Study of Complex Formation between N-Phenylaza-15-Crown-5 with Mg2+, Ca2+, Ag+ and Cd2+ Metal Cations in Some Binary Mixed Aqueous and Non-aqueous Solvents using the Conductometric Method

The complexation reactions between Mg²⁺, Ca²⁺, Ag⁺ and Cd²⁺ metal cations with N-phenylaza-15-crown-5 (Ph-N15C5) were studied in acetonitrile (AN)–methanol (MeOH), methanol (MeOH)–water (H₂O) and propanol (PrOH)–water (H₂O) binary mixtures at different temperatures using the conductometric method. The conductance data show that the stochiometry of all of the complexes with Mg²⁺, Ca²⁺, Ag⁺ and Cd²⁺ cations is 1:1 (L:M). The stability of the complexes is sensitive to the solvent composition and a non-linear behaviour was observed for variation of log K _f of the complexes versus the composition of the binary mixed solvents. The selectivity order of Ph-N15C5 for the metal cations in neat MeOH is Ag⁺>Cd²⁺>Ca²⁺>Mg²⁺, but in the case of neat AN is Ca²⁺>Cd²⁺>Mg²⁺>Ag⁺. The values of thermodynamic parameters (ΔH _c ^o , ΔS _c ^o ) for formation of Ph-N15C5–Mg²⁺, Ph-N15C5–Ca²⁺, Ph-N15C5–Ag⁺ and Ph-N15C5–Cd²⁺ complexes were obtained from temperature dependence of stability constants and the results show that the thermodynamics of complexation reactions is affected by the nature and composition of the mixed solvents.     

Ion specificity and viscosity of rutile dispersions

The isoelectric point (IEP) of rutile is shifted to higher pH values in the presence of greater than 10⁻⁴ mol dm⁻³ Ba²⁺, Ca²⁺ and Mg²⁺, and when a critical concentration (5 × 10⁻⁴ mol dm⁻³ for Ba²⁺ and 1 × 10⁻³ mol dm⁻³ for Ca²⁺) is exceeded there is no IEP at all and the ζ potential is always positive. A common intersection point for the ζ-potential curves of the different concentrations of salt is found, but for the various salts the point is shifted from ζ = 0 mV for Mg²⁺ up to ζ = 20 mV for Ba²⁺. Between the IEP and the charge-reversal point a rheologically unstable region is discovered. The shear stress of rutile dispersions (2.5 g rutile + 4 g electrolyte solution) at shear rates of 116 s⁻¹ shows the same pH dependence irrespective of the concentration of alkaline-earth metal cations up to 10⁻² mol dm⁻³. The shear stress is less than 1 Pa below pH 3.8 and in the pH range 5–12 it assumes a value between 50 and 80 Pa at 116 s⁻¹ with some scatter; however, no systematic trend with concentration of alkaline-earth metal cations and a rather insignificant decrease with pH at pristine conditions are observed. The acidic branch of the yield stress (pH) and low shear rate viscosity (pH) curves is insensitive to the presence of alkaline-earth metal cations, and the same behaviour is found for the ζ potential. The alkaline-earth metal cations induce an increase in viscosity in the basic region and a shift in the pH of maximum viscosity to high pH values. It was also discovered that the effect different alkaline-earth metal cations have on the rheological properties at the same concentration is different from the effect induced by indifferent electrolytes. When the ζ potential increases the viscosity at high pH is increased in a series which follows the increase in size of the cation. Received: 9 September 1998 Accepted in revised form: 12 January 1999     

A Thermodynamic Study Between 18-Crown-6 with Mg2+, Ca2+, Sr2+and Ba2+ Cations in Water–Methanol and Water–Ethanol Binary Mixtures Using The Conductometric Method

