Influence of the type of titration and of data treatment methods on metal complexing parameters determination of single and multi-ligand systems measured by stripping voltammetry

A set of simulated experiments was analysed in order to compare the influence of the titration type and of data treatment methods on the accuracy of metal complexing parameters determination for one- and two-ligand systems. The simulated data corresponded to those obtained by anodic stripping voltammetry and were chosen to represent experiments in linear, logarithmic and decade titration modes. The values of preset complexing parameters for one- and two-ligand systems were chosen to fit into the expected experimental range. Random noise was added to the data prior to the treatment. Five different data treatments were applied: Chau-Buffle, Ru?i?-van den Berg and Scatchard linearisations, and non-linear fitting and PROSECE optimisations.The investigation has shown that even in the case of a one-ligand system, logarithmic and decade titrations are much better compared to the linear ones. Linearisation methods are in many cases inferior to those using optimisation algorithms. Random noise has a significant influence on the results of linearisation methods as well. For linearisation methods, in the case of a one-ligand system, high correlation has been found for the confidence interval of the calculated parameters and the difference between the preset and the calculated values. This correlation is proposed to be used as an estimation for the results quality in real experiments. PROSECE is by far superior to other methods in most of the cases due to its flexible and powerful mathematical background. It is highly recommended as a tool for data treatment. Construction of “contour-graphs” enables error prediction of the calculated complexing parameters. PROSECE is proposed as an orientation and valorisation tool in real samples analyses.     

Significance of data treatment and experimental setup on the determination of copper complexing parameters by anodic stripping voltammetry

Different procedures of voltammetric peak intensities determination, as well as various experimental setups were systematically tested on simulated and real experimental data in order to identify critical points in the determination of copper complexation parameters (ligand concentration and conditional stability constant) by anodic stripping voltammetry (ASV). Varieties of titration data sets (Cu_measuredvs. Cu_total) were fitted by models encompassing discrete sites distribution of one-class and two-class of binding ligands (by PROSECE software). Examination of different procedures for peak intensities determination applied on voltammograms with known preset values revealed that tangent fit (TF) routine should be avoided, as for both simulated and experimental titration data it produced an additional class of strong ligand (actually not present). Peak intensities determination by fitting of the whole voltammogram was found to be the most appropriate, as it provided most reliable complexation parameters.Tests performed on real seawater samples under different experimental conditions revealed that in addition to importance of proper peak intensities determination, an accumulation time (control of the sensitivity) and an equilibration time needed for complete complexation of added copper during titration (control of complexation kinetics) are the keypoints to obtain reliable results free of artefacts.The consequence of overestimation and underestimation of complexing parameters is supported and illustrated by the example of free copper concentrations (the most bioavailable/toxic specie) calculated for all studied cases. Errors up to 80% of underestimation of free copper concentration and almost two orders of magnitude overestimation of conditional stability constant were registered for the simulated case with two ligands.     

Structural parameters of concentrated aqueous solutions of LiCl under extreme conditions,as calculated by the method of integral equations. Effect of temperature

Peculiarities of structure formation of aqueous LiCl solutions at different salt : water molar ratios (LiCl : n H₂O, n = 3.15, 8.05, 14.90) under conditions of isobaric heating (p = 100 bar, T = 298, 323—523 K, T = 50 K) were studied by the method of integral equations. Heating of LiCl : 14.90H₂O solution was found to lead to disappearance of tetrahedral ordering of solvent molecules, appreciable weakening of the coordination abilities of both ions, and to an increase of the number of contact ion pairs and a decrease of the number of solvent-separated ion pairs. For the LiCl : 8.05H₂O system, the tetrahedral structure of the solvent disappears at a lower temperature and heating has a less pronounced effect on the coordination and associative abilities of the ions. In the LiCl : 3.15H₂O solution, tetrahedral ordering of the solvent molecules disappears at 298 K and the number of contact ion pairs decreases as temperature increases. Other structural changes in this system upon heating are similar to those found for the LiCl : 14.90H₂O and LiCl : 8.05H₂O solutions.     

