Specific features of the dynamic mode of differential dissolution as of a method of phase analysis

The author considers the specific features of the separation properties of the stoichiographic method of differential dissolution (DD) and the role of thermodynamic and kinetic factors in the processes of the separation of solid mixtures. It is shown that the separation properties of DD should be characterized by selectivity and efficiency, as it is accepted in chromatography. The separation possibilities of DD are considered in characterizing uniformity, stoichiometry-nonstoichiometry of the elemental composition of phases, and also of their macro- and microstructures. The comparison of the principles and characteristics of DD, voltammetry, titrimetry, mass spectroscopy, and chromatography has led to the conclusion that the place of DD among the other methods is determined, primarily, by its unique property, namely, its standardless nature.     

Analytical Chemistry of Phases of Variable Composition and the Principles of Stoichiographic Determination of Such Phases in Multielement Multiphase Samples

The general problems of determining the stoichiometry and content of solid phases of variable composition in multielement multiphase substances are discussed. The possibilities and conditions of application of a reference-free stoichiographic method based on the differential dissolution (DD) of components for the analysis of samples containing spatially homogeneous and inhomogeneous phases are considered. The results of mathematical modeling of the DD process for a mixture of phases of constant and variable composition are presented. These data substantiate the use of the stoichiogram affinity criterion for the identification of spatially inhomogeneous phases of variable composition. Examples of applying the DD method to the phase analysis of various substances containing phases of variable composition are given.     

Determination of the chemical composition of supported vanadium-containing oxide catalysts by the differential dissolution method

The results of the application of the stoichiographic method of differential dissolution (DD) in the determination of the chemical composition of vanadium-containing catalysts are presented. In the studied catalyst series, amounts of vanadium were deposited onto TiO₂, SiO₂, Al₂O₃, ZrO₂, and Nb₂O₅. The catalysts were prepared by the impregnation method or by the spray drying method and thermally treated at different temperatures. The DD method was used for the precise correction of the phase composition of the V₂O₅/TiO₂ catalyst samples in order to determine the nature of the active component of these catalysts and obtain the correct information on their structure using the NMR method.     

Determination of the chemical composition of modified fiber glass silicate materials by differential dissolution

The paper presents the results of application of the stoichiographic method of differential dissolution (DD) to the analysis of modified aluminosilicate and zirconium silicate fiberglass materials (FGM) and related catalysts. Na-Al-Si and Na-Zr-Si FGM have been studied after sodium leaching-out and subsequent modification with Al, Co, and Pt additives. The conditions of detection, identification, and determination of different forms of FGM modifiers and the analysis of the surface of these samples are provided. The DD method has been used for the precise correction of the composition of the fiber glass sample surface.     

Calculations and interpretation of the results of stoichiographic analysis of solid multi-element multi-phase compounds and materials

Methods and approaches to the calculations and interpretation of the results of stoichiographic analysis of solid multi-element multi-phase compounds and materials using the flow mode of the differential dissolution (DD) method are reported. Special features of calculationls as well as the objective reasons for the necessity of obtaining more or less precise results of stoichiographic calculations are discussed. The details of the procedure of stoichiographic calculations, refinement, and interpretation of the results are considered on an example of the DD-analysis of the Co-Ni-Fe-Bi-K-P-Mo-O/SiO₂-catalyst for the selective oxidation and oxidative ammonolysis of hydrocarbons. The specific details and methods used for the estimation of the performance characteristics of the DD analytical procedure are discussed.     

Methodology and procedure of the stoichiographic analysis of solid inorganic substances and materials

The technical and experimental aspects of creating the conditions of differential dissolution in a flow stoichiographic system are discussed; the scheme of the experimental apparatus, stoichiograph, is presented. The construction, operation conditions of the main units of the stoichiograph, and sample treatment issues are considered. The principles of the creation and optimization of the conditions of the dynamic differential dissolution for the analysis of compounds and materials of the known and unknown phase composition are discussed: the composition of solvents and temperature, and the principles of their variation in time, including those in the processes of stoichiographic titration.     

