A model developed for acid dissolution thermodynamics of a Turkish bentonite

A model was proposed to calculate some thermodynamic parameters for the acid dissolution process of a bentonite containing a calcium-rich smectite as clay mineral along with quartz, opal and feldspar as impurities. The bentonite sample was treated with H₂SO₄ by applying dry method in the temperature range 50–150°C for 24 h. The acid content in the dry bentonite-sulphuric acid mixture was 45 mass%. The total content (x) of Al₂O₃, Fe₂O₃ and MgO remained in the undissolved sample after treatment was taken as an equilibrium parameter. An apparent equilibrium constant, K _a, was calculated for each temperature by assuming K _a=(x _m−x)/x where x _m is the total oxide content of the natural bentonite. Also, an apparent change in Gibbs free energy, ΔG _a^o, was calculated for each temperature by using the K _a value. The graphs of lnK _a vs. 1/T and ΔG _a^o vs. T were drawn and then the real change in both the enthalpy, ΔH ^o and the entropy, ΔS ^o, values were calculated from the slopes of the straight lines, respectively. Inversely, real ΔG ^o and K values were calculated from the real ΔH ^o and ΔS ^o values through ΔG ^o = −RT ln K = ΔH ^o − TΔS ^o equation. The best ΔH ^o and ΔS ^o fittings to this relation were found to be 65687 J mol⁻¹ and 164 J mol⁻¹K⁻¹, respectively.     

Evaluation of Multiplexed CE with UV Detection for Rapid pK a Estimation of Active Pharmaceutical Ingredients

The acid dissociation constant (pK _a) is a key physicochemical parameter for characterizing active pharmaceutical ingredients (APIs). Early determination of pK _a values is highly desirable in drug discovery, pharmaceutical process research and formulation design. To overcome the challenges of limited sample availability and potential low purity of API samples at early stages of drug development, as well as to increase sample analysis throughput, a multiplexed 96-channel capillary electrophoresis with UV detection was evaluated as a practical approach for high throughput pK _a estimation of proprietary APIs in support of pharmaceutical research. Proprietary APIs with diverse structures were examined using the approach. The pK _a values were successfully determined with good accuracy and precision. System robustness was demonstrated and analysis of at least eight samples can be completed within 1 h. A rapid pK _a estimation procedure for marginally soluble APIs was proposed by performing single-point multiplexed CE–UV measurement without extrapolation using 10 or 20% methanol as co-solvent. Direct pK _a estimation of APIs using DMSO solution samples and crude reaction samples containing a large amount of solvents and reagents and high level of impurities was also demonstrated using the multiplexed CE–UV approach.     

Energy dependence of photon-induced Kα and Kβ x-ray production cross-sections for some elements with 38≤Z≤51 in the energy range 20–50 keV

The energy dependence of photon-induced Kα and Kβ x-ray production (or x-ray fluorescence) cross-sections for Sr, Y, Mo, Ru, Pd, Ag, In and Sb elements has been studied in the energy range of 20–50 keV using Energy Dispersive X-Ray Fluorescence (EDXRF) Spectrometry. The photon energy dependence of K x-ray production cross-sections was measured with secondary excitation method. A radioisotope point source of ²⁴¹Am was employed to excite the K x-rays of secondary exciter elements. The L x-ray yields from Th and U were measured to determine I_oG (the intensity of exciter K x-rays falling on primary target). The measurements have been made by observing the x-ray emissions with the help of HPGe detector coupled with a multichannel analyzer. The areas of the Kα and Kβ spectral peaks, as well as the net peak areas, have been determined by a fitting process. The measured Kα and Kβ x-ray production cross-sections have been compared with calculated theoretical values in this energy regime. The present experimental results for all the elements were in general agreement with the theoretical values calculated using photoionization cross-sections, fractional rates (based on Hartree–Slater potentials) and fluorescence yields.     