A numerical method of computing the kinetic parameters

A numerical method of computing the kinetic parameters (the activation energy (E), the preexponential constant (A) and the reaction order (n)) of exothermic decomposition of energetic materials via the exothermic rate equation is presented. The values ofE, A, andn are reported for the exothermic decomposition of six typical energetic materials, 1,6-diazido-2,5-dinitrazahexane (I), 1,5-diazido-3-nitrazapentane (II), 2,2,4,7,9,9-hexanitro-5-methyl-4,7-dinitrazadecane (III), 2,2,2-trinitroethyl-4,4,4-trinitrobutyrate (IV), 1,4-dinitro-2,3-dioxo-1,4-dinitrazacyclohexane (V) and 1,3,5-trianitro-1,3,5-triazafurazano[3,4-f]cycloheptane (VI).

     

Precision of integral methods for the determination of the kinetic parameters

In this paper, a systematic analysis of the errors involved in the determination of the kinetic parameters (including the activation energy and frequency factor) from five integral methods has been carried out. The integral methods analyzed here are Coats-Redfern, Gorbachev, Wanjun-Yuwen-Hen-Zhiyong-Cunxin, Junmeng-Fusheng-Weiming-Fang, Junmeng-Fang and Junmeng-Fang-Weiming-Fusheng method. The results have shown that the precision of the kinetic parameters calculated by the different integral methods is dependent on u (E/RT), that is, on the activation energy and the average temperature of the process.     

Spectroscopic studies and determination of the formation constants and thermodynamic parameters for a new water-soluble Co(II) Schiff-base complex and some imidazole derivatives

A new water-soluble Co(II) Schiff-base complex, sodium[{N,N′-bis(5-sulfosalicylidene)-1,8-diamino-3,6-dioxaoctan}cobalt] dihydrate, abbreviated as Na₂[Co(II)L], was synthesized and characterized. The formation constants and thermodynamic parameters for the interaction of this complex with imidazole (Im) and 1-methylimidazole (MeIm) were determined spectroscopically in aqueous solution, ethanol/water (10/90), and methanol/water (10/90) under physiological conditions (pH?=?7), constant ionic strength (I?=?0.1?mol?dm^?3 KNO₃), and various temperatures ranging from 294 to 310?K. Our spectroscopic and thermodynamic results show that this adduct formation is endothermic and the positive values of ΔS _f° make ΔG _f° negative. The trend in variation of ΔH _f° and ΔS _f° for Im is in the order water?>?methanol?>?ethanol, but for MeIm it is in the opposite order which is related to the hydrogen bonding between solvents and these donors. Formation constants between MeIm and Na₂[Co(II)L] in these three solvents are larger than for Im which depends on the electron donation of methyl on MeIm.     

On the determination of order parameters for homogeneous and twisted nematic liquid crystals from Raman spectroscopy

Traditional approaches to the use of Raman spectroscopy as an aid to the determination of local order parameters in liquid crystalline materials have employed polarizations of the excitation source and/or the analyser which are orthogonal to the liquid crystalline director. The present paper describes a Raman study, which seeks to take advantage of the additional information available from examining the complete range of orientations of the director in relation to these polarization directions. A theory is developed which shows how it is possible to use this additional information to derive more reliable values of the P₂ and P₄ local order parameters in homogeneous and twisted nematic liquid crystal cells.     