The complexation reactions between Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺ cations with the macrocyclic ligand, 18-Crown-6 (l8C6) in water–methanol (MeOH) binary systems as well as the complexation reactions between Ca²⁺ and Sr²⁺ cations with 18C6 in water–ethanol (EtOH) binary mixtures have been studied at different temperatures using conductometric method. The conductance data show that the stoichiometry of all the complexes is 1:1. It was found that the stability of 18C6 complexes with Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺ cations is sensitive to solvent composition and in all cases, a non-linear behaviour was observed for the variation of log K _f of the complexes versus the composition of the mixed solvents. In some cases, the stability order is changed with changing the composition of the mixed solvents. The selectivity order of 18C6 for the metal cations in pure methanol is: Ba²⁺ > Sr²⁺ > Ca²⁺ > Mg²⁺. The values of thermodynamic parameters (Δ H _c ° and Δ S _c °) for formation of 18C6–Mg²⁺, 18C6–Ca²⁺, 18C6–Sr²⁺ and 18C6–Ba²⁺complexes were obtained from temperature dependence of the stability constants. The obtained results show that the values of (Δ H _c ° and Δ S _c °) for formation of these complexes are quite sensitive to the nature and composition of the mixed solvent, but they do not vary monotonically with the solvent composition.This revised version was published online in July 2005 with a corrected issue number.     

A Simple Fluorometric Method Using Chlorophyll a for Determination of Hg2+ Ion

A simple and green analytical procedure based on chlorophyll a is presented for the determination of Hg²⁺ ion. Chlorophyll a was extracted and purified from the leaves of pea and is employed as a reagent for analysis of Hg²⁺ ion. It displays remarkable fluorescence emission at 674 nm when excited at 412 nm. The emission intensity decreased significantly on exposure to various concentrations of Hg²⁺ ion. This forms the basis for the determination of Hg²⁺ ion. The proposed method was evaluated for sensitivity and selectivity. The linear concentration range was found to be 2.0–10 μM with r² = 0.997 and the limit of detection for Hg²⁺ ion was 1.3 μM. Ions including Pb²⁺, Cd²⁺, Ag⁺, Zn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Mg²⁺, Mn²⁺, Ru³⁺, Er³⁺, K⁺, Na⁺, NH₄⁺, Cl⁻, NO₃⁻, CH₃COO⁻ and SO₄²⁻ did not interfere with the measurement of Hg²⁺ ion even at 500-fold excess. Since chlorophyll a is widely available in the leaves of most plants, and the extraction and purification process is simple, this technique can provide an alternative, sensitive and economical way to determine Hg²⁺ ion.     

Ca2+- and Mg2+-doped covalent organic frameworks exhibiting high hydrogen and acetylene storage

A multiscale theoretical investigation has been performed to study the hydrogen and acetylene storage in Ca²⁺- and Mg²⁺-doped COFs (COF-105 and COF-108). The first-principles calculations show that the Ca²⁺ and Mg²⁺ can be immobilized at the COFs surfaces, and the doped Ca and Mg cations can adsorb five H₂ molecules and three C₂H₂ molecules with ideal binding energies. The Grand Canonical Monte Carlo (GCMC) simulations were carried out to obtain the hydrogen and acetylene uptakes of Ca²⁺- and Mg²⁺-doped COFs at room temperature in the different pressure ranges. Our results demonstrate that, at T = 298 K and p = 100 bar, the total gravimetric uptakes of H₂ in Ca²⁺-doped COF-105 and COF-108 reach 6.78 and 6.54 wt%, respectively, and a higher uptakes of 7.14 and 7.27 wt% have been reached for Mg²⁺-doped COF-105 and COF-108, respectively. At T = 298 K and p = 1 bar, the acetylene uptakes of Ca²⁺-doped COF-105, Ca²⁺-doped COF-108, Mg²⁺-doped COF-105, and Mg²⁺-doped COF-108 are 406.42, 366.24, 308.07, and 319.88 cm³/g (corresponding to the excess uptakes of 358.37, 316.38, 236.7109, and 245.42 cm³/g), respectively. The Ca²⁺-doped COF-105 displays a highest acetylene storage capacity among all materials reported. The Ca²⁺- and Mg²⁺-doped COFs can be very practical hydrogen or acetylene storage medium in the future.     

