This paper reviews the literature on rare earth(III) and actinide(III) hydroxide thermodynamics, in particular the determination of their enthalpies of formation at 25{degree}C. The hydroxide unit-cell volumes, lanthanide/actinide ion sizes, and solid-solution stability trends have been correlated with a generalized acid-base strength model for oxides to estimate properties for heterogeneous equilibria that are relevant to nuclear waste modeling and to characterization of potential actinide environmental interactions. Enthalpies of formation and solubility-product constants of actinide(III) hydroxides are estimated

Morss, L. R.

Williams, C. W.

UNT Digital Library

Enthalpies of formation of rare earths and actinide(III) hydroxides: Their acid-base relationships and estimation of their thermodynamic properties

" ' ANL/CP--74763DE92 005235ENTHALPIES OF FORMATION OF RARE EARTH AND ACTINIDE(III) HYDROXIDES;"II-IEIR ACID-BASE RELATIONSHIPS AND ESTIMATION OF THEIRTHERMODYNAMIC PROPERTIESLESTER R. MORSS AND CLAYTON W. WILLIAMS .... "I Chemisu31 Division, Argonne National Laboratory, Argonne, Illinois 60439, U.S.A.'! ABSTRACT!This paper reviews the literature on rare earth(III) and actinide(III) hydroxid'.thermodynamics, in particular the determination of their enthalpies of fonnation at 25 °C. Thehydroxide unit-cell volumes, lantha_nide/actinide ion sizes, and soEd-solution stability trends havebeen correlated with a generaEzed acid-base strength model for oxides to estimate properties forheterogeneous equilibria that are relevant to nuclear waste modeling and to characterization ofpotential actinide environmental interactions. Enthalpies of formation and solubility-productconstants of actinide(III) hydroxides are estimated.ENTRODUCTIONi_The thermodynamic properties of crystalline rare earth and actinide hydroxides areneeded to determine the free ener_es for reactions between these compounds and near-neutralaqueous solutions. Raising the pH of an aqueous rare earth or actinide solution producesgelatinous hydroxide precipitates that are not crystalline and that approach equilibrium exuemelyslowly. For some rare earths, long-term equilibrium studies have been made to determine the=2 equilibrium constant of the reaction,ai M(OH)3(crystalline) = M3+(aq) + 30H-(aq). (1)These studies, using "aged" and presumably crystalline hydroxides, are believed to yieldthermodynamically valid solubi!ity products Kbsl0 [1].IImA rigorous way to determine the standard free energy of reaction (1) is to prepare! crystalIine M(OH)3 and to determine its enthalpy of formation and standard entropy. Among_7. hydroxides, enthalpies of' formation have been measured by solution calorimetry only for the_ lanthanides La(OH)3 [2, 3] and Nd(OH)3 [4]. Entropies have been determined for several rarei earth hydroxides [5]. These enthalpies and entropies have been combined with literature valuesfo M3"(aq) and OH-(aq) to yield solubility product K = Kbsl0 The agreement between• .SP • ,,.equilibrium measurements for reacnon (1) and calculanons (from enthalpy and entropymeasurements) is good enough to piace a high level of confidence in the equilibriummeasurements for rare earth hydroxides.dil!m The only thermodynamic properties have been determined for actinide hydroxides areiN equilibrium constants for solubility of Pu(OH)3 [6], Am(OH)3 [7], and "Pu(OH)4" [8]. The:!