Stoichiographic Methods in the Analysis of Substances of Unknown Composition

A hierarchic structure of concepts of the chemical composition of substances, including substances of unknown composition, was proposed on the basis of physicochemical criteria. The latest achievements in the theory and practice of standardless stoichiographic methods for determining the molecular composition of substances (differentiating dissolution and ion chromatography) were considered.     

Stoichiographic determination of the composition and concentration of phases encapsulated in the bulk of a solid matrix phase

A two-step method of stoichiographic calculations used in the determination of the formulas of phases encapsulated in the bulk of matrices of another composition is proposed. At the first stage of calculations, the stoichiometric composition of the encapsulated phase is determined and, at the second step, the concentrations of this phase and the matrix are found. The use of this method in the analysis of oxide La–Cu catalysts by the method of differential dissolution ensures the detection of the phase La₁Cu₁ (La₂Cu₂O₅), incorporated within the matrix La₁Cu_0.5 (La₂CuO₄) phase.     

Stoichiography for investigation of the functional materials composition,structure, and properties

Principles of the stoichiography and differential dissolution method (DD) providing deep insight into the composition of mixtures containing unknown chemical compounds are considered. Compounds can be identified using their primary feature — the stoichiometry of elemental composition; reference samples of the compounds are not needed. The DD method makes it possible to analyze mixtures of crystalline and/or amorphous phases with constant and/or variable composition in the form of dispersed powders, ceramics, crystals, thin films and nanosize objects. Various aspects of the stoichiography and DD method application to investigation of composition, structure and properties of the functional materials are discussed.     

Stoichiography and fanlike separation of substances in dynamic mass-transfer processes

For the first time, the concept of fanlike separation of substances in dynamic processes of homo- and heterophase mass transfer was introduced, which enables one to quantitatively determine the composi tions of mixtures of chemical compounds by stoichiographic calculations without selective separation of the mixtures.     

A simple and rapid method for measuring dissolved oxygen in waters with gold microelectrode

Dissolved oxygen (DO) is an essential indicator in chemical, biological and biochemical processes and needs to be rapidly measured in many cases. In this work, a rapid, sensitive and simple electrochemical method, first derivative linear sweep voltammetry (FD-LSV), was established for the determination of DO. The peak height of FD-LSV was found to be influenced by scan rate and microelectrode surface area. An empirical formula was proposed to describe the relationship between the FD-LSV peak height and the influencing factors. Compared with other electrochemical methods for DO determination, the FD-LSV method needed lesser time and was more accurate. This method was successfully applied for the determination of the DO levels of various environmental water samples.     

Optimization and validation of an automated voltammetric stripping technique for ultratrace metal analysis

A new automated batch method for the determination of ultratrace metals (nanogram per liter level) was developed and validated. Instrumental and chemical parameters affecting the performance of the method were carefully assessed and optimized. A wide range of voltammetric methods under different chemical conditions were tested. Cadmium, lead and copper were determined by anodic stripping voltammetry (ASV), while nickel, cobalt, rhodium and uranium by adsorptive cathodic stripping voltammetry (AdCSV). The figures of merit of all of these methods were determined: very good precision and accuracy were achieved, e.g. relative percentage standard deviation in the 4-13% for ASV and 2-5% for AdCSV.The stripping methods were applied to the determination of cadmium, lead, copper, nickel, cobalt, rhodium and uranium in lake water samples and the results were found to be comparable with ICP-MS data.     

Adsorptive cathodic stripping voltammetry determination of ultra trace levels of cobalt

The complexation of Co(II) with 4-(2-pyridylazo)resorcinol (PAR) was studied by spectroscopy and voltammetry. In addition, an adsorptive stripping voltammetric method was developed for the determination of ultra trace levels of Co(II) using a hanging mercury drop electrode. The method is based on accumulation of Co(II) on a mercury electrode using PAR as the complexing agent. The effects of instrumental and chemical parameters on the sensitivity of the method were investigated. The detection limit of the method is 0.003 ng/mL. Most foreign species do not interfere with the determination. The high sensitivity and selectivity of this method were demonstrated by determination of cobalt in blood and water samples.     

