Evaluation of Speciation Technology - OECD Nuclear Energy Agency

A calorimetric chamber must have other properties: a low specific heat relative to that of the
contained aqueous solution in order to gain sensitivity, an excellent thermal conductivity to
compensate the poor agitation of the designed system, and finally a chemical inertness against the
contained solution. An 18-carat gold alloy was selected for a new calorimetric chamber while
essentially conserving the previous design and system support. The new system has a heat capacity of
ca. 50 J•K -1 when filled with 6.5 cm 3 of water.
The performance of the instrument was verified by using two test reactions. Firstly, the molar
enthalpy of solution of TRIS [tris(hydroxymethyl)-aminomethane(cr)] in 0.100 mol•dm -3 HCl was
measured. The resultant effect, ∆slnHm =-(29.90 ± 0.18) kJ⋅mol -1 *, is comparable to the value recommend
by the National Bureau of Standards (NBS), ∆slnHm = -(29.769 ± 0.029) kJ•mol -1 [5]. A second test with a
reaction leading to CO2(g) formation was also carried out by dissolving Na2CO3(cr) in 1.00 mol dm -3
HCl. The resultant molar energy of solution of Na2CO3(cr) (P.A. Merck, heated at 623 K under
vacuum) is ∆slnEm = -(51.94 ± 0.55) kJ•mol -1 . This value can be compared with that obtained using a
suitable Hess cycle and auxiliary data**, ∆slnEm = -(53.3 ± 0.8) kJ•mol -1 [1]. The difference could be
attributed to approximations used for the calculation, such as ∆fHm 0 (CO2, sln) = ∆fHm 0 (CO2, aq).
The accuracy of the new calorimeter chamber ranges between 0.6 and 1% for chemical effects ranging
from 1 to 5 Joules, which is appropriate since limited amounts of samples are handled (a few mg).
Characterisation of lanthanide hydroxycarbonates
If orthorhombic A-form, A-M(OH)CO3(cr) containing variable amounts of water, is usually
isolated from aqueous solution, the hexagonal B-form, B-M(OH)CO3(cr), is obtained best under
hydrothermal conditions by decomposition of the hydrated sesquicarbonate according to Eq. (1) [6].
M2(CO3)3 nH2O(cr) → 2 B-M(OH)CO3(cr) + (n-1) H2O(g) + CO2(g) M = Nd, Sm. (1)
Each compound was prepared in triplicate and carefully characterised. Each prepared sample was
analysed by gravimetry of the lanthanide sesquioxides. For each of them, all observed diffraction lines
were indexed in the hexagonal system and intensities were consistent with the space group P(-6)
(No. 174). All the result, lattice parameters agree well with literature data. They closely relate to those
published for B-Am(OH)CO3(cr) [3]. The FT-IR spectra of all samples in KBr pellets show the two
typical well-defined double bands of the same intensity in the region of OH-stretching (νOH) vibration
near 3 500 cm -1 [7]. Absence of DTA signals below 400°C confirms that the samples do not contain
water.
Thermochemistry of lanthanide hydroxycarbonates
The standard molar enthalpy of formation of lanthanide hydroxycarbonates can be obtained by
using the thermodynamic cycles shown in Table 1. The first reaction corresponds to the energy of
solution of B-M(OH)CO3(cr) in 6.00 mol•dm -3 HCl and measured with the new calorimetric chamber.
Within the uncertainty limits, no systematic differences could be detected between the various
preparations of a given compound. In each cycle, the second reaction involves the molar enthalpy of
dissolution of the lanthanide metals of high purity in the same medium, 6.00 mol•dm -3 HCl [9]. In this
table, the last two reactions (3 and 4) represent the partial molar enthalpies of formation of H2O and
* Uncertainty limits are reported for the 95% confidence level using standard statistical methods [11].
** Unless specified, auxiliary thermodynamic data are those recommended by CODATA [9].
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