EXAFS study of aqueous Zr(IV) and Th(IV) complexes in alkaline CaCl(2) solutions: Ca(3)[Zr(OH)(6)](4+) and Ca(4)[Th(OH)(8)](4+)

A hitherto unknown type of aqueous complex, ternary Ca-MIV-OH complexes (M = Zr and Th), causes unexpectedly high solubilities of zirconium(IV) and thorium(IV) hydrous oxides in alkaline CaCl2 solutions (pHc = 10-12, [CaCl2] > 0.05 mol.L(-1), and pHc = 11-12, [CaCl2] > 0.5 mol.L(-1), respectively). The dominant aqueous species are identified as Ca3[Zr(OH)6]4+ and Ca4[Th(OH)8]4+ and characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy. The number of OH- ligands in the first coordination sphere detected by EXAFS, NO = 6 (6.6 +/- 1.2) for Zr and NO = 8 (8.6 +/- 1.2) for Th, are consistent with the observed slopes of 2 and 4 in the solubility curves log [M]tot vs pHc. The presence of polynuclear hydrolysis species and the formation of chloride complexes can be excluded. EXAFS spectra clearly show a second coordination shell of calcium ions. The [Zr(OH)6]2- and [Th(OH)8]4- complexes with an unusually large number of OH- ligands are stabilized by the formation of associates or ion pairs with Ca2+ ions. The number of neighboring Ca2+ ions around the [Zr(OH)6]2- and [Th(OH)8]4- units is determined to be NCa = 3 (2.7 +/- 0.6) at a distance of RZr-Ca = 3.38 +/- 0.02 A and NCa = 4 (3.8 +/- 0.5) at a distance of RTh-Ca = 3.98 +/- 0.02 A. The Ca3[Zr(OH)6]4+ and Ca4[Th(OH)8]4+ complexes have first (M-O) and second (M-Ca) coordination spheres with the Ca2+ ions bound to coordination polyhedra edges.     

Solubility of Zr(OH)<Subscript>4</Subscript>(am) and the Formation of Zr(IV) Carbonate Complexes in Carbonate Solutions Containing 0.1–5.0 mol·dm<Superscript>−3</Superscript> NaNO<Subscript>3</Subscript>

The solubility of amorphous zirconium hydroxide [Zr(OH)₄(am)] was investigated in carbonate solutions containing various concentrations of sodium nitrate. The observed dependences of Zr(IV) solubility on the hydrogen ion concentration (pH_c) and carbonate concentration suggested the formation of \( {\text{Zr}}({{\text{CO}}}_{3} )_{4}^{4 - } \), \( {\text{Zr}}({{\text{CO}}}_{3} )_{5}^{6 - } \), and \( {\text{Zr(OH)}}_{ 2} ( {{\text{CO}}}_{3} )_{2}^{2 - } \) as the dominant species in the neutral to weakly alkaline pH regions. The solubility of Zr(IV) at certain pH_c values and carbonate concentrations was observed to increase slightly with increasing ionic strength, while the solid phase was determined to be Zr(OH)₄(am) at all ionic strengths by using thermal analysis. By applying the specific ion interaction theory, the solubility data at different pH_cs, carbonate concentrations, and ionic strengths were analyzed to determine the formation constants of the Zr(IV) carbonate complexes and their ion interaction coefficients. The obtained values explain well the solubility data, which are discussed in comparison with those of analogous tetravalent actinide carbonates.     

Studies on the Hydrolytic Behavior of Zirconium(IV)

