Solutions Of Constant Ionic Strength Research Articles

PH dependence of the voltammetric response of Polyaniline

Voltammetric studies of Polyaniline (Pani) films were carried out in solutions of 3.7M sulphate solutions of constant ionic strength and pH values comprised in the range between −0.6 and 3.0. The results show that if the voltammetric results are properly corrected by capacitive currents, according to Feldberg's model, it will be possible to obtain the potential dependence of the faradaic oxidation degree.The integrated anodic and cathodic charges are practically equal and independent of the electrolyte pH, in the range −0.6<pH<2.0. The pH dependence of the current peaks is qualitatively explained by the change in the interactions between the redox centres as a consequence of the changes in their protonation.

Photophysics and halide quenching of Soret-excited ZnTPPS4− in aqueous media

Steady state S2–S0 and S1–S0 absorption and emission spectra and picosecond S2 decay and S1 fluorescence rise times have been measured for the model porphyrin ZnTPPS4− in water and in aqueous iodide solutions of constant ionic strength. The dynamics of S1 quenching by iodide are well-modeled by a Stern–Volmer mechanism yielding kQ=1.75×109M−1s−1. The S2 state is quenched on a ps time scale by a static electron-transfer mechanism at an average donor–acceptor separation of 0.38nm. The implications of these data for dye-sensitized solar cells that employ I−/I3− as the redox medium are discussed.

Thermodynamic studies on cobalt extraction by 2-heptyl-2-methylnonanoic acid in benzene from chloride solutions

Solvent extraction of Co(II) from aqueous chloride solutions of constant ionic strength (I = 1 M) with a long-chain fatty acid, 2-heptyl-2-methylnonanoic acid (HA), in benzene was studied at four temperatures: 278, 288, 298, and 313 ± 1 K. The influence of various parameters on the extraction equilibrium was studied. The effect of mixing HA with TBP in benzene on the extraction process was examined, and a synergistic effect was revealed. The stoichiometry of the extraction equilibria was determined, and the thermodynamic studies were made for both extraction systems. The thermodynamic functions ΔG0, ΔH0, and ΔS0 were calculated, and the related data were discussed.

Modified model of electrical double layer for mixed solutions of constant ionic strength

A model is suggested, which generalized the Alekseev-Popov-Kolotyrkin model for the case of mixed solutions of constant ionic strength of the type mc KA*− + (1 − m)c KA, where A*− is the surface-active anion that lowers the electrical double layer capacitance as compared with that observed in the presence of surface-inactive anion A−. Calculations made for a real system mc NaNO3 + (1 − m)c NaF showed that at 0.2 ≤ m ≤ 1 a transition occurred from quadratic to linear dependence of adsorption energy on the electrode potential (or charge). The conclusions are in agreement with experimental data obtained for the studied system.

Structures and Energetics of Erbium Chloride Complexes in Aqueous Solution

A series of aqueous, constant ionic strength solutions containing 0.5 m Er is studied as a function of chloride/perchlorate ion concentration using high-energy X-ray scattering (HEXS) to probe the metal ion coordination environment. Perchlorate is seen to form an outer-sphere complex only in the end member (4.0 m perchlorate) of the series. Chloride ions are seen to bind as both inner- and outer-sphere complexes. A quantitative analysis of the scattering data is used to determine stability constants that depend on whether the complexation with chloride ions is assumed to add to or to replace the waters bound in the first coordination sphere. Published stability constants obtained from liquid-liquid extraction clarify the mechanism involved and, together with the HEXS data, present a consistent picture of the chemistry. The stability constants determined from the HEXS data, beta(1) = 0.38(8) and beta(2) = 0.014(9), are small, less than the available thermal energy in solution at room temperature. The combination of chemical separations and HEXS data for the analysis supports the argument that the complexes formed are real under the experimental conditions. Details of the structural correlations and interactions are discussed for applications including separations and environmental modeling.

Monovalent cations affect the free solution mobility of DNA by perturbing the hydrogen‐bonded structure of water

The free solution mobilities of single- and double-stranded DNA molecules of various molecular weights have been measured by capillary electrophoresis in solutions of constant ionic strength containing a common anion and fifteen different monovalent cations. In solutions with the same ionic composition, the mobilities of different DNA molecules can vary by up to 20%, depending on molecular weight, the number of strands, and the presence or absence of A-tracts, runs of four or more contiguous adenine residues. Importantly, the mobilities observed for the same DNA sample can vary by up to 40% in solutions containing different cations. The mobility differences observed for the same DNA in solutions containing different cations cannot be rationalized by differences in the anhydrous radii or intrinsic conductivities of the various cations, or by the sequence-dependent binding of certain cations to A-tracts. Instead, the observed mobilities are linearly correlated with the average number of water-water hydrogen bonds that are present in solutions containing different cations. The mobilities are also correlated with the viscosity B coefficients of the various cations and with the rotational correlation times frictional coefficients observed for water molecules in solutions containing different cations. Hence, monovalent cations modify the free solution mobility of DNA primarily by perturbing the hydrogen-bonded structure of water, affecting the friction experienced by the migrating DNA molecules during electrophoresis.

