The study of the electrochemical oxidation mechanism of diphenylamine derivatives in dipolar aprotic solvents: III. Effect of acids and bases of different strengths

The effect of addition of different acids and bases in a wide range of concentration on the voltammetric oxidation of both Variamine blue (VB) and 4-aminodiphenylamine (4-ADPA) was examined in acetonitrile (AcN). The oxidation potential values of either wave of both depolarizers were found to depend on the acid or base strength of the additive—relative to the depolarizer and solvent—and on their concentration in solution. Addition of strong acid as CH₃SO₃H resulted in a single oxidation wave at potentials either equal to or more positive than that of the original second wave. Relatively weak acids as picric and pyridinium ion only shifted the oxidation potential of the first wave to more positive values without affecting the general oxidation pattern.On the other hand, strong bases like ethanolamine, resulted in the formation of a preoxidation wave at more negative potential when present in low concentration and a single wave at identical negative potential at high enough concentrations. Bases of comparable strengths, as that of the depolarizer, like pyridine do not form prewaves but only shifted the final single oxidation wave to more negative potential values. Weaker bases like water and ethanol mainly shifted the oxidation potential of the second wave to less positive values.     

Chronology and characteristics of groundwater along the United Arab Emirates-Oman arid region: a guide for regional sustainability

Journal of Radioanalytical and Nuclear Chemistry - Estimating groundwater recharge rate is essential for water sustainability of the arid region of the United Arab Emirates that receives most...     

Open problems in some competition models

We present open problems and conjectures for some two-dimensional competition models, namely the logistic competition model and a Ricker-type competition model.     

The \bar \partial -problem on q-pseudoconvex domains with applications

For a q-pseudoconvex domain Ω in ? ⁿ , 1 ≤ q ≤ n, with Lipschitz boundary, we solve the $\bar \partial $ -problem with exact support in Ω. Moreover, we solve the $\bar \partial $ -problem with solutions smooth up to the boundary over Ω provided that it has smooth boundary. Applications are given to the solvability of the tangential Cauchy-Riemann equations on the boundary.     

Spin transition in a chainlike supramolecular iron(II) complex

A one-dimensional supramolecular head-to-tail N+ -H...N-type hydrogen-bonded chain of the complex [FeII(L)2H](ClO4)3.MeOH [L = 4'-(4'-pyridyl)-1,2':6'1'-bis(pyrazolyl)pyridine] exhibits a reversible, thermally driven spin transition at 286 K with a hysteresis loop of ca. 2 K.     

Molecularly imprinted polymer Based PVC-Membrane-Coated graphite electrode for the determination of metoprolol

A PVC-based membrane containing metoprolol molecularly imprinted polymer (MIP) coated directly on graphite electrode for the determination of metoprolol in real samples is reported. This potentiometric sensor was designed by dispersing the MIP particles in dioctylphthalate plasticizer as solvent mediator and then embedded in polyvinyl chloride matrix. The electrode exhibited a near-Nernstian slope of 55.4 ± 1 mV decade^-1 for metoprolol over a wide concentration range between 2.0 × 10^-7-8.0 × 10^-3 M and a detection limit of 1.3 × 10^-7 M. With a response time of about 14 s it could be used for at least 6 months without any divergence in potential. The proposed electrode can be used in the pH range of 3.5-10.5 and can reveal good selectivities for metoprolol over a wide variety of ions. Finally, the designed sensor was successfully applied as an indicator electrode to determine the concentration of metoprolol in tablets, human urine and plasma. The results were compared favorably with those obtained by HPLC method and showed satisfactory agreements with them.     

Solid-Phase Extraction of Metoprolol onto(Methacrylic acid-ethylene glycol dimethacrylate)-based Molecularly Imprinted Polymer and Its Spectrophotometric Determination

A new adsorbent for molecularly imprinted solid phase extraction (MISPE) of metoprolol was synthesized using methacrylic acid (MAA) as functional monomer and ethylene glycol dimethacrylate (EGDMA) as cross‐linking agent causing a non‐covalent, bulk, thermal radical‐polymerization. Control polymer (non‐imprinted polymer) was prepared under well defined conditions without the use of metoprolol. The synthesized polymers were characterized by IR spectroscopy, X‐ray diffraction and thermal analysis techniques. This polymer was used for the rapid extraction and preconcentration of metoprolol from real samples prior to spectrophotometric determination. Extraction efficiency and the influence of flow rates of sample and stripping solutions, pH, type of eluent for elution of metoprolol from polymer, break through volume and limit of detection were studied. The detection limit of the proposed method is 0.4 ng·mL^?1. The method was applied successfully to the recovery and determination of metoprolol in tablets, human urine and plasma samples.     

Vibrations of balanced fault-free ball bearings

This paper investigates the vibrations of balanced fault-free ball bearings. A lumped mass-damper-spring model is adopted including the use of the Hertzian contact theory to represent the stiffness of the bearing rolling elements. We found that the equilibrium point of the bearing undergoes a supercritical pitchfork bifurcation as the bearing internal clearance increases. We developed closed-form expressions for the frequency-response functions of the horizontal and vertical motions of bearings with small internal clearance (below the bifurcation point). We also developed a chaos map to describe the locations and intensity of chaos in the internal clearance-shaft speed parameter space for bearings with larger internal clearance (beyond the bifurcation point).