The stability constants of zirconium(IV) hydrolysis species have been measured at 15, 25, and 35 °C [in 1.0 mol-dm^–3 (H,Na)ClO₄] using both potentiometry and solvent extraction. In addition, the solubility of [Zr(OH)₄(am)] has been investigated in a 1 mol-dm^–3 (Na,H)(ClO₄,OH) medium at 25 °C over a wide range of –log [H⁺] (0-15). The results indicate the presence of the monomeric species Zr(OH)³⁺, Zr(OH)₂ ²⁺, Zr(OH)₃ ⁺, and Zr(OH)₄ ⁰(aq) as well as the polymeric species Zr₄(OH)₈ ⁸⁺ and Zr₂(OH)₆ ²⁺. The solvent extraction measurements required the use of acetylacetone. As such, the stability constants of zirconium(IV) with acetylacetone were also measured using solvent extraction. All stability constants were found to be linear functions of the reciprocal of temperature (in kelvin) indicating that H ^o and S ^o are both independent of temperature (over the temperature range examined in the study). The results of the solubility experiments have shown four distinctly different solubility regions. In strongly acidic solutions, the solubility is controlled by the formation of polynuclear hydrolysis species in solution whereas in less acidic solution the formation of mononuclear hydrolysis species becomes dominant. The largest portion of the solubility curve is controlled by equilibrium with aqueous Zr(OH)₄ ⁰(aq) where the solubility is independent of the proton concentration. In alkaline solutions, the solubility increases due to formation of the zirconate ion. The middle region was used to determine the solubility constant (log *K _s10) of Zr(OH)₄(s). From the data in the alkaline region, a value of the stability of the zirconate ion has been determined. This is the first time that the possible evidence for the zirconate ion has been identified in aqueous solution that has previously been found only in the solid phase.     

Thermodynamic Model for BiPO4(cr) and Bi(OH)3(am) Solubility in the Aqueous Na+–H+–\mathrm{H}_{2}\mathrm{PO}_{4}^{-}–\mathrm{HPO}_{4}^{2-}–\mathrm{PO}_{4}^{3-}–OH−–Cl−–H2O System

Prior to this study no data for the solubility product of BiPO₄(cr) or the complexation constants of Bi with phosphate were available. The solubility of BiPO₄(cr) was studied at 23±2?°C from both the over- and under-saturation directions as functions of a wide range in time (6–309 days), pH values (0–15), and phosphate concentrations (reaching as high as 1.0 mol?kg^?1). HCl or NaOH were used to obtain a range in pH values. Steady state concentrations and equilibrium were reached in <6 days. The data were interpreted using the SIT model. These extensive data provided a solubility product value for BiPO₄(cr) and an upper limit value for the formation of BiPO₄(aq). Because the aqueous system in this study involved relatively high concentrations of chloride, reliable values for the complexation constants of Bi with chloride were required to accurately interpret the solubility data. Therefore as a part of this investigation, existing Bi–Cl data were critically reviewed and used to obtain values of equilibrium constants for various Bi–Cl complexes at zero ionic strength along with the values for various SIT ion interaction parameters. Predictions based on these thermodynamic quantities agreed closely with our experimental data, the chloride concentrations of which ranged as high as 0.7 mol?kg^?1. The study showed that BiPO₄(cr) is stable at pH values <9.0. At pH values >9.0, Bi(OH)₃(am) is the solubility controlling phase. Reliable values for the Bi(OH)₃(am) solubility reactions involving Bi(OH)₃(aq) and $\mathrm{Bi}(\mathrm{OH})_{4}^{-}$ and the formation constants of these aqueous species are also reported.     

Polarographic investigation of Cu(II) complexes with N,N,N',N'-tetrakis-(2-hydroxypropyl)-ethylenediamine

During investigation of the formation of Cu(2+) ion complexes with N,N,N',N'-tetrakis-(2-hydroxypropyl)-ethylenediamine (Quadrol-Q) by means of constant current polarography (20 degrees C, ionic strength J = 3 mol l(-1)), the possibility of the formation of two complex compounds; CuQ(2+) and CuQ(2+)(2), was shown within the pH range from 6 to 8. The logarithms of the stability constants for these compounds are 10.6 +/- 0.5 and 14.6 +/- 0.4 respectively. Cu(II) complexation increases sharply when the pH increases from 8 to 10. It was shown that the data at a pH of greater than 10 are in accordance with the existence of the hydroxy complexes CuQ(OH)(2) and CuQ(2)(OH)(2), the logarithms of the stability constants being 26.9 +/- 0.5 and 29.1 +/- 0.3.     