Calculations of chemical equilibria in heparin-arginine-H<Subscript>2</Subscript>O-NaCl and MCl<Subscript>2</Subscript>-heparin-arginine-H<Subscript>2</Subscript>O-NaCl systems (M = Ca<Superscript>2+</Superscript>, Mg<Superscript>2+</Superscript>) in a physiological solution

Chemical equilibria in the high-molecular-weight heparin (Na₄hep)-arginine (HArg)-H₂O-NaCl and MCl₂-Na₄hep-HArg-H₂O-NaCl systems of electrolytes (M = Ca²⁺, Mg²⁺) were calculated by the method of mathematical simulation of chemical equilibria from representative planned pH-metric titration experiment at 2.30 ≤ pH ≤ 10.50 in a physiological solution medium in the presence of 0.154 M NaCl as a background electrolyte at 37°C. The initial concentrations of the basic components were n × 10⁻³ M (n ≤ 4).     

Sensing method based on impedance variation of minicolumn packed with cation-exchanger under electric field

Voltage-induced impedance variation of the minicolumn (i.d. 0.53 mm, length 2 mm) packed with cation exchanger was investigated to develop a sensing method. An aqueous sample solution containing the metal cations was continuously supplied to the minicolumn during the impedance measurement with the simultaneous application of both alternating current voltage (amplitude, 1.0 V; frequency, 200 kHz to 6 Hz) and direct current (DC) offset voltage (0.1 to 1.0 V). On a complex plane plot, the profile of the column impedance consisted of a semicircle (200 kHz to 100 Hz) and a straight line (<100 Hz), of which slope varied with the magnitude of the applied DC offset voltage (V _DC). The slope–V _DC relation depended on the kind of the metal cation and its concentration; in particular, the slope–V _DC relations of monovalent cations (Na⁺ and K⁺) and divalent ones (Mg²⁺ and Ca²⁺) were significantly different. With the change in the concentration of minor divalent salt of MgCl₂ or CaCl₂ (60 to 140 μM) in the sample solution containing 10 mM NaCl, the slopes showed almost linear relationships between those with application of V _DC = 0.1 V and 1.0 V both for magnesium and calcium additions. In the case of plural addition of both MgCl₂ and CaCl₂ to the solution, the data points in the slope_{0.1  V}–slope_{1.0  V} plot were located between the two proportional lines for single additions of magnesium and calcium, reflecting both the mixing ratio and net concentrations of the divalent cations. Thus, simulations determination of Mg²⁺ and Ca²⁺ can be attained on the basis of the slope_{0.1  V}–slope_{1.0  V} relation obtained by the impedance measurements of the minicolumn. Actually, the contents of both magnesium and calcium cations in the bottled mineral waters determined simultaneously using the proposed method were almost equivalent to those obtained by the atomic absorption spectrometric measurement.     

Thermodynamic parameters of TI+, Ra2+, Bi3+and Ac3+ adsorption onto polyhydroxyethylmethacrylate-hydroxyapatite composite

In this research, a new composite, poly (hydroxyethylmethacrylate-hydroxyapatite) [P(HEMA-Hap)], was synthesized and its adsorptive features for natural radionuclides (TI⁺, Ra²⁺, Bi³⁺and Ac³⁺ in a leaching solution) were investigated at differing initial pH, concentration and temperature ranges. The natural radionuclides were counted by gamma spectrometer using a type NAI (Tl) detector. The adsorption data obtained were well represented by Langmuir and Freundlich type isotherms. The magnitude of determined monolayer adsorption capacities (X _L) for the adsorbed radionuclides were TI⁺ = Ac³⁺ > Ra²⁺ = Bi³⁺. These results demonstrated that P(HEMA-Hap) had high affinity to the natural radionuclide. The thermodynamic parameters indicated that the adsorption mechanisms were spontaneous (ΔG < 0) in terms of adsorption free enthalpy, and changes in the enthalpy and entropy values showed that the overall adsorption process was endothermic (ΔH > 0), thus increasing entropy (ΔS > 0).     

