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Vol 13, No 1 (2018)
View or download the full issue PDF (Russian)
https://doi.org/10.32362/2410-6593-2018-13-1

ANALYTICAL METHODS IN CHEMISTRY AND CHEMICAL TECHNOLOGY

5-21 413
Abstract
Chemical sensors are one of the most demanded tools of modern analytical chemistry. Recently, devices based on the registration of color changes upon reflecting visible irradiation from the surface of so-called "photonic crystals" (PC) have begun to be used for analytical chemistry purposes. Some advantages of this method are the possibility of visual detection of substances, relatively high sensitivity, and the ability to change the properties of such sensors by varying the element base of the PC. The effect of various mechanical, electrical, optical, chemical and other factors on the objects under study leads to additional changes in the spectral responses from the PC surface with deposited materials. A sufficiently short response time allows the use of such sensors for the operational control of various substances with a high degree of hazard. In the long term, such devices can be used as test systems for the detection and analysis of a wide class of chemical and biological substances. This review is devoted to various types of sensors based on photonic crystals. It deals with: photonic crystals of natural and synthetic origin; various possible structures of PC; causes of the appearance of characteristic optical properties; detection of mechanical, thermal, electrical, magnetic and optical effects on the PC, as well as effects on organic compounds of various classes; areas of application of sensors based on PC.
22-32 166
Abstract
A method for fabricating a copper microdisk electrode of an original design based on 50 μm diameter wire sealed in borosilicate glass is described. The electrochemical properties of the copper microelectrode were studied by the method of steady-state voltammetry in a 2 M NaOH solution in the potential range from -1.1 to 0.8 V (versus saturated Ag/AgCl-electrode). In order to improve the electrochemical response a method for two-stage electrode activation based on a copper dissolution / redeposition procedure followed by polarization in an alkaline medium is suggested. Morphological and physico-chemical changes on copper surface after activation were examined by atomic force microscopy and X-ray photoelectron spectroscopy. After this procedure, the electrode showed a heterogeneous morphology with coarse texture and high roughness parameters, and a layer of catalytically active Cu(III) species was formed on copper surface. The best results were achieved with an activation time of 60 s and a polarization potential of -0.3 V. The effectiveness of the activation procedure was tested during the chronoamperometric determination of methanol, ethanol and ethylene glycol. Factors affecting the formation of the analytical signal of alcohols were studied, and optimal conditions of amperometric measurements were selected on their basis. Under optimal conditions, the metrological characteristics of the method were determined. The peak current response increases linearly with alcohols concentration over the range 0.01 - 0.45 M (0.04 - 3% v/v). The repeatability of the electrode response was evaluated as 3.8% (n = 10). The activated copper microelectrode was used for the determination of ethanol in pharmaceutical and other products.

