Adsorption of urea on a polycrystalline silver electrode; comparison of electrochemical and radiometric methods

The adsorption of urea on a polycrystalline silver electrode was studied by radiometry and impedance spectroscopy. The differential capacity of the silver electrode in 0.01 M NaClO₄ solution containing urea in concentrations from 10^–6 to 5×10^–4 M has been determined. The isotherms of urea adsorption, found from the capacitance and radiometric measurements have been compared. The experimental data were described by the Langmuir isotherm, and the Gibbs energy of adsorption was calculated. The urea adsorption takes place in the entire range of the applied potential. The process is reversible with respect to the electrode potential and the bulk urea concentration.Dedicated to Professor Gyorgy Horanyi on the occasion of his 70th birthday     

Studies of particle-particle interactions using polystyrene latices and time-average light scattering

Dilute dispersions of monodisperse negatively-charged polystyrene latex particles, radii 161 Å, have been examined by time-average light scattering at various latex volume fractions and electrolyte concentrations. The latter were varied from the low value produced by maintaining mixed bed ion-exchange resin beads in the systems (ca. 10^–5 mol dm^–3) to the value of 5×10^–3 mol dm^–3 obtained by the addition of sodium chloride. From angular scattering measurements determinations of the structure factors were made; these were produced as a consequence of the particle-particle interactions in the system. By extrapolation of the structure factor to zero scattering angle, values were obtained for the osmotic compressibility and hence the osmotic pressure of the systems as a function of the latex volume fraction. It was found that the experimental data obtained could be interpreted in terms of a hard-sphere model for the particle-particle interaction. Good agreement was obtained provided that the particles were assigned a hard-sphere radius which was determined by the electrostatic repulsion between the particles.     

Electrokinetic control of fluid in plastified laser-printed poly(ethylene terephthalate)-toner microchips

The application of plastified laser-printed poly(ethylene terephthalate)(PET)-toner microchips to capillary electrophoresis was investigated. Electroosmotic flow was observed in the direction of the cathode for the buffer system studied (phosphate, pH 3–10). Average electroosmotic mobilities of 1.71×10^–4 to 4.35×10^–4 cm² V^–1 s^–1 were observed from pH 3 to 10. This variation suggests that silica fillers in the toner and on the surface of the polymer dominate the zeta potential of the material, which is also confirmed by XPS measurements. Dopamine and catechol were used as model analytes for microchip electrophoresis in combination with electrochemical detection. Results show that these two analytes can be efficiently separated and detected electrochemically with the plastified laser-printed PET-toner microchips.     

Voltammetric trace determination of ubiquinones at mercury electrodes

Summary Voltammetric methods were developed for the determination of ubiquinones on the basis of their adsorption and redox behaviour at mercury electrodes. Linear calibration plots were obtained in the concentration range between 10^–4 mol/l and 2.3·10^–5 mol/l from differential pulse voltammetric measurements under experimental conditions avoiding significant adsorption of the analyte. A remarkable decrease of the detection limit down to 10^–7 mol/l was achieved with the help of adsorptive accumulation of ubiquinone molecules at the mercury electrode followed by differential pulse detection. The resulting non-linear calibration plot was explained with a model, which takes into account both adsorption and diffusion of the analyte.     

Kinetics and mechanism in the decomposition of CCl4 in a radio-frequency pulse discharge

In the decomposition of CCl₄ in an r.f. pulse discharge the observation of time-resolved absorption spectra of the CCl radical allowed concentration measurements and a kinetic and mechanistic investigation of the system. Kinetic spectroscopy and end product analysis, with computer simulation of proposed mechanisms, indicates that the major decomposition reaction is CCl₄»CCl+3Cl (Cl₂+Cl), with a minor contribution from CCl₄»CCl₂+2Cl. Radical concentrations were of the order of 10^–7 mol dm^–3 (10¹⁴ molec. cm^–3). CCl removal was kinetically second order with a rate coefficient value of (3.7±8)×10¹⁰ mol^–1 dm^–3 s^–1 at 295±3 K at gas pressure 0.1 torr.     