Isotachophoretic determination of mobility and pKa by means of computer simulation : V. Evaluation of mo and pKa of twenty-eight dipeptides and assessment of separability

Isotachophoretic qualitative indices, R_E, for twenty-eight dipeptides were measured in the range pH 7.4–9.6. The absolute mobility, m_o, and pK_a values were evaluated by the use of the least-squares method, utilizing a simulation of the isotachophoretic steady state. The m_o values were newly evaluated and the pK_a values were in good agreement with literature values. By comparison of the evaluated m_o and pK_a values of the dipeptides with those of the constituent amino acids, simple relationships were found which may be used to estimate the m_o and pK_a values of other dipeptides. The separability of the dipeptides was also evaluated by considering the differences between their simulated effective mobilities. It is concluded that isotachophoresis is very convenient for the separation of dipeptides and their constituent amino acids.     

Dissolution of Al-Fe materials and analysis by inductively coupled plasma – atomic emission spectrometry

A technique was developed for the dissolution of Al-Fe materials containing difficult to dissolve Al₂O₃. The developed procedure uses HCl and HNO₃ for initial sample attack followed by digestion with a mixture of H₃PO₄ and H₂SO₄ at 200?°C. This procedure was employe to dissolve Al-Fe material samples before the determination of Al and Fe. Minor and trace elements (B, Cr, Cu, Mo, Si, Zr) were determined after dissolution in HCl and HNO₃. Results of a round robin study verified the procedure accuracy. The developed methods have the required accuracy and precision to be used as a quality control procedure for Al-Fe materials analysis.     

Thermal and epithermal neutron activation analysis using the monostandard method

A new approach is presented for neutron activation analysis using the monostandard (single comparator) method. Elements to be determined are classified into two groups; those with σ_o>I_o are activated with whole neutron spectrum (without Cd-cover), using a standard of the same group (e.g. Sc), while elements with I_o>σ_o are activated with epithermal neutrons (under Cd-cover) using a standard of the same group (e.g. Au or Co). Epithermal activation increases the number of determinable elements and its coupling with the monostandard method has the advantage of using a small Cd-cover. The σ_o and I₀ values of some elements were determined to test this approach.     

Insight into molecular interactions and solvation properties of methylammonium perchlorate in acetonitrile-dimethylsulfoxide binary mixtures: Compressibility,viscometric and dissolution thermodynamic studies

Densities (ρ), ultrasonic velocities (u) and viscosities (ƞ) of Methylammonium perchlorate (CH₃NH₃ClO₄) in binary solvent mixtures of Acetonitrile (AN) and Dimethylsulfoxide (DMSO) containing 0.0, 0.20, 0.40, 0.60, 0.80 and 1.0 ​mol fraction of DMSO at various temperatures ranging from 298K–328K at atmospheric pressure, have been measured. From the experimental data important parameters, namely, apparent molal volume (V_Φ), isentropic compressibility (K_s), limiting apparent molal isentropic compressibility (K^o_s,ɸ), A and B-viscosity coefficients from Jones-Dole equation, B/V_ɸ^o values and viscous flow related thermodynamic properties have been derived. Interpretation of the obtained thermodynamic parameters was done with regards to solute-solvent and solute-solute interactions and structure breaking and making potential of the solutes in solution. To evaluate the extent of preferential solvation, K^o_s,ɸ values and B-coefficients for the electrolytes have been further split up into the contribution of individual ion (K^o_s,ɸ± and B_± values) by reference electrolyte method.     

Gas-chromatographic headspace analysis of phenol and cresols in soils by direct acetylation

A quick, simple and selective procedure has been developed for the determination of phenol, o-cresol, m-cresol, and p-cresol in soil samples. The method is based on the gas chromatographic headspace analysis of phenols as acetate derivatives. These acetates were prepared directly in the wetted soil samples by acetic anhydride in the presence of KHCO_3˙ With this procedure sample handling is reduced to a minimum as desorption and derivatization of the analytes are developed from the wetted samples inside the vial of the head space sampler. The acetate esters were analyzed using a 60 m × 0.56 mm DIIDP capillary column (di-isodecyl-phthalate), using 2-chlorophenol as an internal standard. The detection limit was between 0.03–0.08 μg g⁻¹ for the different species considered. Soil samples having carbon contents lower than 5 % can be analyzed by the proposed procedure.     

