Corrections for volume changes in describing diffusion in electrolyte solutions

It is possible to avoid certain difficulties in describing the diffusion in electrolyte solutions by taking into account the process of spatial rearrangement of species that accompanied the concentration variations and was recently introduced into consideration. The experimental substantiation of taking into account this process is the different concentrations of solvent, i.e., water, observed in solutions of different concentrations.     

THE INFLUENCE OF FLUORESCENCE CONCENTRATION QUENCHING ON THE EMISSION ANISOTROPY OF FLAVINS IN GLYCERINE-WATER SOLUTIONS

The measurements of the emission anisotropy r/r0 of flavomononucleotide (FMN) within a range of concentrations of 10(-5)-10(-1) mol/L in glycerine-water solutions of different viscosities--0.056 Pa/s (system I) and 0.256 Pa/s (system II) have been carried out. In the range of high concentrations the repolarization effect due to the sharp drop of the quantum yield has been observed. The experimental results have been compared with theoretical expressions evaluated by taking into account both concentration and rotation depolarization. A good agreement on the values of the theoretical parameters obtained from independent measurements has been found. It has been stated that in the investigated systems the excitation energy transfer may be treated as a Markov process.     

Copper-Selective Electrode in the Analysis of Dye-Containing Solutions

The effect of dye concentrations in solutions on the potential of a copper-selective electrode with a chalcogenide membrane was studied. It was found that the electrode potential depends on the dye concentration in solutions containing no copper ions. Conventional selectivity coefficients of the electrode with respect to dye anions were determined. The selectivity coefficients ambiguously changed with the dye nature and their concentration in solutions. The use of the calibration graph method without taking into account the composition of the test solution gave rise to great errors in the results of determinations.     

Kinetics of N-glutaryl-L-phenylalanine p-nitroanilide hydrolysis catalyzed by alpha-chymotrypsin in aqueous solutions of dodecyltrimethylammonium bromide

The rate of hydrolysis of N-glutaryl-L-phenylalanine p-nitroanilide (GPNA) catalyzed by alpha-chymotrypsin (alpha-CT) has been measured in aqueous solutions of dodecyltrimethylammonium bromide (DTAB) at concentrations below and above the critical micelle concentration, as well as in the absence of surfactant. Under all the conditions employed, the reaction follows a Michaelis-Menten mechanism. The presence of the surfactant leads to superactivity below and above the critical micelle concentration (CMC), with a maximum reaction rate taking place near the CMC when the results are treated in terms of the analytical concentration of the substrate. A similar behavior was observed by working with the enzyme partially deactivated in the presence of 4 M urea. After correction to take into account the partitioning of the substrate between the micelles and the external media, the activity of the enzyme tends to remain almost constant above the corresponding CMCs. This results from a compensation of a decrease in the catalytic constant (k(cat)) and a decrease in the Michaelis constant (K(M)). The behavior of alpha-CT in the hydrolysis of GPNA in DTAB solutions is at variance with that previously reported for the hydrolysis of 2-naphthyl acetate in solutions of the same surfactant (E. Abuin, E. Lissi, R. Duarte, Langmuir 19 (2003) 5374). An explanation of the different effects of the surfactant on the behavior of the enzyme with both substrates is advanced, taking into account the complexity of the mechanism of the alpha-CT-mediated reaction, more specifically, in terms of different rate-limiting steps for the formation of the measured products.     

Kinetics of chlorine-induced polyethylene degradation in water pipes

The presence of chlorine in drinking water supplies in many countries creates the undesirable side effect of causing a relatively under investigated degree of polymer degradation in the polyethylene pipes used for transport. In order to predict pipe lifetimes and ensure safe water supplies, a kinetic model using data for the degradation rates of polyethylenes immersed in chlorine solutions, was developed. In order to replicate phenomena that normally occur very slowly at low concentrations of chlorine, accelerated ageing studies were necessary. These were carried out at high chlorine concentrations under well-defined experimental conditions (70, 400 and 4000 ppm). Results showed that, for the chlorine concentrations studied, a chain scission process associated with carbonyl formation is occurring. It was also shown that the rate of this degradation does not depend on the presence of stabilizer. A kinetic model, taking into account the chlorine concentration, is proposed in order to simulate the molar mass changes occurring. This will facilitate the prediction of the degree of polyethylene embrittlement and ultimately the lifetime.     