Motion of protons in the central opening of porphyrins

The distribution of the electrostatic potential of the molecule of porphine (P) and the anions formed by the successive elimination of one and two central protons from it has been studied by the SCF-MO-LCAO method in the all-valence-electron CNDO/2 approximation. The electrostatic potential of the potential of the (P-2HO^2– dianion is characterized by the presence of four minima located at a distance of about 1 Å from the nitrogen atoms. The potential in them is equal to –991 kJ/mole. The value of the potential at the center of the opening is –978 kJ/mole. The distribution of the electrostatic potential of the (P-H)^– anion with a fixed position of the proton near one of the nitrogen atoms is characterized by the presence of a deep valley situated at the oppositely lying nitrogen atom perpendicularly to the N-H bond. The potential of the molecule of P in the plane of the ring does not have negative values, attesting to the energetic unfavorability of the planar conformations of the (P+H)⁺ cation.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 2, pp. 216–220, March–April, 1986.     

Adsorptive stripping voltammetric measurements of trace amounts of platinum(II) and ruthenium(III) in the presence of 1-(2-pyridylazo)-2-naphthol

An extremely sensitive stripping voltammetric procedure for low level measurements of platinum (II, IV) or ruthenium (III, IV) is reported. The method is based on the interfacial accumulation of the platinum (II) or ruthenium (III)-1-(2-pyridylazo)-2-naphthol complex on the surface of a hanging mercury drop electrode, followed by the reduction of the adsorbed complex during the cathodic scan. The peak potential was found to be –0.8 V vs. Ag/AgCl electrode and the reduction current of the adsorbed complex ions of platinum (II) or ruthenium (III) was measured by differential pulse cathodic stripping voltammetry. The optimum experimental conditions were: 1.5×10^–7 mol/l of 1-(2-pyridylazo)-2-naphthol solution of pH 9.3, preconcentration potential of –0.2 V, accumulation time of 3 min and pulse amplitude of 50 mV with 4 mV s^–1 scan rate in the presence of ethanol-water (30% v/v) — sodium sulphate (0.5 mol/l). Linear response up to 6.4 × 10^–8 and 5.1 × 10^–8 mol/l and a relative standard deviation (at 1.2×10^–8 mol/l) of 2.4 and 1.6% (n=5) for platinum (II) and ruthenium (III) respectively were obtained. The detection limits of platinum and ruthenium were 3.2×10^–10 and 4.1×10^–10 mol/l, respectively. The electronic spectra of the Pt(II) — PAN and Ru(III) — PAN complexes were measured at pH 9.3 and the stoichiometric ratios of the complexes formed were obtained by the molar ratio method. The effects of some interfering ions on the proposed procedure were critically investigated. The method was found suitable for the sub-microdetermination of ruthenium (IV) and platinum (IV) after their reduction to ruthenium (III) and platinum (II) with sulphur dioxide in acid media. The applicability of the method for the analysis of binary mixtures of ruthenium (III) and (IV) or platinum (II) and (IV) has also been carried out successfully. The method is simple, rapid, precise, and promising for the determination of the tested metal ions at micro-molar concentration level.     

Measurement and Modeling of Th Sorption onto TiO(2)

The sorption and desorption of Th onto TiO₂ (10 g/L) was studied as a function of pH (1–11), ionic strength (0.005–0.1 M NaCl, NaClO₄), thorium concentration (from 1 × 10⁻⁹ to 3 × 10⁻² M), and carbonate concentration (up to 10⁻² M) using ²³⁴Th tracer. The results indicate the reversible formation of an inner sphere complex with a strong pH dependence. No effect of the carbonate on sorption could be measured in the pH range investigated (5–10.5). The data were fitted with five different conceptual models (constant capacitance, diffuse layer, Stern, triple layer, and a nonelectrostatic model) using FITEQL. Different hydroxy complex surface reactions were tried, one at a time, varying the site density (1–12 sites/nm²). The reaction that provided the best fit depended strongly on the surface site density and the model used. To be able to fit a reaction to the pH-dependent data with the compact layer models, the effect of the electrostatic term had to be decreased by decreasing the site density below 5 sites/nm². None of the electrostatic models could be used to fit all of the isotherm data. The nonelectrostatic model could be used to reasonably fit both the pH-dependent and isotherm data.     

