An Aqueous Thermodynamic Model for the Complexation of Sodium and Strontium with Organic Chelators Valid to High Ionic Strength. II. N-(2-Hydroxyethyl)ethylenedinitrilotriacetic Acid (HEDTA)

This is the second paper in a two part series on the development of aqueous thermodynamic models for the complexation of Na⁺ and Sr²⁺ with organic chelators. In this paper, the development of an aqueous thermodynamic model describing the effects of ionic strength, carbonate concentration, and temperature on the complexation of Sr²⁺ by HEDTA under basic conditions is presented. The thermodynamic model describing the Na⁺ interactions with the HEDTA^3–chelate relies solely on the use of Pitzer ion-interaction parameters. The exclusive use of Pitzer ion-interaction parameters differs significantly from our previous model for EDTA, which required the introduction of a NaEDTA^3– ion pair. Estimation of the Pitzer ion-interaction parameters for HEDTA^3– and SrHEDTA^– with Na⁺ allows the extrapolation of a standard-state equilibrium constant for the SrHEDTA^– species, which is one order of magnitude greater than the 0.1 M reference state value available in the literature. The overall model is developed from data available in the literature on apparent equilibrium constants for HEDTA protonation, the solubility of salts in concentrated HEDTA solutions, and from new data on the solubility of SrCO₃(c) obtained as part of this study. The predictions of the final thermodynamic model for the Na-Sr-OH-CO₃-NO₃-HEDTA-H₂O system are tested by application to chemical systems containing competing metal ions (i.e., Ca²⁺).     

Determination of thermodynamic properties of aqueous mixtures of RbCl and Rb2SO4 by the EMF method at T=298.15 K

The thermodynamic properties, including activity coefficients, osmotic coefficients and excess Gibbs free energy for RbCl and Rb₂SO₄ aqueous mixtures at T=298.15 K and in 0.01 mol · kg⁻¹ to 5 mol · kg⁻¹ ionic strength, were determined by emf measurements. The Rb–ISE and Ag–AgCl electrodes used in this work were prepared in our laboratory and had a reasonably good Nernst response. The experimental data were fitted by using the Harned rule and Pitzer model. The Harned coefficients and the Pitzer binary and ternary interaction parameters for the system have been evaluated. The experimental results obey the Harned rule. The Pitzer model can be used to describe this aqueous system satisfactorily.     

Water Activities, Osmotic and Activity Coefficients and Some Correlation of Aqueous Mixtures of Alkaline-Earth and Ammonium Nitrates, NH4NO3-Y(NO3)2-H2O with Y ≡ Ba2+, Mg2+ and Ca2+ at T = 25 °C

The thermodynamic properties of the mixed aqueous electrolyte of ammonium and alkaline earth metal nitrates have been studied using the hygrometric method at 25?°C. The water activities of these {yNH₄NO₃+(1?y)Y(NO₃)₂}(aq) systems with Y ≡ Ba²⁺, Mg²⁺ and Ca²⁺ were measured at total molalities ranging from 0.10 mol?kg^?1 to saturation for different NH₄NO₃ ionic-strength fractions of y=0.20, 0.50 and 0.80. These data allow the calculation of osmotic coefficients. From these measurements, the ionic mixing parameters are determined and used to calculate the solute activity coefficients in the mixtures at different ionic-strength fractions. The results of these ternary solution measurements are compared with those for binary solutions of the alkaline earth nitrates of magnesium, calcium and barium with ammonium nitrates. The behavior of the aqueous electrolyte solutions containing mixtures of barium or calcium or magnesium with ammonium nitrates are correlated and show that ionic interactions are more important for the system containing Mg²⁺ than for Ca²⁺ or Ba²⁺. The trends are mainly due to the effects of the ionic size, polarizability and the hydration of the ions in these solutions.     