XAFS and LIBD investigation of the formation and structure of colloidal Pu(IV) hydrolysis products

Pu(IV) oxyhydroxide colloid growth is investigated with XAFS and LIBD. From combined results a model of colloid formation is proposed, which leads to a face-centered cubic Pu sublattice having cation defects, as observed with EXAFS, and a linear dependency of log [Pu(IV)] on -log [H+] with slope -2, in accord with LIBD. The solubility for Pu(IV) measured with LIBD is close to the lower limit of the solubility curve from previously reported data.     

Structure of the Aqua Ions and Fluoride Complexes of Uranium(IV) and Thorium(IV) in Aqueous Solution an EXAFS Study

The structures of aqueous M(4+)(aq) and MF(3+)(aq), where M is uranium(IV) or thorium(IV), have been determined by L(III) edge EXAFS using data from solutions of 1.5 M HClO(4) in which the M(IV) concentrations ranged from 0.03 to 0.3 M. A least-squares refinement of the data for the aqua ions indicated 10.8 +/- 0.5 water molecules in the first hydration sphere of both ions and M-O bond distances for U(IV) and Th(IV) of 2.42 +/- 0.01 and 2.45 +/- 0.01 ?, respectively. By considering both previous structure information and the EXAFS data, we selected N = 10 +/- 1 as the most likely coordination number of both M(IV) aqua ions. EXAFS measurements from acidic aqueous uranium(IV) and thorium(IV) solutions containing fluoride show that large changes in the first coordination sphere occur. The experimental data indicates an asymmetrical distribution of the distances, probably as a result of differing M-F and M-O bond lengths. These can be described by a model that contains two different bond distances, one M-F distance at 2.10 ? and one M-O distance at 2.45 ? for U(IV); for Th(IV), the corresponding distances are 2.14 and 2.48 ?. The total coordination number in this model is unchanged from the aqua ions, i.e., 10 +/- 1.     

Pu(VI) hydrolysis: further evidence for a dimeric plutonyl hydroxide and contrasts with U(VI) chemistry

A significant fraction of plutonium that is soluble in environmental waters and other aqueous solutions can be present as complexes of plutonyl, PuO2(2+). Few thermodynamic data are available for this ion, representing a problematic gap in plutonium chemistry and in the forecasting of radionuclide behavior under contamination and nuclear repository conditions. To address this need and more accurately determine the stoichiometry and stability of the basic hydrolytic products, we completed complimentary potentiometric and spectrophotometric studies of plutonium(VI) hydrolysis over the concentration range of 10(-2) to 10(-5) M Pu(VI). Dinuclear hydroxide species (PuO2)2(OH)2(2+) and (PuO2)2(OH)4(0)(aq) with hydrolysis constants log beta(2,2) = -7.79 +/- 0.20 and log beta(4,2) = -19.3 +/- 0.5 are indicated in all experiments of millimolar Pu(VI), 0.10 M NaNO3 solutions at 25 degrees C. At lower Pu(VI) concentrations, at and below 10(-4) M, the monomeric species PuO2OH+ and PuO2(OH)2(0)(aq) form with hydrolysis constants of log beta(1,1) = -5.76 +/- 0.07 and log beta(2,1) = -11.69 +/- 0.05, respectively. Distinct optical absorbance bands at 842 and 845 nm are reported for the mononuclear and dinuclear first hydrolysis species. Standard hydrolysis constants at zero ionic strength were calculated from the experimentally determined constants using the specific ion interaction theory. The Pu(VI) hydrolysis species and constants are compared with results from previous studies for plutonium and uranium. Major differences between uranyl and plutonyl hydrolysis are described.     

The thermodynamic model of inorganic arsenic species in aqueous solutions Potentiometric study of the hydrolitic equilibrium of arsenic acid

Protonation constants of arsenic acid were determined at different ionic strengths in NaClO(4) (0.1, 0.5, 1.0, 3.0 mol dm(-3)), NaCl (0.5 and 1.0 mol dm(-3)) and KCl (0.5, 1.0 and 3.0 mol dm(-3)) ionic media by means of a potentiometric study. The distribution of arsenate species was defined depending on two important variables in natural environments: pH and composition. All the experimentation was performed at 25 degrees C. The differences found in the protonation constants for different medium compositions, were explained by the different behaviour of the interaction parameters of the species considered in the different media and ionic strengths. These parameters were reported for all hydrolitic As(V) species and were calculated using the Modified Bromley's Methodology (MBM). The corresponding thermodynamic stepwise formation constants were also determined (log degrees K(1)=11.58+/-0.01, log degrees K(2)=7.06+/-0.01, log degrees K(3)=2.25+/-0.01). All the results obtained showed not only the importance of the ionic strength but also of the composition of the ionic medium on the distribution of the acid-base species of As(V) as a function of pH in natural waters.     

