample. The outputs of these procedures are complete concentration profiles of equilibrium system, spectral profile of intermediate components, and good estimation of conditional formation constants. The reliability of the method is evaluated using model data. Spectrophotometric studies of murexide–calcium, dithizone–nickel, methyl thymol blue (MTB)–copper, and competition of murexide and sulfate ions for complexation with zinc, are used as experimental model systems with different complexation stoichiometries and spectral overlapping of involved components.

Sequence-selective DNA cleavage by a chimeric metallopeptide.

A chimeric metallopeptide derived from the sequences of two structurally superimposable motifs was designed as an artificial nuclease. Both DNA recognition and nuclease activity have been incorporated into a small peptide sequence. P3W, a 33-mer peptide comprising helices alpha2 and alpha3 from the engrailed homeodomain and the consensus EF-hand Ca-binding loop binds one equivalent of lanthanides or calcium and folds upon metal binding. The conditional formation constants (in the presence of 50 mM Tris) of P3W for Eu(III) (K(a) = (2.1 +/- 0.1) x 10(5) M(-1)) and Ce(IV) (K(a) = (2.6 +/- 0.1) x 10(5) M(-1)) are typical of isolated EF-hand peptides. Circular dichroism studies show that 1:1 CeP3W is 26% alpha-helical and EuP3W is up to 40% alpha-helical in the presence of excess metal. The predicted helicity of the folded peptide based on helix length and end effects is about 50%, showing the metallopeptides are significantly folded. EuP3W has considerably more secondary structure than our previously reported chimeras (Welch, J. T.; Sirish, M.; Lindstrom, K. M.; Franklin, S. J. Inorg. Chem. 2001, 40, 1982-1984). Eu(III)P3W and Ce(IV)P3W nick supercoiled DNA at pH 6.9, although EuP3W is more active at pH 8. CeP3W cleaves linearized, duplex DNA as well as supercoiled plasmid. The cleavage of a 5'-(32)P-labeled 121-mer DNA fragment was followed by polyacrylamide gel electrophoresis. The cleavage products are 3'-OPO(3) termini exclusively, suggesting a regioselective or multistep mechanism. In contrast, uncomplexed Ce(IV) and Eu(III) ions produce both 3'-OPO(3) and 3'-OH, and no evidence of 4'-oxidative cleavage termini with either metal. The complementary 3'-(32)P-labeled oligonucleotide experiment also showed both 5'-OPO(3) and 5'-OH termini were produced by the free ions, whereas CeP3W produces only 5'-OPO(3) termini. In addition to apparent regioselectivity, the metallopeptides cut DNA with modest sequence discrimination, which suggests that the HTH motif binds DNA as a folded domain and thus cleaves selected sequences. The de novo artificial nuclease LnP3W represents the first small, underivatized peptide that is both active as a nuclease and sequence selective.

Holo- and hemidirected lead(II) in the polymeric [Pb(4)(mu-3,4-TDTA)2(H2O)2]*4H2O complex. N,N,N',N'-tetraacetate ligands derived from o-phenylenediamines as sequestering agents for lead(II).

The coordinating ability of the ligands 3,4-toluenediamine-N,N,N',N'-tetraacetate (3,4-TDTA), o-phenylenediamine-N,N,N',N'-tetraacetate (o-PhDTA), and 4-chloro-1,2-phenylenediamine-N,N,N',N'-tetraacetate (4-Cl-o-PhDTA) (H4L acids) toward lead(II) is studied by potentiometry (25 degrees C, I = 0.5 mol x dm(-3) in NaClO4), UV-vis spectrophotometry, and 207Pb NMR spectrometry. The stability constants of the complex species formed were determined. X-ray diffraction structural analysis of the complex [Pb4(mu-3,4-TDTA)4(H2O)2]*4H2O (1) revealed that 1 has a 2-D structure. The layers are built up by the polymerization of centrosymmetric [Pb4L2(H2O)2] tetranuclear units. The neutral layers have the aromatic rings of the ligands pointing to the periphery, whereas the metallic ions are located in the central part of the layers. In compound 1, two types of six-coordinate lead(II) environments are produced. The Pb(1) is coordinated to two nitrogen atoms and four carboxylate oxygens from the ligand, whereas Pb(2) has an O6 trigonally distorted octahedral surrounding. The lead(II) ion is surrounded by five carboxylate oxygens and a water molecule. The carboxylate oxygens belong to four different ligands that are also joined to four other Pb(1) ions. The selective uptake of lead(II) was analyzed by means of chemical speciation diagrams as well as the so-called conditional or effective formation constants K(Pb)eff. The results indicate that, in competition with other ligands that are strong complexing agents for lead(II), our ligands are better sequestering agents in acidic media.

