Synthesis,characterization, speciation and biological studies on metal chelates of 1-benzoyl(1,2,4-triazol-3-yl)thiourea

Proton-ligand association constants of 1-benzoyl(1,2,4-triazol-3-yl)thiourea (BTTU) and its complex formation constants with some bivalent metal ions Ni(II), Co(II), Mn(II), Zn(II), and Cu(II), have been determined potentiometrically in 50% EtOH–H₂O and 0.1 M NaNO₃. The complexes formed in solution have a stoichiometry of 1:1 and 1:2 [M:L], where M represents the metal ion and L the BTTU ligand. The corresponding thermodynamic parameters are derived and discussed. The complexes are stabilized by enthalpy changes and the results suggest that complexation is an enthalpy-driven process. The effects of metal ion, ionic radius, electronegativity, and nature of ligand on the formation constants are discussed. The formation constants of the complexes with 3d transition metals follow the order Mn²⁺ < Co²⁺ < Ni²⁺ < Cu²⁺ > Zn²⁺. The metal complexes were synthesized and characterized by elemental analyses, conductance, IR, ¹H NMR, and magnetic measurements. The low magnetic moment of 0.11 BM for the Cu(II) complex is suggestive of dimerization through Cu–Cu interaction. The concentration distribution diagrams of the complexes were evaluated. The ligands and their metal complexes have been screened in vitro against some bacteria and fungi.     

Simultaneous separation and extraction of Ag(I), Pb(II) and Pd(II) ions by solid phase method and determination of these ions by flame atomic absorption spectrometry

A simultaneous preconcentration and separation method for determination of trace amount of dissolved Ag⁺, Pb²⁺ and Pd²⁺ ions by modified octadecyl silica membrane disks with DBzDA18C6 was developed. The adsorbed metal complexes were eluted from disk with 10?mL of 4?M KCl and determined by flame atomic absorption spectroscopy. Several parameters such as anion effect, pH of sample solution, type of eluent, amount of ligand, sample and elution flow rate were evaluated. The effect of diverse ions on preconcentration was also investigated. A precocentration factor of 110 can easily be achieved depending on the volume of the sample. For 100?mL of the solution the linear dynamic rang were found to be 30–1000, 140–6000, 60–900?μg?l^?1 for Ag⁺, Pb²⁺ and Pd²⁺, respectively. Based on three standard deviation of the blank the detection limit was obtained as 1.8, 8.0 and 4.2?μg?L^?1 for Ag⁺, Pb²⁺, Pd²⁺, respectively. The formation constants of Ag⁺ and Pb²⁺ ions with DBzDA18C6 at 25?°C were determined from the molar conductance–mole ratio data. This method was applied for the determination of Ag⁺, Pb²⁺ and Pd²⁺ in environmental water, tea and soil samples.     

Ultra trace level square wave anodic stripping voltammetric sensing of mercury(II) ions in environmental samples using a Schiff base-modified carbon paste electrode

ABSTRACT

In this approach, a new carbon paste electrode modified with N,N′-bis(5-bromo-2-hydroxybenzylidene)-2,2-dimethylpropane-1,3-diamine Schiff base ligand (L) was synthesised for selective and effective determination of Hg²⁺ ions in aqueous environmental samples using cyclic and square wave anodic stripping voltammetric methods. First, the selective detection of mercury ion was confirmed by evaluating the stability constants of metal complexes formed between the Schiff base ligand (L) and some desired cations by conductometric measurements. Afterwards, by preparing an effective carbon paste electrode modified with L, the experimental and instrumental parameters affecting the performance of modified electrode were investigated. Square wave anodic stripping voltammograms were obtained after applying an accumulation potential ?0.5 V and accumulation time 150 s in Britton–Robinson buffer solution at pH 2.0. The optimal square wave parameters found are pulse amplitude 75 mV, frequency 50 Hz and step potential 6 mV. The procedure exhibited linear range from 0.4 to 120 μg L^?1 Hg²⁺ with a limit of detection of 0.042 μg L^?1. The proposed electrode was proved to be highly selective in the presence of various cations and anions and was successfully used for determination of mercury in tobacco and several water samples.     

