Solution Stirring Research Articles

Electropolymerization of 2,5-di-(-2-thienyl)-pyrrole in ethanolic medium. Effect of solution stirring on doping with perchlorate and chloride ions

The electrochemical behavior of 2,5-di-(-2-thienyl)-pyrrole (SNS) on Pt has been studied from a 10 mM monomer solution in 0.2 M LiClO 4+ethanol or in 0.2 M LiCl+ethanol by cyclic voltammetry, chronopotentiometry and chronoamperometry. The monomer exhibits two similar consecutive oxidation processes. Uniform, adherent and electroactive films of dark-blue poly(SNS) doped with ClO 4 − or with Cl − are obtained at low potentials related to the first process. Reproducible film weights are found at 0.700 V versus Ag|AgCl during 360 s. The increase in transport rate of reactants by stirring the solution with a magnetic bar at 150 rpm accelerates the SNS electropolymerization, allowing to collect much more polymer weight than from the quiescent solution. The productivity of poly(SNS) doped with Cl − determined by ‘ex situ’ ultramicrogravimetry increases notably under stirring, although its percentage in Cl − is similar to that found under quiescent conditions. This is ascribed to the production of a major proportion of longer linear molecules in polymer, consistent with its higher conductivity when it is synthesized under stirring. This effect is not so clear for the poly(SNS) doped with ClO 4 − due to the little influence of stirring upon its productivity and conductivity. The detection of short linear oligomers in the soluble fractions of polymers in thioglycerol by mass spectrometry-fast atom bombardment allows to propose a radical polycondensation as initial electropolymerization mechanism.

Growth of mercuric iodide platelets for X-ray room temperature detectors in the HgI 2–HI–H 2O system

Mercuric iodide platelets were grown in the HgI 2–HI–H 2O system by solvent evaporation at constant temperature. Different solution temperatures, growing times and stirring conditions were studied. In order to determine the best growing parameters the chemistry of the HgI 2–HI–H 2O system was studied. Complexes [HgI 3] − and [HgI 4] −2 were identified and quantified in the solution by visible and UV spectrometry, according to the R ratio ([HgI 2]/[HI]). Solution oxidation and I 3 − interference were eliminated performing growth in high purity Argon atmosphere. Good mercuric iodide platelets, up to 6.0×7.0×1.5 mm 3 in size, were obtained with appropriate argon flow, temperature and growing time, for several independent experiences. Platelets were observed by optical microscopy and the absence of possible dislocations was confirmed by studying etch pits density after etching with ethanol–trichloroethylene. Non-existence of occluded water and hydrocarbons was confirmed by Fourier transform infrared spectroscopy and was related to results obtained for mercuric iodide crystals grown by the physical vapor transport method. Detectors were made with the platelets and their apparent resistivity, current density and X-ray performance were measured. Discussions about future improvements of growing mercuric iodide platelets in the HgI 2–HI–H 2O system and comparison of this method with others are included.

Morphological changes in porous silicon nanostructures: non-conventional photoluminescence shifts and correlation with optical absorption

In this paper, we show that the photoluminescence (PL) shifts of p-type porous silicon (PS) are mainly attributed to some morphological changes related to anodisation conditions. We discuss how differences in the stirring and nature of the electrolytic solution can lead to morphological changes of the PS layers. It has been found that when PS is formed in pure aqueous HF solution, it can exhibit a non-conventional and reproducible “porosity – PL peak relationship”. By correlating the PL spectral behaviour and PS morphology throughout a quantum-confinement model, we explain both conventional and non-conventional PL shifts. Correlation of PL and optical absorption (OA) shows that the PL peak energy and the optical absorption edge of PS exhibit the same trend with size effect. The spectral behaviour of OA with regard to that of PL is well analysed within the quantum-confinement model throughout the sizes and shapes of the nanocrystallites forming PS. The value of the effective band gap energy determined from the calculated lowest PL energy almost corresponds to that estimated from the optical absorption coefficient. These results suggest that the lowest radiative transition between the valence band and the conduction band corresponds to the largest luminescent wires, and that the radiative recombination process leading to the PL emission occurs in the c-Si crystallite core.