Acid–Base Zeta-Metric Titration of Quartz Dispersions in Ethanol Solutions of Electrolytes

The conductivity of dilute quartz suspensions and electrophoretic mobility of quartz particles in solutions with the concentration C = 10^–5–10^–2 M XBr (X = H, Cs, Na, and Li) and NaOH, as well as in mixed solutions of 10^–4 M XBr (X = Cs, Na, and Li) + 10^–4–10^–2 M HBr and 10^–4 M XBr + 10^–4–10^–2 M XOH (X = Cs, Na, and Li) in ethanol containing 6 vol % of water were measured using conductometry and microelectrophoresis. The values of surface conductivity of quartz were calculated by the Wagner formula and used to calculate zeta potential by the Henry–Booth formula. The resultant dependences (logC) suggest that the value and sign of zeta potential are determined not only by the adsorption of potential-determining ions ⁺ and ^–, but also by the competitive specific adsorption of all ions of the aforementioned electrolytes, the adsorption values increasing in a cation series Li⁺ < Na⁺ < Cs⁺ < H⁺ and an anion series Br^– < OH^–. In particular, it is found that the titration of the above suspensions with XOH bases results in the reversal of zeta potential sign from negative to positive at a concentration depending on the adsorption capacity of alkali cation.     

Voltammetric study of the interaction of lomefloxacin (LMF)–Mg(II) complex with DNA and its analytical application

The voltammetric behavior of the LMF-Mg(II) complex with DNA at a mercury electrode is reported for the first time. In NH₃–NH₄Cl buffer (pH=9.10), the adsorption phenomena of the LMF–Mg(II) complex were observed by linear sweep voltammetry. The mechanism of the electrode reaction was found to be a reduction of LMF in the complex, and the composition of the LMF–Mg(II) complex is 2:1. In the presence of calf thymus DNA (ctDNA), the peak current of LMF–Mg(II) complex decreased considerably, and a new well-defined adsorptive reduction peak appeared at −1.63 V (vs. SCE). The electrochemical kinetic parameters and the binding number of LMF–Mg(II) with ctDNA were also obtained. Moreover, the new peak currents of LMF–Mg(II)–DNA system increased linearly correlated to the concentration of DNA in the 4.00×10⁻⁷–2.60×10⁻⁶ g ml⁻¹ range when the concentrations of LMF–Mg(II) complex was fixed at 5.00×10⁻⁶ mol l⁻¹, with the detection limits of 2.33×10⁻⁷ g ml⁻¹. An electrostatic interaction was suggested by electrochemical method.     

The effect of high pressure on the ion pair equilibrium constant of alkali metal fluorides: A spectrophotometric study

Bromophenol blue indicator was used in UV-visible spectrophotometric measurements to study ion association constants of alkali metal fluorides. The equilibrium constants for the ion pair formation of the alkali metal fluorides were determined as a function of ionic strength at one atmosphere pressure and 25°C. The effect of pressure on these association constants was measured at a constant total ionic strength of 1.0 mol-kg^–1 over a pressure range of 1 to 2000 atmospheres at 25°C. The pressure dependences of the stoichiometric association constants of the alkali metal fluorides are given by: lnK _LiF ^* =0.77–2.47×10^–4P–2.12×10^–8P²; lnK _NaF ^* =0.53–1.08×10^–4P–1.66×10^–8P²; lnK _KF ^* =0.24–4.41×10^–5P–7.15×10^–8P²; lnK _RbF ^* =–0.17–8.65×10^–5P–4.51×10^–8P²; and lnK _CsF ^* = –0.37–1.14×10^–4P–6.82×10^–8P², where P is the pressure in atmospheres. The stoichiometric molar volume and compressibility changes for ion pair formation of the alkali metal fluorides were evaluated from the pressure dependence of K _MF ^* data. The thermodynamic association constants were also calculated making use of activity coefficient data from the Pitzer equations. The partial molal volume and compressibility changes for ion pair formation of each alkali metal fluoride are reported.     

