Thermodynamic characteristics of the complexation between Pb<Superscript>2+</Superscript> and <Emphasis Type="Italic">N</Emphasis>,<Emphasis Type="Italic">N</Emphasis>-Bis(carboxymethyl)aspartic acid in aqueous solution

The enthalpies of complexation between N,N-bis(carboxymethyl)aspartic acid (H₄Y) and the Pb²⁺ ion at 298.15 K were determined from calorimetric data for a wide range of the ionic strengths (KNO₃). The thermodynamic characteristics ΔH, ΔG, and ΔS, of formation of the complexes PbHY⁻ and PbY²⁻ were calculated for zero and fixed ionic strengths. The results obtained were interpreted.     

Effect of background electrolyte concentration on the heat effects in the processes of formation of L-asparagine complexes with Cu<Superscript>2+</Superscript> ions in aqueous solution

A direct calorimetric method was used to measure the heat effects in the reactions of formation of Cu(II) complexes with L-asparagine in aqueous solutions at 298.15 K and ionic strength 0.5, 1.0, 1.5 (KNO₃). The standard thermodynamic characteristics (Δ_r H ⁰, Δ_r G ⁰, Δ_r S ⁰) of the processes of complex formation in the system under study were calculated.     

Influence of pH,ionic strength,foreign ions and FA on adsorption of radiocobalt on goethite

This work contributed to the adsorption of radiocobalt on goethite as a function of contact time, pH, ionic strength and foreign ions in the absence and presence of fulvic acid (FA) under ambient conditions. The results indicated that adsorption of Co(II) was dependent on ionic strength and foreign ions at low pH values (pH < 7.8), and independent of ionic strength and foreign ions at high pH values (pH > 7.8). Outer-sphere surface complexation and/or ion exchange were the main mechanisms of Co(II) adsorption on goethite at low pH values, whereas inner-sphere surface complexation was the main adsorption mechanism at high pH values. The presence of FA enhanced Co(II) adsorption at low pH values, but reduced Co(II) adsorption at high pH values. The thermodynamic data (ΔH ⁰, ΔS ⁰, ΔG ⁰) were calculated from the temperature dependent adsorption isotherms, and the results suggested that adsorption process of Co(II) on goethite was spontaneous and endothermic. The results are crucial to understand the physicochemical behavior of Co(II) in the nature environment.     

Studies on the Reaction Kinetics and Mechanism of Iron(II) Reduction of the <Emphasis Type="Italic">cis</Emphasis>-Halogeno(cetylamine)bis(ethylenediamine)cobalt(III) Complex Ion in Aqueous Perchlorate Medium

The electron-transfer kinetics of the ionic surfactant complex cis-chloro/bromo(cetylamine)bis(ethylenediamine)cobalt(III) by iron(II) in aqueous perchlorate medium at μ=1.0 mol⋅dm⁻³ ionic strength have been studied at 303, 308 and 313 K by spectrophotometry under pseudo-first-order conditions using an excess of the reductant. The effects of [H⁺], ionic strength and [Fe²⁺] on the rate were determined. The reaction was found to be second order and showed to be independence of the acid concentration in the range [H⁺]=0.05–0.25 mol⋅dm⁻³. The second order rate constant increased with surfactant–cobalt(III) concentration and the occurrence of aggregation of the complex itself altered the reaction rate. Activation and thermodynamic parameters have been computed. It is suggested that the reaction of Fe²⁺(aq) with the cobal (III) complex proceeds by an inner-sphere mechanism. The critical micelle concentration (CMC) values of these surfactant–metal complexes were obtained in aqueous solution from conductance measurements. Specific conductivity data (at 303, 308 and 313 K) served for the evaluation of the temperature-dependence of the critical micelle concentration (CMC) and the thermodynamics of micellization (ΔG _m^o,ΔH _m^o and ΔS _m^o).     