Improvement of Crystallinity of Organic Nonlinear Optical Crystal DAST by Using Solution Stirring

Improvement of Crystallinity of Organic Nonlinear Optical Crystal DAST by Using Solution Stirring

Factors affecting the dissociation and aggregation of human interferon gamma

The biologically active form of interferon γ (IFN-γ) is a dimer consisting of two identical non-covalently bound polypeptide chains. We have studied spectroscopically the dimer–monomer dissociation equilibrium of human recombinant IFN-γ and have found that the monomers possess approximately 50% lower Trp quantum yield than the dimers [Boteva et al. Biochemistry 1996;35:14825]. In the present study we characterise the conformational properties of the two states — monomeric and dimeric, and analyse the effects of the salt composition of human blood plasma, physiological cations K +, Na +, Ca 2+ and Mg 2+ and mechanical stress on the dimer–monomer equilibrium. A medium with electrolyte composition of human blood plasma increases both the association and dissociation rate constants without shifting significantly the dimer–monomer equilibrium. The physiological cations shift the equilibrium towards dissociation of dimers into monomers by lowering the activation energy and the free energy of the process thus decreasing the stability of IFN-γ. Mechanical stress caused by stirring of the protein solution reduces irreversibly the Trp fluorescence by 75–80% and decreases significantly the α-helical content and favours the aggregation.

Steady-State Voltammetry with Stationary Disk Millielectrodes in Magnetic Fields: Nonlinear Dependence of the Mass-Transfer Limited Current on the Electron Balance of the Faradaic Process

Generally, faradaic current passing through an electrolytic cell placed in a magnetic field causes stirring of the electrolytic solution and current−voltage characteristics similar to those obtaine...

II. Crystal growth through forced stirring of melt or solution in Czochralski configuration

An original and effective method for stirring a melt or solution during the process of crystal growth in the Czochralski configuration is proposed. To stir a melt or solution in this configuration is very important for producing various crystals. Effective stirring, i.e. preparation of a homogenous crystallization medium, is especially critical for the growth of crystals from viscous melt-solutions, borates for example. The method under consideration was applied to the growth of barium borate (BaB 2O 4) from Na 2O melt-solution. It is based on the rotation of a growth crucible with simultaneous motion of a cylindrical shape-forming mixer with several blades placed at a slope in the melt. The method allows easy addition of nutrient for steady-state growth.

Colloidal gas aphrons generated from the anionic surfactant AOT for the separation of proteins from aqueous solution

Colloidal gas aphrons are microbubbles 10–100 μm in diameter created by intense stirring of a surfactant solution. They are composed of a gaseous inner core surrounded by a surfactant film. They have a high surface area, are relatively stable and have the ability to interact with proteins through electrostatic interactions. These characteristics make colloidal gas aphrons ideally suited for use as a downstream processing technique. In this study the effect of different process parameters on the recovery of three proteins (conalbumin, ribonuclease A and α-chymotrypsinogen A) was studied for a protein–AOT (sodium bis-(2 ethylhexyl) sulfosuccinate) system. High enrichment ratios (up to 8) and high recovery of the protein (85%) are found under optimum conditions. © 1999 Society of Chemical Industry

Membrane potentials and electrolyte permeation in a cation-exchange membrane

A method has been developed which permits one to determine the electrolyte permeation velocities and the membrane system permeability from measurements of the membrane potential as a function of time. The method has been applied to a cation-exchange membrane separating two aqueous KCl solutions, at the same pressure and temperature, but of different concentrations. The experiments have been carried out in two concentration ranges and at different solution stirring rates. The obtained results showed that the membrane system permeability depends notably on the stirring rate, this dependence being greater for the highest concentration range. The intrinsic permeability of the membrane was determined from considerations about the concentration polarization effect, its value being greater in the lower concentration range.

Effect of nitrite anions on the rate of iron dissolution in acid electrolytes

The effect of the concentration of nitrite ions, potential, pH and solution stirring on the dissolution rate of iron was studied. Nitrite ions were shown to accelerate active dissolution. The accelerating action of the oxidant depends on the ratio of the rates of electroreduction at the electrode surface and homogenous chemical reduction in the reaction layer adjacent to the electrode. A possible mechanism of oxidant action on the iron dissolution is considered. The electroreduction process is assumed to generate at the iron surface an activated complex accelerating metal dissolution.

Study on Some Properties of the Electrolyte Solution in the Electrodeposition of Palladium

SUMMARYSome properties of Pd electroplating solution were studied. The results showed that current efficiency and velocity of palladium deposition were decreased and increased respectively by raising the current density, which indicated that the stirring of plating solution was an effective route to raise current efficiency. The choice of concentrations of potassium citrate and ammonium oxalate in solution was mainly determined by the complexing ability of potassium citrate for the palladium ion and the solubility of ammonium oxalate. The electrolyte was shown to have a good throwing power and excellent covering power. Hull cell tests showed that an adherent, smooth, bright and white Pd electrodeposit could be obtained.