Reaction potential map analysis of chemical reactivity—III

Reaction potential maps (RPM) have been introduced as a new tool for the study of molecular reactivity. The equipotential energy maps, which are created on given planes around a molecule, define reaction contours towards specific counter-reagent models and are evaluated by perturbation theory. Since the calculated interaction energy involves electrostatic, polarization, exchange, and charge transfer energies, the RPM's can be used to predict site selectivity in a variety of chemical reactions. We found that the calculated RPM's of the SCN^– anion explained well the experimental observations that it reacts at the S atom with soft electrophiles and at the N atom with hard electrophiles. The difference in reactivity between SCN^– and OCN^– was clearly shown by the RPM's of these anions. The ambident nucleophilic nature of the NO ₂ ^– and the CH₂CHO^– anions was also well represented by their RPM's.     

Search for neutrons from cold nuclear fusion

An attempt was made to detect neutrons from the so-called cold nuclear fusion of deuterium in palladium and titanium, both saturated with deuterium: the palladium electrolytically and the titanium from gas phase. The measurements were performed in a tunnel located 30 m deep in limestone, using³He filled proportional counters surrounded by water for neutron moderation. In all cases the detected neutron flux was practically equal to the background level. Very low upper limits to the neutron source strength were obtained from this experiment: 2×10^–4 n.s^–1g^–1 Pd and 4.3×10^–4 n.s^–1g^–1 Ti on the 1 level.     

Ion channels of biological membranes: prediction of single channel conductance

The Poisson-Boltzmann equation was solved numerically for models of the pore regions of the Shaker K⁺ channel and of two glycoporins (LamB and ScrY) to yield electrostatic potential profiles along the pore axes. From these potential profiles, single-channel current-voltage (I–V) relations were calculated. The importance of a proper treatment of the ionisation state of two rings of aspartate sidechains at the mouth of the K⁺ channel pore emerged from such calculations. The calculated most likely state, in which only two of the eight aspartate sidechains were deprotonated, yielded better agreement with experimental conductance data. An approximate calculation of single-channel conductances based simply on pore geometry yielded very similar conductance values for the two glycoporins. This differed from an␣experimentally determined conductance ratio of ScrY:LamB=10:1. Preliminary electrostatics calculations appeared to reproduce the observed difference in conductance between the two glycoporins, confirming that single-channel conductance is determined by electrostatic as well as geometric considerations. Received: 25 May 1998 / Accepted: 4 August 1998 / Published online: 2 November 1998     

MD studies of electrolyte solution|liquid mercury interfaces

We report molecular dynamics (MD) computer simulations of a single lithium or iodide ion near a water|liquid mercury interface. The ion–mercury and the water–mercury potentials are derived from ab initio calculations of an ion or a water molecule and a mercury cluster consisting of seven, nine or 10 atoms. The flexible BJH water model and a mercury–mercury potential derived from pseudopotential theory are employed. The ion–water potentials are also based on ab initio calculations. The structural properties at the interfaces are described in terms of various density profiles and the ion–mercury radial distribution functions (RDF). An analysis of the induced rearrangements of the mercury atoms at and below the surface is also performed. Finally, the spectral densities of the hindered translational motions of the ions parallel and perpendicular to the mercury surface are reported. We conclude that, while the I⁻-ion is contact adsorbed on the mercury surface, the Li⁺-ion is not.     

