Lanthanides / Actinides / Chlorides / Ions / Thermochemistry
SummaryThe determination by solution calorimetry of thermochemical data associated with the formation of environmentally important compounds such as lanthanide and actinide hydroxides, hydroxycarbonates and carbonates... requires the knowledge of the molar enthalpies of formation of the relevant cations in the medium -often hydrochloric acid solutions -selected for the dissolution of these compounds. Surprisingly enough, for a number of lanthanides, the data in such media are fragmentary and sometimes contradictory. In this paper, are reported new results on the enthalpy of solution of lanthanum, neodymium, gadolinium, erbium and lutetium, in hydrochloric acid solutions at concentrations up to 6 mol · dm \ using ultra high purity metal samples. The evolution of these enthalpies of solution as a function of the HCl concentration is discussed and compared to the corresponding literature data for trivalent actinides (mainly plutonium and americium).From the measurement of the enthalpies of solution of GdCl·, (cr) and LuCl·, (cr) in media ranging from 6.00 mol · dm"' HCl to 10~3 mol · dm -3 HCl, new values are reported for the standard molar enthalpies of formation of these compounds and of the gadolinium and lutetium ions.Based on these and other determinations which appeared recently in the literature, an evaluation of the status of the standard molar enthalpies of formation of several lanthanide trichlorides and ions is made.
Lanthanide and actinide carbonates and hydroxycarbonates are, together with the hydroxides, important compounds in the problematics of high level waste disposal, since such and related compounds may appear as solid phases as a result of the interaction of the waste with natural waters. Experimental results on the stability and relationships between these compounds, particularly the hydroxycarbonates, are relatively fragmentary and arise mainly from solubility measurements, and, to a lesser extent, from enthalpy of formation determinations.We report here the enthalpies of formation of a number of well characterized hydroxycarbonates of trivalent lanthanides, namely those of neodymium, samarium, dysprosium, ytterbium, and of americium, as well as that of neodymium sesquicarbonate, using solution calorimetry in acidic media. The obtained values are discussed in the light of literature data on existing solubility and enthalpy of formation of related compounds.Our results have been obtained with a newly conceived sealed calorimeter, briefly described in the paper, in which the C0 2 produced during the dissolution reaction is maintained in solution, thus eliminating the problem of the kinetically slow evolution of the C0 2 (g) from the medium and the irregularities in the associated thermal effect.
The enthalpies of solution of Np 2 O s (cr) and Nd(OH) 3 (cr) in 6.00 mol-dm" 3 HCl, and of Np0 2 0H • 2.5H 2 0(am) and Nd(OH) 3 · H 2 0(am) in 1.00 mol · dm" 3 HCl have been measured at 298.15 K. From these measurements the following standard molar enthalpies of formation, A f H°m (kJ · mol -1 ) have been calculated as: Np 2 0 5 (cr), -(2162.7±9.3); Np0 2 0H • 2.5H 2 0(am), -(1939.9±4.7); Nd(OH) 3 · H 2 0(am), -(1712.4±2.6); Nd(OH) 3 (cr), -(1404.2±3.3). The stability of these phases is discussed in relation with literature data on solubilities and standard molar enthalpies of formation.
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