Palladium(II) and platinum(II) complexes of 2-hydroxy acetophenone N(4)-ethylthiosemicarbazone – crystal structure and description of bonding properties

Reaction of H 2PtCl 4 and K 2PdCl 4 with 2-hydroxyacetophenone N(4)-ethylthiosemicarbazone, H 2Ap4Et, afforded [Pt(Ap4Et)(H 2Ap4Et)] and [Pd(Ap4Et)(H 2Ap4Et)]. Their crystal and molecular structures are reported and represent the first 1:2 thiosemicarbazone complexes with ligands having both different formal charge and denticity. The dianion, Ap4Et 2−, coordinates in a planar conformation to palladium(II) or platinum(II) via the phenolato O, imine N and thiolato S atoms, while the neutral molecule exhibits monodentate coordination by the thione S atom. Intra-, intermolecular hydrogen bonds and C–H⋯π contacts lead to aggregation and a supramolecular assembly. Electronic, IR, and NMR spectral data, as well as electrochemical measurements, are included. The p K a values of the poorly water soluble H 2Ap4Et were obtained spectrophotometrically in aqueous solutions of constant ionic strength.

Electrochemical Properties of Poly-o-Phenylenediamine Films in Solutions with Variable Concentration of Hydronium Ions

Electrochemical behavior of poly-o-phenylenediamine (PoPhD) films in lithium perchlorate and perchloric acid solutions of different pH and constant ionic strength is studied using cyclic voltammetry, low-amplitude chronoamperometry, chronopotentiometry, and faradaic-impedance spectroscopy. The experimental results point to the diffusion–migration kinetics of charge transfer processes in redox-active PoPhD films and show that two such processes occur during oxidation–reduction of PoPhD. The processes are separated most fully at low concentrations of hydronium ions. Effect of the electrode potential and electrolyte composition on these processes is examined. Different methods yield similar results and permit their more reliable interpretation.

Pulp Potential Control in Flotation: The Effect of Hydrogen Peroxide Addition on the Extent of Xanthate Oxidation

The effect of increasing the redox potential by means of hydrogen peroxide additions upon the extent of oxidation of potassium ethyl xanthate (KEX: C2H5OCS2K) was investigated. The study was carried out at pH 7 in solutions of constant ionic strength (0.01 M NaNO3) using an initial surfactant concentration of 100 mg/L (6.176 × 10−4 M KEX). The Eh was varied in the range of 260 to 450 mV (vs SHE). The concentration of surfactant was determined by means of UV spectrometry at a wavelength of 280 nm. At shorter wavelengths it was observed that the absorbances of NaNO3 and H2O2 interfere with the measurement. It was determined that the main product of the oxidation of xanthate was perxanthate; no dixanthogen was detected regardless of the Eh of the solution.

Organotin-Drug Interactions. Organotin Adducts of Tenoxicam: Synthesis and Characterization of the First Organotin Complex of Tenoxicam

The synthesis and spectral characterization of the novel organotin complexes [SnBu2(ten)] (1) and [SnBu2(Hten)2] (2) of the potent and widely used anti-inflammatory drug tenoxicam (H2ten) are reported. A crystal-structure determination of 1 showed that, in this complex, the ligand is doubly deprotonated at the hydroxy O-atom and the amide N-atom and is coordinated to the SnBu2 fragment via four- and six-membered chelate rings. An extended network of Sn−O−Sn, C−H⋅⋅⋅O and C−H⋅⋅⋅π contacts lead to aggregation and a supramolecular assembly. Potentiometric titrations in nonaqueous solutions support the ionization of the drug by removal of the second H-atom, the amide H-atom, in the presence of the diorganotin(IV) fragment. The Ka values of the poorly H2O-soluble drug tenoxicam were obtained spectrophotometrically in aqueous solutions of constant ionic strength.