H2O2: a Ca2+ or Mg2+‐sensing function in statin passive diffusion

In a previous paper Guillaume's group demonstrated that magnesium (Mg²⁺ concentration range 0.00–2.60 mm ) increased the passive diffusion of statins and thus played a role in their potential toxicity. In order to confirm an increase in this passive diffusion by divalent salt cations, the role of calcium chloride (CaCl₂) on the statin–immobilized artificial membrane (IAM) association was studied. It was demonstrated that calcium supplementation (Ca²⁺ concentration range 0.00–3.25 mm ) increases the statin passive diffusion. In addition, it was shown that the Ca²⁺ effect on the statin–IAM association is higher than that of Mg²⁺. These results show that Ca²⁺ enhances the passive diffusion of drugs into biological membranes and thus their potential toxicity. Also, addition of H₂O₂ to the medium showed a hyperbolic response for the statin passive diffusion and this effect was enhanced for the highest Ca²⁺ or Mg²⁺ concentrations in the medium. H₂O₂ is likely to interact with the polar head groups of the IAM through dipole–dipole interactions. The conformational changes in H₂O₂–IAM result in a higher degree of exposure of hydrophobic areas, thus explaining why the binding of pravastatin, which showed the lowest logP value, was less affected by H₂O₂. This result shows the significant contribution of H₂O₂ and thus the oxidative stress on the statin passive diffusion. Much of the sensitivity derives from the action of Ca²⁺ or Mg²⁺, in turn supported the idea that H₂O₂ may serve a Ca²⁺ or Mg²⁺ sensing function in statin passive diffusion Copyright © 2015 John Wiley & Sons, Ltd.     

Purification and Characterization of a Novel Collagenase from <Emphasis Type="Italic">Bacillus pumilus</Emphasis> Col-J

The collagenase, produced extracellular by Bacillus pumilus Col-J, was purified by ammonium sulfate precipitation followed by two gel filtrations, involving Sephadex G-100 column and Sepharose Fast Flow column. Purified collagenase has a 31.53-fold increase in specific activity of 87.33 U/mg and 7.00% recovery. The collagenase has a relative molecular weight of 58.64 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The optimal temperature for the enzyme reaction was 45 °C. More than 50% of the original activity still remained after 5 min of incubation at 70 °C or 10 min at 60 °C. The maximal enzyme activity of collagenase was obtained at pH 7.5, and it was stable over a pH range of 6.5–8.0. The collagenase activity was strongly inhibited by Mn²⁺, Pb²⁺, ethylenediamine tetraacetic acid, ethylene glycol tetraacetic acid, and β-mercaptoethanol. However, Ca²⁺ and Mg²⁺ greatly increased its activity. The collagenase from B. pumilus Col-J showed highly specific activity towards the native collagen from calf skin. The K _m and V _max of the enzyme for collagen were 0.79 mg/mL and 129.5 U, respectively.     

Solvothermal Syntheses, Crystal Structures, and Properties of New [Mn( dien )2]2+ Complexes

 The two new compounds Mn(dien)₂[MoS₄] (1) and Mn(dien)₂[Mo₂O₂S₆] (2) (dien = diethylenetriamine) were prepared under solvothermal conditions. Both compounds were obtained as phase-pure products. The structures consist of new [Mn(dien)₂]²⁺ cations and isolated tetrahedral [MoS₄]²⁻ (1) or [Mo₂O₂S₆]²⁻ (2) anions. Between the anions and the cations, hydrogen bonding is observed. Compound 1 crystallizes in the tetragonal space group I (a = 10.219(2), c = 9.259(2) ?, Z = 2), whereas 2 crystallizes in the monoclinic space group P2₁/c (a = 8.703(2), b = 18.390(4), c = 14.603(3) ?, β = 103.18(3)°, Z = 4). The thermal behaviour of the thiomolybdates was investigated using difference thermoanalysis (DTA) and thermogravimetry (TG). Both compounds decompose under argon with a single endothermic signal in the DTA curve (peak maximum: 252 (1) and 242°C (2)).     

Synthesis,Crystal Structure and Raman Spectrum of Hydrazinium(+2) Fluoroarsenate(III) Fluoride,N<Subscript>2</Subscript>H<Subscript>6</Subscript>AsF<Subscript>4</Subscript>F