The effect of natural organic matter on the formation and solubility of M(OH)4 solid phases

In this study the formation of M(OH)₄ solid phases (e.g. Th(IV), Ce(IV) and Zr(IV)) has been investigated as a function of the natural organic matter (NOM) concentration in 0.1 M NaClO₄, in the pH range between 2 and 5, and under normal atmospheric conditions. M(OH)₄ has been prepared by alkaline precipitation and characterized by TGA, ATR-FTIR, XRD, XPS and solubility measurements. According to the experimental data M(OH)₄ is stable and is the solubility limiting solid phase even in the presence of increased humic acid concentration in solution. In contrast to MO₂(OH)₂ and M(OH)₃ solid phases, increasing humic acid concentration does not affect the crystallite size and the solubility product of M(OH)₄. The M(OH)₄ solubility is basically pH depended and governed by the presence of colloidal species. Regarding the effect of NOM on the solid phase formation of redox-sensitive M(IV) ions (e.g. Ce(IV)), it could be shown that the amount of the reduced metal-ion (e.g. Ce(III)) in the solid phase depends linearly from the NOM concentration in the test solutions.     

Solution chemistry of uranyl ion with iminodiacetate and oxydiacetate: A combined NMR/EXAFS and potentiometry/calorimetry study

The solution chemistry of uranyl ion with iminodiacetate (IDA) and oxydiacetate (ODA) was investigated using NMR and EXAFS spectroscopies, potentiometry, and calorimetry. From the NMR and EXAFS data and depending on stoichiometry and pH, three types of metal:ligand complex were identified in solution in the pH range 3-7: 1:1 and 1:2 monomers; a 2:2 dimer. From NMR and EXAFS data for the IDA system and previous studies, we propose the three complex types are [UO(2)(IDA)(H(2)O)(2)], [UO(2)(IDA)(2)](2)(-), and [(UO(2))(2)(IDA)(2)(mu-OH)(2)](2)(-). From EXAFS spectroscopy, similar 1:1, 2:2, and 1:2 complexes are found for the ODA system, although (13)C NMR spectroscopy was not a useful probe in this system. For the 1:1 and 1:2 complexes in solution, EXAFS spectroscopy is ambiguous because the data can be fitted with either a long U-N/O(ether) value (ca. 2.9 A) suggesting 1,7-coordination of the ligand or a U-C interaction at a similar distance, consistent with terminal bidentate coordination. However, the NMR data of the IDA system suggest that 1,7-coordination is the more likely. The stability constants of the three complexes were determined by potentiometric titrations; the log beta values are 9.90 +/-, 16.42 +/-, and 10.80 +/- for the 1:1, 1:2, and 2:2 uranyl-IDA complexes, respectively, and 5.77 +/-, 7.84 +/-, and 4.29 +/- for the 1:1, 1:2, and 2:2 uranyl-ODA complexes, respectively. The thermodynamic constants for the complexes were calculated from calorimetric titrations; the enthalpy changes (kJ mol(-)(1)) and entropy changes (J K(-)(1) mol(-)(1)) of complexation for the 1:1, 1:2, and 2:2 complexes respectively are the following. IDA: 12 +/- 2, 230 +/- 8; 8 +/- 2, 151 +/- 9; -33 +/- 3, -283 +/- 11. ODA: 26 +/- 2, 198 +/- 12; 20 +/- 2, 106 +/- 8; -24 +/- 2; -219 +/- 8.     

