Radiohochspannungsionophoretische untersuchungen der isotopenaustauschreaktionen zwischen polythionaten und sulfit

The isotopic exchange of sulfur-35 between trithionate/sulfit and tetrathionate/pentathionate is examined. High voltage paper ionophoresis is applied for the separation of the reaction mixture. The amount of activity of the reaction products is determined at different times. This enables to calculate the kinetical data of those exchange systems.      

Determination of traces of bismuth by isotopic exchange reaction in the organic phase

A new radioanalytical method has been developed based on homogeneous isotopic exchange in the organic phase. The theory of the method is presented and discussed. This method has been applied for the simple, selective and rapid determination of bismuth based on the isotopic exchange between bismuth diethyldithiocarbamate and bismuth iodide complex labelled with²¹⁰Bi. As little as 0.01 μg of the metal can be determined.     

EPR studies of isotopic exchange involving adsorbed oxygen species on MgO

In the EPR spectra of ¹⁷O₂^? adsorbed on MgO, the hyperfine lines due to ¹⁷O(I = $\text{5}\text{2}$) have been used to monitor the isotopic exchange with gas phase oxygen. The predicted behaviour of the different possible mechanisms (place exchange and/or isotopic exchange) is compared with the experimental results which show that, on MgO at room temperature, isotopic exchange occurs via an intermediate species which is tentatively identified as O₄^?.     

Mass spectrometric investigations of the kinetic stability of chromium and copper complexes with humic substances by isotope-labelling experiments

Isotope-labelling exchange experiments were carried out to investigate the kinetic stability of Cr(III) complexes with humic substances (HS). To compare the results with those of an ion, not expected to form kinetically stable HS complexes with respect to its electron configuration, Cu(II) was investigated under the same conditions. HS solutions of different origin were therefore spiked with ⁵³Cr(III) or ⁶⁵Cu(II) after saturation of HS with chromium and copper of natural isotopic composition. In fractions of metal/HS complexes with different molecular weight, obtained by ultrafiltration and HPLC/ICP-MS using size exclusion chromatography (SEC), respectively, the isotope ratios of chromium and copper were determined by ICP and thermal ionisation mass spectrometry. Distinct differences in the isotopic composition of chromium were found in the permeate of the ultrafiltration compared with the corresponding unseparated solution, which indicates kinetically stable Cr(III)/HS complexes. On the other hand, the copper isotopic composition was identical in the permeate and the unseparated solution, which shows that a total exchange of Cu²⁺ ions took place between free and HS complexed copper ions. The SEC/ ICP-MS experiments also resulted in a different isotopic distribution of chromium in the chromatographically separated complexes whereas the copper complexes, separated by SEC, showed identical isotopic composition. The kinetic stability of Cr(III)/HS complexes could be explained by the d³ electron configuration of Cr³⁺ ions, a fact which is well known from classical Cr(III) complexes, and influences substantially the mobility of this heavy metal in the environment.     

A kinetic study of rearrangement and degradation reactions of tetrathionate and trithionate in near-neutral solutions

The kinetics of reactions of trithionate and tetrathionate via different reaction pathways were studied in near-neutral solutions. In the case of trithionate, the predominant degradation reaction is hydrolysis to thiosulfate and sulfate. The pseudofirst-order rate constant is independent of pH and measured to be (6.2 ± 0.2) × 10(-7) s(-1) for the pH range of 5.5-10.5. With tetrathionate, the reaction in both neutral and alkaline solutions occurs via a thiosulfate catalyzed rearrangement reaction to trithionate and pentathionate followed by their further reactions. The data suggest that when thiosulfate is completely absent, this first step will not occur, and in the presence of thiosulfate, the rate of the first step is independent of pH in the pH range 6-8. The secondary reactions include the hydrolysis of trithionate and the further rearrangement and degradation of pentathionate. This mechanism explains the dominant reaction products for both neutral and alkaline solutions. In the presence of thiosulfate and at near-neutral pH, the rearrangement reactions dominate over the degradation reactions and exhibit second-order kinetics, with the rate constants in the pH range of 6-8 determined to be (4.24 ± 0.26) and (1.89 ± 0.18) × 10(-4) M(-1) s(-1) for tetra- and pentathionates, respectively. At extremely low thiosulfate concentrations, the main secondary reaction is the hydrolysis of trithionate at pH < 7, and at pH > 7, the degradation of pentathionate.     