Thermodynamics of mixed-ligand complex formation of copper(II) ethylenediaminetetraacetate with amino acids in solution

The mixed-ligand complex formation in the systems Cu²⁺–Edta^4?–L (L = His, Lys, Orn, Arg, Im) has been calorimetrically, pH-potentiometrically, and spectrophotometrically studied in aqueous solution at 298.15 K and the ionic strength of I = 0.5 (KNO₃). The thermodynamic parameters of formation of the CuEdtaL, CuEdtaHL, and (CuEdta)₂L complexes have been determined. The probable coordination mode for the complexone and the ancillary ligand in the mixed-ligand complexes was discussed.     

Complexation of managanese(II), cobalt(II), nickel(II), zink(II), and cadmium(II) cations with amoxicillin

The interaction of amoxicillin anions (Axn^?) with Mn²⁺, Co²⁺, Ni²⁺, Zn²⁺, and Cd²⁺ in aqueous solution at 20°C and an ionic strength of 0.1 (KNO₃) has been studied pH-metrically. In a neutral and weak alkaline solution, MAxn⁺ and M(OH)Axn complexes are formed. The formation constants and the pH ranges of existence of these complexes have been determined.     

Composition and stability constants of copper(II) complexes with succinic acid determined by capillary electrophoresis

The advantage of capillary electrophoresis was demonstrated for studying a complicated system owing to the dependence of direction and velocity of the electrophoretic movement on the charge of complex species. The stability constants of copper(II) complexes with ions of succinic acid were determined by capillary electrophoresis, including the 1?:?2 metal to ligand complexes which are rarely mentioned. The measurements were carried out at 25 °C and ionic strength of 0.1, obtained by mixing the solutions of succinic acid and lithium hydroxide up to pH 4.2–6.2. It was shown that while pH was more than 4.5 the zone of copper(II) complexes with succinate moves as an anion. It is impossible to treat this fact using only the complexes with a metal-ligand ratio of 1?:?1 (CuL⁰, CuHL⁺). The following values of stability constants were obtained: log β(CuL) = 2.89 ± 0.02, log β(CuHL⁺) = 5.4 ± 0.5, log β(CuL₂^2?) = 3.88 ± 0.05, log β(CuHL₂^?) = 7.2 ± 0.3.     

Studies of Size-Based Selectivity in Aqueous Ternary Complexes of Americium(III) or Lanthanide(III) Cations

Spectrophotometric and calorimetric titrations were used to determine the equilibrium constants (log₁₀ K ₁₁₁) and enthalpies of formation (ΔH ₁₁₁) for aqueous ternary complexes of the form M(L^a)(L^b) (M = Nd³⁺, Sm³⁺, Tb³⁺, Ho³⁺, Er³⁺, or Am³⁺; L^a = DTPA^5?, DO3A^3?, or CDTA^4?; L^b = oxalate (Ox), malonate (Mal), or iminodiacetate (IDA)). Inner-sphere ternary complexes were readily formed with the septadentate DO3A (1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid) and hexadentate CDTA (trans-1,2-diaminocyclohexanetetraacetic acid) ligands, whose binary complexes have residual metal-coordinated water molecules that are readily displaced by the smaller secondary ligands. The stability constants for the formation of lanthanide–CDTA complexes with Ox, Mal, and IDA generally increase with decreasing ionic radius when steric hindrance is minimal, with the trend in the M(CDTA)^? formation constants overshadowing any size-based reversal in the stepwise ternary complexation constants. Similar ternary complexes with DO3A showed little increase in thermodynamic stability compared to analogous CDTA complexes and no preference for larger Ln cations. The octadentate DTPA (diethylenetriaminepentaacetic acid) ligand proved too large to form ternary complexes to a measurable extent with any of the secondary ligands investigated, despite the presence of one residual inner sphere water molecule.     