_ hydroxides of americium(III) and curium(m) are of particular interest in nuclear waste migration-=!li studies; the lighter actinides thorium, uranium, neptunium, and plutonium exist as An(IV)!l hydrous oxides in near-neutral or basic solutions. Although efforts have been made to minimizeradiolysis errors, to correct for oxidized species, and to remove colloidal particles, there remainsii doubt whether these were true equilibria; independent thermochemical or acid-base assessmentsshould remove such doubt. DISTRIBUTION OF THIS DOCUMENT IS UNLIMITEOThe 1;ubmitted manucript has been authored ,,t @ .?:by a contractor of the U.S. Government (_'f_ 1: ;_ '' ;'/ "under contract No. w-aI-109"ENG-38. _ t_" _ C:"-e,..,;Accordingly, the U. S. Government retains a .,nonexclusive, royalty-free license to publishmor reproduce the published form of this, contribution, or allow others tO dO sO. forU. S. Government _rl::)oses.THERMODYNAMIC AS SESSMENTSHydroxidesRare earth hydroxides (Y, La-Yb)(OH)3 crystallize in trigonal space group P63/m.Baes and Mesmer [1] found a correlation between the solubility products of lanthanidehydroxides and their unit cell a parameters and published a set of recommended "smoothed" Kspvalues. Morss [9,10] followed a similar approach but correlated unit-cell volumes with Kspvalues.Syntheses of crystalline hydroxides, coupled with calorimetric measurements of theirenthalpies of formation, are important because they can be combined with measured entropies todetermine standard free energies of formation for this isostructural class of compounds:AfG°[La(OH)3(s)] = AfHO[La(OH)3(s)] - 298.15 x AfS°[La(OH)3(s)]/1000 -- -1415.5 -298.15x 0.4334 = -(1283 + 2 ) kJ.mo1-1 and AfGO[Nd(OH)3(s)] = AfHO[Nd(OH)3(s)] - 298.15 xAfS°[Nd(OH)3(s)]/IO00 =-1403.8 -298.15 x 0.4453 =-(1271.0 + 1.0) kJ-mo1-1. For thesolubility reactions we calculate, using AfGO[La3+(aq) = -688 kJ tool -I, AfGO[Nd3+(aq) = -672+ 2 kJ tool -l, and AfG°(OH-,aq) = -157.244 Icl tool -1, we calculate forLa(OH)3(s) = La3+(aq) + 30H-(aq) (2)ArG"=+(123+ )kJ.mol -land forNd(OH)3(s) + Nd3+(aq) + 30H-(aq) (3)AG° = +(127.2 + 3) kJ.mo1-1The corresponding solubility products (log Ksp = -AGO/RT)are given in Table I along withliterature values.Actinide hydroxides (Am, Cm)(OH)3 also crystallize in trigonal space group P63/m.Baes and Mesmer [1] recommended that the solubility products of actinide(HI) hydroxides beestimated from the structural correlation that they noted for lanthanide hydroxides. Morss carriedout such an estimation [10]. However, his values for Pu(OH)3 and Am(OH)3 differ by about 2units in log Ksp from those derived from equilibrium measurements [6,7]. Thus it appears thatthere may be an acid-base factor that distinguishes the 5f (actinide) hydroxide basicities fromthose of the 4f (lanthanide) hydroxides.Se.squioxide8Smith [I 1] developed an empirical set of oxide acidity parameters based upon enthalpiesof formation of oxy-salts and the relationshipacidity(ACID) - acidity(BASE) = [-AH(reaction)] 1/'2 (4)where AH refers to the transfer of one mole of oxide ion from basic oxide to acidic oxide. Henormalized the relationship by setting the acidity of the very weak acid H20 equal to zero. Thusthe enthalpy change of the hydrolysis reaction (normalized to one mole of oxide ion transfer)H20(1) + 1/3 M203(s) = 2/3 M(OH)3(s) (5)generates a useful acidity parameter a for an oxide if the enthalpy of formation of the oxide isknown: [-AH(reaction 5)] 1/2 = a(M203), since a(H20) = 0. Values of a calculated from thisX_.,tUL.,UtlOltXIa _vi x..,aZ_..,, 3 txalu ._,uZ,,..s 3 are _.re g_._A_ A AAA A _=sQ.JLA, _Ae_I those of Smith."i• PTable I. Solubility Products of Lanthanide