Summary.  Hydrazinium(+2) fluoroarsenate(III) fluoride was prepared by the reaction of hydrazinium(+2) fluoride and liquid arsenic trifluoride. N₂H₆AsF₄F is stable at 273 K, but decomposes slowly at room temperature. N₂H₆AsF₄F crystallizes in the orthorhombic space group Pnn2 with a = 774.0(2) pm, b = 1629.2(4) pm and c = 436.6(1) pm; V = 0.5506(3) nm³, Z = 4 and d _c  = 2.461 g cm⁻³. The structure consists of N₂H₆ ²⁺ cations, AsF₄ ⁻ anions, and F⁻ anions and is interconnected by a hydrogen bonding network. Distorted trigonal-bipyramidal AsF₄ ⁻ units are very weakly interconnected and form chains along the b axis. Bands in the Raman spectrum are assigned to the vibrations of N₂H₆ ⁺² cations and AsF₄ ⁻ anions. Corresponding author. E-mail: adolf.jesih@ijs.si Received April 18, 2002; accepted July 15, 2002     

Comparison of Ca2+ and Mg2+ binding to calmodulin. An enthalpy,entropy, and gibbs free energy analysis

A consistent set of G _B , H _B , and S _B parameters have been determined from ion specific electrode, calorimetric, and spectrophotometric studies for the binding of Ca²⁺ and Mg²⁺ to bovine calmodulin at pH=7.0 and an ionic strength I of 0.113M. A non-linear least squares analysis of calcium specific ion electrode data yields, on a molar basis, four calcium dissociation constants: 10^–7 for the first site, 10^–5 for the fourth site, and two constants between these values. Both calorimetric experiments and an indicator method provide evidence that Mg²⁺ binds to calmodulin, probably at the same sites as Ca²⁺, but with affinities about 100 times smaller: 4×10^–5 for the first site and 2×10^–3 for the fourth. Calorimetric titrations on Ca²⁺ binding to calmodulin in three buffers are consistent with 0.46 protons released upon binding at all four sites and yield an average H _B per site of 5.6 and 7.9 kJ-mol^–1 for Ca²⁺ and Mg²⁺, respectively. The entropy of the system increases by 524 and 361 J-K^–1-mol^–1 when Ca²⁺ and Mg²⁺, respectively, bind to four sites on calmodulin, i.e., the selectivity of calmodulin for Ca²⁺ is primarily derived from entropy effects. Further analysis based on elimination of the entropy term for the metal ions demonstrates that calmodulin bound to Ca²⁺ has a larger entropy than the unbound calmodulin; the opposite is true for calmodulin bound to Mg²⁺. These analyses are consistent with the hypothesis that Ca²⁺ forms tight complexes at all sites on calmodulin and that release of waters of hydration upon binding is the source of the increase of entropy in the system.     

Complexation of 4′-nitrobenzo-15-crown-5 with Mg<Superscript>2+</Superscript>, Ca<Superscript>2+</Superscript>, Sr<Superscript>2+</Superscript> and Ba<Superscript>2+</Superscript> metal cations in acetonitrile-methanol binary solutions

The complex formation reactions between Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺ metal cations with macrocyclic ligand, 4′-nitrobenzo-15C5, were studied in acetonitrile (AN)-methanol (MeOH) binary mixtures at different temperatures using conductometric method. The results show that 4′-nitrobenzo-15C5 forms 1:1 [ML] complexes with Mg²⁺, Ca²⁺ and Sr²⁺ metal cations in solutions. But in the case of Ba²⁺ cation a 1:2 [ML₂] complex is formed in these solvent systems. The stability of the complexes is sensitive to the solvent composition and a non-linear behavior was observed for variation of logK _f of the complexes versus the composition of the binary mixed solvents. The stability constants of complexes decrease suddenly with increasing the concentration of methanol in this binary system. The values of thermodynamic parameters (ΔH _c^° and ΔS _c^°) for formation of (4′-nitrobenzo-15C5.Mg)²⁺, (4′-nitrobenzo-15C5.Ca)²⁺ and (4′-nitrobenzo-15C5.Sr)²⁺ complexes were obtained from temperature dependence of the stability constants and the results show that these parameters are affected by the nature and composition of the mixed solvents. A non-linear behavior is observed between the ΔS _c^° and the composition of the mixed solvents.     