Synthesis and characterization of some potential antitumor palladium(II) complexes of 2-aminomethylbenzimidazole and amino acids

The stoichiometry and stability constants of complexes formed between [Pd(AMBI)(H₂O)₂]²⁺ (AMBI?=?2-(aminomethyl)-benzimidazole) with some selected bio-relevant ligands containing different functional groups were investigated at 25°C and 0.1?mol?L^?1 ionic strength. The ligands used are imidazole, cysteine, glutathione (GSH), threonine, aspartic acid, 1,1-cyclobutane dicarboxylic acid (CBDCA) and lysine. The stoichiometry and stability constants of the formed complexes were reported and the concentration distribution of the various complex species was evaluated as a function of pH. The results show ring opening of CBDCA and monodentate complexation of the DNA constituent with the formation of [Pd(AMBI)(CBDCA–O)DNA], where (CBDCA–O) represents cyclobutane dicarboxylate coordinated by one carboxylate oxygen. The equilibrium constant of the displacement reaction of coordinated inosine, as a typical DNA constituent, by glutathione, as a typical thiol ligand, was investigated. The effect of dioxane on the formation constant of CBDCA with Pd(AMBI)²⁺ is reported. Five new palladium(II) complexes of the formula [Pd(AMBI)(AA)] ^{n +} (where AMBI?=?2-aminomethyl benzimidazole, AA is an anion of glycine, alanine, cysteine, methionine, and serine) have been synthesized. These palladium(II) complexes have been ascertained by elemental, molar conductance, infrared and ¹H-NMR spectroscopy. The isolated Pd(II) complexes were screened for their antibacterial and cytotoxic activities and the results are discussed.     

Conductometric studies and application of new Schiff base ligand as carbon paste electrode modifier for mercury and cadmium determination

A new chemically modified carbon paste electrode by 2,2?-((pyridine-2,6-diylbis(azanylylidene))bis(methanylylidene))diphenol (L) ligand has been made and used as a sensor for determination of trace mercury and cadmium ions with cyclic voltammetry (CV) and differential pulse voltammetry (DPV) methods. Complexation studies of the ligand with Cu²⁺, Zn²⁺, Hg²⁺, Ni²⁺ and Cd²⁺ ions by conductometric method in acetonitrile–ethanol mixture at 25°C show that the ML complexes have formed. The formation constants of complexes were calculated from the computer fitting of the molar conductance–mole ratio data, and the stability of the resulting complexes varied in order of Cd²⁺ > Hg²⁺ > Cu²⁺ > Zn²⁺ > Ni²⁺. Then a simple and effective chemically modified carbon paste electrode with L was prepared, and the electrochemical properties and applications of the modified electrode were investigated. Under the optimal conditions, the detection limit was 0.0494 μg L^?1 and 0.0782 μg L^?1 for cadmium and mercury ions, respectively, and the linear range for both metal ions were from 1 to 100 μg L^?1. The electrode shows high sensitivity, reproducibility and low cost, and was successfully applied to determination of Cd²⁺ and Hg²⁺ ions in water samples with recovery in the range of 97–101%.     

Conductometric and 1H NMR studies of thermodynamics of complexation of Zn2+, Cd2+ and Pb2+ ions with tetrathia-12-crown-4 in dimethylsulfoxide-nitrobenzene mixtures

The complex formation between Zn²⁺, Cd²⁺ and Pb²⁺ ions with macrocyclic ligand, tetrathia12-crown-4 (12S4) was studied in dimethylsulfoxide (DMSO)–nitrobenzene binary mixtures at different temperatures using conductometric and ¹H NMR methods. In all cases, 12S4 found to form 1:1 complexes with these cations. The formation constants of the resulting 1:1 complexes in different solvent mixtures were determined by computer fitting of the resulting molar conductance- and chemical shift-mole ratio data. There is an inverse relationship between the complex stability and the amount of DMSO in the solvent mixtures. The stability of the resulting M²⁺-12S4 complexes found to decrease in the order Pb²⁺ > Cd²⁺ > Zn²⁺. The values of ?H°, ?S° and ?G° for complexation reactions were evaluated from the temperature dependence of formation constants via van’t Hoff method. The obtained results revealed that, in all cases, the complexes are enthalpy stabilized, but entropy destabilized and the values of ?H° and ?S° are strongly depend on the nature of medium. There is also a linear relationship between all ΔH° and TΔS° values indicating the existence of entropy–enthalpy compensation in complexation of the three cations and ligand in the solvent systems studied.     