Mass spectrometric investigations of the kinetic stability of chromium and copper complexes with humic substances by isotope-labelling experiments

Isotope-labelling exchange experiments were carried out to investigate the kinetic stability of Cr(III) complexes with humic substances (HS). To compare the results with those of an ion, not expected to form kinetically stable HS complexes with respect to its electron configuration, Cu(II) was investigated under the same conditions. HS solutions of different origin were therefore spiked with ⁵³Cr(III) or ⁶⁵Cu(II) after saturation of HS with chromium and copper of natural isotopic composition. In fractions of metal/HS complexes with different molecular weight, obtained by ultrafiltration and HPLC/ICP-MS using size exclusion chromatography (SEC), respectively, the isotope ratios of chromium and copper were determined by ICP and thermal ionisation mass spectrometry. Distinct differences in the isotopic composition of chromium were found in the permeate of the ultrafiltration compared with the corresponding unseparated solution, which indicates kinetically stable Cr(III)/HS complexes. On the other hand, the copper isotopic composition was identical in the permeate and the unseparated solution, which shows that a total exchange of Cu²⁺ ions took place between free and HS complexed copper ions. The SEC/ ICP-MS experiments also resulted in a different isotopic distribution of chromium in the chromatographically separated complexes whereas the copper complexes, separated by SEC, showed identical isotopic composition. The kinetic stability of Cr(III)/HS complexes could be explained by the d³ electron configuration of Cr³⁺ ions, a fact which is well known from classical Cr(III) complexes, and influences substantially the mobility of this heavy metal in the environment. Received: 7 December 1998 / Revised: 25 March 1999 / Accepted: 27 March 1999     

Determining Counterion Exchange Selectivities at Micelle Surfaces from Fluorescence Decay Measurements*

A method for determining counterion exchange selectivities at the surface of ionic micelles from time-resolved fluorescence quenching measurements is developed. This method is employed to determine selectivity coefficients for thiosulfate/chloride exchange at the surface of cati-onic hexadecyltrimethylammonium chloride micelles (KTS/CI, ⁼ 1.1 ± 0.4) and for copper (II)/sodium exchange at the surface of anionic sodium dodecyl sulfate micelles (K_Cu/Na= 1.3 ± 0.3). In both cases, the selectivity coefficients are found to be independent of detergent and added salt concentration.     

Analysis of Total Non-Volatile Non-Sulfide Reduced Sulfur in Potable and Untreated Surface and Groundwaters

Abstract

An analytical method was developed to measure the total concentration of non-volatile non-sulfide reduced sulfur in treated and untreated surface and groundwater. The method was based on the alkaline reduction by Raney nickel (prepared in situ from Raney alloy) of organic and inorganic sulfur compounds (in oxidation states below +6) to sulfide. Sulfide was swept out of the reflux apparatus under nitrogen into a trap of zinc acetate and determined colorimetrically as ethylene blue. The recoveries obtained from solutions of elemental sulfur, sulfide, sulfite, thiosulfate, tetrathionate, cysteine, cystine, methionine, glutathione, allylthiourea, sulfanilamide and thiocyanate ranged from 84–102% with typical recoveries of approximately 90%. Sulfate was not detectable by the procedure. Chlorine present in potable water samples interfered in the procedure and was removed with sodium borohydride. The relative standard deviation of the method varied from 0.25 to 5.6% and averaged 3%. The detection limit based on a 500 mL sample was 3 mg m^?3.     

The effect of sodium cation concentration on the electrochemical reduction of copper(I) thiosulfate complexes

The electrochemical behavior of copper(I) thiosulfate complexes from aqueous solutions containing different amounts of sodium perchlorate NaClO₄ is studied by the methods of hydrodynamic voltammetry and potentiometry with a Na⁺-selective electrode. The electrochemical reaction orders p with respect to Na⁺ cations are determined from the dependences of exchange currents and direct reaction currents at fixed potential on the equilibrium concentration of Na⁺ cations. The reaction order p is close to 1 in the Na⁺ concentration range of 0.06–0.12 M and drops to zero for $c_{Na^ + } $ > 0.12 M. The ion pair (IP) {Na[Cu(S₂O₃)₂]}_2? the formation of which precedes the electron transfer reaction is the electrochemically active species for the reduction of copper(I) thiosulfate complexes. The stability constant of IP is determined (K = 27.0 ± 2.4) as well as the rate constants of IP formation and decomposition.     