and ActinideIII) Hydroxides and Acidity Parametersof SesquioxidesM pKsp, a(M203) b M pKsp, a(M203) bM(OH)3 a M(OH)3 aY -6.5?La 21.7 -6.1 Ac 20.921.6 c -7.5 eCe 22.1 -5.8Pr 22.1 -5.8Nd 23.1 -5.7 U 22.222.3 [41 -6.9 ePm (24) Np 22.4Sm 25.2 -5.1 Pu 22.6 -6.9 d26.2 [6] -5.1 eEu 26.5 -5.1 Am 24.0 -5.6 e25.7 [7]25.7 fGd 26.9 -5.0 Cm 24.0 -5.8 e25.0 fTb 26.3 -4.3 Bk 25.0 -4.2 eDy 25.9 -4.7 Cf 25.5 -4.2 eHo 26.5 -4.5 Es 26.0Er 26.6 -4.3Tm 26.6 -4.2Yb 26.6 -4.5Lu 27.0 -3.3a Ref 1 (lanthanides); ref 10 (actinides) unless otherwise stated.b Ref 11 unless otherwise stated.c This work, calculated from refs 3 and 9.d This work, equations 4 and 5.e This work, a(M203) = [AH(6) -102 kJ] 1/2.fThis work (see text).The enthalpies of formation of all rare-earth sesquioxides have been determined fromsolution and combustion calorimetry, and "best values" have been assessed [9,10]. Enthalpies offormation of Am, Cm, .a_dCf sesquioxides have been measured and that of Pu has beenassessed [12]. Lanthanide-actinide enthalpy comparisons and actinide enthalpy-of-formationestimations have been made [12].We generalize Smith's acid-base correlation by noting that the enthalpies of solution oflanthanide oxides in infinitely-dilute aqueous acid vary. systematically [12] with ionic size:2H+(aq) + 1/3 Ln203(s) = 2/a Ln3+(aq) + H20(1) (6)This generalization is useful for elements, such as the actinides, that have no enthalpy-of-formation data tor oxy-salts. We adjust AH(6) to zSFI(5)by noting that AH(6) - zM-I(5)= 103 and100 kJ mo1-1 respectively for La and Nd. Thus a(M203) = [zM--I(6)-102 kJ] 1/2.Acidities ofactinide(III) oxides generated from this relationship are shown in Table I.ACID-BASE CORRELATIONSWe accept the value of pKsp = 26.2 for Pu(OH)3 [6] and calculate AfG"[Pu(OH)3,s] =-1200.3 kJ tool -I We use the most-recently assessed value of AfH"(Pu203, s, 298 K) = -1656kJ tool -z [10] along with the known S"(Pu203, s, 298 K) = 163.0___0.6 J tool -1K -1 [13] andour estimate tor S"[Pu(OH)3, s, 298 K = 131.6 J K-1 mo1-1 [14] to derive AfH°[Pu(OH)3, s, 298K) = -1327.9 Icl tool-1. From this value we calculate the Smith acidity, parameter a[Pu203] =-6.9. Note that this acidity parameter is the same as mat of the structurally-similar Nd203.In a paper on the enthalpy of formation of Cf203, Morss [12] noted that the enthalpies ofsolution (in infinitely dilute acid) of actinide(III) oxides are systematically less exothermic thanthe enthalpies of solution of structurally-similar lanthanide oxides. We interpret this difference torepresent the _eater acidity of the actinide(III) oxides and hydroxides compared to structurally-similar lanthanide oxides and hydroxides in terms of reaction (6). This interpretation isconsistent with the smaller Ksv (more negative log Ksp, greater pKsv) values of actinide(III)hydroxides compared to strucfurally-similar lanthanide hydroxides.-It has traditionally been assumed that hydrolysis of trivalent actinide ionsAn3+(aq) + H20(1) An(OH)2+(aq) + H+(aq) K 13"= = 11 (7)parallels that of lanthanide ions of the same ionic radius [15]. Recent measurements, e.g. [7],give evidence for a larger 13"11(more negative log 13"11,more positive pl3*l 1) value for Am 3+.These recent measurements imply, as above, that actinide(III) aquo-ions are more acidic thanstructurally-similar lanthanide aquo-ions.ACTINIDE(HI) HYDROXIDE THERMODYNAMICSFrom the acidity parameters a developed for actinide(III) oxides, and the a developed forPu203 above, we use Smith's approach to set a parameters for Pu203, Am203, and