Solvothermal Syntheses,Crystal Structures,and Properties of New [Mn(<Emphasis Type="BoldItalic">dien</Emphasis>)<Subscript>2</Subscript>]<Superscript>2+</Superscript> Complexes

Summary.  The two new compounds Mn(dien)₂[MoS₄] (1) and Mn(dien)₂[Mo₂O₂S₆] (2) (dien = diethylenetriamine) were prepared under solvothermal conditions. Both compounds were obtained as phase-pure products. The structures consist of new [Mn(dien)₂]²⁺ cations and isolated tetrahedral [MoS₄]²⁻ (1) or [Mo₂O₂S₆]²⁻ (2) anions. Between the anions and the cations, hydrogen bonding is observed. Compound 1 crystallizes in the tetragonal space group I (a = 10.219(2), c = 9.259(2) ?, Z = 2), whereas 2 crystallizes in the monoclinic space group P2₁/c (a = 8.703(2), b = 18.390(4), c = 14.603(3) ?, β = 103.18(3)°, Z = 4). The thermal behaviour of the thiomolybdates was investigated using difference thermoanalysis (DTA) and thermogravimetry (TG). Both compounds decompose under argon with a single endothermic signal in the DTA curve (peak maximum: 252 (1) and 242°C (2)). Received November 5, 2001. Accepted December 27, 2001     

A combined experimental and DFT study on the complexation of Mg<Superscript>2+</Superscript> with beauvericin

From extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium Mg²⁺(aq) + 1·Sr²⁺(nb) ⇆ 1·Mg²⁺(nb) + Sr²⁺(aq) taking place in the two-phase water–nitrobenzene system (1 = beauvericin; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log K _ex (Mg²⁺, 1·Sr²⁺) = 0.0 ± 0.1. Further, the stability constant of the 1·Mg²⁺ complex in nitrobenzene saturated with water was calculated for a temperature of 25 °C as log β_nb (1·Mg²⁺) = 9.1 ± 0.2. By using quantum mechanical DFT calculations, the most probable structures of the non-hydrated 1·Mg²⁺ and hydrated 1·Mg²⁺·3H₂O complex species were predicted.     

Polymorphism and Pressure Driven Thermal Spin Crossover Phenomenon in [Fe( abpt )2(NC X )2] ( X =?S, and Se): Synthesis, Structure and Magnetic Properties

Summary.  The monomeric compounds [Fe(abpt)₂(NCX)₂] (X = S (1), Se (2) and abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole) have been synthesized and characterized. They crystallize in the monoclinic P2₁/n space group with a = 11.637(2) ?, b = 9.8021(14) ?, c = 12.9838(12) ?, β = 101.126(14)°, and Z = 2 for 1, and a = 11.601(2) ?, b = 9.6666(14) ?, c = 12.883(2) ?, β = 101.449(10)°, and Z = 2 for 2. The unit cell contains a pair mononuclear [Fe(abpt)₂(NCX)₂] units related by a center of symmetry. Each iron atom, located at a molecular inversion center, is in a distorted octahedral environment. Four of the six nitrogen atoms coordinated to the Fe(II) ion belong to the pyridine-N(1) and triazole-N(2) rings of two abpt ligands. The remaining trans positions are occupied by two nitrogen atoms, N(3), belonging to the two pseudo-halide ligands. The magnetic susceptibility measurements at ambient pressure have revealed that they are in the high-spin range in the 2 K–300 K temperature range. The pressure study has revealed that compound 1 remains in high-spin as pressure is increased up to 4.4 kbar, where an incomplete thermal spin crossover appears at around T _1/2 = 65 K. Quenching experiments at 4.4 kbar have shown that the incomplete character of the conversion is a consequence of slow kinetics. Relatively sharp spin transition takes place at T _1/2 = 106, 152 and 179 K, as pressure attains 5.6, 8.6 and 10.5 kbar, respectively. Corresponding author. E-mail: jose.a.real@uv.es Received June 12, 2002; accepted July 1, 2002     

Direct Synthesis of Nanocrystalline Hydroxyapatite by Hydrothermal Hydrolysis of Alkylphosphates