Influence of contact time, pH, soil humic/fulvic acids, ionic strength and temperature on sorption of U(VI) onto MX-80 bentonite

The sorption of U(VI) from aqueous solution on MX-80 bentonite was studied as a function of contact time, pH, ionic strength, solid contents, humic acid (HA), fulvic acid (FA) and temperature under ambient conditions using batch technique. The results indicate that sorption of U(VI) on MX-80 bentonite is strongly dependent on pH and ionic strength. The removal of U(VI) to MX-80 bentonite is rather quick and the kinetic sorption data is simulated well by a pseudo-second-order rate equation. The presence of HA enhances the sorption of U(VI) on MX-80 bentonite obviously, but the influence of FA on U(VI) sorption is not obvious. The thermodynamic parameters (ΔH ⁰, ΔS ⁰, and ΔG ⁰) for the sorption of U(VI) calculated from temperature dependent sorption suggest that the sorption reaction is endothermic and spontaneous.     

Th(IV) adsorption on mesoporous molecular sieves: effects of contact time,solid content,pH, ionic strength,foreign ions and temperature

The mesoporous molecular sieves (Al-MCM-41) are synthesized with montmorillonite as silica–alumina source by hydrothermal method. The application of Al-MCM-41 for the adsorption of Th(IV) from aqueous solution is studied by batch technique. The effects of contact time, solid content, pH, ionic strength, foreign ions, and temperature are determined, and the results indicate that the adsorption of Th(IV) to Al-MCM-41 is strongly dependent on pH values but independent of ionic strength. The adsorption isotherms are simulated by D–R and Freundlich models well. The thermodynamic parameters (ΔH ⁰, ΔS ⁰, ΔG ⁰) are calculated from the temperature dependent adsorption isotherms at 293, 313 and 333 K, respectively, and the results suggest that the adsorption of Th(IV) on Al-MCM-41 is a spontaneous and endothermic process. Al-MCM-41 is a suitable material for the preconcentration of Th(IV) from large volumes of aqueous solutions.     

Sorption of radiocobalt(II) onto Ca-montmorillonite: effect of contact time, solid content, pH, ionic strength and temperature

In this paper, the sorption of Co(II) from aqueous solution to Ca-montmorillonite was studied under ambient conditions by using batch technique. The effects of contact time, solid content, pH, ionic strength and temperature on the sorption of Co(II) to Ca-montmorillonite was also investigated. The results indicated that the sorption of Co(II) was strongly dependent on pH values. The sorption was dependent on ionic strength at low pH values, but independent of ionic strength at high pH values. Outer-sphere surface complexes were formed on the surface of Ca-montmorillonite at low pH values, whereas inner-sphere surface complexes were formed at high pH values. The Langmuir, Freundlich and Dubinin–Radushkevich (D–R) models were used to simulate the sorption isotherms of Co(II) at three different temperatures. The thermodynamic parameters (ΔH ⁰, ΔS ⁰ and ΔG ⁰) were calculated from the temperature dependent sorption isotherms, and the results indicated that the sorption reaction of Co(II) to Ca-montmorillonite was an endothermic and spontaneous process. The high sorption capacity of Co(II) on Ca-montmorillonite suggests that the Ca-montmorillonite is a suitable material for the preconcentration and solidification of radiocobalt from aqueous solutions.     

Effect of pH, ionic strength, foreign ions, fulvic acid and temperature on 109Cd(II) sorption to γ-Al2O3

The sorption of Cd(II) from aqueous solution on γ-Al₂O₃ was investigated under ambient conditions. Experiments were carried out as a function of contact time, solid content, pH, ionic strength, foreign ions, fulvic acid and temperature. The results indicated that the sorption of Cd(II) was strongly dependent on pH and ionic strength. At low pH, the sorption of Cd(II) was dominated by outer-sphere surface complexation and ion exchange with Na⁺/H⁺ on γ-Al₂O₃ surfaces, whereas inner-sphere surface complexation was the main sorption mechanism at high pH. The Langmuir, Freundlich and Dubinin–Radushkevich (D–R) models were used to simulate the sorption isotherms at three different temperatures. The thermodynamic data (ΔG ⁰, ΔS ⁰, ΔH ⁰) calculated from the temperature dependent sorption isotherms suggested that the sorption of Cd(II) on γ-Al₂O₃ was an spontaneous and endothermic process.     