An Aqueous Thermodynamic Model for the Complexation of Nickel with EDTA Valid to High Base Concentration

An aqueous thermodynamic model is developed which accurately describes the effects of high base concentration on the complexation of Ni²⁺ by ethylenedinitrilotetraacetic acid (EDTA). The model is primarily developed from an extensive dataset on the solubility of Ni(OH)₂(cr) in the presence of EDTA and in the presence and absence of Ca^{2 +} as the competing metal ion. The solubility data for Ni(OH)₂(cr) were obtained in solutions ranging in NaOH concentration from 0.01 to 11.6 mol-kg^–1, and in Ca^{2 +} concentrations extending to saturation with respect to portlandite, Ca(OH)₂. Owing to the inert nature of the Ni-EDTA complexation reactions, solubility experiments were approached from both the oversaturation and undersaturation direction and over time frames extending to 413 days. The final aqueous thermodynamic model is based upon the equations of Pitzer, accurately predicts the observed solubilities to concentrations as high as 11.6 mol-kg^–1 NaOH, and is consistent with UV–Vis spectroscopic studies of the complexes in solution.     

Cooperativity and Anticooperativity in Ion-Water Interactions: Implications for the Aqueous Solvation of Ions

Non-additive effects in hydrogen bonds (HB) take place as a consequence of electronic charge transfers. Therefore, it is natural to expect cooperativity and anticooperativity in ion-water interactions. Nevertheless, investigations on this matter are scarce. This paper addresses the interactions of (i) the cations Li⁺, Na⁺, K⁺, Be²⁺, Mg²⁺, and Ca²⁺ together with (ii) the anions F⁻, Cl⁻, Br⁻, NO₃⁻ and SO₄²⁻ with water clusters (H₂O)_n, n=1–8, and the effects of these ions on the HBs within the complete molecular adducts. We used quantum chemical topology tools, specifically the quantum theory of atoms in molecules and the interacting quantum atoms energy partition to investigate non-additive effects among the interactions studied herein. Our results show a decrease on the interaction energy between ions and the first neighbouring water molecules with an increment of the coordination number. We also found strong cooperative effects in the interplay between HBs and ion-dipole interactions within the studied systems. Such cooperativity affects considerably the interactions among ions with their first and second solvation shells in aqueous environments. Overall, we believe this article provides valuable information about how ion-dipole contacts interact with each other and how they relate to other interactions, such as HBs, in the framework of non-additive effects in aqueous media.     

Crystallization kinetics of calcium carbonate at a stoichiometric ratio of components

The formal kinetics of calcium carbonate crystallization in aqueous solutions is studied at a stoichiometric ratio of Ca²⁺ and CO₃^2- ions. The kinetics of the process was monitored by convenient and reliable methods (complexometric analysis for calcium in an aqueous solution and energy dispersive and microscopic measurement of solid particle sizes). The effect the temperature and degree of supersaturation have on the periods of induction and mass crystallization and the equilibrium concentration of calcium ions in solution is estimated at continuously controlled pH and solution ionic strength. The kinetic parameters (n, k, τ_1/2, E_a) of calcium carbonate crystallization are calculated. It is shown that calcium carbonate with a calcite structure formed at a stoichiometric ratio of reagents, and changes in the temperature (25–45°C) and the solution’s degree of supersaturation (2–6) within the considered range had no effect on the characteristics of the solid phase.     

Isopiestic study of (calcium chloride + water) and (calcium chloride + magnesium chloride + water) atT = 313.15 K

The osmotic coefficients of aqueous calcium chloride solutions were experimentally determined atT =  313.15 K by the isopiestic method. Magnesium chloride served as the isopiestic standard for the calculation of osmotic coefficients. The molality range covered in this study correspond to about 0.1mol · kg ^− 1to 3.0mol · kg ^− 1. In addition, the osmotic coefficients of aqueous mixtures of calcium chloride and magnesium chloride were determined over the range of ionic strength levels of about 0.1mol · kg ^− 1to 9mol · kg ^− 1and at various mole fractions. The results obtained were correlated by the Pitzer equation.     

