Voprosy khimii i khimicheskoi tekhnologii (Issues of Chemistry and Chemical Technology)

23-29

Authors: V.I. Goleus

Density, molar volume and thermal expansion of borate and borosilicate melts are properties that significantly affect the process of forming composite coatings by slip-firing technology. Based on experimental data on the density of glass-forming melts, adequate generalizing mathematical models have been developed by using the method of multiple correlation; these models describe the dependence of the molar volume (VT) of alkaline borate, alkaline silicate and multicomponent melts on their oxide composition and temperature with a standard deviation of (0.22–0.27) cm3 mol–1. The regression equation has the following general form: VT=A(xi)–B(xi)/T, where xi is the content of an oxide, T is the temperature, subscript "i" stands for the number of an oxide concerned. The changes in volumetric coefficient of thermal expansion (CTE) of the molten glasses are analyzed as functions of both temperature and chemical composition. It should be noted that borate melts in comparison with silicate ones have much higher values of CTE. The highest value of CTE of experimental melts is observed at glass transition temperatures (Tg). An increase in the temperature above Tg contributes to a significant decrease in CTE. An increase in the content of alkali metal oxides in most glass-forming melts results in an increase in the values of CTE.

39-48

Authors: E.S. Duvanova, A.V. Popova, A.V. Rysich, S.V. Radio, G.M. Rozantsev

The method of pH-potentiometric titration and mathematical simulation were used to study the equilibrium processes in aqueous solutions of the WO42––CH3COOH–H2O system in the acidity range Z=(CH3COOH)/(Na2WO4)=0.8–1.7 at СW=0.01 mol L–1 and T=2980.1 K, a constant ionic strength being maintained by sodium nitrate as a background electrolyte ((NaNO3)=0.10 mol L–1). We developed the models of polyoxotungstate anions formation and the equilibrium transformation processes, which adequately describe experimental pH vs. Z dependences. It was found that acetic acid using to create the solution acidity that is necessary for the formation of isopoly tungstate anion contributes only to the formation of protonated paratungstate B anions Нх[W12O40(ОН)2](10–х)– (where x=0–4). We calculated the logarithms of the concentration equilibrium constants of the polyanion formation and plotted the distribution diagrams. Double sodium-manganese(II) paratungstate B Na8(H2O)28Mn(H2O)2[H2W12O42]4H2O was synthesized at Z=1.00 to confirm the results of the mathematical modeling. The chemical composition of the prepared salt was established by chemical elemental analysis, thermal analysis, FTIR spectroscopy, and single crystal X-ray analysis. The stepwise process of salt dehydration was studied by means of differential thermal analysis.

56-64

Authors: V.A. Litvin, R.A. Njoh

A simple, fast and effective method for producing synthetic substances with properties similar to natural humic substances has been proposed. The synthesis method is based on the oxidation of quercetin by molecular oxygen in an alkaline medium, followed by conversion to the acid form by passing through a cation exchange column. Study of elemental and functional compositions, spectral properties (UV/Vis and IR range) and redox characteristics allowed qualifying the resulting product as a synthetic fulvic acid. The enhanced antibacterial properties of the obtained synthetic product were established. The minimum concentration of inhibition of synthetic fulvic acid derived from quercetin is 25 g mL–1, which is in 100 times less than for natural humic substances.

73-80

Authors: B. Minaev, О. Aksimentyeva, O. Lut, O. Shevchenko, O. Pogrebniak

We studied the electrochemical process of salicylic acid oxidation in acidic solutions on the nanostructured anodes prepared on the basis of nickel with additionally precipitated platinum. The investigation was carried out by means of linear and cyclic voltammetry methods in combination with quantum chemical calculations. The results of voltammetry study showed that clearly expressed current maxima were observed in the range of 0.43 V to 1.15 V in the anode sections of the voltammograms, in contrast to the background electrolyte. The oxidation potentials of the depolarizer almost coincided with those typical of a smooth platinum electrode, but the current density was 1.8 times higher on the electrode under study. Based on the analysis of voltammetric curves, the effective rate constants of heterogeneous charge transfer, activation energy and diffusion coefficient were calculated. The calculation of the thermodynamic parameters of the intermediates of salicylic acid electrooxidation showed that the heat of formation of 2,3-dihydroxybenzoic acid is higher than that of 2,5-dihydroxybenzoic acid, which indicates a higher stability of the latter. Analysis of the calculated thermodynamic characteristics of each stage of salicylic acid electrochemical oxidation showed that the process is endothermic, occurs with an increase in entropy and the Gibbs energy decreases. The rate of the overall process is determined by the rate of the first stage.

90-94

Authors: Yu.P. Pavlovskii, S.I. Gerasimchuk, V.V. Sergeev, G.V. Melnyk, V.B. Vostres

We developed a method for determination of the crystallization enthalpy of organic substances in a heat-conducting calorimeter at a temperature close to 298 K. Crystallization was conducted in the MID-200 calorimeter using an ampoule technique. The volume of ampoules, which contain the supercooled liquids, was approximately 0.2 cm3. The residual pressure was 650 Pa to lessen the heat loss. A few crystals of the material under investigation were employed as the crystallization centers. Based on the results of the experiment, the crystallization enthalpies of dicumyl peroxide, peroxyoctanoic acid and di-tert-butylperoxy isophthalate were determined. The vaporization and sublimation enthalpies of peroxyoctanoic acid were assessed using an ampoule method in the calorimeter. It was established that dicumyl peroxide which was supercooled 14 K below the fusion temperature exhibited a 20% increase in crystallization enthalpy as compared with that calculated from the difference between vaporization and sublimation enthalpy. At the same time, the crystallization enthalpy of peroxyoctanoic acid supercooled by 5 K was equal to that calculated from the difference between vaporization and sublimation enthalpy within the limiting experimental error. The received data show that the proposed method is promising for determining the heat of crystallization of liquids that are able to exist in a supercooled state during some time.