Substitution kinetics of chelate complexes as studied by isotopic exchange

The fact that isotopic exchange reactions take place at chemical equilibirum makes this type of reaction useful for kinetic studies. In the case of chelate complexes [M(chel)_n] isotopic exchange reactions can be classified as metal exchange reactions. [M(chel)_n]+^*M⇆[^*M(chel)_n]+M and ligand exchange reactions [M(chel)_n]+^*chel⇆[M(chel)_n−1 ^*chel]+chel The literature on type (a) and type (b) reactions of mainly transition metal complexes is reviewed with respect to the kinetic information obtained. The value and the limitations of isotopic exchange studies as a kinetic technique are discussed.     

The determination of235U/238U ratios by activation analysis utilizing high resolution γ-spectrometry

The application of high resolution γ-spectrometry to the direct determination of²³⁵U/²³⁸U isotopic ratios in small uranium samples after neutron activation is reported. The consistency obtained relative to mass spectrometrically determined isotopic ratios is better than 5% for isotopic ratios between 0.06885 and 1.095. The method is not subject to the main disadvantages of the known radiochemical methods for²³⁵U/²³⁸U isotopic ratio determinations, and should be appreciably more rapid compared to the solid-source mass spectrometric method giving the same precision.     

Detailed Description of Pulse Isotopic Exchange Method for Analyzing Oxygen Surface Exchange Behavior on Oxide Ion Conductors

A novel pulse ¹⁸O-¹⁶O isotopic exchange (PIE) technique for measurement of the rate of oxygen surface exchange of oxide ion conductors was presented. The technique employs a continuous flow packed-bed micro-reactor loaded with the oxide powder. The isothermal response to an ¹⁸O-enriched pulse passing through the reactor, thereby maintaining chemical equilibrium, is measured by on-line mass spectrometry. Evaluation of the apparent exchange rate follows from the uptake of ¹⁸O by the oxide at given reactor residence time and surface area available for exchange. The developed PIE technique is rapid, simple and highly suitable for screening and systematic studies. No rapid heating/quenching steps are required to facilitate ¹⁸O tracer anneal or analysis, as in other commonly used techniques based upon oxygen isotopic exchange. Moreover, the relative distribution of the oxygen isotopologues ¹⁸O₂, ¹⁶O¹⁸O, and ¹⁶O₂ in the effluent pulse provides insight into the mechanism of the oxygen exchange reaction. The PIE technique has been demonstrated by measuring the exchange rate of selected oxides with enhanced oxide ionic conductivity in the range of 350?900 oC. Analysis of the experimental data in terms of a model with two consecutive, lumped steps for the isotopic exchange reaction shows that for mixed conductors Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ(BSCF) and La₂NiO_4+δ the reaction is limited by the apparent rate of dissociative adsorption of O₂ molecules at the oxide surface. For yttria-stabilized zirconia (YSZ), a change-over takes place, from rate-limitations by oxygen incorporation below ∽800 oC to rate-limitations by O₂ dissociative adsorption above this temperature. Good agreement is obtained with exchange rates reported for these materials in literature.     

Kinetics of Heterogeneous Isotopic Exchange Reaction in Finite Bath

A general method to find the rate constant and particle self-diffusion coefficient is suggested for a heterogeneous isotopic exchange reaction which is controlled by surface mass reaction or controlled by a combination of surface mass reaction and intraparticle diffusion. The values of the kinetic parameter, ξ₁ (ratio of the forward surface mass reaction rate to the intraparticle diffusion rate), particle self-diffusion coefficient D and rate constant k are obtained by the proposed method for the isotopic exchange reaction systems CaCO₃(s)/Ca²⁺(aq) and CaC₂O₄(s)/Ca²⁺(aq).     

