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  • Author or Editor: G. Duplâtre x
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Abstract  

Ion association has been studied by positron lifetime spectroscopy in aqueous solutions containing the Ni2+ and SO 4 2– ions at 294 K with the double aim of assessing the reliability of the method for quantitative determination of complex formation constants and of probing the validity of various expressions to calculate single-ion activity coefficients at high ionic strength. The existence of two complexes, identified as NiSO4 and Ni2SO 4 2+ , is shown by the data analysis. Considering the formation constant of the former, KI=(196±10)M–1, determined in previous works leads to discarding several of the expressions commonly used for activity corrections. Two possible values are retained for KI, (193±20)M–1 and (179±20)M–1, while KII related to Ni2SO 4 2+ is better defined, as (2.57±0.14)M–1.

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Abstract  

For a better understanding of isotope exchange in solid Tl4Cl6 the effects of crushing the crystals were investigated by conductivity and by positron annihilation lifetime measurements. As in untreated Tl4Cl6, the conductivity variation with temperature shows a break at about 450 K. The activation energies, 0.53 eV and 0.70 eV, respectively, below and above 450 K, are very close to those in the untreated material but the absolute values of conductivity are lower after crushing which is attributed to the trapping of the mobile defects of dislocation. The positron lifetime variation with increasing temperature shows some contribution of extrinsic defects, annealing at about 413 K. On further heating or cooling cycles, the lifetime changes are controlled by the production of intrinsic cation vacancies, whose formation enthalpy, 0.39 eV, is close to that derived for untreated Tl4Cl6. The shape of the initial part of the curves would indicate that crushing does not directly create appreciable concentrations of cation vacancies but would rather produce annealable defects, possibly dislocations, favouring the formation and/or trapping of such vacancies.

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Abstract  

The solvent extraction of uranium(VI) with a di-carboxylated calix[4]arne (LH2) in chloroform and 1,2-dichloroethane has been studied in the presence or absence of alkali ions (M=Na+,K+). For UO 2 2+ when studied alone, a 12 (metal: ligand) extracted species is evidenced, with a rather low associated extraction equilibrium constant. The efficiency of extraction increases drastically in the presence of alkali ions, due to the formation of heteronuclear complexes. In all cases, the extracted species are found to be both 122 and 112 (UO 2 2+ MLH2) mixed complexes, except in chloroform with K+, where only the latter is formed. In the case of Na+, mass spectrometry spectra confirm the existence of both homo and heteronuclear complexes as determined in the extraction studies.

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Abstract  

The rate constants for the reactions of positronium with I 3 , Br, and I in dimethylsulphoxide (DMSO) and for I 3 in water have been determined, and the orthopositronium lifetime variations have been used for studying polyhalide formation in the (I2+Br)/DMSO system. It is found that the formation of more than one polyhalide is needed to account for the data. On basis of this new evidence, previous results in methanol and in DMSO are re-examined. Besides the primary I2X (X: halide), I2X 2 2− complexes have to be invoked. Trial absorption measurements confirm the presence of solvolysis equilibria for I2 in DMSO. Taking these facts into account, much higher, albeit poorly defined, values than previously reported are found for the I2X formation constants. The reliable Kc values deduced are 28M−1, 0.57M−1 and 2.5M−1 respectively for I2Cl, I2Cl 2 2 and I2Br 2 2− in methanol, and 1.4M−1 and 0.8M−1, respectively for I2Cl 2 2− and I2Br 2 2− in DMSO.

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Abstract  

The formation of positronium in various solvents and the inhibition and enhancement of positronium formation by a wide variety of solutes are discussed. The studies contribute to the investigation of localisation processes in liquids.

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Abstract  

The rate constants for the reactions of positronium with halogen molecules and with halide ions in methanol (I2, Br2, Cl, Br) and in dimethylsulphoxide (I2, Cl) have been measured. The variations of the ortho-positronium lifetime in mixtures of these solutes have been used for quantitative determination of the formation constants of the corresponding polyhalides. These were found to be 2.9 and 5.4 M−1, respectively, for I2Cl and I2Br in methanol and 2.0 M−1 for I2Cl in dimethylsulphoxide. Experiments on acidified AgClO4 solutions confirmed the formation, at molar concentrations, of the Ag2 ClO 4 + species, very probably a triplet ion association.

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Abstract  

The effect of CCl4 on positronium (Ps) formation is examined in isooctane through positron lifetime spectroscopy experiments, in the framework of a series of studies on AOT-based reverse micellar systems. This solute appears as a rather strong Ps formation inhibitor, as expected in view of its well-known electron scavenging properties. The reciprocal of the triplet Ps intensity (1/I 3) varies linearly with CCl4 concentration (C), up to about 0.08M, allowing to derive a high inhibition constant,k=36.6M–1. Above 0.08M, however, the inhibition slackens out, which is attributed to an anti-inhibition effect. On the basis of previous reports on halocompounds, a general scheme of reactions is proposed to occur in the positron spur, including, besides the primary Ps formation through thee +/e reaction and the recombination reactions, electron scavenging by CCl4 as the direct inhibiting reaction, followed by the reaction of the thus formed anion with the positron. The latter reaction may lead either to a [CCl4Ps] bound-state or, if the positron picks off the electron from the anion, to some additional Ps formation. By using a simple quantitative approach to this scheme, an equation descriptive of the variation ofI 3 withC is derived which agrees well with the experimental data.

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Abstract  

Several attempts have been made to synthesize mixed-valence compound of antimony at low temperature in view of studying the isotopic exchange in solids. The experimental procedure for the synthesis, at 263 K of Cs4(SbIIICl6)(SbVCl6) has been established. This compound seems very promising for isotopic exchange studies in the solid state.

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Journal of Radioanalytical and Nuclear Chemistry
Authors:
G. Duplâtre
,
Zs. Kajcsos
,
T. Goworek
,
L. Varga
,
L. Liszkay
,
I. Billard
, and
K. Lázár

Abstract  

Positron annihilatioin data collected for zeosil (a zeoilite-type solid allowing abundant Psformation) are discussed, with particular attention payed to the longest-lived positronium states as observed in lifetime spectroscopy experiments. To unravel the nature of these components, additional information was sought by applying an extermal magnetic field and by reecording the total energy distributioin of the annihilation radiation. From both the lifetime and the extended Doppler spectra, it is shown that positronium presents a very long-lived component with a relative intensity of ca. 30% and a lifetime close to the intrinsic tripler lifetime (140ns), therefore undergoing almost no pick-off annihilation. From the magnetic field experiments, the contact density parameter has a value of essentially 1, which is characteristic of an unpoerturbed triplet Ps state in vacuo. This positronium state therefore does not appear to interact with the medium.

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