Dissolution of Al-Fe materials and analysis by inductively coupled plasma – atomic emission spectrometry

A technique was developed for the dissolution of Al-Fe materials containing difficult to dissolve Al₂O₃. The developed procedure uses HCl and HNO₃ for initial sample attack followed by digestion with a mixture of H₃PO₄ and H₂SO₄ at 200 °C. This procedure was employe to dissolve Al-Fe material samples before the determination of Al and Fe. Minor and trace elements (B, Cr, Cu, Mo, Si, Zr) were determined after dissolution in HCl and HNO₃. Results of a round robin study verified the procedure accuracy. The developed methods have the required accuracy and precision to be used as a quality control procedure for Al-Fe materials analysis. Received: 9 February 1998 / Revised: 1 April 1998 / Accepted: 4 April 1998     

ko-measurements and related nuclear data compilation for (n, γ) reactor neutron activation analysis

k_o-factors of 35 isotopes used in reactor neutron activation analysis were measured with a high degree of accuracy (1–2%). To minimize systematic errors, measurements were carried out using different reactor types, irradiation conditions (18 < Φ_s/Φ_e), Ge(Li) detectors, sample detector geometry, etc. Analyst-oriented tabulations including all necessary nuclear data, “best values”, as well as recommended k_o-values are given to facilitate analytical work with the new method. Some practical aspects as well as limitations of the k_o-method are also outlined together with the applied neutron flux and cross-section conventions. Research associate of the National Fonds voor Wetenschappelijk Onderzoek, Belgium     

Thermodynamic properties of peptide solutions. Part 15. Partial molar isentropic compressibilities of some glycyl dipeptides in aqueous solution at 15 and 35°C

The partial molar isentropic compressibilities at infinite dilution,K _s,2 ^o , have been obtained for eight glycyl dipeptides of sequence gly-X (X is an amino acid) in aqueous solution at the temperatures 15 and 35°C. The results have been combined with those obtained at 25°C, that were reported earlier, to evaluate the temperature dependences ofK _s,2 ^o for the dipeptides in the temperature range 15 to 35°C. TheK _s,2 ^o values for all the dipeptides are negative and increase (become more positive) with an increase in temperature. The slopes of the temperature dependences ofK _s,2 ^o for the dipeptides with typically hydrophobic side-chains are significantly larger than those for dipeptides with hydrophilic side-chains.     

Solid phase extraction of Pb(II) and Cd(II) as 2,9 dimethyl-4,7-diphenyl-1,10-phenanthroline chelates on activated carbon cloth in environmental samples and their determination by flame atomic absorption spectrometry

A solid-phase extraction procedure for Pb(II) and Cd(II) as 2,9 dimethyl-4,7-diphenyl-1,10-phenanthroline complexes on activated carbon cloth (ACC) has been established. In the determination step, flame atomic absorption spectrometry (FAAS) was used. The optimum conditions for pH, type and volume of eluent, volume of sample solution, flow rates of eluent, sample solution and matrix effect were determined. For quantitative recovery of the analyte ions, refereed optimum values are as follows: amount of ACC, 0.4 g; pH, 6.0 and eluent, 10 mL 3 M HNO₃. To test the accuracy of the method, a certified reference material (CRM) analysis and add-recovery methods were performed. The developed method was applied for the determination of the analyte elements in water and vegetable samples.     

Estimation of Dimerisation Constants from Complexatin-Induced Displacements of 1H-NMR Chemical Shifts: Dimerisation of Caffeine

The determination of the dimerisation constant (K^D) for the weak self-association of a compound C in dilute solution according to the equilibrium, 2C?C₂ is described. The method uses chemical shifts measured on a series of solutions of C at different concentrations: the optimum K^D is defined by a linear regression best-fit procedure, which simultaneously determines optimum values for δ_o and also for δ_∞, the intrinsic chemical shifts for nuclei in the monomer and dimer species. The dimerisation of caffeine in D₂O is used as a model to demonstrate the working of the method and the quality of results obtained. The most probable value of K_D for caffeine at 30.5° is found in the range 5.5–6.0 kg solution · mol^?1, and the enthalpy and entropy of dimerisation are found to be ΔH^? = ?15.1 kJ · mol^?1 and ΔS^? = ?35.3 J · °C^?1 · mol^?1, respectively. The influence of small errors in δ_o on the confidence limits of K^D is discussed.     