Studies of oxygen transport mechanism in electrolytes based on doped lanthanum silicate with apatite structure using techniques of oxygen isotopic heteroexchange and impedance spectroscopy

The work presents the results of studying the mechanism of oxygen transport for a new promising class of oxygen-containing electrolytes based on lanthanum silicate with an apatite structure using impedance spectroscopy and isotopic oxygen heteroexchange. At 1000 K, in the case of samples with an optimum composition including codoped Fe and Al, σ ～ 3 × 10^?3 to 10^?2 S/cm and D* reaches ～10^?8 cm²/s, which is close to the values of YSZ and Ce_0.9Gd_0.1O_{2 ? δ} (GDC). Lower energies of conductivity activation and oxygen diffusion for doped apatites (～0.5–0.8 eV instead of ～1 eV for GDC) and also equivalence as regards exchange of all oxygen atoms within apatite agree with the model, in which oxygen mobility is determined by a nonlinear cooperative migration process of oxygen atoms with fast exchange between interstitial and regular sites.     

Elemental,Isotopic, and Pesticide Analysis of Wild and Cultivated Berries

ABSTRACT

The focus of this study was to investigate differences in isotopic, elemental, and trace pesticide concentrations of wild and cultivated berries from Transylvania. To emphasize differences based on geographical origin, stable isotopic ratios of ²H/¹H, ¹⁸O/¹⁶O, and ¹³C/¹²C were determined by isotopic ratio mass spectrometry. Elemental fingerprinting of berries was performed by inductively coupled plasma mass spectrometry. The determination of trace pesticides in berries was performed by gas chromatography–mass spectrometry. Differences between wild and cultivated berries were evaluated using multivariate statistical analysis. The results suggest that multielemental, isotopic, and trace pesticide fingerprinting is feasible for sample differentiation.     

Exchange reactions in selectively excited Br2(B3Π)

Laser-controlled bimolecular isotopic exchange reactions in bromine leading to alterations in natural isotopic abundance ratios have been directly observed using laser-modulated molecular beams and mass analysis. Several instances of selective excitation and subsequent rapid isotope exchange are observed within the 7 GHz gain curve of the exciting 514.5 nm line of an argon-ion laser. These exchange processes are attributed to the direct photochemical participation of excited molecular states in the B³Π_ou⁺ system of Br₂.     

Crystal hydration and vacancy enhanced isotopic exchange reactions in heterogeneous solid-liquid systems

The rate of some heterogeneous isotopic exchange reactions can be enhanced by the simultaneous hydration of the solid participating in the exchange process. The results presented here show that the concomitant hydration of anhydrous sodium sulphate enhances the Na₂SO₄–²²Na⁺ isotopic exchange reaction in a water-acetone medium. The reaction rate is further enhanced by creating a controlled number of vacancies in the crystal lattice of sodium sulphate by doping.     

Exchange and decomposition reactions of isobutyl iodide with iodine

In the presence of elementary iodine, isobutyl iodide (2-methyl-1-iodopropane) undergoes isotopic exchange and also decomposes with production of additional iodine. Both reactions are approximately first order in isobutyl iodide and half order in iodine molecules. In degassed hexachlorobutadiene at 160°, the rate constants for exchange and decomposition are 7.5 × 10^?6 and 11.4 × 10^?6 (liter/mole)^1/2sec^?1, respectively. The decomposition is probably initiated by iodine atom abstraction of a β hydrogen atom, but comparison with rates for related compounds indicates that this hydrogen abstraction does not contribute significantly to the mechanism of exchange.     

Massenspektrometrische Bestimmung von ppm-Mengen Thorium in Uranerzen durch Isotopenverdünnungsanalyse

Thorium is chemically separated from uranium ores in a non-quantitative way. In one sample the isotopic ratio ²³⁰Th/²³²Th is determined in a mass spectrometer. In another sample ionium is added as indicator for the isotopic dilution; the thorium of this particular sample is also analysed in a mass spectrometer. The content of ²³⁰Th and ²³²Th is calculated from the measurements of the isotopic ratios and the amount of the ionium indicator. The results are compared with the results obtained by other methods.     

The kinetics of the15N/14N isotopic exchange between nitric oxide and nitric acid

The rate of the¹⁵N/¹⁴N isotopic exchange between NO−HNO₃ at high nitric acid concentration (2–10M) have been measured. The experimental data were obtained by contacting nitric oxide at atmospheric pressure with nitric acid solution labelled with¹⁵N, in a glass contactor.