Potentiometry, Stability and Thermodynamics of Diethyltin(IV) Dichloride with Some Selected Biomolecules

The interaction of diethyltin(IV), DET, with selected bioligands having a variety of model functional groups was investigated at 25 °C and ionic strength 0.1 mol·dm^?3 NaCl using the potentiometric technique. The hydrolysis constants of the DET cation and the formation constants of the complexes formed in solution were calculated using the MINIQUAD-75 program. The stoichiometry and stability constants for the complexes formed are reported. The results show the formation of 1:1 and 1:2 complexes with amino acids and DNA constituents. The dicarboxylic acids form 1:1 complexes. The peptides form both 110 complexes and the corresponding deprotonated amide species [Et₂Sn(LH_?1)] (11-1). The participation of different ligand functional groups in binding to organotin is discussed. The concentration distributions of the various complex species were evaluated as a function of pH. The standard thermodynamic parameters ΔH° and ΔS°, calculated from the temperature dependence of the equilibrium constants, were investigated for the interaction of DET with thymine as a representative example of DNA constituents. The effect of ionic strength and solvent on hydrolysis constants of DET, protonation equilibria of thymine and its complex formation with DET were investigated and discussed.     

Complex Formation Equilibria of Unusual Seven Coordinate Fe(III) Complexes with DNA Constituents

Seven-coordinate Fe(III) complexes [Fe(dapsox)(H₂O)₂]⁺, where [dapsox = 2,6-diacetylpyridine-bis(semioxamazide)] is an equatorial pentadentate ligand with five donor atoms (2O and 3N), were studied with regard to their acid–base properties and complex formation equilibria. Stability constants of the complexes and the pK _a values of the ligands were measured by potentiometric titration. The interaction of [Fe(dapsox)(H₂O)₂]⁺ with the DNA constituents, imidazole and methylamine·HCl were investigated at 25 °C and ionic strength 0.1 mol·dm^?3 NaNO₃. The hydrolysis constants of the [Fe(dapsox)(H₂O)₂]⁺ cation (pK _a1 = 5.94 and pK _a2 = 9.04), the induced ionization of the amide bond and the formation constants of the complexes formed in solution were calculated using the nonlinear least-squares program MINIQUAD-75. The stoichiometry and stability constants for the complexes formed are reported. The results show the formation of 1:1 and 1:2 complexes with DNA constituents supporting the hepta-coordination mode of Fe(III). The concentration distributions of the various complex species were evaluated as a function of pH. The thermodynamic parameters ΔH° and ΔS° calculated from the temperature dependence of the equilibrium constants were investigated for interaction of [Fe(dapsox)(H₂O)₂] with uridine.     

Evaluation of Vanadium(IV) as a Non-radioactive Surrogate for Technetium(IV) by Comparison of Stability Constants for Polyamino Polycarboxylate Ligand Complexation

The stability constants for the Tc(IV) and V(IV) complexation with the polyamino polycarboxylate ligands IDA, NTA, HEDTA and DTPA were determined using liquid–liquid extraction techniques. These stability constants were then used to evaluate the validity of using V(IV) as a chemical analogue for Tc(IV). Results suggest that Tc(IV), as TcOOH⁺, will form β _1?11 complexes with the selected ligands, while V(IV), as VO²⁺, will form β ₁₀₁ complexes. The values for these determined stability constants are (in log₁₀ unit) 10.9 ± 0.1, 11.4 ± 0.1, 14.9 ± 0.1, and 20.1 ± 0.1 for Tc(IV) in 0.5 mol·L^?1 NaCl at 25 °C, for IDA, NTA, HEDTA and DTPA, respectively, they are 9.3 ± 0.1, 11.6 ± 0.2, 15.8 ± 0.1, and 20.8 ± 0.1 for V(IV) in 0.5 mol·L^?1 NaCl at 25 °C, for the same suite of ligands. The incorporation of a hydroxide into the metal ligand complexes formed by Tc(IV) is proposed as the largest factor differentiating the apparent stability constants of Tc(IV) and V(IV). This work shows that V(IV) is a poor analog for Tc(IV); however, despite the differences in complexation mechanism between V(IV) and Tc(IV), V(IV) still appears to have some use for predicting Tc(IV) complexation behavior.     