Thermochemical investigation of natural antlerite

Thermal and thermochemical investigations of natural hydroxyl-bearing copper sulfate Cu₃SO₄(OH)₄??antlerite have been carried out. The stages of its thermal decomposition have been studied employing the Fourier-transform IR spectroscopy. The enthalpy of formation of antlerite from the elements ??_f H _m ^o (298.15?K)?=?(?1750?±?10)?kJ?mol^?1 has been determined by the method of oxide melt solution calorimetry. Using value of S _m ^o (298.15?K), equal to (263.46?±?0.47)?J?K^?1?mol^?1, obtained earlier by the method of adiabatic calorimetry, the Gibbs energy value of ??_f G _m ^o (298.15?K)?=?(?1467?±?10)?kJ?mol^?1 has been calculated.     

Relevance of NH4F in acid digestion before ICP-MS analysis

In order to implement a simpler, less expensive and more safe sample dissolution procedure, we have substituted the HF-HClO₄ mixture by NH₄F. By testing three certified reference materials, lichen 336, basalt BE-N, soil 7, it was found that the three-reagents digestion without HF and HClO₄ (HNO₃ + H₂O₂ + NH₄F was used) was very effective for the pretreatment of ICP-MS measurement. The comparison was based on the measurement results and their uncertainties. All are reference material for amount contents of different trace elements. The accuracy and precision of the developed method were tested by replicate analyses of reference samples of established element contents. The accuracy of the data as well as detection limits (LODs) vary among elements but are usually very good (accuracy better than 8%, LODs usually below 1 μg/g in solids). ICP-MS capabilities enable us to determine routinely 13 and 16 minor and trace elements in basalt and soil.     

Headspace gas-chromatographic determination of formaldehyde in urine

A method is proposed for determining formaldehyde by headspace gas chromatography using an o-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine (PFBHA) derivatizing agent. Formaldehyde in urine was derivatized to o-(pentafluorobenzyl)oxime and extracted by heating an urine sample with PFBHA in a sealed vial. Gas-chromatographic analysis of the headspace was performed in the temperature-programmed mode using an HP-5 capillary column with a flame ionization detector. The limit of detection is 3.5 μg/L. The accuracy of the procedure, equivalent to an extended relative uncertainty, does not exceed 21%. The examination of the procedure using urine samples of the population of the Irkutsk region has shown that the regional reference levels of formaldehyde concentration are in the range 44–83 μg/L.     

Element concentrations in candidate biological and environmental reference materials by ko-standardized INAA

Summary k_o-Based Neutron Activation Analysis (k_oINAA) was used to analyze the candidate reference materials Apple Leaves and Peach Leaves, and Oriental Tobacco Leaves and Virginia Tobacco Leaves. Concentration values for 27 elements were measured. The accuracy was ascertained by analysis of two certified reference materials, NIST 1572 Citrus Leaves and 1573 Tomato Leaves. The homogeneity test of the IAEA Evernia prunastri candidate reference material in aliquots 100 mg is extended to the elements Sc, Cr, Fe, Co, Zn, Rb, Sb, Cs, Ba, Ce and Th.     

Modification of the titrimetric method of analyzing pectin substances

A potentiometric variant of the titrimetric method of analyzing pectin substances has been developed which is distinguished by adequate accuracy and good reproducibility. The modified method has been tested on methyl (methyl 2,3-di-O-methyl-α,β-galactopyranosid)uronate and has been used for determining the free (K_f) and esterified (K_e) carboxy groups of the pectin substances of mint. The methods of isolating, purifying, and fractionating the pectin substances and determining their K_f and K_e values are given. Formulas are presented for calculating the percentage values of K_f and K_e.     