The Synthesis and Properties of the Chemically Oxidized Perovskite, La1–xSrxCoO3−δ(0.5≤x≤0.9)

The series of La1−xSrxCoO3−δ(0.5≤x≤0.9) perovskites have been prepared using the Pechini gel technique. The products were chemically oxidized by stirring in a sodium hypobromite solution. The samples have been characterized by powder X-ray diffraction, thermal analysis, iodometric titration, Co K edge X-ray absorption spectroscopy, temperature-dependent SQUID magnetic susceptibility, and temperature-dependent electrical resistivity. The Sr-rich samples (x>0.7) have the brownmillerite-type structure prior to oxidation and the cubic perovskite structure after treatment with sodium hypobromite. Iodometric titration shows as much as a ∼14% increase in Co(IV) concentration in the Sr-rich samples after chemical oxidation. The Co K edge spectra show that there is very little change in the formal cobalt valence with increasing Sr content. The effective magnetic moments of the oxidized compounds level off with increasing Sr concentration. All of the chemically oxidized samples exhibit small-bandgap semiconducting behavior. The data lead to the postulation of an equilibrium between Co(IV) and O−in the series.

329 Effects of hypothalamically elicited restlessness behavior on the distribution and surface expression of the adhesion molecules of leukocytes in the cat

Controlling the nucleation and growth processes is important for growing large, high-quality crystals of oxides, proteins, and nitrides from a solution. We must understand how the solution conditions should be controlled, and determine a universal principle that is applicable to the growth of various high-quality crystals.In this report, we will present the effects of solution stirring and nucleation control on the solution growth of CsLiB6O10 (CLBO), proteins, and GaN crystals with respect to the following issues: (1) effect of solution stirring on the growth of high-quality CLBO crystals, (2) high-quality protein crystal growth by means of forced nucleation induced by laser irradiation and the solution-stirring method, and (3) high-quality 2-in GaN crystal by Na flux liquid-phase epitaxy method.

Crystal growth through forced stirring of melt or solution in Czochralski configuration

An original and effective method for stirring a melt or solution during the process of crystal growth in the Czochralski configuration is proposed. The efficiency of the method was supported by a model experiment. The method allows easy addition of nutrient for steady-state growth.

Modified Composite Electrode Containing HgO as Built-In Mercury Precursor for Anodic Stripping Analysis of Trace Metals

A HgO-modified composite electrode was prepared by adapting a composite electrode technique, which is compatible to a mercury thin-film electrode in anodic stripping analysis of heavy metals. The preparation method and utilization of the HgO-modified composite electrode in anodic stripping analysis are described. The built-in mercury precursor HgO could be utilized feasibly for generation of surface mercury droplets by in situ electrochemical treatment or pretreatment without the necessity of a Hg2+ solution. Mercury microdroplets generated on the electrode surface were stable to meet ordinary requirements for stirring of solution or rotation of electrode. Hydrogen gas evolution perturbed the stability of the surface mercury severely. The hydrogen reduction overpotential was directly dependent on the content of mercury oxide in the electrode matrix. The electrodes containing more than 10 wt % mercury oxide were fairly stable so that the deposition potential could be extended up to −1.2 V for most applicat...

Electrochemistry with Stationary Disk and Ring−Disk Millielectrodes in Magnetic Fields

The most well-known effect of the magnetic field on electrochemical systems is hydrodynamic convection (stirring) of the electrolytic solution. The basic hydrodynamic equations governing mass trans...

Leaching behavior of rhenium in ammonium iodide/iodine solutions

The leaching behavior of rhenium in ammonium iodide solutions with iodine as oxidant was studied. Experiments were carried out in an autoclave to investigate the effects of stirring speed, temperature, solution pH, and iodine and iodide concentrations on the dissolution of rhenium. It was found that a high temperature and a high solution pH were essential for the efficient leaching of rhenium. The dissolution rate was found to be dependent upon the concentration of iodine and iodide. It was concluded that the dissolution of rhenium under the conditions studied was controlled by the mass transfer of iodine with an apparent activation energy of 15.8 kJ/mole.