Development of polyphosphate parameters for use with the AMBER force field

Accurate force fields are essential for reproducing the conformational and dynamic behavior of condensed-phase systems. The popular AMBER force field has parameters for monophosphates, but they do not extend well to polyphorylated molecules such as ADP and ATP. This work presents parameters for the partial charges, atom types, bond angles, and torsions in simple polyphosphorylated compounds. The parameters are based on molecular orbital calculations of methyldiphosphate and methyltriphosphate at the RHF/6-31+G* level. The new parameters were fit to the entire potential energy surface (not just minima) with an RMSD of 0.62 kcal/mol. This is exceptional agreement and a significant improvement over the current parameters that produce a potential surface with an RMSD of 7.8 kcal/mol to that of the ab initio calculations. Testing has shown that the parameters are transferable and capable of reproducing the gas-phase conformations of inorganic diphosphate and triphosphate. Also, the parameters are an improvement over existing parameters in the condensed phase as shown by minimizations of ATP bound in several proteins. These parameters are intended for use with the existing AMBER 94/99 force field, and they will permit users to apply AMBER to a wider variety of important enzymatic systems.     

An updated version of the computational package SIMPRE that uses the standard conventions for Stevens crystal field parameters

The crystal field approach used by SIMPRE is analyzed, verifying the exactness of the results concerning energy levels and magnetic properties calculated by the package. To coincide with the prevailing conventions, we reformulate the presentation of the crystal field parameters, so that the results are now, also from a formal point of view, strictly correct. New calculations are presented to test the influence of neglecting the excited J states, a common but critical approximation employed by SIMPRE. For that, we examine the case of Er(trensal) complex (H₃trensal = 2,2′,2″‐tris(salicylideneimino)triethylamine) where the influence of this approximation is found to be minimal. A patched version of the code, SIMPRE 1.1, and an updated version of the user manual are now available. Finally, we comment on “Software package SIMPRE – revisited,” which apparently revisits a software package without inspecting or using the code. © 2014 Wiley Periodicals, Inc.     

Advances in numerical methods for the solution of population balance equations for disperse phase systems

Accurate prediction of the evolution of particle size distribution is critical to determining the dynamic flow structure of a disperse phase system. A population balance equation (PBE), a non-linear hyperbolic equation of the number density function, is usually employed to describe the micro-behavior (aggregation, breakage, growth, etc.) of a disperse phase and its effect on particle size distribution. Numerical solution is the only choice in most cases. In this paper, three different numerical methods (direct discretization methods, Monte Carlo methods, and moment methods) for the solution of a PBE are evaluated with regard to their ease of implementation, computational load and numerical accuracy. Special attention is paid to the relatively new and superior moment methods including quadrature method of moments (QMOM), direct quadrature method of moments (DQMOM), modified quadrature method of moments (M-QMOM), adaptive direct quadrature method of moments (ADQMOM), fixed pivot quadrature method of moments (FPQMOM), moving particle ensemble method (MPEM) and local fixed pivot quadrature method of moments (LFPQMOM). The prospects of these methods are discussed in the final section, based on their individual merits and current state of development of the field. Supported by the National Basic Research Program of China (Grant No. 2004CB720208), the National Natural Science Foundation of China (Grant Nos. 40675011 & 10872159), and the Key Laboratory of Mechanics on Disaster and Environment in Western China     

Advantage of the universal equation over the linear equation for the calculation of retention parameters of homologous series in capillary chromatography

A procedure is suggested for preparation of reproducible and stable galss capillary Apiezon LH+KF columns for the determination of organic compounds containing different functional groups. The advantage of the universal equation over the linear equation for calculation of retention indices for any homologs, including the first members of the series, is shown using the following five homologous series as examples: fatty acid methyl esters, methyl ketones, secondary and tertiary aliphatic amines, and N-alkyl piperidines. The accuracy of the calculation enables the universal equation to be used in computer-assisted identification.     

On statistical mechanical equations of state for simple fluids : Effective hard spheres and quantum corrections

Nordholm, S., Greberg, H. and Penfold, R., 1991. On statistical mechanical equations of state for simple fluids. Effective hard spheres and quantum corrections. Fluid Phase Equilibria, 90: 307-332.

A comparison is made of the mean field generalised van der Waals theory, based on a variationally determined hard sphere diameter, with the recent equation of state proposed by Song and Mason incorporating a temperature-dependent hard sphere diameter and correlation effects through the second virial coefficient. The simple cell theory ansatz of the former is less accurate but permits a wide range of applications including the estimation of quantum effects on the bulk properties of light fluids at low temperatures. Results for the critical parameters of ³He, ⁴He, H₂, Ne, CH₄ and Ar are examined. The relevance to the corresponding theory of non-uniform fluids is noted.     