Effect of the form and the parameters of intermolecular interaction potential on calculated values. Henry constant for adsorption of noble gases on graphitized black and the specific surface area determined therefrom

The effect of the form and the parameters of the atom-atom potential of intermolecular interaction, as well as that of the approximations used in its summation over the atoms of a solid, on the values of the Henry constant $\text{K}$₁ for the adsorption of noble gases on the basal face of graphite has been examined. At fixed values of the depth and of the position of the atom-atom potential minimum, $\text{K}$₁ values are only slightly sensitive to the form of this potential, if its summation is performed taking into account the layer structure of graphite. Disregarding the laminated structure of the graphite lattice leads to highly underestimated values of $\text{K}$₁. The values of the parameter $\text{c}$₁ of the attractive forces of the potential (6, 8, exp) for the intermolecular interaction of the atoms of noble gases with the carbon atom of graphite have been determined, using experimental values of $\text{K}$₁ for the adsorption of these gases on graphitized thermal carbon blacks. The corresponding changes in internal energy, entropy and heat capacity have been calculated.The effect of the form and the parameters of the potential function φ of the intermolecular interaction of a monoatomic molecule with the basal face of graphite on the energy φ_o in the minimum of the potential curve φ($\text{z}$), and on the product $\text{sz}$_o of the specific surface area $\text{s}$, and the equilibrium distance between the molecule and the surface $\text{z}$_o, has been studied. φ_o and $\text{sz}$_o were determined from experimental values of the Henry adsorption constant $\text{K}$₁ measured at different temperatures. From the experimental values of $\text{K}$₁ for the adsorption of noble gases, the value of $\text{s}$ for graphitized thermal carbon black Sterling PT, 2700 (P-33), has been determined. This value of $\text{s}$ does not essentially depend on the adsorbate and lies within the limits of error of the $\text{s}$ value, determined for this carbon black by the BET method. When determining $\text{s}$ from $\text{K}$₁, it is necessary to take into account the structure of the solid so as to choose the form of potential φ correctly. Values of $\text{z}$_o should be determined from the atom-atom potential function of the adsorbate-adsorbent intermolecular interaction.     

Tetraalkylammonium Picrates in the Dichloromethane–Water System: Ion-Pair Formation and Liquid–Liquid Distribution of Free Ions and Ion Pairs

Equilibria concerning picrates of tetraalkylammonium ions (Me₄N⁺, Et₄N⁺, Pr₄N⁺, Bu₄N⁺, Bu₃MeN⁺) in a dichloromethane−water system have been investigated at 25 ^∘C. The 1:1 ion-pair formation constants (K _IP,o ^o) in dichloromethane at infinite dilution were conductometrically determined. The distribution constants (K _D ^o) of the ion pairs and the free cations between the solvents were determined by a batch-extraction method. The K _IP,o ^o value varies in the cation sequence, Bu₄N⁺ ≈ Pr₄N⁺ ≈ Et₄N⁺ < Bu₃MeN⁺ < < Me₄N⁺; this trend is explained by the electrostatic cation−anion interaction taking into account the structures of the ion pairs determined by density functional theory calculations. For the ion pairs of the symmetric R₄N⁺ cations, there is a linear positive relationship between log₁₀ K _D ^o and the number of methylene groups in the cation (N _{CH 2}). The ion pair of asymmetric Bu₃MeN⁺ has a higher distribution constant than that expected from the above log₁₀ K _D ^o versus N _{CH 2} relationship. These cation dependencies of log₁₀ K _D ^o for the ion pairs are explained theoretically by using the Hildebrand-Scatchard equation. For all the cations, the log₁₀ K _D ^o value of the free cation increases linearly with N _{CH 2}; the variation of log₁₀ K _D ^o is discussed by decomposing the distribution constant into the Born-type electrostatic contribution and the non-Born one, and attributed to the latter that is governed by the differences in the molar volumes of the cations. The cation dependencies of the ion-pair extractability and ion pairing in water are also discussed. An erratum to this article can be found at