Distribution of Pb(II) species in aqueous solutions

Turbidity and zeta potential measurements were made on solid precipitates formed after raising the pH of a 10(-3) mol/l lead nitrate aqueous solutions containing 10(-2) mol/l KCl with KOH. Distribution diagrams of Pb(II) species in aqueous solutions at a total Pb(II) concentration of 10(-3) mol/l and 10(-2) mol/l KCl was constructed to explain the results. Several solubility products for solid Pb(OH)(2) were found from the literature and used to construct the diagrams. It was observed that distribution diagrams constructed using a solubility product of 1.43x10(-20) explained the experimental results better than those with other reported solubility products.     

Quantitative adsorption and local structures of gallium(III) at the water-alpha-FeOOH interface

The adsorption of Ga(III) at the water-alpha-FeOOH (goethite) interface has been investigated by means of quantitative adsorption experiments, extended X-ray absorption fine structure (EXAFS) spectroscopy, and surface complexation modeling. Under the conditions studied, pH range 3-11 and surface coverages of 0.9-3.2 micromol/m2, Ga(III) was found to adsorb strongly to alpha-FeOOH, and the surface species were more resistant toward hydrolysis and formation of soluble Ga(OH)4- than either solid gallium hydroxides or soluble polynuclear complexes. The EXAFS measurements revealed the presence of octahedral Ga(III) complexes at the water-alpha-FeOOH interface, with practically no structural variations as a function of pH or total gallium concentration. Analysis of the first coordination shell required an anharmonic model indicating a distorted geometry of the GaO6 octahedra, with mean Ga-O distances at 1.96-1.98 angstroms. A method based on the continuous Cauchy wavelet transforms (CCWT) was used to identify backscattering atoms in the higher coordination shells. This analysis indicated predominately Fe backscattering, and the quantitative data fitting resulted in three Ga-Fe paths at 3.05, 3.2, and 3.55 angstroms, which correspond to two edge-sharing and one corner-sharing linkage, respectively. The collective results from EXAFS spectroscopy showed that Ga(III) adsorbs to Fe equivalent sites at the surface alpha-FeOOH as an extension of the rows of Fe octahedra in the bulk structure. This interpretation was further corroborated by a Ga-Fe-Fe multiple scattering path at 6.13 angstroms. The quantitative adsorption and proton data were modeled using a surface complexation formalism based on a 1 pK(a) constant capacitance model. In agreement with the EXAFS results, the model obtained included one predominating surface complex with the stoichiometry [triple bond]FeOGa(OH)2(-0.5) and the stability constant log beta(intr.) = -2.55 +/- 0.04 ([triple bond]FeOH(-0.5) + Ga3+ + 2H2O <--> [triple bond]FeOGa(OH)2(-0.5) + 3H+).     

The acidic constants of 2-hydroxybenzohydroxamic acid in NaClO4 solutions at 25 degrees C

The protolysis equilibria of 2-hydroxybenzohydroxamic acid, H2SAX, have been studied at 25 degrees C in different ionic media by potentiometric titration with a glass electrode. The media were 0.513, 1.05, 2.21 and 3.5 mol/kg NaClO4. The constants beta(-p)(H2SAX<==>H(2-p)SAX(-p)+pH+), combined with salting effects of NaClO4 on H2SAX deduced from solubility determinations, were processed by the specific interaction theory, SIT, to give equilibrium constants at infinite dilution, log beta(-1)(o) = -7.655 +/- 0.013 and log beta(-2)(o) = -17.94 +/- 0.04, as well as specific interaction coefficients b(HSAX-,Na+) = 0.12 +/- 0.01 and b(SAX2-,Na+) = 0.17 +/- 0.02, molal(-1).     