An Aqueous Thermodynamic Model for the Complexation of Sodium and Strontium with Organic Chelates Valid to High Ionic Strength. I. Ethylenedinitrilotetraacetic Acid (EDTA)

An aqueous thermodynamic model is developed, which accurately describes the effects of Na⁺ complexation, ionic strength, carbonate concentration, and temperature on the complexation of Sr²⁺ by ethylenedinitrilotetraacetic acid (EDTA) under basic conditions. The model is developed from the analysis of literature data on apparent equilibrium constants, enthalpies, and heat capacities, as well as on an extensive set of solubility data on SrCO₃(c) in the presence of EDTA obtained as part of this study. The solubility data for SrCO₃(c) were obtained in solutions ranging in Na₂CO₃ concentration from 0.01 to 1.8 m, in NaNO₃ concentration from 0 to 5 m, and at temperatures extending to 75C. The final aqueous thermodynamic model is based upon the equations of Pitzer and requires the inclusion of a NaEDTA^3– species. An accurate model for the ionic strength dependence of the ion-interaction coefficients for the SrEDTA^2– and NaEDTA^3–aqueous species allows the extrapolation of standard state equilibrium constants for these species, which are significantly different from the 0.1 m reference state values available in the literature. The final model is tested by application to chemical systems containing competing metal ions (i.e., Ca²⁺) to further verify the proposed model and indicate the applicability of the model parameters to chemical systems containing other divalent metal-EDTA complexes.     

Kosmotropic and chaotropic behavior of hydrated ions in aqueous solutions in terms of expansibility and compressibility parameters

Hydrated ions have fundamental applications in chemical and biological processes. Kosmotropic and chaotropic nature of hydrated ions affect the water structure in solutions depending upon their hydrophobicity or hydrophilicity nature. In present study Kosmotropic and chaotropic behavior of hydrated ions have been explained in terms of volumetric and acoustic parameters like apparent molar volume (V_ϕ), expansibility and compressibility factors for aqueous electrolytic solutions provide useful information about interactions among ions and water molecules. Results of V_ϕ showed that SO₄²⁻ ions due to stronger H-bonding with water molecules are termed as kosmotropes while Cl⁻ and HCO₃^– are chaotropes due to their weaker H-bonding with water molecules. More compressible structure of solutions in the presence of SO₄²⁻ ions indicated its kosmotropic behavior and comparatively less compressible structure of solutions in the presence of Cl⁻¹ and HCO₃^– ions renders them chaotropes. Results obtained from expansibility factor showed the dominance of electrostatic interactions over hydrophobic hydration of ions at higher temperatures. Greater values of expansibility factor for SO₄²⁻ ions as compared to Cl⁻¹ and HCO₃^– ions renders them kosmotropic ion while later are termed as chaotropes. Hence, thermo-acoustic parameters could be effectively used to describe the hydrogen bonding character of ionic solutions in terms of kosmotropic and chaotropic behavior of solutions.     

Biogenic synthesis of calcium oxalate crystal by reaction of calcium ions with spinach lixivium

In this paper, we report a biogenic synthesis protocol for preparation of calcium oxalate (CaC₂O₄, CaOx) crystal at room temperature by a simple protein-mediated reaction of aqueous Ca²⁺ ions with the C₂O₄²⁻ ions spontaneously released from spinach. The aggregation of calcium oxalate monohydrate (COM) and calcium oxalate dihydrate (COD) with a rod-like morphology was mainly formed in the spinach root lixivium, and the proportion of COM crystal in the aggregation increased with the concentration of Ca²⁺ ions increasing, however, spindle-shaped crystal was mainly obtained in the spinach leaf lixivium and the content of COM in the product was higher than that obtained in the root lixivium with the similar concentration of Ca²⁺ ions. COM phase disappeared and only COD crystal with morphology of tetragonal bipyramidal prisms presented in the product when the leaf lixivium was replaced by the leaf broth. The biomolecules such as proteins with molecular weight of 31 kDa liberated from the spinach root are negative-charged, which played important roles for the control of CaOx crystal growth in the root lixivium corresponding to the changes of protein secondary structures after reaction with Ca²⁺ ions. This research was potentially important for unraveling the biomineralization mechanism of CaOx crystal.     

