[en] Formation constants for NpO₂(CO₃)_i^l2i (i = 1, 2 and 3), NaNpO₂CO_3(s) and Na₃NpO₂(CO₃)_2(s) are deduced from Simakin's et al. (1977), Maya's (1983), and Vitorge's et al. data, who also found evidence for a mixed Np(V)-OH-CO₃ soluble complex. Simakin (1977) found NpO₂(CO₃)₃^-4, it was confirmed by Riglet (1989), and by Offerle, Capdevilla and Vittorge (1995). Temperature influence was studied by Ullman and Schreiner (1988), and by Offerle, Capdevila and Vittorge (1995). Grenthe, Riglet and Vitorge (1986 and 1989) proved the existence of the trinuclear species (NpO₂)₃(CO₃)₆^-6. Maya (1984) mis-interpreted his data; nevertheless they show evidence of a new polynuclear mixed species, certainly (NpO₂)₂(OH)₃CO₃^-1, as initially proposed by Maya. No other Np(V) or Np(VI) soluble complex could be detected, the proposed ones quantitatively account for all published works. Unpublished data allowed to estimate the stability of intermediary mononuclear complexes and NpO₂CO_3(s) solubility product. M₄NpO₂(CO₃)_3(s) (M⁺ = K⁺ or NH₄⁺) ones are deduced from Gorbenko-Germanov and Klimov (1966), and Moskvin (1975) data as respectively interpreted and reinterpreted by this review. Thermodynamic data determined in this report are under discussion within OECD-NEA-TDB. (author)

[en] A 1.5 M Na₂CO₃ solution of Np(V) is electrolysed to Np(IV) at -2.0 V/SHE. -1g(H⁺) is decreased from 10.4 to 7.2 by bubbling CO₂ in these solutions, where Np(IV) spectra can be interpreted with the only lost of one CO₃^2- anion from the Np(IV) limiting complex. From these spectral changes, the following parameters are fitted: 20.5 ± 2.1, 8.44 ± 0.9 and 28.9 ± 2.9 l/mol./cm for the Np(CO₃)₅^6- molar absorptivity at 823, 990 and 1013 nm respectively, and 54.5 ± 5.5, 40.6 ± 4.1 and 8.53 ± 0.9 for the Np(CO₃)₄^4- ones, and log((Np(CO₃)₅^6-) / ((Np(CO₃)₄^4-)(CO₃^2-))) = 1.47 ± 0.08, 1.63 ± 0.05, 1.80 ± 0.04, 1.79 ± 0.10 and 2.21 ± 0.03 at the half point reaction in initially 0.2, 0.3, 0.4, 0.5 and 0.6 M Na₂CO₃ solutions. These values are extrapolated to 0 ionic strength by using the Specific Interaction Theory (SIT). The redox potential of 0.3, 0.6, 1 and 1.5 M Na₂CO₃ solutions of Np(V) and Np(IV) mixtures, is stable usually after three hours at T from 5 to 60 deg C, and then for up to three weeks at 21.5 deg C. At 25 deg C, its values are 0.247, 0.234, 0.244 and 0.228 V/SEH in 0.3, 0.6, 1 and 1.5 M Na₂CO₃ solutions. When the tonic strength is equal to 0: E = 0.52 ± 0.1 V/SEH and ΔS/F = -1.1 ± 0.7 mV. deg C^-1. Assuming this potential is controlled by the NpO₂(CO₃)₃^5- + 2 CO₂ + e^- ↔ Np(CO₃)₅^6- equilibrium, the formation constant of the limiting complex is deduced by using published values of the other needed equilibria: log Β₅^deg = 38 ± 4. Qualitative results on the preparation and on the spectra of Np(IV) are used to explain the apparent contradictions between some published results. (authors). 39 refs., figs., tabs

[en] Coprecipitation is often understood as the incorporation of elements at trace concentrations into -initially pure- solid compounds. Coprecipitation has typically been used to identify radioactive isotopes. Coprecipitation can result in lowering solubility as compared to the solubility, when controlled by pure compounds. For this reason it is also important for geochemistry, waste management and de-pollution studies. The solid obtained with coprecipitation is a new homogeneous solid phase called solid solution. The 2 formula needed to calculate the aqueous solubility when controlled by the ideal AB_b(1-x)C_cx solid solutions are K_s,B^1-x*K_s,C^x equals [A^z_A]*[B^z_B]^b(1-x)*[C^z_C]^cx/((1-x)^b(1-x)x^cx) and K_s,C/K_s,B equals (1-x)^b*[C^z_C]^c/[B^z_B]^b*x^c), where K_s,B and K_s,C are the classical constant solubility products of the AB_b and AC_c end-members, the b and c values are calculated from the (z_i) charges of the ions and from charge balance. This report is essentially written to provide a thermodynamic demonstration of the law of mass action in attempts to confirm scientific bases for solubility calculations in geosciences (as typically retention of radio-nuclides by co-precipitation), and to facilitate such calculations. Note that the law of mass action is here a set of 2 equations (not only 1) for the ideal or near ideal systems. Since they are consistent with the phase rule, no extra formula (beside mass balance) is needed to calculate the concentrations of all the species in both phases, namely: [A^z_A], [B^z_B], [C^z_C] and specially x