THEORETICAL BASES OF CHEMICAL TECHNOLOGY

33-44 141
Abstract
To construct a generalized dependency, a scale for the unknown quantities and variables must be selected. The states of points are located in P-V-T (pressure-volume-temperature) space. The scale for the construction of generalized dependences of the studied properties and variables of the problem must be sustainable. In order to find a sustainable transition from liquid to vapor the behavior of the characteristic function of free energy is investigated. Since the phase transition occurs at a constant temperature, free energy is equal to expansion work. In the analysis of the liquid-vapor transition, the curve of the temperature dependence of expansion work for all investigated substances has a maximum. The temperature corresponding to the maximum expansion work is denoted by Tm. It was noted that temperature Tm associated with Tc (critical) with the simple relation Tm =0.76Tс with a spread of 2%. Naturally, this state corresponds to the free energy minimum value, and in accordance with the principle of minimality of characteristic functions of this process is stable. Therefore, the parameters of this process were chosen as the bringing scale in the construction of dimensionless dependencies. In this paper we use the method of constructing generalized dependencies in the reduced form, based on the characteristic functions minimality principle. Approximating formulas were obtained for calculating reduced heat of evaporation from reduced density, entropy, and freons surface tension. The reduction scale is considered on the liquid and vapor saturation line under substances consideration. The characteristic functions minimality principle is used. In the course of the analysis, calculation formulas were derived for pure freons both individually and in a combined form. The interrelation between the differences of these thermodynamic properties on the saturation line during a liquid-vapor phase transition is shown. This makes it possible to determine some properties using other methods by calculation.
45-54 160
Abstract
In the present work, an analysis of the mutual arrangement of a number of iso-manifolds in the concentration tetrahedron formed by the components of the separated mixture and an additional substance, an extractive agent, was carried out to estimate the possibility of separating three-component mixtures containing biazeotropic constituents by extractive rectification. The objects of research were four-component systems formed by ternary constituents (butyl propionate (BP) - propionic acid (PA) - butyl butyrate (BB), butyl propionate - propionic acid - butyric acid (BA), butyl butyrate - butyric acid -butyl propionate, butyl butyrate - butyric acid -propionic acid) of the commercially important BP-PA-BB-BA system, and the extractive agent sulfolane. Using the results of the computational experiment based on mathematical model NRTL-HOC we obtained complete data on the vapor-liquid equilibrium in binary, three- and four-component systems. The structures of liquid-vapor phase diagrams were obtained, and thermodynamic-topological analysis of all four-component systems was carried out. The mutual arrangement in the concentration tetrahedron of manifolds of relative volatility (equal to 1) of the components, reflecting the evolution of pseudo-azeotropes in sections with a constant concentration of the extractive agent; pseudo-ideal manifolds along which the activity coefficients of the components of the base (separated) mixture are equal to each other; isothermo-isobaric manifolds generated by Bancroft points in binary azeotropic constituents were determined. We established that it is possible to separate the studied three-component mixtures by extractive rectification using sulfolane.
55-65 150
Abstract
The prospects of using heat pumps for the separation of mixtures by combination of different mass transfer processes are considered, which is associated with the supply and removal of thermal energy. The heat released in some stages can be successfully used at other stages of separation. Using heat pumps makes it possible to change the temperature potentials of heat fluxes, which greatly improves the efficiency of regenerative heat transfer. If a vapor phase forms when carrying out mass transfer processes, it can be used as the working fluid of the heat pump of the open type. If mass transfer processes are carried out without the formation of vapor phases, it is necessary to apply heat pumps of the closed type, in the circuit of which a variety of intermediate heat transfer fluids circulate. The article briefly describes the features of mixtures separation by a combination of fractional crystallization by evaporation, dissolution, fractional melting, distillation and rectification with the use of compressor heat pumps of open and closed type. In order to assess the energy efficiency of different variants of combined processes and to compare them with traditional separation it is suggested to use the relative equivalent consumption, which makes it possible to take into account the different cost of thermal and electric energy required for carrying out the considered process of separation. Different variants of organizing the combined separation are compared. It was found that their energy efficiency significantly depends on the composition of the initial mixture, on requirements for the separation products, as well as on the position of the eutectic point on the liquid - solid phase charts. It was shown that the use of heat pumps allows reducing several times energy costs, as well as the consumption of refrigerants for combined separation.
66-74 139
Abstract
Development and improvement of industrial technologies based on the principle of combining reaction and mass exchange processes is an urgent task. This is due to their advantage over the traditional sequential way of performing the chemical transformation and separation of the resulting reaction mixture. In reaction-rectification processes, the conversion of the process, the rate of reaction and selectivity may be increased due to continuous withdrawal of the formed products from the reaction zone. In addition, capital and energy costs in such processes are significantly reduced due to the reduction or even complete absence of external recycling in them. The modern method of developing reaction-rectifying processes is the analysis of statics, which allows isolating the limiting stationary states corresponding to the maximum yield of the target product. An essential disadvantage limiting the possibility of using this method for solving practical problems is the consideration of a single chemical reaction in its framework. At the same time, when passing to real processes, as a rule, this restriction is violated. The article offers a number of original approaches designed in the form of an algorithm that allows the analysis of statics to be extended to reaction-rectification processes with an unlimited number of components forming the reaction mixture and an unlimited number of chemical reactions taking place between them. On the basis of this algorithm, a ChIM program was developed in the SciLab environment, which makes it possible to single out sets of limiting stationary states of a combined process characterized by maximum reagent conversion values, selectivity and yield of the target product. The procedure for using the ChIM program is illustrated by the example of the industrial process for obtaining mesityl oxide from acetone. Calculations conducted using the software package Aspen Plus proved the possibility of practical implementation of limiting stationary state providing the maximum yield of mesityl oxide, which was predicted by using of ChIM.

SYNTHESIS AND PROCESSING OF POLYMERS AND POLYMERIC COMPOSITES

75-92 173
Abstract
The temperature dependences of conductivity at alternating current (a frequency of 1 kHz) were studied for thin polychloroprene films formed from a solution on metal electrodes having different polarities: anode, cathode and zero potential. It was found that the nature and form of the temperature dependence of the conductivity of the investigated polymer film depends on the method of sample formation. Temperature regions of maximum conductivity were detected. The realization of the heating cycle followed by the film cooling in a limited region between the electrodes under the action of an alternating electric field leads to an increase in the specific conductivity and a shift of the extreme values to the region of lower temperatures. This effect is manifested to the greatest extent for films formed at the cathode. The process of heating and subsequent cooling is of hysteresis nature both for permittivity and for the dielectric loss tangent. The dependence of dielectric loss tangent on permittivity in a rather wide temperature range is inversely proportional. During the analysis of temperature dependences the mechanism of conductivity was studied for polychloroprene thin films at direct and alternating current. The determined activation energies of the polymeric system conductivity change enable concluding that the mechanisms of electric conductivity at direct and alternating current are of similar nature. The extreme values of permittivity may be due to structural alterations in the process of heating. This is confirmed by the data on the temperature dependence of dielectric loss tangent. In order to explain conductivity at direct current a model of dipole traps is offered.


ISSN 2410-6593 (Print)
ISSN 2686-7575 (Online)