Conversion of phosphogypsum to potassium sulfate

Summary Solubility of calcium sulfate in concentrated aqueous chloride solutions is of particular significance in chloride hydrometallurgy and various crystallization processes, such as the production of potassium sulfate from phosphogypsum and potassium chloride. This paper examines an example of the second type of application in which gypsum and potassium chloride are reacted to form K₂SO₄. The solubility of phosphogypsum in aqueous solutions of KCl, HCl, and mixtures of both has first been measured at various temperatures and concentrations. The parameters investigated are HCl concentration up to 6M, KCl concentration up to 180 g L^-1 and temperature from 25 to 80°C. In addition, the influence of co-existing chloride salts, such as (HCl+KCl), on the solubility of calcium sulfate is estimated from 25 to 80°C. The solubility increases obviously with the temperature increment as it does initially with acid concentration, reaching a maximum of about 3M HCl, 130 g L^-1 KCl and then drops. At the same time, the solubility of CaSO₄·2H₂O decreases with increasing KCl concentration.     

Removal of phosphate from solution by adsorption and precipitation of calcium phosphate onto monohydrocalcite

The sorption behavior and mechanism of phosphate on monohydrocalcite (CaCO₃?H₂O: MHC) were examined using batch sorption experiments as a function of phosphate concentrations, ionic strengths, temperatures, and reaction times. The mode of PO₄ sorption is divisible into three processes depending on the phosphate loading. At low phosphate concentrations, phosphate is removed by coprecipitation of phosphate during the transformation of MHC to calcite. The sorption mode at the low-to-moderate phosphate concentrations is most likely an adsorption process because the sorption isotherm at the conditions can be fitted reasonably with the Langmuir equation. The rapid sorption kinetics at the conditions is also consistent with the adsorption reaction. The adsorption of phosphate on MHC depends strongly on ionic strength, but slightly on temperature. The maximum adsorption capacities of MHC obtained from the regression of the experimental data to the Langmuir equation are higher than those reported for stable calcium carbonate (calcite or aragonite) in any conditions. At high phosphate concentrations, the amount of sorption deviates from the Langmuir isotherm, which can fit the low-to-moderate phosphate concentrations. Speciation–saturation analyses of the reacted solutions at the conditions indicated that the solution compositions which deviate from the Langmuir equation are supersaturated with respect to a certain calcium phosphate. The obtained calcium phosphate is most likely amorphous calcium phosphate (Ca₃(PO₄)₂?xH₂O). The formation of the calcium phosphate depends strongly on ionic strength, temperature, and reaction times. The solubility of MHC is higher than calcite and aragonite because of its metastability. Therefore, the higher solubility of MHC facilitates the formation of the calcium phosphates more than with calcite and aragonite.     

Infrared spectroscopic study on Ca2+ binding to Akazara scallop troponin C in comparison with peptide analogues of its Ca2+-binding Site IV

Fourier-transform infrared spectroscopy (FT-IR) was applied to study the coordination structure of Ca²⁺ bound in Akazara scallop troponin C (TnC) and its site-directed mutant possessing inactivated Site IV (E142D mutant) in D₂O solution. The COO⁻ antisymmetric stretching region provides information about the coordination modes of a COO⁻ group to a metal ion. The wild type exhibits a band at 1543 cm⁻¹ in the Ca²⁺-bound state, indicating that the side-chain COO⁻ group of Glu142 (the position 12 of Site IV) serves as the ligand for Ca²⁺ in the bidentate coordination mode [F. Yumoto, M. Nara, H. Kagi, W. Iwasaki, T. Ojima, K. Nishita, K. Nagata, M. Tanokura, Eur. J. Biochem. 268 (2001) 6284–6290]. However, the E142D mutant showed no band around 1543 cm⁻¹ in the Ca²⁺-loaded state, indicating that the side-chain COO⁻ group of Asp142 does not bind to Ca²⁺ in the bidentate coordination mode. This result suggests that the absence of a methylene group is critical for the Ca²⁺ coordination structure of Akazara scallop TnC. The Ca²⁺-ligand interaction at Site IV is discussed in comparison with the results of synthetic peptide analogues of Site IV of Akazara scallop TnC.     