Interaction of lead(II) with sediment particles: a mercury microelectrode study

The usefulness of microelectrodes for trace metal speciation in the presence of particles is tested using anodic stripping voltammetry (ASV) at a mercury microelectrode. Discrimination between dissolved and particulate Pb(II) concentrations is carried out in the presence of particles from a freeze-dried sediment sample from Tejo (Portugal) estuary (zone not influenced by fresh water) as well as in the presence of SiO 2 particles, chosen to mimic the major component of the sediment. Different sets of experiments were done with the sediment samples: before and after decomposition of organic matter and in the presence and absence of the dissolved major macroconstituents such as calcium, magnesium and chloride in order to assess the influence of organic matter and the competition of those ions for lead complexation with the sediment surface. In all situations, the ionic strength was kept constant at 0.70 M and the pH=7.4±0.1. For the different experimental conditions used complexometric capacities for Pb(II) and average conditional surface complex formation constants were estimated from ASV voltammetric titration curves.

Infrared spectra of phthalic acid, the hydrogen phthalate ion, and the phthalate ion in aqueous solution

The infrared spectra of a series of aqueous solutions containing phthalic acid (1,2-benzenedicarboxylic acid) and varying pH were examined using attenuated total reflection Fourier transform infrared spectroscopy and potentiometry. The basis spectra of phthalic acid, the hydrogen phthalate ion, and the phthalate ion were isolated using a factor analysis in which the absorbance of these species varies with pH and total phthalate concentration according to equilibrium and mass balance relations. Assignments of these basis spectra were made by comparison with spectra calculated ab initio. The conditional formation constants of phthalic acid and the hydrogen phthalate ion were determined at 25.0±0.1°C in 0.6 M NaCl ionic media using infrared spectroscopy and in 1.5 M NaCl ionic media using both infrared spectroscopy and potentiometry.

Formation constants in the ternary systems: Co(II)‐L‐Cysteine ‐ Adenine & Zn(II)‐L‐Cysteine‐Adenine2

A potentiometric titration technique has been used to determine stability constants for the various complexes of Co(II) and Zn(II) with L‐cysteine and from DNA bases i.e. Adenine. Stability constants of ternary systems have been evaluated by the method suggested by Irwing‐Rossotti. In addition, the conditional constants were calculated as a function of pH. The maximum values of the conditional formation constants were found to be in accordance with the mixed‐ligand complex formation constants at a given pH region. Furthermore, the molar fractions of different species from mixed complexes were calculated by means of formation constants. The values of stability constants of mixed‐ligand complexes at 25°C are as follows: log K=9.50 for Co(II)‐L‐cysteine‐Adenine; log K 1 = 6.85 and log K 2 = 6.00 for Zn(II)‐L‐cysteine‐Adenine2. The ionic strength was kept constant at I = 0.20 with NaClO4.

Conditional formation constants or chemical speciation data?

For decades conditional constants have been used by industry, in promotional literature and in patents, to assess the efficiency of a chelating agent in complexing a metal ion at a set pH. Speciation computer programs now permit the complete system to be modelled and to take into account interfering side reactions and variations in pH. This work compares conditional constant calculations with speciation modelling and shows that such modelling can produce a more efficient and accurate assessment of the power of a chelating agent.