Effect of Buffer Species on the Complexation of Basic Drug Terfenadine with β-Cyclodextrin

The complexation of terfenadine (Terf) with β-cyclodextrin (β-CD) in solution and solid state has been investigated by phase solubility diagram (PSD), differential scanning calorimetry (DSC), powder X-ray diffractometry (PXD) and proton nuclear magnetic resonance (¹H-NMR). The PSD results indicated that the salt saturation with the buffer counter ion (citrate⁻², H₂PO₄⁻¹ and Cl⁻¹ ions) of Terf (pK _a = 9.5) and the hydrophobic effect play in tandem to increase the value of the complex formation constant (K₁₁) measured at different conditions of pH, ionic strength, buffer type and buffer concentration. The correlation of the free energy of complex formation (ΔG₁₁) with the free energy of inherent solubility of Terf (ΔG_So) obtained by changing the pH, ionic strength and buffer concentration was used to measure the contribution of the hydrophobic effect (desolvation) to complex formation. The hydrophobic effect was found to constitute 57.8% of the driving force for complex stability, while other factors including specific interactions contribute −13.4 kJ/mol. ¹H-NMR spectra of Terf–citrate and Terf–HCl salts gave identical chemical shift displacements (ΔΔ) upon complexation, thus indicating that the counter anions are positioned somewhere outside of the β-CD cavity. DSC, XRPD and ¹H-NMR proved the formation of solid Terf/acid/β-CD ternary complexes.     

Potentiometric Determination of the Stability Constants of Trimethyltin(IV) Chloride Complexes with Imino-bis(Methylphosphonic Acid) in Water and Dioxane–Water Mixtures

The interaction of trimethyltin(IV) (TMT) with imino-bis(methylphosphonic acid) (IDP), abbreviated as H₄L, was investigated at 25 °C and at ionic strength 0.1 mol⋅dm⁻³ (NaNO₃) using a potentiometric technique. The formation constants of the complexes formed in solution were calculated using the nonlinear least-squares program MINIQUAD-75. The stoichiometry and stability constants are reported for the complexes formed. The results show the formation of 110, 111, 112 and 11-1 complexes for the TMT–IDP system. The concentration distribution of the various complex species was evaluated. The effect of dioxane as a solvent, on both the protonation constants and the formation constants of trimethyltin(IV) complexes with IDP, is discussed. The thermodynamic parameters ΔH ^∘ and ΔS ^∘ calculated from the temperature dependence of the equilibrium constants were evaluated. The effect of ionic strength on the protonation constants of IDP is also discussed.     

Effect of pH,ionic strength and humic substances on the adsorption of Uranium (VI) onto Na-rectorite

In this study, the adsorption of U(VI) from aqueous solution on Na-rectorite was studied as a function of various environmental conditions such as contact time, pH, ionic strength, soil humic acid (HA)/fulvic acid (FA), solid contents, and temperature under ambient conditions by using batch technique. The kinetic adsorption is fitted by the pseudo-second-order model very well. The adsorption of U(VI) on Na-rectorite was strongly dependent on pH and ionic strength. A positive effect of HA/FA on U(VI) adsorption was found at low pH, whereas a negative effect was observed at high pH. The presence of HA/FA enhanced the U(VI) adsorption at low pH values, but reduced U(VI) adsorption at high pH. The thermodynamic parameters (ΔH ⁰, ΔS ⁰, and ΔG ⁰) were also calculated from the temperature dependent adsorption isotherms, and the results suggested that the adsorption of U(VI) on Na-rectorite was a spontaneous and endothermic process.     

Removal of Zn(II) from aqueous solution by natural halloysite nanotubes

Clay minerals have been widely used in wastewater disposal due to their strong sorption and complexation ability towards various environmental pollutants. In this study, the removal of Zn(II) from aqueous solution by natural halloysite nanotubes (HNTs) was studied as a function of various solution chemistry conditions such as contact time, pH, ionic strength, coexisting electrolyte ions and temperature under ambient conditions. The results indicated that the removal of Zn(II) by HNTs was strongly dependent on pH and ionic strength. Langmuir and Freundlich models were used to simulate the sorption isotherms of Zn(II) at three different temperatures of 293, 313 and 333 K. The thermodynamic parameters (ΔH ⁰, ΔS ⁰ and ΔG ⁰) calculated from the temperature dependent sorption isotherms indicated that the removal process of Zn(II) by HNTs was endothermic and spontaneous. At low pH, the removal of Zn(II) was dominated by outer-sphere surface complexation and/or cation exchange with Na⁺/H⁺ on HNT surfaces, whereas inner-sphere surface complexation was the main removal mechanism at high pH. From the experimental results, one can conclude that HNTs may have a good potentiality for the disposal of Zn(II)-bearing wastewaters.     