Vapor pressure measurements on non-aqueous electrolyte solutions. Part 2. Tetraalkylammonium salts in methanol. Activity coefficients of various 1-1 electrolytes at high concentrations

Precise vapor pressure data for solutions of Et₄NBr, Bu₄NBr, Bu₄Nl, Bu₄NClO₄, and Am₄NBr in methanol at 25°C in the concentration range 0.04–1]<1.6 are communicated and discussed. Polynomials in molalities are given which may be used for calculating precise vapor pressure depressions of these solutions. Osmotic coefficients are calculated by taking into account the second virial coefficient of methanol vapor. Discussion of the data at low concentrations is based on the chemical model of electrolyte solutions taking into account non-coulombic interactions; ion-pair association constants are compared to those of conductance measurements. Pitzer equations are used to reproduce osmotic and activity coefficients at high concentrations; the set of Pitzer parameters b=3.2, ₁ = 2.0, and ₂ = 20.0 is proposed for methanol solutions.     

Adsorption of selected herbicides from aqueous solutions on activated carbon

The adsorption of MCPA and 2,4-D on the activated carbon Filtrasorb 300 was studied. The adsorption isotherms of herbicides from aqueous solutions were measured over a wide range of solute concentrations and at different temperatures. The experimental equilibrium data were analyzed by the Langmuir–Freundlich isotherm taking into account the energetic heterogeneity of adsorption system. The effect of temperature and herbicide properties on its uptake was discussed. The thermal analysis was applied in order to find the differences in herbicide interactions with carbon surface. The kinetic dependences were measured and the relations between solute properties and adsorption rate were discussed.     

Van der Waals model and equation of state of nitrobenzene-alkanes solutions near the critical consolute temperature

On the basis of the fluctuation theory of phase transitions a system close to the critical point is the ideal gas of order parameter fluctuations. An extended equation of state for binary solutions close to the critical consolute temperature has been proposed taking into account the properties of a real Van der Waals gas in this model. This equation has been used to analyze the temperature dependences of the concentrations of a series of nitrobenzene + alkane binary solutions in terms of different order parameters. It has been shown that the molar concentration of the solution should be used as an order parameter of the analyzed systems. It has been determined that the parameters of the extended equation of state are linear functions of (a) the number of carbon atoms in alkanes and (b) the compressibility factor of the solution components.     

Kinetics of a bimolecular chemical reaction in the liquid phase with association of both reactants

Mathematical modeling of a bimolecular chemical reaction in the liquid phase was carried out taking account of association of both reactants. The formally simple chemical reaction is thus transformed into a multistage process, which results in the formation of temporal dissipative structures. It was found that, apart from the usual dynamic modes, there appear oscillations of the concentrations of intermediates, characterized by two considerably differing periods.     

Factors determining the direction of the stereoselection process in the generalized Frank model

The Frank model for spontaneous chiral stereoselection of biomolecules is extended by taking into account the influence of electroweak effects on the rates of enantiomeric chemical reactions. Explicit analytical solutions of the respective differential 'equations are obtained, describing the behaviour of the generalized Frank model immediately after the beginning of the process, when the direction of the stereoselection is decided. The direction of the stereoselection is affected by both stochastic and deterministic factors, whose complex interrelations are examined.received by the Publisher 20 September 1989     

Spectrophotometric determination of polymeric flocculants in waters of different types

Previously studied reactions of the formation of ion associates between polyallyldimethylammonium chloride (PADAC) and triphenylmethane dyes were used in the determination of the residual concentration of polymeric flocculants. Rapid procedures were developed for analyzing hydrochemical objects of different types without the separation of PADAC and the effect of interferents was taken into account. The analysis of process solutions of water treatment was performed using the reaction with Xylenol Orange; the analysis of potable water and purified sewage waters was based on the reaction with Erythrosine. The precision of the procedures is at a level of RSD ≤ 20%. The systematic error does not exceed the tolerable level if the concentrations of metals and surfactants in the studied water are lower than their maximum permissible concentrations.     

Thermodynamics of electrostatic forces interaction in concentrated electrolyte solutions

The potential of interactions of particles in concentrated solutions of electrolytes in the steady state is calculated taking into account the electrostatic and solvation (hydration) interactions and weak chemical bonding between the hydration shells.     

Vapor pressure measurements on non-aqueous electrolyte solutions. Part 3: Solutions of sodium lodide in ethanol, 2-propanol,and acetonitrile

Precise vapor pressure data for solutions of Nal in ethanol from 0.04 to 1.9m, and 2-propanol and acetonitrile from approximately 0.06 to 1.5m are communicated and discussed. Polynomials in molalities are given for calculating precise reference values. Osmotic coefficients were calculated by taking into account the second virial coefficients of solvent vapors. Discussion of the data at low concentrations is based on the chemical model of electrolyte solutions and ion-pair association constants are compared to those obtained from other properties of sodium iodide solutions. Pitzer equations are used to reproduce osmotic and activity coefficients at high concentrations; the set of Pitzer parameters for methanol solutions b=3.2, ₁ = 2.0, and ₂ = 20.0 may be used for ethanol, 2-propanol, and acetonitrile solutions.     