Synthesis of Silver Nanoparticle‐Graphene Composites for Electroanalysis Applications using Chemical and Electrochemical Methods

An electrochemical device was developed for the simultaneous determination of sulfamethoxazole (SMX) and trimethoprim (TMP) using differential pulse voltammetry and glassy carbon (GC) electrodes modified with reduced graphene oxide (rGO) and silver nanoparticle (AgNP) composites, synthesised using both chemical and electrochemical methods. The morphology and electrochemical behaviour of the GC electrodes modified with the rGO/AgNP (chemical method) and rGO‐AgNP (electrochemical method) composites were characterised by scanning electron microscopy and cyclic voltammetry. These techniques demonstrated that, in both methods, the graphene oxide was modified by the AgNPs, and the composite synthesised by the electrochemical method showed a better dispersion of the nanoparticles, resulting in an increase in the surface area compared to the rGO/AgNP composite. The GC/rGO‐AgNP electrode was evaluated and optimised for the simultaneous determination of SMX and TMP, achieving detection limits of 0.6 μmol L⁻¹ for the SMX and 0.4 μmol L⁻¹ for the TMP. The proposed GC/rGO‐AgNP electrochemical device was successfully applied to the simultaneous determination of SMX and TMP in wastewaters samples.     

Electrochemical deposition and corrosion stability of Zn–Co alloys

Electrochemically deposited Zn–Co alloys under various deposition conditions were investigated using anodic linear sweep voltammetry for phase structure determination, scanning electron microscopy for surface morphology analysis, atomic absorption spectroscopy for determination of chemical composition, and polarization measurements and open circuit potential measurements for determination of corrosion properties. The influence of deposition current density, temperature, and composition of deposition solution on the phase structure and corrosion properties of Zn–Co alloys was studied. It was shown that the ratio of cobalt to zinc ions in the plating bath strongly affects the chemical content and phase structure, as well as corrosion stability, of Zn–Co alloys. Zn–Co alloys deposited from plating baths with the lowest and the highest ratios of cobalt and zinc ions exhibited the lowest corrosion rate.     

CuO Nanostructures Based Electrochemical Sensor for Simultaneous Determination of Hydroquinone and Ascorbic Acid

A simple, sensitive and reliable electrochemical sensor has been developed based on CuO nanostructures modified glassy carbon electrode for simultaneous determination of hydroquinone (HQ) and ascorbic acid (AA). The CuO nano material was synthesized by aqueous chemical growth method using different sources of OH. The characterization of nano material was performed by Fourier transform infrared spectroscopy, X‐ray diffraction, field emission scanning electron microscopy and energy dispersive X‐ray spectroscopy. The glassy carbon electrode was modified by CuO nano material using drop cast method and studied by cyclic voltammetry. The CuO/GCE exhibited excellent electrocatalytic activity towards the oxidations of HQ and AA in borate buffer solution (pH 8.0) and the corresponding electrochemical signals have appeared as two well resolved oxidation peaks with significant peak potential differences of (0.21V vs. Ag/AgCl). Differential pulse voltammetry was used for simultaneous determination of HQ and AA using the CuO/GCE. At the optimum conditions, for simultaneous determination by synchronous change of the analyte concentrations, the linear response ranges were between 0.0003–0.355 mM for HQ and 0.0001–0.30 mM for AA respectively. Furthermore, CuO/GCE was successfully applied for the independent determination of AA in fruit juices as well as for the simultaneous determination of HQ and AA in cosmetic samples.     