Application of symmetry coupling coefficients in the evaluation of the electronic spectra of d 7 ions

The optical absorption spectrum of cobalt chloride thiourea has been studied at room and liquid air temperatures. The results have been interpreted using group theoretical methods taking into account all the quartet states and several doublet states. Crystal field, electrostatic, and spin-orbit coupling interactions were included to yield the allowed transitions with the relative transition probabilities. The experimental results were interpreted for the following set of parameters: B = 750cm^–1, C = 3100cm^–1, = 7370cm^–1, = -200 cm^–1, = 500cm^–1, and = 450 cm^–1.     

Investigation of the complex formation of technetium with thiourea by spectrophotometry and electromigration

TcO ₄ ^– is reduced by thiourea /tu/ to Tc/III/ in 6N HCl. By capillar electrophoresis three different cations have been identified with electrophoretic mobilities of u=/4.3–4.8/x10^–4 u=/3.2–3.8/x10^–4, and u=/2.2–2.8/x10^–4 cm² v^–1s^–1. These species were assigned to complex cations [Tc/tu/₆]³⁺, [Tc/tu/₅Cl]²⁺, and [Tc/tu/₄Cl₂]⁺, respectively. [Tc/tu/₆]³⁺ was identified by spectrophotometry according to recently published data. [Tc/tu/₅Cl]²⁺ was isolated and chemically characterized. The formation of the monovalent cation [Tc/tu/₅Cl₂]⁺ was concluded from spectrophotometrical measurements.     

Influence of molar ratios of precursor,catalyst, solvent and water on monolithicity and physical properties of TMOS silica aerogels

In continuation to our earlier work on aerogels, the experimental results on the monolithicity and physical properties of silica aerogels as a function of the molar ratios of tetramethoxysilane (TMOS) precursor, catalyst (NH₄OH), methanol (MeOH) solvent and water, are reported. The molar ratios of NH₄OH/TMOS, MeOH/TMOS and H₂O/TMOS were varied from 7.1 × 10^–6 to 9.6 × 10^–1, 1 to 90 and 1 to 18 respectively. It has been found that larger molar ratios of NH₄OH/TMOS (10^–2), MeOH/TMOS (13 to 60) and H₂O/TMOS (>10) resulted in transparent but cracked aerogels, and very low molar ratios of these combinations gave monolithic but less transparent or opaque aerogels. The best quality silica aerogels, in terms of monolithicity, transparency and low density, have been obtained with TMOS:MeOH:H₂O:NH₄OH in the molar ratio of 1:12:4:3.7 × 10^–3 respectively. The aerogels have been characterized by density, optical transmission, surface area and porosity measurements. The results have been discussed by taking into account the hydrolysis and condensation reactions, and syneresis effects.     

Die kinetische polarographische Stufe des Urans in Gegenwart von Chlorat-Ionen

An increase of the 2nd polarographic uranium(VI) wave has been observed in the presence of chlorate ions in HClO₄–NaClO₄, or HClO₄–NaClO₄–NaCl supporting electrolyte, resp. The polarographic measurements at different temperatures and at various perchloric acid concentrations show that this increase is due to a kinetic U(III)-U(IV) current. The activation parameters of the U(III)-U(IV) oxidation reaction with ClO₃ ^– have been calculated usingKautecky's method.The approximately 5fold increase of the 2nd polarographic wave allows the determination of small amounts of uranium (10^–5–10^–6 mole/l).

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Mit 2 Abbildungen     

Analysis of High-Purity Volatile Compounds

The current status of the analysis of high-purity volatile substances is considered. Two types of impurities in high-purity volatile substances were distinguished: molecularly dissolved substances and suspended particles. The main factors that restrict the limiting capabilities of analytical techniques were revealed. The attained detection limits were 10^–8–10^–10% for metal impurities, 10^–4–10^–8% for organic substances, 10^–5–10^–9% for water, and 10^–5–10^–7 for permanent gases. Suspended particles of 0.04–0.003 m in size were determined by light scattering.