Summary.  The influence of reaction conditions (temperature, type of catalyst, time) on the base-catalyzed reaction of mono-, di-, and trialkylphosphates (alkyl = methyl, ethyl, i-propyl, n-butyl) with Ca²⁺ ions and on the structure and composition of the reaction products was studied. The composition of the calcium phosphates depends mainly on the reaction temperature. At temperatures below 100°C, a nanocrystalline solid product transforming into dicalcium phosphate by heating (calcination) was found. Pure nanocrystalline hydroxyapatite was prepared by hydrothermal synthesis at 160°C from mono- and dialkylphosphates. The size of hydroxyapatite crystallites was about 1 nm, the particle size about 150 nm. Agglomerated particles of hydroxyapatite larger than 2 μm were prepared at 200°C. Hydrothermal reaction of trialkylphosphates with Ca²⁺ ions at 200°C produced CaHPO₄. The experimental results were used to propose a reaction mechanism for base-catalyzed hydrothermal reactions of alkylphosphates with Ca²⁺ ions. Received October 4, 2001. Accepted (revised) November 19, 2001     

Conductometric and microcalorimetric analysis of the alkaline-earth/alkali-metal ion exchange onto polyacrylic acid

The specificity of the exchange between divalent (Di²⁺ = Ca²⁺ or Ba²⁺) and monovalent (M⁺ = Li⁺, Na⁺ or K⁺) ions onto a polyacrylic chain is examined using conductometric and microcalorimetric techniques. Assuming the formation of a bidentate complex between the Di²⁺ and the carboxylate groups, the conductometric data give the exchange ratio (Di²⁺/M⁺) and the speciation of the acrylic groups. No significant difference is observed between the three alkali-metal ions for a given Di²⁺ ion. Comparisons between Ca²⁺ and Ba²⁺ show a stronger hydrophobicity of the former as it precipitates at a complexation ratio r = 0.33 versus r = 0.45 for the Ba²⁺ salt. Microcalorimetric data show that all Di²⁺/M⁺ exchange energies are positive and depend significantly on the type of cations. The largest displacement energy (the more positive) is found for the binding of Ca²⁺ with sodium polyacrylate (8.13 kJ · mol⁻¹) and the smallest for Ba²⁺ with lithium polyacrylate (1.88 kJ · mol⁻¹). The interpretation of the data leads to the conclusion that specificity of the Di²⁺ binding originates in the dehydration phenomenon and specificity between the three alkali-metal ions is due to the decrease in the electrostatic bond strength with an increase in the ionic radii. The Di²⁺/M⁺ exchange is entropically driven. Received: 28 January 1999 Accepted in revised form: 7 April 1999     

Quantification of H, B and F in Kornerupine: Accuracy of SIMS and SREF (X-Ray Single-Crystal Structure-Refinement) Data

 We use a multiple-analytical approach based on secondary-ion mass-spectrometry (SIMS), X-ray single-crystal structure refinement (SREF) and electron-probe micro-analysis (EPMA) to derive the complete crystal-chemical formula of a B-rich kornerupine-group mineral, prismatine, from Hrarigahy, Madagascar: (Ca_0.01Li_0.02Mg_0.20Fe²⁺ _0.10) (Mg_3.57Fe²⁺ _0.06 Al_5.37) (Si_3.84B_0.91Al_0.26)O₂₁ (OH_1.08F_0.07). SIMS matrix effects related to crystal structure were investigated by analyzing two grains with a known crystallographic orientation relative to the ion beam. Boron orders at the T3 site. The refined site-scattering for T3, 6.33 eps (electrons per site) agrees well with the mean bond-length for this site (1.512 ?), which indicates nearly complete occupancy by B (85% rel.). B₂O₃ (∼ 4 wt%), derived by SREF, agrees with the SIMS data within analytical uncertainty using Si as the inner reference for the matrix. The occupancy of the X site obtained by combining the SIMS and EPMA data (5.30 eps; electrons per site) agrees with the refined site-scattering value (5.75 eps). Trace quantities of Li and Ca are ordered at this site. SIMS data for H₂O is in accord with the stoichiometric value, indicating complete occupancy at O10 by OH. Fluorine (∼ 0.17 wt%) orders at O10: it corresponds to ∼ 0.07 atoms per formula unit (apfu) vs. 0.15 apfu (atoms per formula unit) by SREF, indicating a slight overestimation of F with SREF, as previously observed in fluoborite. Our data show that SIMS chemical matrix effects are well-calibrated, and emphasize the usefulness of independent micro-analytical techniques in testing the mutual accuracy and consistency of experimental data.