Equilibria and kinetics of complex formation between copper(II) and the polyamine Me2octaen

The kinetics and the equilibria of complex formation between Cu(II) and 1,25-dimethyl-1,4,7,10,13,16,19,22,25-nonaazapentacosane (Me₂octaen) have been investigated in acidic aqueous solution, at an ionic strength of 0.1 mol dm⁻³ and 25°C, by the stopped-flow method and UV spectrometry. Me₂octaen is a linear polyamine made by the union of eight en units bearing two methyl residues at the ends of the chain. Spectrophotometric titrations and kinetic experiments indicated that below pH 2.4 the binding of Cu(II) to the ligand gives rise to several 1:1 complexes differing for their degree of protonation, whereas above pH 2.4 binuclear complexes are formed as well. Concerning the mononuclear species, the ratios of the formation to the dissociation rate constants are in agreement with the equilibrium constants measured by static spectrometry. The analysis of the kinetic and spectrophotometric data reveals that, in the range of pH between 1.7 and 2.4, the most reactive forms of the ligand (denoted in its fully protonated form as H₉L⁹⁺) are H₇L⁷⁺, H₆L⁶⁺, and H₅L⁵⁺. The analysis of the results shows that Cu²⁺ and H₆L⁶⁺ may react according to the internal conjugate base mechanism (ICB), although the observed internal conjugate base effect is more modest than that displayed by the Ni²⁺–Me₂octaen system. The small magnitude of the ICB effect is explained in terms of the enhanced reactivity of copper(II) owing to the Jahn–Teller distortion occurring for Cu²⁺ complexes.     

Kinetics of the reaction of nitric oxide with polypyridylamine iron(II) complexes

The reactions of three polypyridylamine ferrous complexes, [Fe(TPEN)]²⁺, [Fe(TPPN)]²⁺, and [Fe(TPTN)]²⁺, with nitric oxide (NO) (where TPEN = N,N,N′,N′-tetrakis(2-pyridylmethyl)ethylenediamine, TPPN = N,N,N′,N′-tetrakis(2-pyridylmethyl)-1,2-propylenediamine, and TPTN = N,N,N′,N′-tetrakis(2-pyridylmethyl)trimethylenediamine) were investigated. The first two complexes, which are spin-crossover systems, presented second-order rate constants for complex formation reactions (k_f) of 8.4 × 10³ and 9.3 × 10³ M^?1 s^?1, respectively (pH 5.0, 25 °C, I = 0.1 M). In contrast, the [Fe(TPTN)]²⁺ complex, which is in low-spin ground state, did not show any detectable reaction with NO. k_f values are lower than those of high-spin Fe(II) complexes, such as [Fe(EDTA)]^2? (EDTA = ethylenediaminetetraacetate) and [Fe(H₂O)]²⁺, but higher than low-spin Fe(II) complexes, such as [Fe(CN)₅(H₂O)]^3? and [Fe(bipyridine)₃]²⁺. The release of NO from the [Fe(TPEN)NO]²⁺ and [Fe(TPPN)NO]²⁺ complexes were also studied, showing the values 15.6 and 17.7 s^?1, respectively, comparable to the high-spin aminocarboxylate analogs. A mechanism is proposed based on the spin-crossover behavior and the geometry of these complexes and is discussed in the context of previous publications.     

Potentiometric Studies of Ternary Complexes Involving Palladium(II) with Fluoxetine Drug and Some Bio-ligands

The complex formation equilibria of [Pd(FLX)(H₂O)₂]²⁺, where FLX = the drug fluoxetine, with the bio-ligands (L), glycine, serine, cysteine, histidine, glycylglycine and glycylvaline, were studied and their formation constants were determined. Stoichiometries and stability constants of the complexes were determined at 25 °C and at constant 0.10 mol·dm^?3 NaNO₃ ionic strength. The concentration distribution of the complexes in solution was evaluated. Palladium(II) complexes were synthesized and characterized by the ¹H NMR and electronic spectral studies. The structures consist of monomeric units in which the Pd(II) atoms exhibit square planar geometry.     

Complexation of Am(III) and Nd(III) by 1,10-Phenanthroline-2,9-Dicarboxylic Acid

The complexant 1,10-phenanthroline-2,9-dicarboxylic acid (PDA) is a planar tetradentate ligand that is more preorganized for metal complexation than its unconstrained analogue ethylendiiminodiacetic acid (EDDA). Furthermore, the backbone nitrogen atoms of PDA are aromatic, hence are softer than the aliphatic amines of EDDA. It has been hypothesized that PDA will selectively bond to trivalent actinides over lanthanides. In this report, the results of spectrophotometric studies of the complexation of Nd(III) and Am(III) by PDA are reported. Because the complexes are moderately stable, it was necessary to conduct these titrations using competitive equilibrium methods, competitive cation complexing between PDA and diethylenetriaminepentaacetic acid, and competition between ligand protonation and complex formation. Stability constants and ligand protonation constants were determined at 0.1 mol·L^?1 ionic strength and at 0.5 mol·L^?1 ionic strength nitrate media at 21 ± 1 °C. The stability constants are lower than those predicted from first principles and speciation calculations indicate that Am³⁺ selectivity over Nd³⁺ is less than that exhibited by 1,10-phenanthroline.     