Computer simulation of polymer networks: mesoscopic heterogeneity of the structure and topological parameters

Samples of networks of reacting bi- and trifunctional monomers have been obtained by the dynamic Monte Carlo method for the two dimensional lattice model. Systems with various densities and numbers of chemical junctions have been studied. The steady-state structural factor, the number of the degree of conversion, and topological parameters of samples appear to depend strongly on the initial (random) spatial configuration and the size of the Monte Carlo base cell. Deviations of structural parameters calculated by ensemble and time averaging are statistically significant, which makes it possible to classify networks of this type as so-called non-ergodic fortuitous media that exhibit this property for scales of 10² nm. Newly developed facilities for topological analysis allow one to observe the kinetics of changes in individual components of the system: trees, circuits, and other more complex structures.Papers presented in the section Physical Chemistry of this issue are based on the materials delivered at the XII Seminar on Intermolecular Interactions and Conformation of Molecules, Khar'kov, October 3–8, 1994.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1226–1232, July, 1995.The work was financially supported by the Russian Foundation for Basic Research (Project No. 9503-08170a).The authors are grateful for Professor J. G. H. Joosten of Leiden University for helpful critical comments.     

Equation for the direct determination of the density matrix: Time-dependent density equation and perturbation theory

The density equation proposed previously for the direct determination of the density matrix, i.e. for the wave mechanics without wave, is extended to a time-dependent case. The time-dependent density equation has been shown to be equivalent to the time-dependent Schr?dinger equation so long as the density matrix, included as a self-contained variable, is N-representable. Formally, it is obtainable from the previous time-independent equation by replacing the energy E with . The perturbation theory formulas for the density equation have also been given for both the time-dependent and time-independent cases. Received: 16 June 1998 / Accepted: 2 September 1998 / Published online: 8 February 1999     

Existence, uniqueness, and calculation of the physically acceptable solution of a particular case of the Sherman equations

When a chemical sample made of N elements is analyzed by using sequential selective excitation by monochromatic X-ray beams and selective measurement of the characteristic X-rays, the production of secondary fluorescence does not interfere with the measurements. This experimental situation leads to a particular simple case of the Sherman equations which can be written in this instance as linear equations. The linear equations thus obtained are shown to be very similar to the equations appearing in the classical models of Beattie and Brissey and of Lachance and Traill. The linear algebra proves the existence of N different sets of solutions, but the Perron Frobenius theorem ensures that there is one and only one physically feasible solution, and also leads to the method for obtaining it. This equation solution method can be extended to the equations appearing when standard samples of pure elements are also measured.The propagation of the errors in the measurements to the errors in the sample concentrations has been calculated and simulated, and the results have shown that the solution is well conditioned.     

On the usefulness of generalised quantum chemical valence parameters in monitoring the course of a chemical reaction: A case study of the photochemical decomposition of HNO in excited states

A variant of the orthogonal gradient method of orbital optimization in the INDO-MCSCF framework has been used to study the photochemical decomposition of the HNO molecule into H + NO in the lowest^1.3A″ states. A complete geometry optimization has been carried out at all points of the reaction path which appears to be almost barrierless. The one-electron density matrix extracted from the optimized wavefunction at each point has been used to generate the relevant sets of quantum chemical valence parameters. A sharp transition is noted in the N-H bond order and hydrogen free valence index when plotted as functions of r_NH. This enables us to locate the transition region easily.     