Cm203.From these parameters we calculate enthalpies of formation of these hydroxides from AH(5) asfollows: AfH*[Pu(OH)3, s, 298 K] = -1328 kJ mol-1; AfH*[Am(OH)3, s, 298 K] = -i346 kJtool-l; andAfH°[Cm(OH)3, s, 298 K] = -1341 kJ tool-1. With entropy estimates, we calculatepKsp[Am(OH)3] = 25.7 and pKsp[Cm(OH)3] = 25.0. Clearly it will be beneficial to measureenthalpies of formation for these hydroxides and pKa for Cm(OH)3 to test this extension of theacid-base systematics developed in this paper and to provide more reliable thermodynamicparameters for these and heavier actinide hydroxides.• , , &oAcknowledgementThis work was sponsored by the U.S. Department of Energy, Office of Basic EnergySciences, Divisions of Chemical and Materials Sciences, under Contract W-31-109-ENG-38.REFERENCES1. C. F. Baes, Jr., and R. E. Mesmer, The Hydrolysis of Cations (Wiley-Interscience, NewYork, 1976). (The present paper follows the terminology and notation of this classical work.)2. H. von Wartenberg, Z. anorg, allg. Chem. 29.__9,228(1959).3. E. H. P Cordfunke, R. J. M. Konings, and W. Ouweltjes, J. Chem. Thermodynamics "_'_449 (1990).4. L. R. Morss, C. M. Haar, and S. Mroczkowski, J. Chem. Thermodynamics 2_..!,1079(1989).5. R. D. Chirico and E. F. Westrum, Jr. J. Chem. Thermodynamics 12, 71 (1980); _ 311.6. A. R. Felmy, D. Rai, J. A. Schramke, and J. L. Ryan, Radiochimica Acta 4___8_8,29 (1989).7. S. Stadler and J. I. Kim, Radiochimica Acta 44/45, 39 (1988).8. J. I. Kim and B. Kanellakopulos, Radiochimica Acta 48, 145 (1989).9. L. R. Morss, in Standard Potentials in Aqueous Solution, edited by A. J. Bard, R. Parsons,and J. Jordan (Marcel Dekker, New York, 1985).10. L. R. Morss, chapter 17 in The Chemistry of the Actinide Element.s, edited by J. J. Katz, G.T. Seaborg, and L. R. Morss (Chapman & Hall, 1986).11. D. W. Smith, J. Chem. Education 6__A,480(1987).12. L. R. Morss, J. Fuger, J. Goffart, and N. Edelstein, J. Less-Common Metals 12___7_7,251(1987).13. H. E. Flotow and M. Tetenbaum, J. Chem. Phys. 7J, 5269 (1981).14. S°[Pu(OH)3] is estimated by difference as S"[Nd(OH)3] + 1/2 [S°(Pu203 _ - S°(Nd203)].This estimaticn takes into account cation mass effect and cation magnetic entropy, assumingthe magnetic entropy contributions of sesquioxide and hydroxide are the same.15. S. Ahrland, chapter 21 in The Chemistry of the Actinide Elements., edited by J. J. Katz, G.T. Seaborg, and L. R. Morss (Chapman & Hall, 1986).DISCLAIMERThis report was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United States Government nor any agency thereof, nor any of theiremployees, makes any warranty, express or implied, or assumes any legal liability or responsi-bility for the accuracy, completeness, or usefulness of any information, apparatus, product, orprocess disclosed, or represents that its use would not infringe privately owned rights. Refer-ence herein to any specific commercial product, process, or service by trade name, trademark,manufacturer, or otherwise does no' necessarily constitute or imply its endorsement, recom-mendation, or favoring by the United States Government or any agency thereof. The viewsand opinions of authors expressed herein do not necessarily state or reflect those of theUnited States Government or any agency thereof.iI

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Enthalpies of formation of rare earths and actinide(III) hydroxides: Their acid-base relationships and estimation of their thermodynamic properties

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