Formation of Zinc(II) and Cadmium(II) Complexes with Pyridine Oxime Ligands in Aqueous Solution

Complex formation equilibria involving pyridine-2-carboxaldehyde oxime (1), 1-(2-pyridinyl)ethanone oxime (2) and 6-methylpyridine-2-carboxaldehyde oxime (3), HL, with zinc(II) and cadmium(II) ions were studied in aqueous 0.1 M NaCl solution at 25° C by potentiometric titrations with a glass electrode. Experimental data were analysed with the least-squares computer program SUPERQUAD to determine the complexes formed and their stability constants. With Ligands 1 and 2 the sets of complexes for Zn(II) and Cd(II) are essentially the same, mono- and dinuclear oxime complexes and their deprotonated/hydrolysed products H_pM_q(HL)^2q+p _r. Owing to the steric requirements of the 6-methyl group, sets of complexes formed with 3 are distinctly different. For zinc(II), only dinuclear oximato species H_pZn₂(HL)^4q+p ₂ ( p = ? 2, ? 3, ? 4) are found, while for the larger cadmium(II) ion mononuclear oximato species CdL⁺ and CdL₂ are detected in addition to the dinuclear complex H_pCd₂(HL)^4q+p ₂ ( p = ? 3).     

Synthesis, electrochemical, magnetic, catalytic and antimicrobial studies of N-functionalized cyclam based trinuclear copper(II) and nickel(II) complexes

Trinuclear copper(II) and nickel(II) complexes have been prepared by using Schiff base ligands derived from 1,8-[bis(3-formyl-2-hydroxy-5-methyl) benzyl]-4,11-dimethyl-l,4,8,11-tetraazacyclotetradecane, and 1,8-[bis(3-formyl-2-hydroxy-5-bromo)benzyl]-4,11-dimethyl-l,4,8,11-tetraazacyclotetradecane with aliphatic and aromatic diamines. All the complexes were characterized by elemental and spectroscopic analysis. Electrochemical studies of the copper(II) complexes in DMF solution show three irreversible one electron reduction process around E _pc  ¹ = ?0.59 to ?0.80 V, E _pc  ² = ?0.89 to ?1.14 V and E _pc  ³ = ?1.17 to ?1.29 V, and for nickel(II) complexes it is around E _pc  ¹ = ?0.63 to ?0.77 V, E _pc  ² = ?1.20 to ?1.35 V and E _pc  ³ = ?1.60 to ?1.74 V. ESR spectra and magnetic moments of the trinuclear Cu(II) complexes show the presence of antiferromagnetic coupling. Cryomagnetic investigation of the trinuclear copper(II) complexes show that the observed ?2J values are in the range of 116–178 cm^?1. The rate constants for hydrolysis of 4-nitrophenylphosphate by the complexes are in the range of 2.68 × 10^?2 to 9.81 × 10^?2 min^?1. The rate constants values for the catecholase activity of the copper(II) complexes fall in the range of 3.03 × 10^?2 to 9.32 × 10^?2 min^?1. All the complexes.     

Coordination properties of bidentate (N,O) and tridentate (N,O,O) heterocyclic alcohols with dimethyltin(IV)

The complex-formation equilibria of dimethyltin(IV) (DMT) with 4-hydroxymethyl imidazole (HMI) and 2,6-dihydroxymethyl pyridine (PDC) have been investigated. Stoichiometry and stability constants for the complexes formed were determined at different temperatures and 0.1?mol?L^?1 NaNO₃ ionic strength. The concentration distribution of the complexes in solution was evaluated as a function of pH. The effect of dioxane as a solvent on both protonation constants and formation constants of DMT complexes with HMI and PDC are discussed. The thermodynamic parameters ΔH° and ΔS° calculated from the temperature dependence of the equilibrium constants were investigated.