Microwave dielectric properties of potassium hydroxide aqueous solutions

Microwave dielectric properties of potassium hydroxide aqueous solutions were studied at frequencies of 13 to 25 GHz over a wide range of concentrations and temperatures of 288 to 308 K. Experimental electrical conductivity data were used to determine ionic losses at high frequencies. The static dielectric permittivity σ_s, dielectric relaxation time τ, and thermodynamic activation parameters of dielectric relaxation (ΔH _ɛ⁺⁺, ΔG _ɛ⁺⁺, and ΔS _ɛ⁺⁺) were calculated. These quantities decrease in transfer from water to solutions. Thereby we demonstrate that potassium hydroxide has a disturbing effect on the initial hydrogen-bond network of water.     

Dissociation Quotients for Citric Acid in Aqueous Sodium Chloride Media to 150°C

The three molal dissociation quotients for citric acid were measured potentiometrically with a hydrogen-electrode concentration cell from 5 to 150°C in NaCl solutions at ionic strengths of 0.1, 0.3, 0.6, and 1 molal. The molal dissociation quotients and available literature data at infinite dilution were fitted by empirical equations in the all-anionic form involving an extended Debye-Hückel term and up to five adjustable parameters involving functions of temperature and ionic strength. This treatment yielded the following thermodynamic quantitites for the first dissociation equilibrium at 25°C: logK _1a=−3.127±0.002, ΔH _1a ^o =4.1±0.2 kJ-mol⁻¹, ΔS _1a ^o =−46.3±0.7 J-K⁻¹-mol⁻¹, and ΔCp _1a ^o =−162±7 J-K⁻¹-mol⁻¹; for the second acid dissociation equilibrium at 25°C: logK _2a =−4.759±0.001, ΔH _2a ^o =2.2±0.1, ΔS _2a ^o =−83.8±0.4, and ΔCp _2a ^o =−192±15, and for the third dissociation equilibrium at 25°C: logK _3a=−6.397±0.002, ΔH _3a ^o =−3.6±0.2, ΔS _3a ^o =−134.5±0.7, and ΔCp _3a ^o =−231±7.     

Nonaqueous Solution Studies on the Protonation Equilibria of some Phenolic Acids

The protonation equilibria for some phenolic acids in nonaqueous solutions have been studied by pH-potentiometry. The dissociation constants, pK _a, of these phenolic acids and the thermodynamic functions, ΔG ^o,ΔH ^o and ΔS ^o, for the successive and overall protonation processes of these phenolic acids have been derived at different temperatures in three different mixtures of water and dioxane (mole fractions of dioxane were 0.083, 0.174 and 0.33). Titrations were also carried out in (water + dioxane) with ionic strengths of 0.15, 0.20 and 0.25 mol⋅dm⁻³ NaNO₃, and the resulting dissociation constants are reported. A detailed thermodynamic analysis of the effects of organic solvent, dioxane, temperature and ionic strength on the protonation processes of phenolic acids is presented and discussed to determine the factors which control these processes. Ahmed E. Fazary; previous address: Egyptian Organization for Biological Products and Vaccines, 51 Wezaret El-Zeraa Street, Agouza, Giza, Egypt. Tel. +2010-3017357.     

Thermochemical investigation of acid-base interactions in peptide solutions

Heat effects of interaction of D,L-α-alanyl-D,L-α-alanine, glycil-γ-aminobutyric acid, glycil-L-asparagine and D,L-α-alanyl-D,L-asparagine with KOH, LiOH and HNO₃ solutions were measured by the direct calorimetry method at 288.15, 298.15, 308.15, 318.15 K and at several values of the ionic strength created by adding KNO₃ and LiNO₃. The standard dissociation enthalpies of the investigated ligands were obtained by the extrapolation to zero ionic strength. The standard thermodynamic characteristics (ΔG ⁰ , ΔH ⁰ , ΔS ⁰ , ΔC _p ⁰) of the processes of acid-base interaction in dipeptide solutions were calculated. Several peculiarities of acid-base interaction reactions in the solutions of biologically important ligands were found. The correlations between the thermodynamic characteristics of the protolytic equilibria in the dipeptide and aminoacids solutions and the structure of these compounds were determined.     