[en] The protonation constants of TPTZ (tripyridyl (2) - 2,4,6 triazine 1,3,5) have been measured: pKa₁ = 3.8 and pKa₂ = 2.7. (I = 1M, KCl). TPTZ can be autoassociated as (HTPTZ)sub(x)sup(x+) (x=3 or 4). The Am TPTZ³⁺ formation constant (log β = 4.22) is more stable than the lanthanides ones: log β₁ = 2.23/3.16/2.81/3.35/3.11/3.00/2.50/2.43/2.43/2.03./2.00/2.09 and 2.3 respectively for La/Pr/Nd/Sm/Eu/Gd/Tb/Dy/Ho/Er/Tm/Yb and Lu. The selectivity of TPTZ is applied to investigate the groups separation actinides (III)-lanthanides by a liquid-liquid extraction procedure, from nitric acid into several diluents. Acidic extractants dibutylthiophosphoric, di-2 ethylhexyldithiophosphoric, α-bromocapric (H α B Cr₁₀) or dinonylnaphtalensulfonic (HDNNS) acid were used to insure the organic complexes electroneutrality. Am(III) and Cm(III) and lanthanides are extracted into decanol as M(α Br C₁₀)₃ and MTPZ (α Br C₁₀)₃ this last complex is more stable with actinides (III) than with lanthanides (log Kew = -3,1 and -3,9 respectively). HDNNS-TPTZ mixtures form inverted micelles in t-butylbenzene and can extract the actinides 20 times better than the lanthanides from 0.3 M HNO₃. We explained qualitatively and quantitatively the extraction data, by assuming that HDNNS-TPTZ micelles behave like a 3rd phase

[en] The reversible redox potentials of the Plutonium couples are measured by using cyclic voltammetry, in perchloric media at ionic strength, I from 0,5 M to 3M, and temperature, T, from 5 deg C to 65 deg C. At each T, experimental results, E(T,I), are extrapolated to I = O by applying the Specific Interaction Theory (S.I.T.) to get interaction coefficients, Δ is element of (T), and E(T,O) (e.g. standard potentials when T = 25 deg C). At T = 25 deg C the numerical values of the potentials of all the Pu couples are nearly the same. It is then not easy to detect a systematic error due to disproportionation or redox impurity. This can explain some discrepancy on numerical values already published. We finally propose ''recommended values'' of the reversible redox potentials. As a first approximation, the variations of these potentials seem to be quite linear versus temperature: entropy variation versus T is small. But taking into account heat capacity that is involved in the E(T,I) second order derivative, usually improves the fitting. A second order expansion of ε(T) and of the Debye Huckel term, D(T) are used to propose equations that account for simultaneous ionic strength and temperature influences on G, S, Cp, H, and lg K. These equations, in particular those modelling the ionic strength influence on ΔS, ΔCp, and ΔH are first checked for published mean activity coefficients of HCI and NaCI. Small discrepancy between the numerical values of entropy changes of actinides redox couples, deduced from electrochemical and calorimetric techniques are discussed. (authors). 27 refs., 6 tabs., 10 figs

[en] By using spectrophotometric and potentiometric methods it is shown that, in media of high ionic strength (NaClO₄ 3M) and total concentrations of hexavalent actinoids higher than 10^-3M, trimers are formed. The equilibrium constants for the reactions 3MO₂(CO₃)₃^4- reversible (MO₂)₃(CO₃)₆^6- + 3 CO₃^2- are log K (U) = -11,3; log K (Np) = -10,1 and log K (Pu) = -7,4. It is demonstrated that one of the cations of the trimer can be exchanged with another actinoid cation: the equilibrium constants for the reactions 2UO₂ (CO₃)₃^4- + MO₂ (CO₃^4- reversible (UO₂)₂ MO₂ (CO₃)₆^6- + 3CO₃^3- are log K = -11,3; -10,0 and -8,8 respectively for M: U, Np, Pu. Then, polynuclear complexes can be efficient solution ''carriers'' for other hexavalent actinides in a waste disposal. Some properties of the solid phases MO₂CO₃(s) are discussed. Experimental studies of chemical equilibria of americium (III, IV) are reviewed: in carbonate media Am(III) species are Am CO₃⁺, Am (CO₃)₂^- Am (CO₃)₃^3-, (Am₂ CO₃)sub(s) and (NaAm(CO₃)₂)sub(s); for Am(IV), log B₅ approximately equal to 40. In NaClO₄ (3M) medium solubility measurements of Np(V) show that logβ₁ = 5,09, logβ₂ = 8,15, logβ₃ = 10,46, log Ks(Na Np O₂ CO₃) = 10,56 and log Ks (Na₃ NpO₂(CO₃)₂) = 12,44; ionic strength corrections are proposed