Thermodynamic properties of (NaCl + KCl + LiCl + H2O) at T = 298.15 K: Water activities,osmotic and activity coefficients

The water activities of aqueous electrolyte mixture (NaCl + KCl + LiCl + H₂O) were experimentally determined at T = 298.15 K by the hygrometric method at total ionic-strength from 0.4 mol · kg⁻¹ to 6 mol · kg⁻¹ for different ionic-strength fractions y of NaCl with y = 1/3, 1/2, and 2/3. The data allow the deduction of new osmotic coefficients. The results obtained were correlated by Pitzer’s model and Dinane’s mixing rules ECA I and ECA II for calculations of the water activity in mixed aqueous electrolytes. A new Dinane–Pitzer model is proposed for the calculation of osmotic coefficients in quaternary aqueous mixtures using the newly ternary and quaternary ionic mixing parameters of this studied system. The solute activity coefficients of component in the mixture are also determined for different ionic-strength fractions y of NaCl.     

Gelation mechanism of ionic polysaccharides

Experimental results obtained by membrane equilibria, osmotic pressure, viscosity and circular dichroism measurements on alginate and pectate solutions in the presence of Ca²⁺ ions are presented. From equilibrium dialysis data both electrostatic and cooperative interactions seem to describe the binding process of Ca²⁺ ions onto polymer chains. An increase of the number-average molecular weight M̄_n for both poly-saccharides with calcium ion concentration is observed. An increase of polymer dimensions can well account for the observed increase of the intrinsic viscosity [η] with bound Ca²⁺ ion concentration at several ionic strengths.     

Measurement and modelling of solubility for calcium sulfate dihydrate and calcium hydroxide in NaOH/KOH solutions

The solubility of calcium sulfate dihydrate (CaSO₄·2H₂O) and calcium hydroxide (Ca(OH)₂) in alkali solutions is essential to understand their desilication behavior from Bayer liquor. In this work, solubilities of calcium sulfate dihydrate and calcium hydroxide for the ternary systems of CaSO₄·2H₂O–NaOH–H₂O, CaSO₄·2H₂O–KOH–H₂O, and Ca(OH)₂–NaOH–H₂O were measured by using the classic isothermal dissolution method over the temperature range of 25–75 °C. The Pitzer model embedded in Aspen Plus platform was used to model the experimental solubility data for these systems. The experimental solubility data was employed to obtain the new binary interaction parameters for Ca(OH)⁺–OH⁻, Ca(OH)⁺–Ca²⁺ and Ca(OH)⁺–K⁺, suggesting that the species Ca(OH)⁺ is a dominant species in simulated solubility for alkali systems. Validation of the parameters was performed by predicting the solubility for the ternary systems of Ca(OH)₂–NaOH–H₂O, CaSO₄·2H₂O–NaOH–H₂O and CaSO₄·2H₂O–KOH–H₂O with the overall average relatively deviation (ARD) of 2.12%, 0.75% and 1.63%, respectively.     

Nd3+ and Am3+ ion interactions with sulfate ion and their influence on NdPO4(c) solubility