Mixed hydroxide complex formation and solubility of bismuth in nitrate and perchlorate medium

From the precipitation borderlines in the pBi'-pH diagram, determined experimentally under CO(2)-free conditions, the stability constants of bismuth hydroxide, bismuthoxynitrate and bismuthoxyperchlorate have been established. The following values have been found Nitrate-medium: Perchlorate-medium: log *K(SO)(OH) = 5.2, log *K(SO)(OH) = 5.2; log *K(SO)(NO(3)) = -1.2, log*K(SO)(ClO(4)) = -0.9; log *beta(2) = -4.0, log *beta(2) = -4.1; log *beta(3) = -10.0, log *beta(3)= -9.9; log *beta(4) = -21.5, log *beta(4) = -21.5; log *beta(1,0,1) = 1.2, log *beta(1,0,1) = 3.5. The constants refer to precipitates equilibrated for 30 min, prepared at room temperature (23 +/- 0.5 degrees) in sodium perchlorate or sodium nitrate medium with an ionic strength of 1.00 +/- 0.01. Concerning error propagation it is stated that pBi' values calculated with these constants will have a standard deviation of about 0.1 log unit.     

The removal efficiency of chestnut shells for selected pesticides from aqueous solutions

The removal of selected pesticides such as carbofuran (CF) and methyl parathion (MP) using low-cost abundant sorbent chestnut shells from aqueous solutions has been investigated in the present study. The sorption parameters, i.e., contact time, pH, initial pesticide solution concentration and temperature have been studied. Maximum percent sorption (99+/-1%) was achieved for (0.38-3.80) x10(-4) and (0.45-4.5) x10(-4) mol dm(-3) of MP and CF pesticide solutions respectively, using 0.4 g of sorbent in 100 ml of solution for 30 min agitation time at pH 6. The Freundlich, Langmuir and Dubinin-Radushkevich (D-R) models have been applied, and their constants for methyl parathion and carbofuran, sorption intensity 1/n (0.55+/-0.02 and 0.54+/-0.04), multilayer sorption capacity C(m) (28.3+/-0.5 and 16.4+/-0.7) x10(-3) mol l(1-1/n)dm(3/n)g(-1), monolayer sorption capacity Q (22.5+/-0.5 and 10.8+/-0.3) x10(-6) mo lg(-1), binding energy, b (2.9+/-0.2 and 5.2+/-0.5) x10(4) dm(3)mol(-1), and sorption energy E (11.2+/-0.1 and 11.5+/-0.2 kJ mol(-1)) have been evaluated respectively. Lagergren, Morris-Weber and Reichenberg equations were employed to study kinetics of sorption process. Thermodynamic parameters DeltaH (-5.09+/-0.1 and 22.8+/-0.4 kJ mol(-1)), DeltaS (-4.33+/-0.0003 and 0.09+/-0.001 kJ mol(-1)K(-1)) and DeltaG((303K)) (-2.9 and -3.8 kJ mol(-1)) have been calculated for methyl parathion and carbofuran, respectively. The developed sorption procedure has been employed to environmental samples.     

Hydroxo sulfate complexes of iron (III) in solution

The ternary Fe (III)-OH(-)-SO4(2-) complexes have been investigated at 25 degrees C in 3 M NaClO4 by potentiometric titration with glass electrode. The metal and sulfate concentrations ranged from 2.5 x 10(-3) to 0.03 M and from 5.10(-3) to 0.060 M, respectively. [H+] was decreased from 0.05 M to incipient precipitation of basic sulfate which occured at log[H+] between -2.3 and -2.5 depending on the concentration of the metal. For the interpretation of the data stability constants of HSO4(-), of binary hydroxo complexes (FeOH2+, Fe(OH)2+, Fe2(OH)2(4+), Fe3(OH)4(5+), Fe3(OH)5(4+)) and of sulfate complexes (FeSO4+, FeHSO4(2+), Fe(SO4)2-) were assumed from independent sources. The data are consistent with the presence of FeOHSO4, log beta 1-11 = -0.49 +/- 0.03. Equilibrium constants are defined as beta pqr for pFe3+ +qH+ +rSO4(2-) [symbol: see text] FepHq(SO4)r3p+q-2r. No substantial better fit could be found by adding a second mixed complex. Only a slightly smaller agreement factor resulted introducing as minor ternary complex Fe3(OH)6(SO4)3(3-) with log beta 3-63 = -5.8 +/- 0.5. Its evidence, however, cannot be considered conclusive.     