Thermochemical study of processes of complex formation of Cu<Superscript>2+</Superscript> ions with L-glutamine in aqueous solutions

Heats of the interaction of Cu(NO₃)₂ solutions with L-glutamine solutions were measured directly by calorimetry at a temperature of 298.15 K and ionic strength values of 0.5, 1.0, and 1.5 (KNO₃). Using RRSU universal software, the experimental data were subjected to rigorous mathematical treatment with allowances made for several concurrent processes in the system. The heats of formation of the CuL⁺ and CuL₂ complexes were calculated from the calorimetric measurements. The standard heats of the complex formation of Cu²⁺ with L-glutamine were obtained by extrapolation to zero ionic strength. The complete thermodynamic characteristic (Δ_r H ^o, Δ_r G ^o, Δ_r S ^o) of the complex formation processes in a Cu²⁺—L-glutamine system was obtained.     

Thermodynamics of citrate complexation with Mn<Superscript>2+</Superscript>, Co<Superscript>2+</Superscript>, Ni<Superscript>2+</Superscript> and Zn<Superscript>2+</Superscript> ions

Isothermal titration calorimetry has been used to determine the stoichiometry, formation constants and thermodynamic parameters (ΔG ^o, ΔH, ΔS) for the formation of the citrate complexes with the Mn²⁺, Co²⁺, Ni²⁺ and Zn²⁺ ions. The measurements were run in Cacodylate, Pipes and Mes buffer solutions with a pH of 6, at 298.15 K. A constant ionic strength of 100 mM was maintained with NaClO₄. The influence of a metal ion on its interaction energy with the citrate ions and the stability of the resulting complexes have been discussed.     

Impact of environmental conditions on the sorption behavior of radionuclide 63Ni(II) onto hierarchically structured γ-MnO2

A novel hierarchically structured γ-MnO₂ has been synthesized using a simple chemical reaction between MnSO₄ and KMnO₄ in aqueous solution without using any templates, surfactants, catalysts, calcination and hydrothermal processes. As an example of potential applications, hierarchically structured γ-MnO₂ was used as adsorbent in radionuclide ⁶³Ni(II) treatment, and showed an excellent ability. The effects of pH, ionic strength, temperature, humic acid (HA) and fulvic acid (FA) on the sorption of radionuclide ⁶³Ni(II) to hierarchically structured γ-MnO₂ have been investigated by using batch techniques. The results indicated that the sorption of ⁶³Ni(II) on γ-MnO₂ is obviously dependent on pH values but independent of ionic strength. The presence of HA/FA strongly enhances the sorption of ⁶³Ni(II) on γ-MnO₂ at low pH values, whereas reduces ⁶³Ni(II) sorption at high pH values. The sorption of ⁶³Ni(II) on γ-MnO₂ is attributed to inner-sphere surface complexation rather than outer-sphere surface complexation or ion exchange. The thermodynamic parameters (ΔH ⁰, ΔS ⁰, ΔG ⁰) are also calculated from the temperature dependent sorption isotherms, and the results suggest that the sorption of ⁶³Ni(II) on γ-MnO₂ is a spontaneous and endothermic process.     

Determination of four trace preservatives in street food by ionic liquid-based dispersive liquid-liquid micro-extraction

A rapid and simple ionic liquid-based dispersive liquid-liquid micro-extraction (DLLME) method has been developed to pre-concentrate four paraben preservatives (methylparaben, ethylparaben, propylparaben, and butylparaben) from street food (pancakes). Several potentially influential factors such as the type of ionic liquid and disperser solvent, extraction time, sample pH, ionic strength, and the volume of the ionic liquid and disperser solvent were investigated. The optimum experimental conditions for the proposed micro-extraction process were: 0.1 mL of 1-octyl-3-methylimidazolium hexafluorophosphate ([C₈MIM][PF₆]) as an extraction solvent, 0.1 mL of acetonitrile as a disperser solvent, 5 min extraction time, and sample ionic strength of 30 % sodium chloride in water sample at pH 6.0. The LODs and LOQs were in the range of 1.0–1.5 ng g⁻¹ and 3.5–4.5 ng g⁻¹, respectively. Spiking recoveries were in the range of 60.1–79.5 % and the associated RSDs were all in the range of 1.8–7.0 %. The results show that DLLME is a suitable method for the determination of parabens in pancake samples and ionic liquid is a good extractant in this process.