The effects of Nd(III)/Am(III) complexation with sulfate were studied by 1) re-examining existing data for the Am–SO₄ system using more, advanced aqueous electrolyte models valid to high concentration to obtain reliable thermodynamic data for SO ₄ ^2– complexes or ion interactions with Nd³⁺ and Am³⁺ and 2) conducting experimental solubility studies of NdPO₄(c), an analog phase of AmPO ₄ (c), a possibly important phase in high level nuclear wastes, in the presence of SO ₄ ^2– to test the newly developed thermodynamic model and show the possible influence of sulfate in a repository environment. The data showed that the increase in the solubility of NdPO ₄ (c) resulted primarily from the increase in ionic strength. Slightly higher observed Nd concentrations in the presence of sulfate, as compared with concentrations predicted at the experimental ionic strengths, resulted from the weak complexes or ion interactions involving Nd ³⁺ –SO ₄ ^2– . The Pitzer ion interaction parameters, applicable to 0.5m sulfate, were obtained for Am ³⁺ –SO ₄ ^2– from a reinterpretation of known solvent extraction data. These parameters are also consistent with literature data for Am ³⁺ /Na⁺ exchange and solvent extraction in the presence of sulfate. When used for the analogous Nd ³⁺ –SO ₄ ^2– system to predict NdPO ₄ (c) solubility in the presence of sulfate, they provided excellent agreement between the predicted and the observed solubilities, indicating that they can be reliably used to determine Nd ³⁺ or Am ³⁺ ion interactions with SO ₄ ^2– in all ground waters where SO ₄ ^2– is less than 0.5m     

Metal⋅⋅⋅F−C Bonding in Low-Coordinate Alkaline Earth Fluoroarylamides

A set of calcium and barium complexes containing the fluoroarylamide N(C₆F₅)₂⁻ is presented. These compounds illustrate the key role of stabilising M⋅⋅⋅F−C secondary interactions in the construction of low-coordinate alkaline earth complexes. The nature of Ca⋅⋅⋅F−C bonding in calcium complexes is examined in the light of structural data, bond valence sum (BVS) analysis and DFT computations. The molecular structures of [Ca{N(C₆F₅)₂}₂(Et₂O)₂] ( 4 ′), [Ca{μ-N(SiMe₃)₂}{N(C₆F₅)₂}]₂ ( 5₂ ), [Ba{μ-N(C₆F₅)₂}{N(C₆F₅)₂}⋅toluene]₂ ( 6₂ ), [{BDI^DiPP}CaN(C₆F₅)₂]₂ ( 7₂ ), [{N^N^DiPP}CaN(C₆F₅)₂]₂ ( 8₂ ), and [Ca{μ-OB(CH(SiMe₃)₂)₂}{N(C₆F₅)₂}]₂ ( 9₂ ), where {BDI^DiPP}⁻ and {N^N^DiPP}⁻ are the bidentate ligands CH[C(CH₃)NDipp]₂⁻ and DippNC₆H₄CNDipp⁻ (Dipp=2,6-iPr₂-C₆H₃), are detailed. Complex 6₂ displays strong Ba⋅⋅⋅F−C contacts at around 2.85 Å. The calcium complexes feature also very short intramolecular Ca−F interatomic distances at around 2.50 Å. In addition, the three-coordinate complexes 7₂ and 8₂ form dinuclear structures due to intermolecular Ca⋅⋅⋅F−C contacts. BVS analysis shows that Ca⋅⋅⋅F−C interactions contribute to 15–20 % of the bonding pattern around calcium. Computations demonstrate that Ca⋅⋅⋅F−C bonding is mostly electrostatic, but also contains a non-negligible covalent contribution. They also suggest that Ca⋅⋅⋅F−C are the strongest amongst the range of weak Ca⋅⋅⋅X (X=F, H, C_π) secondary interactions, due to the high positive charge of Ca²⁺ which favours electrostatic interactions.     

Hygrometric determination of the water activities and the osmotic and activity coefficients of (ammonium chloride + sodium chloride + water) atT = 298.15 K

The mixed aqueous electrolyte system of ammonium and sodium chlorides has been studied by the hygrometric method at the temperature 298.15 K. The relative humidities of this system were measured at total molalities from 0.3mol · kg ^− 1 to 6 mol · kg ^− 1for different ionic-strength fractions of NH ₄Cl with y =  (0.33, 0.50, and 0.67). The data obtained allow the deduction of new water activities and osmotic coefficients. The experimental results are compared with the predictions of the extended composed additivity model proposed in our previous work, the Robinson–Stokes, Reilly–Wood–Robinson, and Lietzke–Stoughton models. From these measurements, the new Pitzer mixing ionic parameters were determined and used to predict the solute activity coefficients in the mixture.