Heats of formation and singlet-triplet separations of hydroxymethylene and 1-hydroxyethylidene

Thermochemical parameters of hydroxymethylene (HC:OH) and 1-hydroxyethylidene (CH3C:OH) were evaluated by using coupled-cluster, CCSD(T), theory, in conjunction with the augmented correlation consistent, aug-cc-pVnZ, basis sets, with n = D, T, Q, and 5, extrapolated to the complete basis set limit. The predicted value at 298 K for Delta Hf(CH2O) is -26.0 +/- 1 kcal/mol, as compared to an experimental value of -25.98 +/- 0.01 kcal/mol, and for Delta Hf(CH:OH) it is 26.1 +/- 1 kcal/mol. The hydroxymethylene-formaldehyde energy gap is 52.1 +/- 0.5 kcal/mol, the singlet-triplet separation of hydroxymethylene is Delta E(ST)(HC:OH) = 25.3 +/- 0.5 kcal/mol, the proton affinity is PA(HC:OH) = 222.5 +/- 0.5 kcal/mol, and the ionization energy is IEa(HC:OH) = 8.91 +/- 0.04 eV. The predicted value at 298 K for Delta Hf(CH3CHO) is -39.1 +/- 1 kcal/mol as compared to an experimental value of -40.80 +/- 0.35 kcal/mol, and for Delta Hf(CH3C:OH) it is 11.2 +/- 1 kcal/mol. The hydroxyethylidene-acetaldehyde energy gap is 50.6 +/- 0.5 kcal/mol, the singlet-triplet separation of 1-hydroxyethylidene is Delta E(ST)(CH3C:OH) = 30.5 +/- 0.5 kcal/mol, the proton affinity is PA(CH3C:OH) = 234.7 +/- 0.5 kcal/mol, and the ionization energy is IEa(CH3C:OH) = 8.18 +/- 0.04 eV. The calculated energy differences between the carbene and aldehyde isomers, and, thus, the heats of formation of the carbenes, differ from the experimental values by 2.5 kcal/mol.     

Synthesis of uniform anatase TiO2 nanoparticles by gel-sol method. 1. Solution chemistry of Ti(OH)(4-n)+(n) complexes

The mole fractions of hydroxo complexes of titanium(IV) ion in an aqueous solution with 0.10 mol dm(-3) NaClO4 at 25 degrees C have been determined as a function of pH by a newly developed analytical procedure based on UV spectrophotometry, using a metastable homogeneous solution of 1.25 x 10(-4) mol dm(-30 in total concentration of Ti(IV). Also, the total concentration of the hydroxo complexes in equilibrium with Ti(OH)4 solid, or the solubility of Ti(OH)4 solid in an inhomogeneous system, has been obtained by ICP measurement for the solution phase. A combination of these data yielded the absolute concentration of each complex species in equilibrium with Ti(OH)4 solid. Finally, Ti(OH)+3 complex has been assigned to the precursor for the formation of anatase TiO2 nanoparticles transformed from Ti(OH)4 gel from a comparison between the above equilibrium data and a kinetic study on the formation rate of the anatase TiO2 particles in the gel-sol system.     

The system aluminium(III)-dipicolinic acid in aqueous 0.5M sodium erchlorate medium

A potentiometric and spectrophotometric investigation on the formation of aluminium(III) complexes with dipicolinic (2,6-pyridinedicarboxylic) acid at 25 degrees in aqueous 0.5M NaClO(4) medium is reported. The values of the cumulative formation constants of the two acid species HL(-) and H(2)L are log ss(1) = 4.532 +/- 0.004 and log ss(2) = 6.624 +/- 0.006. At pH < 4 and in the investigated concentration range (0.242 C(m) 0.975 mM,3.16 C(l) 5.27 mM), aluminium(III) forms two mononuclear complexes, one positively charged, with a metal/ligand molar ratio of 1:1, and the other negatively charged, with a metal/ligand molar ratio of 1:2. The two methods of investigation have yielded the following values for the cumulative formation constants: log beta(1(pot)) = 4.87 +/- 0.02; log beta(2(pot)) = 8.32 +/- 0.02 log beta(1(sp)) = 4.85 +/- 0.03. A precipitate occurs at pH 5-6. A paper electrophoretic investigation and comparison with the behaviour of the well-known iron(III) complexes, supports these findings.