Fitting elephants in the density functionals zoo: Statistical criteria for the evaluation of density functional theory methods as a suitable replacement for counting parameters

Counting parameters has become customary in the density functional theory community as a way to infer the transferability of popular approximations to the exchange‐correlation functionals. Recent work in data science, however, has demonstrated that the number of parameters of a fitted model is not related to the complexity of the model itself, nor to its eventual overfitting. Using similar arguments, here, we show that it is possible to represent every modern exchange‐correlation functional approximations using just one single parameter. This procedure proves the futility of the number of parameters as a measure of transferability. To counteract this shortcoming, we introduce and analyze the performance of three statistical criteria for the evaluation of the transferability of exchange‐correlation functionals. The three criteria are called Akaike information criterion, Vapnik‐Chervonenkis criterion, and cross‐validation criterion and are used in a preliminary assessment to rank 60 exchange‐correlation functional approximations using the ASCDB database of chemical data.     

Separation behavior of U(VI) and Th(IV) on a cation exchange column using 2,6-pyridine dicarboxylic acid as a complexing agent and its application for the rapid separation and determination of U and Th by ion chromatography

The retention behavior of U and Th as their 2,6-pyridine dicarboxylic acid (PDCA) complexes on a cation exchange column was investigated under low pH conditions. Based on the observed retention characteristics, an ion chromatographic method for the rapid separation of uranium and thorium in isocratic elution mode using 0.08 mM PDCA and 0.24 M KNO(3) in 0.22 M HNO(3) as the eluent was developed. Both uranium and thorium were eluted as their PDCA complexes within 2 min, whereas the transition and lanthanide metal cations were eluted as an unresolved broad peak after thorium. Under the optimized conditions both U and Th have no interference either from alkali and alkaline earth elements up to a concentration ratio of 1:500 or from other elements up to 1:100. The detection limits (LOD) of U and Th were calculated as 0.04 and 0.06 ppm, respectively (S/N=3). The precision in the measurement of peak area of 0.5 ppm of both U and Th was better than 5% and a linear calibration in the concentration range of 0.25-25 ppm of U and Th was obtained. The method was successfully applied to determine U and Th in effluent water samples.     

Development and validation of an HPLC method for the determination of process-related impurities in pridinol mesylate, employing experimental designs

A simple high performance liquid chromatographic method for the determination of process-related impurities in bulk drug of the central anticholinergic compound pridinol mesylate, has been developed and validated. Spectroscopically characterized synthetic impurities were used as standards. The chromatographic separation was optimized employing an experimental design strategy, and was achieved on a C₁₈ column with a mobile phase containing 50 mM potassium phosphate buffer (pH 6.4), MeOH and 2-propanol (20:69:11, v/v/v), delivered at a flow rate of 1.0 mL min⁻¹. UV detection was performed at 245 nm. The optimized method was thoroughly validated, demonstrating to be selective, when the chromatogram was recorded with a diode-array detector and peak purities were evaluated (>0.9995). The method is robust and linear (r² > 0.99) over the range 0.05-2.5% (5-250% with regards to the 1% specification limit for both process-related impurities); it is also precise, regarding repeatability (RSD ≤ 1.5% for all of the analytes) and intermediate precision aspects and LOQ values for the impurities are below 0.01%. Method accuracy, evidenced by low bias of the results and analyte recoveries in the range of 99.1-102.7%, was assessed at five analyte concentration levels. The usefulness of the determination was also demonstrated through the analysis of different lots of pridinol mesylate bulk substance. The results indicate that the method is suitable for the quality control of the bulk manufacturing of pridinol mesylate drug substance.     

Kinetic analysis of solid-state reactions: errors involved in the determination of the kinetic parameters calculated by one type of integral methods

The integral methods are extensively used for performing the kinetic analysis of solid-state reactions. As the Arrhenius integral function p(u) does not have an exact analytical solution, many approximations have been proposed. One popular type of approximations is called the exponent approximation which can be put in the form . In this study, a systematic analysis of the errors involved in the determination of the kinetic parameters calculated by the integral methods based on the exponent approximations for p(u) has been carried out. The results have shown that the precision of the kinetic parameters computed from the integral methods analyzed in this paper depends on u and the errors of the kinetic parameters determined from Doyle approach are the largest.