Kinetics of N-glutaryl-L-phenylalanine p-nitroanilide hydrolysis catalyzed by alpha-chymotrypsin in aqueous solutions of alkyltrimethylammonium bromides

The rate of N-glutaryl-L-phenylalanine p-nitroanilide hydrolysis catalyzed by alpha-chymotripsin has been measured in aqueous solutions of cetyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, and dodecyltrimethylammonium bromide at concentrations below and above their critical micellar concentrations (CMC). For the three surfactants considered superactivity was observed, with maximum catalytic efficiencies taking place near the corresponding CMCs. The effect of the surfactants after the CMCs is mostly due to a decreased thermodynamic activity of the substrate due to its incorporation into the micelles. After addition of the surfactants, the Michaelis constant values (corrected to take into account the free substrate concentration) tend to decrease, passing through an ill defined minimum, afterwards reaching a constant value. The catalytic rate constants show the same profiles that the catalytic efficiency, being maxima near the surfactants CMCs. This maximum is more important for the surfactant having the shorter tail. This result is explained by considering that the hydrophobicity of the surfactant influences more the CMC than its association to the enzyme.     

Evaluation of sorption efficiency coefficients in passive sampling of Nitrogen(IV) oxide

Nitrogen(IV) oxide concentration in the atmospheric air was measured by passive sampling with two sorbent solutions in order to evaluate their efficiency in absorption of NO₂. A model taking into account the sorption efficiency of the sorbent solutions used was suggested.     

Adsorption and Aggregation Properties of Some Polysorbates at Different Temperatures

Measurements of the surface tension of aqueous solutions of polysorbates (Tween 20, Tween 60 and Tween 80) at 293, 303 and 313 K were made. On the basis of the obtained results the Gibbs surface excess concentration of the Tweens at the water–air interface and critical micelle concentrations were determined. Knowing the Gibbs surface excess concentration and taking into account the difference between the limiting area occupied by water and Tween molecules at the water–air interface, the fraction occupied by Tween molecules was established. The limiting area occupied by the Tween molecule was calculated by applying the Joos equation. The area determined in such a way was confirmed by the calculations of cross section of Tween molecules based on the bond lengths and the angles between them as well as the average distance between the molecules, taking into account their different conformations. This area was used for calculation of the standard Gibbs energy of adsorption using the Langmuir equation. The standard Gibbs energy of Tweens adsorption at the water–air interface was also calculated from the hydrophobic part of Tween molecule–water interface tension and that of hydrophobic part. Using the determined values of standard Gibbs energy of adsorption at different temperatures, the standard enthalpy and entropy values were deduced. The standard thermodynamic functions of micellization were also determined and compared to the Gibbs energy of Tween molecules interactions through the water phase.     

Mutual interferences of elements in the atomic-absorption analysis of human hairs

Mutual interferences of elements in the atomic-absorption analysis of solutions of human hairs were estimated using mathematical procedures of experimental design. The effects of Na, Cl, Ca, Fe, Zn, and Pb, whose concentrations in hairs vary within a wide concentration range, on the results of the determination of Cu, Cd, K, Fe, Zn, and Pb were examined. An interpretation of the obtained models was given. Two methods for taking into account mutual interferences were proposed and approved: (1) the use of multielement reference samples whose chemical compositions are prescribed based on the proposed models and (2) the representation of calibration curves in the form of multidimensional polynomials     

Dirt in cane removal influenced by soil characteristics

Dirt level in sugar cane consignments delivered to the factory is dependent on soil type, in association with harvesting system and weather conditions. Efforts for reducing soil in harvested cane have been made by sugar cane millers, especially improving the washing system installed before crushing. Instrumental neutron activation analysis has shown its potential for assessing dirt reductions in the washed material. Knowledge of elemental concentrations in the different soil fractions highlights the reliability of such measurements especially when taking into account the soil characteristics.     

The Development of the Theory of Strong Electrolyte Solutions

The expression for Gibbs energy of electrolyte solutions has been derived taking into account the contribution of interactions of the hydrated shells of ions. It is in good agreement with data for the dependence of the hydration number on concentration. The contribution of different interactions to the activity coefficient depending on concentration has been analyzed. The possibility of determining parameters which characterize the composition of hydrated shells of individual atoms from experimental data has been considered.     

Evaluation of Ion Concentrations in Laser Mass-Spectrometric Analysis with Photographic Recording

Methods for evaluating concentrations in analysis with the use of an EMAL-2 laser energy–mass analyzer are described. Analytical signals are calculated using the integrated intensity of spectral lines. The characteristic curve of a photographic emulsion is approximated by the function . The possibility of taking into account doubly and triply charged ions in the evaluation of concentrations is analyzed. It is shown that taking into account multiply charged ions reduces the relative root-mean-square deviation from 0.18 to 0.03.