Crystal structure of the 3-D complex [(H2O)Cd(μ-3,4-TDTA)Cd(H2O)]. Potentiometric and 113Cd NMR studies in aqueous solution (3,4-TDTA = 3,4-toluenediamine-N,N,N ′,N ′-tetraacetate)

The coordinating ability of the ligands 3,4-toluenediamine-N,N,N′,N′-tetraacetate (3,4-TDTA), ortho-phenylenediamine-N,N,N′,N′-tetraacetate (o-PhDTA) and 4-chloro-1,2-phenylenediamine-N,N,N′,N′-tetraacetate (4-Cl-o-PhDTA) (L4−) towards cadmium(II) was studied by potentiometry (25 °C, I = 0.5 mol dm−3 in NaClO4) and 113Cd NMR spectroscopy. The analysis of the potentiometric data showed the formation of complexes with the ligand ∶ metal ratio 2 ∶ 1 (identified for the first time for these ligands with any metal), [CdH3L2]3−, [CdH2L2]4−, [CdHL2]5− and [CdL2]6−, with ratio 1 ∶ 1, [Cd(H2L)], [Cd(HL)]− and [CdL]2−, and with ratio 1 ∶ 2, [Cd2H2L]2+, and [Cd2L]. The stability constants were determined. The formation of these complexes was confirmed by 113Cd NMR. X-Ray diffraction structural analysis of the complex [(H2O)Cd(μ-3,4-TDTA)Cd(H2O)] 1 revealed a polymeric 3-D structure in which two types of cadmium environments are produced, Cd1 and Cd2. The 3-D structure is built-up by the self-assembly of [Cd1(3,4-TDTA)]2− units sharing Cd2(II) ions giving interlocked infinite chains that run along the 3 directions. Cd1 is coordinated to two N atoms and four O atoms of the same ligand and one water molecule, Cd2 to six carboxylate-oxygen atoms from four different ligands and to a water molecule. The selective uptake of cadmium(II) was analyzed by means of chemical speciation diagrams as well as so-called conditional or effective formation constants KeffCd. The results indicate that, in competition with other ligands that are strong complexing agents for cadmium(II), the present ligands are better sequestering agents in acidic media.

The surfactant-sensitized analytical reaction of niobium with some thiazolylazo compounds

Cationic surfactants, such as cetylpyridinium bromide (CPB), sensitize the color reaction of Nb(V) with 1-(2-benzothiazolylazo)-2-hydroxy-3-naphthoic acid (I a), 5-(benzothiazolylazo)2,5-naphthalenediol (I b), 5-(2-benzothiazolylazo)8-hydroxyquinoline (I c) and 4-(2- benzothiazolylazo)2, 2 ̄ -biphenyldiol (I d) reagents. The formation of a ternary complex of stoichiometric ratio 1:2:2 (Nb-R-CPB) is responsible for the observed enhancement in the molar absorptivity and the Sandell sensitivity of the formed complex, when a surfactant is present. The ternary complex exhibits absorption maxima at 649, 692, 661 and 612 nm, (ε=3.35×10 4, 3.59×10 4, 4.46×10 4 and 2.79×10 4 l mol −1 cm −1) on using reagent I a, I b, I c, and I d, respectively. Beer’s law is obeyed between 0.05 and 2.50 μg ml −1, while applying the Ringbom method for more accurate results is in the range from 0.20 to 2.30 μg ml −1. Conditional formation constants in the presence and absence of CPB for niobium complexes have been calculated. On the basis of a detailed spectrophotometric study, the nature of the chromophoric reagent–surfactant interaction and the peculiar features of the sensitization by CPB are discussed.

The effect of pH on the complexation of Cd, Ni and Zn by dissolved organic carbon from leachate-polluted groundwater

Complexation of cadmium (Cd), nickel (Ni) and zinc (Zn) by dissolved organic carbon (DOC) in leachate-polluted groundwater was measured using a resin equilibrium method. Metal-DOC complexation was measured at different DOC concentrations over a range of pH values (5–8). The results were compared to simulations made by two speciation models (WHAM and MINTEQA2). Of these models, WHAM came closest to simulating the experimental observations although it systematically overestimated the pH dependence of metal-DOC complexation. Accepting a variation in the free metal ion activity of a factor of 3–4 the WHAM model provided useful predictions of the complexation of Cd and Zn by DOC in the pH range 5–8, and of Ni in the pH range 5–7. At pH 8, however, the model overestimates the extent of Ni-DOC complexation to an unacceptable degree. The MINTEQA2 model predicts virtually no pH dependence for DOC complexation of Cd, Ni and Zn and is thus in very poor agreement with the experimental results. As an alternative approach, relations between the conditional complex formation constant (log K c) and pH were estimated for each metal. Using these relations for estimating the complexation of Cd, Ni and Zn by DOC a deviation in the free metal ion activity up to a factor of 2 can be expected.