Folding property of di-block polyampholytes

1.1. Monolayers of polyvinyl benzoic acid (P.V.B.A.), of poly ortho- vinyl pyridine, and poly para- vinyl pyridine as well as of copolymers containing varying proportions of vinyl benzoic acid (V.B.A.) and 2-vinyl pyridine (2-V.P.) were investigated. These polymers were spread on the air-water interface of buffer solutions of constant ionic strength and of various pH values, and their surface pressure and potentials were measured.2.2. The measured surface potentials and pressures of poly 4-vinyl pyridine (4 P.V.P.) were smaller than the corresponding potentials and pressures of poly 2-vinyl pyridine (2 P.V.P.) at the same surface concentration. From these results it was concluded that there is a higher probability for the trans- than for the cis-configuration of the pyridine residues in the polymeric chain.3.3. In the case of polyvinyl benzoic acid and the polyampholytes, strong intermolecular forces are evident. As a result, equilibrium pressure values were reached very slowly after a certain surface area was established. The forces acting in the case of P.V.B.A. are due to hydrogen bonds and, in the case of the polyampholytes, in the neighborhood of the isoelectric point, are due to both hydrogen bonds and electrostatic attraction of oppositely charged monomeric residues.4.4. The pH of the solution has only negligible influence on the surface pressures and potentials of 2-P.V.P. and 4-P.V.P. until a pH value is reached where the film starts to dissolve. At this point both the pressure and the potential drop suddenly. The influence of pH is more pronounced in the case of P.V.B.A. and is quite striking in the case of polyampholyte films. The contractive forces prevent the dissolution of the hydrophilic units in the surface. They are balanced by repulsive forces which arise from the net charge of the surface film.

Synthesis and characterisation of the first organotin complex of piroxicam. An extended network system via non-hydrogen, hydrogen bonding linkages and C–H · · · π contacts

The organotin complex, [SnBu2(pir)]n, of the potent and widely used anti-inflammatory drug piroxicam, H2pir, was obtained and a crystal structure determination showed that in this complex the ligand is doubly deprotonated at the oxygen and amide nitrogen atoms. There are two similar molecules in the asymmetric unit. The isolated molecules of Sn(1) or Sn(2) are arranged in polymers in a head to tail fashion with a stacking of alternate parallel chains. Extended networks of Sn–O–Sn, C–H· · ·O and C–H· · ·π contacts lead to aggregation and a supramolecular assembly. Real concentration protonation constants for the zwitterionic form (pyridyl group) and the protonated piroxicam (enolic group) were determined spectrophotometrically in pure aqueous solutions of constant ionic strength. It is the first example where piroxicam is proved to act as a doubly deprotonated tridentate ligand.

Polyelectrolyte adsorption on charged surfaces: study by electrokinetic measurements

In order to describe the influence of cationic polyelectrolytes on flocculation of disperse systems the adsorption of poly (diallyldimethylammonium chloride) (PDADMAC) onto silica, mica and acidic polymer latex was investigated. The plateau value of the adsorption isotherms grows with increasing surface charge density of the substrates and electrolyte concentration. The adsorbed layer of the polycation was characterized by zeta potential measurements with KCl solutions of constant ionic strength and varied pH. The zero point of the charge as well as the shape of the zeta potential–pH plot depends on the coverage of the surface with polycations. For fully covered substrates the zero point of the charge as well as the pK A and pK B values calculated by a stochastic search programme are independent of the substrate. Maximum flocculation was observed at about 30% of the plateau value of the adsorption isotherms.

Adsorption of aquatic humic substances on colloidal-size aluminum oxide particles: Influence of solution chemistry

The adsorption of Suwannee River humic substances (HS) on colloidal-size aluminum oxide particles was examined as a function of solution chemistry. The amount of humic acid (HA) or fulvic acid (FA) adsorbed decreased with increasing pH for all solutions of constant ionic strength. In NaCl solutions at fixed pH values, the adsorption of HA and FA increased with increasing ionic strength. The presence of Ca 2+ enhanced the adsorption of HA but had little effect on FA. For identical solution conditions, the amount (by mass) of HA adsorbed to alumina was always greater than FA. Adsorption densities for both HA and FA showed good agreement with the Langmuir equation, and interpretations of adsorption processes were made from the model parameters. For FA, ligand exchange appears to be the dominant adsorption reaction for the conditions studied here. Ligand exchange is also a major adsorption reaction for HA; however, other reactions contribute to adsorption for some solution compositions. At high pH, cation and water bridging become increasingly important for HA adsorption with increasing amounts of Na + and Ca 2+, respectively. At low to neutral pH values, increases in these same two cations make hydrophobic bonding more effective. Calculations of HS carboxyl group densities in the adsorbed layer support the proposed adsorption reactions. From the adsorption data it appears that fewer than 3.3 HS-COO − groups per nm 2 can be bound directly as inner-sphere complexes by the alumina surface. We propose that the influence of aqueous chemistry on HS adsorption reactions, and therefore on the types of HS surface complexes formed, affects the formation and nature of organic coatings on mineral surfaces.

Stress-generated potentials in bone: effects of bone fluid composition and kinetics.