104-109

Authors: O.S. Pogrebnyak, B.P. Minaev, O.P. Shevchenko, O.A. Lut

International Association for Cancer Research included bromates in the group of carcinogenic chemicals. Therefore, it is important to determine the content of bromates in different objects. The purpose of this study is to develop a simple, express and sensitive method for the determination of bromates. We propose the method of spectrophotometric determination of bromate in the concentration range of 0.5 to 45 mg L–1, this technique being based on the reaction of bromate with 2-bromo-N,N-dimethylaniline in the presence of an excess of KBr in the medium of sulfuric acid, followed by photometry of 2,4-dibromo-N,N-dimethylaniline at 240 nm. The optimal concentrations of reagents are determined and the influence of some other conditions on the sensitivity of determination is established. The detection limit of bromate (in compliance with 3s-criterion) is 0.39 mg L–1. The relative error of determination does not exceed 0.10. The influence of foreign ions is studied when determining bromate (5.010–5 mol L–1). Such components of natural water as phosphates, fluorides, chlorides and NH4+ ions have no effect on the bromide determination. Fe3+ ions in comparable quantities impede the determination of bromates. The developed method was tested for the determination of bromate in standard solutions and waters of different categories. The used reagents are stable over time and the proposed method is simple and suitable as an alternative for the determination of bromates in different objects.

117-125

Authors: A.V. Redkina, N.V. Kravchenko, N.D. Konovalova, V.V. Strelko

In order to create alternative vanadium oxide-supported catalysts for the process of non-oxidative propane dehydrogenation to propylene, we studied the effect of the increased content of vanadium oxide in the V2O5–ZrO2–SiO2 composition on its structure and catalytic properties. Zirconium silicate hydrogel in the form of finished spherical granules with the SiO2 content of more than 50% was prepared by direct sol-gel synthesis from zirconium oxychloride and sodium metasilicate using the droplet coagulation technology. Catalysts were fabricated by impregnation of hydrogel with an aqueous solution of vanadyl sulfate salt, hydrothermal treatment and calcination in air. By using scanning electron microscopy, X-ray diffraction analysis and low-temperature nitrogen adsorption/desorption method, we showed that amorphous samples with a developed mesoporous structure (with the pore diameter of ~6 nm and the specific surface area of ~300 m2 g–1) were formed when the content of the supporting V2O5 on zirconium silicate was 10, 20, 25, and 30 wt.%. In the course of temperature increase in the propane dehydrogenation reaction, the catalyst samples crystallized in the reaction mixture propane–inert gas with the formation of tetragonal zirconia. When the content of V2O5 was 25% or 30%, additional phases of reduced vanadium oxides and traces of the zirconium vanadate phase were formed. After the reaction, the specific surface area of the catalysts decreased significantly and the average pore size of the samples with 25% and 30% V2O5 increased to ~30 nm. The propylene yield reproducibly observed on the samples with 25% and 30% V2O5 was lower than that on the samples with 10% V2O5; however, it remained quite high, which was probably due to the expanded diameter of the pores and the appearance of additional ZrV2O7 sites that are active with respect to the dehydrogenation of light alkane.

132-138

Authors: Yu.V. Tamarkina, V.O. Kucherenko, O.O. Velychko

The purpose of this work was to evaluate the influence of alkaline activation temperature (t) on the pore structure of activated carbons produced from brown coal (ACs) and their adsorption capacity towards the sorption of phenol from aqueous solutions. The ACs were prepared by heating (t=400–8500C, 1 hr) of brown coal impregnated with KOH; the prepared samples were designated as AC(t). Based on the nitrogen adsorption-desorption isotherms (77 K), the total volume and surface (S) of all pores and separately micro- and subnanopores were determined. Maximum phenol adsorption capacities of ACs (Am) were registered at the temperature of 250С. The activation temperature was stated to be the most significant factor influencing the ACs capacity to adsorb phenol. The growth of temperature from 4000С to 8500С in the case of AC(800) results in an exponential increase in the value of Аm from 27 mg g–1 to 240 mg g–1 (a maximal value) according to the following equation: Аm=6.038exp(0.0045t) (R2=0.952). The kinetics of phenol adsorption obeys the second order model (R20.982). The AC(800) adsorption isotherm is described by the Freundlich equation (R2=0.988) within the equilibrium concentrations Ce=0.001–2.5 mg cm–3 with the heterogeneity factor nF=3.23 indicating physical adsorption. The same model for Ce0.001 mg cm–3 (R2=0.951) gives nF=0.52, which suggests chemical adsorption. The specific adsorptive capacity АS=Аm/S sharply decreases from 2.11 mg m–2 to 0.21 mg m–2 with increasing the temperature from 4000С to 5500С and remains constant (0.200.01 mg m–2) for ACs prepared at higher temperatures. The invariability of AS indicates the temperature independence of adsorption centers concentrations of ACs prepared within 550–8500С.