Modulating factors of calcium-free contraction at low [MgATP]: a physiological study on the steady states of skinned fibres of frog skeletal muscle.

Factors that modulate Ca(2+)-free contraction at low [MgATP] were examined by analysing steady tension development in skinned fibres of frog skeletal muscle. The commonly accepted bell-shaped relationship between steady tension and log (1/[MgATP]) was found to be highly susceptible to subtle experimental conditions at the higher [MgATP] side (right limb). The limb shifted to the right with increased fibre thickness, interrupted stirring of the bathing solution, increased temperature and fibre extension, although the effects of temperature and extension were marked only in thick fibres (cross-sectional area > 6000 microns 2). The shift of the right limb was reproduced by an addition of ADP to the bathing solution. These results, together with the extreme steepness of the right limb in thick fibres, suggest that a diffusion-dependent self-regenerative activation occurs in thick fibres in which ADP accumulation and ATP depletion positively feed back through further activation of the myofibrillar ATPase. Numerical simulation supported the hypothesis of the self-regenerative activation under poor diffusion conditions, and suggested that a small rise in temperature and fibre extension can trigger the self-regenerative process at the right limb. Consequently, ADP, temperature and fibre extension are deduced to be the primary potentiators of the activation at low [MgATP]. The high efficiency of ADP in shifting the limb suggests that the activating efficiency of the MgADP-bound actomyosin complex is higher than the nucleotide-free actomyosin complex.

Role of water and phase transfer catalysts in the kinetics of condensation of diaryl ketones with acetonitrile initiated by solid potassium hydroxide †

The kinetics of reactions of benzophenone and substituted derivatives with acetonitrile in the presence of excess solid potassium hydroxide (KOH) to give 2,2-diarylacrylonitriles are reported. Reaction rates are strongly temperature dependent, and water, other cosolvents and quaternary ammonium salts all accelerate the reaction; also variation in substituents at one of the para-positions (from H to Cl, F and Me) gives a significant rate effect (ρ = 1.9). In contrast, variations in the amount of solid KOH pellets, the presence of crushed KOH, or an increase in the speed of stirring of the refluxing solution do not have significant kinetic effects. The results are consistent with a mechanism in which the anion [CH2CN]– is transported from the KOH surface before it reacts with the ketone. The rate of disappearance of benzophenone during most of the reactions is predominantly zero order, but variation in the initial concentrations of benzophenone shows that the initial rate is first order in benzophenone; water formed during the reaction does not remain in the acetonitrile (Karl Fischer titration), and the increasingly moist KOH further catalyses the reaction, so the observation of zero order kinetics is fortuitous. The role of solvents as phase transfer catalysts is discussed.

Comparison of direct current, derivative direct current, pulse and square wave voltammetry at single disc, assembly and composite carbon electrodes: stripping voltammetry at thin film mercury microelectrodes with field-based instrumentation

A comparative investigation of normal and stripping forms of voltammetry was undertaken at single and random array microdisc, macrodisc and composite carbon electrodes using dc, derivative dc, square-wave and differential-pulse waveforms. Studies on the reduction of [Fe(CN)6]3– in 0.05 M KNO3 revealed that the dc waveform with derivative readout is superior to the use of square-wave or differential-pulse waveforms at both single disc and random array carbon fibre electrodes. Analytical superiority to that obtained at a conventional glassy carbon macrodisc electrode was also observed. The detection limit for the determination of [Fe(CN)6]3– was 4 × 10–6 M at the carbon fibre random array microdisc electrode and 6 × 10–6 M when using a single disc microelectrode (scan rate = 200 mV s–1). For the determination of Pb and Cd in 0.1 M HCl by stripping voltammetry, the ideal combination of high sensitivity (1 ppb detection limit), avoidance of the need to remove oxygen or utilise solution stirring or electrode rotation could be achieved at mercury-plated thin film single disc or random array microelectrodes or composite electrodes. Furthermore, under these thin film conditions, the analytical performance of each of these three electrode types improved with respect to signal-to-background current ratio as the scan rate increased. Consequently, short potential scanning times as well as optimum sensitivity are achieved with scan rates in the 1–50 V s–1 range. All of the results obtained demonstrate that highly sensitive field-based voltammetric analysis may be accomplished using the combination of an inherently simple two-electrode battery operated instrument, the dc waveform and random array or single carbon fibre microdisc working electrodes. An interface which enables voltammetric studies to be undertaken with a battery operated personal computer is described for this purpose.