It is now generally accepted that stress-generated potentials (SGPs) at low frequencies are due to an electrokinetic phenomenon in the small interior surfaces of bone and are directly proportional to the zeta potential, a property of the poorly characterized bone surface-bone fluid interface. We hypothesized that this interface would be labile and might explain the controversy over whether or not the polarity of SGPs can invert under certain conditions. In this paper, the effects of alterations in the steeping fluid on SGPs for 87 samples from 15 animals were examined in four-point bending for steeping times of < or = 65 h. Calcium and fluoride in distilled-deionized water and constant ionic strength solutions produced concentration-dependent inversions in the SGP sign. A new steady state was reached in approximately 18 h. The effects of the fluoride anion (unlike the calcium cation) apparently were reversible. The results strongly suggest that the zeta potential at the labile bone surface-bone fluid interface can undergo dramatic changes, not only in magnitude but also in sign. The results further suggest that the preparation of bone samples is critical to the understanding of this interface in vivo, and they support the hypothesis that SGPs have a role in bone remodeling.

Adsorption of inorganic ions on bismuth from constant ionic strength solutions in 2-butanol

The adsorption of Cl −, Br −, I − and SCN − ions on a drop-shaped bismuth electrode from solutions in 2-butanol has been investigated by means of differential capacity measurements. The ionic charge due to the specific adsorption was calculated using the Hurwitz-Parsons-Dutkiewicz method. The results were analysed using the virial adsorption isotherm and the classical Grahame-Parsons inner layer model. It was confirmed that the standard Gibbs energy of adsorption of ions in alcohols can be correlated with quantum-chemical indices of reactivity.

Extraction of certain actinide and lanthanide elements from different acid media by polyurethane foams loaded with di(2-ethylhexyl)phosphoric acid (HDEHP)

The extraction of uraninm(VI) from an aqueous HNO 3 phase into an organic phase consisting of a polyurethane foam immobilizing a solution of di(2-ethylhexyl)phosphoric acid (HDEI-1P) in o-dichlorobenzene has been investigated at varying concentrations of nitric acid and HDEHP. The mechanism of the extraction is discussed on the basis of the results obtained. The aggregation number of HDEHP immobilized on the foam was obtained from the analysis of data obtained for the extraction of cerium(IID from acidic perehlorate solutions of constant ionic strength.

Double-layer studies in ethanolic solutions. Part 2.—Adsorption of iodide ion on mercury from solutions of constant ionic strength

The adsorption of iodide ion from ethanolic solutions has been studied at constant ionic strength (0.2 mol dm–3) at a mercury electrode by measuring the interfacial capacity. The data were analysed to obtain the adsorbed charge density as a function of electrode charge density and bulk iodide concentration. These data are shown to obey a virial isotherm. The isotherm parameters obtained for the ethanol system are compared with results reported earlier in eight other solvents. The standard Gibbs energy of adsorption is shown to be a linear function of the Gibbs energy of iodide ion solvation and the entropy of solvent vaporization, the latter quantity being used as a measure of solvent structure. The dielectric properties of the interface in the presence of iodide adsorption are also examined within the context of a non-primitive model based on the mean spherical approximation. Application of this model at the interface is compared with the corresponding model for solvation in the bulk of the solution in the same solvents.

The adsorption of bromide ions on mercury from propylene carbonate solutions of constant ionic strength

The adsorption of Br − anions has been studied from propylene carbonate solutions of low ionic strength (0.16 M) at mercury. A maximum on the capacity curve is observed at low Br − ion concentration, a result which is unusual for strongly adsorbed ions. A new method of analysing the interfacial thermodynamic data, which makes use of data obtained in the absence of Br − ion, is presented. Adsorption is shown to be very strong and can be represented by a virial adsorption isotherm over the range that data are available. The adsorption parameters can be rationalized on the basis of the electrostatic model for adsorption with the proviso that the system is complex because of varying inner layer parameters for a given electrode charge density. When the present data are compared with those obtained in other solvents, it is shown that the standard Gibbs energy of adsorption changes with solvent acidity and basicity due to corresponding changes in anion solvation in the bulk, and the Gibbs energy of solvent adsorption on mercury, respectively, the adsorption parameters at the point of zero charge are examined with respect to non-primitive statistical mechanical models for ionic adsorption, discussed recently in the literature.

SOLVENT EXTRACTION OF SILVER(I) AND MERCURY(lI) BY O,O-DI-n-BUTYL PHENYLAMINO-PHENYLMETHANETHIO PHOSPHONATE FROM CHLORIDE SOLUTIONS

ABSTRACT The extraction of Ag(I)and Hg(II) from chloride solutions of constant ionic strength (lM) by O,O-di-n-butyl a -phenylamino--phenylmethanethiophosphonate dissolved in trichloromethane has been studied. The extraction dependences of metals on the ligand, hydrogen and chloride ions concentrations have been analysed to determine the stoichiometry of the extracted complexes and their equilibrium constants. Evidence was found for the formation of the complexes