光纤折射率传感用于壳聚糖脱乙酰度测定

建立了一种基于光纤折射率传感技术的壳聚糖脱乙酰度测定方法. 利用光纤折射率传感器监测酸碱滴定过程中溶液折射率的变化, 根据折射率变化转折点之间碱的用量来计算壳聚糖的脱乙酰度. 该方法测得的3种不同含量实际样品的脱乙酰度与氢核磁共振波谱(¹H NMR)方法测定结果相符, 验证了方法的可靠性. 该方法具有用量少、 结构简单、 准确、 重复性好和转折点明显等优点, 可应用于工业生产中壳聚糖脱乙酰度的测定.     

The degree of deacetylation of chitosan: advocating the first derivative UV-spectrophotometry method of determination

The degree of deacetylation (DD) is increasingly becoming an important property for chitosan, as it determines how the biopolymer can be applied. Therefore, a simple, rapid and reliable method of determining the DD for chitosan is essential. In this report, the DD of chitosan was determined by nuclear magnetic resonance (NMR), linear potentiometric titration (LPT), ninhydrin test and first derivative UV-spectrophotometry (1DUVS). The DD was calculated on a per mol basis instead of on a per mass basis. This is important as the molecular weights of N-acetyl-d-glucosamine and d-glucosamine are different. By converting the mass of N-acetyl-d-glucosamine and d-glucosamine into mols and calculating for the percentage of d-glucosamine present in the chitosan sample, a more accurate estimation of the DD can be obtained. Of the four methods, there is good correlation between 1DUVS and NMR. The concentration of chitosan solution for 1DUVS analysis was standardised as 0.1000 mg chitosan per ml of 0.0100 M acetic acid solution. The presence of d-glucosamine was corrected for by a reference curve for N-acetyl-d-glucosamine. 1DUVS is easy to perform, sensitive and the interference of other contaminants to the results is minimal compared with the other three methods. Therefore, we advocate 1DUVS to be used as the standard methods for routine determination of DD of chitosan.     

Direct measurement of dynamic frequency shift induced by cross-correlations in 15N-enriched proteins.

We describe a new NMR experimental scheme that allows the direct determination of the dynamic frequency shift induced by chemical shift anisotropy/dipolar interaction (CSA/DD) cross-correlations in 15N-enriched proteins. Its principle consists of comparing two rates of polarisation transfer between the amide proton and nitrogen. The first rate, which is independent of the dynamic frequency shift, is based on a selective Hartmann-Hahn coherence transfer. The second rate, which depends on the dynamic frequency shift, is based on a free evolution of the transverse magnetisation. We report experimental validation of this approach by measuring the average dynamic frequency shift due to CSA/DD cross-correlations in the calcium-binding protein D9k. The method may also be applicable to the measurement of dynamic frequency shift induced by cross-correlations between the Curie spin and dipolar interactions.     

Characterization of a carbon paste electrode modified with tripolyphosphate-modified kaolinite clay for the detection of lead

We report about the use of carbon paste electrode modified with kaolinite for analytical detection of trace lead(II) in domestic water by differential pulse voltammetry. Kaolinite clay was modified with tripolyphosphate (TPP) by impregnation method. The results show that TPP in kaolinite clay plays an important role in the accumulation process of Pb(II) on the modified electrode surface. The electroanalytical procedure for determination of Pb(II) comprised two steps: chemical accumulation of the analyte under open-circuit conditions, followed by electrochemical detection of the pre-concentrated species using differential pulse voltammetry. The analytical performance of this system has been explored by studying the effects of preconcentration time, carbon paste composition, pH, supporting electrolyte concentration, as well as interferences due to other ions. The calculated detection limit based on the variability of a blank solution (3s_b criterion) for 10 measurements was 8.4 × 10⁻⁸ mol L⁻¹, and the sensitivity determined from the slope of the calibration graph was 0.910 mol L⁻¹. The reproducibility (RSD) for five replicate measurements at 1.0 mg L⁻¹ lead level was 1.6%. The results indicate that this electrode is sensitive and effective for the determination of Pb²⁺.