[en] The formal potential, E, of the Np (VI)Np(V) redox couple is measured versus a Ag/AgC1 electrode with junction potential less than 0.002 V, by using cyclic voltammetry in 0.22, 0.55, 1, 1.25 and 1.5 M Na₂CO₃ solutions, and at T = 5, 15, 25, 35, 45, 55 and 60 deg. C. At each T, E is extrapolated to I 0 (I: ionic strength) by using the Specific Interaction Theory (SIT) formula E(I) E(0) + 9 a√I/1+ Ba_i√I + 2 δξm. At each I, E data are fitted to a second order polynomial expression as a function of T, to deduce the entropy change, δS, and the heat capacity change, δCp. The variations of δS and δCp with I calculated by using formulae deduced from the SIT one, are consistent with the data. In the standard conditions E deg.=0.341 ± 0.017 V/ESH, δSdeg. = -190 ± 5 J/K/mol, δCpdeg. -345±750 J/K/mol. δξ = 0,15 + 0,05 -(0,005±0,001)δT + 0,00004 δT² kg /mol, where δT=T-25 deg.C. These numerical values are consistent with the U and Pu ones. The redox potential measured in 1 M Na₂CO₃ solution, is greater by about 0.06 V than the published ones. Junction potentials might account for this difference. Supplementary materials are added, concerning the calculation of activity of water in a weak electrolyte, and on correlation between the numerical values of ξ, or with ionic radius. (authors). 12 refs., 20 figs., 19 tabs

[en] The aim of this work is to propose and to check approximations to calculate from only a few experimental measurements, ionic strength, I, and temperature, T, influences on Gibbs energy, G, redox formal potential, E, and standard equilibrium constant, K. Series expansions versus T are first used: S and Cp/2T_a are typically the -G first and second order terms. In the same way, -ΔH and T²ΔCp/2 are the first and second order terms of R in K expansions versus 1/T. This type of approximation is discussed for the E of the M⁴⁺/M³⁺, MO₂²⁺/MO₂⁺ and MO₂(CO₃)₃^4-/MO₂(CO₃)₃^5-couples (M = U or Pu) measured from 5 to 70 deg C, for the standard ΔG of some solid U compounds, calculated from 17 to 117 deg C, and for ΔCp, ΔG and Ig K of the CO₂(aq)/HCO₃^- equilibrium from 0 to 150 deg C. Excess functions, X^ex, are then calculated from activity coefficients, γ: enthalpy, H, or heat capacity, Cp, adjustment as a function of I changes is needed only when the γ adjustment as a function of T changes is needed. The SIT coefficient, ε, variations with T, are small and roughly linear for the above redox equilibria and for chloride electrolytes mean γ: first order expansion seems enough to deduce ε, and then the excess functions G^ex, S^ex and H^ex, in this T range; but second order expansion is more consistent to estimate Cp^ex. (authors). 25 refs., 3 tabs., 1 fig

[en] This paper examines Americium behavior in Cl^- media at room temperature in connection with environmental and waste disposal programs. Most published values on U, Np, Pu and Am complexation in chloride media have been determined using extraction methods. Spectrophotometric techniques are not sensitive enough to prove actinide complexation by chloride, which is confirmed in this paper for Am(III). Am(OH)_3(s), AmOHCO_3(s), Am₂(CO₃)_3(s) or NaAm(CO₃)_2(s) solid phases can control the Am solubility, depending on the chemical conditions of the aqueous phase (usually P_CO2). ²⁴¹Am solubility is here found to be higher in NaCl 4M media than in NaCl 0.1 M (up to 3 orders of magnitude). Addition of a reducing agent (metallic iron) lowers the solubility. After a week, solubilities in NaCl 0.1 M and 4 M are similar. These results are consistent with Am(III) radiolytic oxidation to Am(V), due to α radiations. Little evidence of Cl^- or mixed Cl^--CO₃^2- complexes is found in these conditions. In Na⁺-OH^--Cl^- media, ²⁴¹Am(III) oxidation had also been proposed. Slow kinetics of precipitation could induce experimental uncertainties

[en] Aqueous complexes of M(III) f-element ions with the inorganic ligands CO₃^2- and SO₄^2- have been investigated using the highly-sensitive speciation techniques TRLFS and ESI-MS. The Eu(CO₃)_i^3-2i (i=1-3) species have been characterized by TRLFS, and the stoichiometry of the limiting complex Eu(CO₃)₃^3- have been confirmed by solubility measurements of NaEu(CO₃)₂(s) at high ionic strength. Temperature effect on Cm(III) carbonate complexes is evidenced by the TRLFS technique. Investigation on sulphate complexation has been done at various ionic strengths by TRLFS on Eu(III) and by ESI-MS on La(III). New thermodynamic data are obtained by both techniques, which are consistent with literature data. (authors)