Electrochemical Characterisation of the Ce(IV) limiting carbonate complex

Transcription

1 Electrocheical Characterisation of the Ce() liiting carbonate coplex Chantal Riglet-Martial 1, Pierre Vitorge 2 and Véronique Calon 1 1 CEA-Cadarache, DCC/DESD/SESD, 118 Saint-Paul-lez-Durance cedex, France.chantal.artial(at)cea.fr) 2 CEA-Saclay, DCC/DESD/SESD, Gif-sur-Yvette Cedex, France.pierre.vitorge(at)cea.fr. Keywords : ceriu (/), actinides (/), carbonate, coplex, activity coefficients, cyclic voltaetry Abstract The stoichioetry and the therodynaic foration constant of the liiting coplex of Ce() were deterined at C by using cyclic voltaetry technique at a hanging ercury drop working electrode in concentrated bicarbonate/carbonate ediu. The Ce(/) redox potential was easured at ph varying 2 fro 9. to 1. and [ CO ] varying fro 1. to 1. M by perforing a CO 2 titration with CO 2 gas. The ionic strength and junction potential effects were taken into account for the potentioetric calibrations and easureents. Quantitative interpretation of the variations of the foral potential E / showed that no polyerisation took place during the redox reaction, and that two CO 2 ligands, but no OH - ligand, were exchanged. As the accepted stoichioetry for the liiting coplex of Ce() is Ce( CO ), the Ce() species is Ce( CO ) 8. In a. olal Na + carbonate/bicarbonate ediu (Ionic strength = 4. ol.kg -1 ), E / Ce( CO ) =.11.8 V/SHE (in olal units) was easured. This value, cobined with the published Ce( CO ) foration constant and the (re-evaluated) (Ce 4+ /Ce + ) standard potential, is used to calculate the foration constant log 1 ( ) = (defined in olal concentration except for Ce 4+ in activity : see table 1) in the sae ediu. The values of E / and log 1 ( ) are ionic strength dependant, e.g. E / = V/SHE and log 1 ( ) = in olar units in a 2.7 M NaClO 4 ediu (Na + olality = olal ionic strength =. ol.kg -1 ). The possible foration of Ce( CO ) is discussed under the experiental conditions used, log 1 ( ) (in olal units). Introduction The actinide cheistry in reducing aqueous solutions is iportant for any waste disposal issues. In a recent bibliographic review [1], we pointed out contradictory interpretations for the speciation of actinide() eleents in bicarbonate/carbonate edia, and showed that the liiting coplex can be a starting point to deterine aqueous speciation in environental conditions. Fro redox easureents involving the known U(VI), Np(V), Pu(VI) and A() liiting carbonate coplexes, the stoichioetry An( CO) has been proposed for the actinide() liiting carbonate coplex ([2, ] for U(), [4] for Np(), [] for Pu() and [, 7, 8] for A()). This stoichioetry has been confired fro the published redox data and the stability of the liiting coplex has been accurately deterined for uraniu only. Recently, this stoichioetry has been confired for the Pu() liiting carbonate coplex fro EXAFS easureents [9]. Surprisingly, aong analogue eleents, both Ce( CO) and Ce( CO) 8 [1, 11, 12, 1, 14, 1] have been proposed for the Ce() liiting coplex. As the cheistry of Ce() [1] and A() [7, 8] in carbonate ediu is well known, the reversible (or quasi-reversible) M(/) redox couple ay be used to study the cheistry of Ce() and A() in concentrated bicarbonate/carbonate edia. Convincing evidence, based on potentioetric easureents, of the esacarbonato ceriu() stoichioetry was proposed in reference [1] in a 8 olal Na + ediu and in a carbonate concentration range fro 2.84 to 4. ol/kg. But since the authors used an Na + specific electrode as a reference electrode, they had to correct their data for the Na + activity variations to deterine the nuber of carbonate ions exchanged during the redox reaction. In the present work, the stoichioetry of the Ce() liiting carbonate coplex was checked by easuring the Ce(/) foral potential in. Na + solutions of uch lower sodiu carbonate concentrations (1. to 1.4 ol/kg) using cyclic voltaetry with a different electrocheical cell and a different ethodology in order to avoid the Na + activity coefficient corrections. The liquid junction potentials were easured and taken into account in the treatent of the data. 4

2 Electocheical Characterisation of the Ce() Carbonate Liiting Coplex. Ch. Riglet-Martial, P. Vitorge, V. Calon 2/1 Experiental All bicarbonate/carbonate solutions, prepared fro crystallised NaHCO (Merck, p.a.) and concentrated NaOH (Merck, p.a., d=1.), were checked by acid-base titration using H 2 SO 4 (Merck Titrisol). A.1 M Ce() stock solution was prepared fro crystallised Ce(ClO 4 ),H 2 O (Johnson Mattey, purity > 99.9%). The test solutions were obtained by adding an aliquot of the Ce stock solution to the previous sodiu bicarbonate/carbonate aqueous solutions. The solutions used in the reference electrodes were prepared fro crystallised NaCl and NaClO 4 (both Merck, p.a.). The response of the glass electrode was checked by using coercial buffer solutions (Merck Titrisol) of ph 2, 7, 9 and 12. Deineralized water delivered by a Millipore Milli-Q plus purifier was used for all the dilutions. The redox potential of the Ce(/) couple was easured by cyclic voltaetry, using a classical three electrode device - including an Ag/AgCl reference electrode (Radioeter XM/D8), a hanging ercury drop electrode (HMDE Metrho E41) as a working electrode and a platinu wire (Radioeter Pt11/CMP) as a counter electrode - connected to an electrocheical analyser (Radioeter Voltalab 2) including a prograable interface with the Voltaaster 2 software. In order to avoid the possible daage of the reference electrode by the carbonate ions, it was isolated fro the test solution by eans of a capillary extension (Radioeter FDL2/CMP) filled with a.2 M NaCl, (I-.2) M NaClO 4 solution, of sae ionic strength I as that of the test solution. The potential of this electrode (noted REF) was checked once a day. Differences higher than.2 V were never observed. The foral potential of the REF electrode was calculated fro Nernst law: E REF = E AgCl/ Ag + A log 1 (a Cl -). Activity coefficients were estiated by applying the SIT forula : log 1 ( i ) = z 2 i D(I ) + (i, X) X for an ion i of charge z i in a X ediu, where X is an ion of charge opposite to that of i [,8, 17]. Nuerical values and definitions used are given in table 1. The voltaogras were recorded between +.24 and -.2 V/SHE at a scanning speed of.2 V/s. The upper liit of +24 V/SHE was iposed by the oxidation wall of ercury. Ten illilitres of the c M Na 2 CO, (I-c ) M NaHCO test solution was first added to a therostated (at C) electrocheical cell (of type Tacussel RM-C + CRSR) and then deoxygenated with argon (HP4 Carboxyque) for one hour. After deoxygenation, a voltaogra of the electrolyte was recorded in order to verify the absence of any electroactive ipurities. In order to deterine whether the redox reaction under study involved a polyer or not, a first set of experients was first carried out with a ceriu total concentration [Ce] t varying fro to M. Two copositions of test solutions P1 (1.42 M Na 2 CO,.1 M NaHCO, ph = 1.) and P2 (1. M Na 2 CO,.7 M NaHCO, ph = 9.8) were investigated. Five deterinations of the E 1/2 potential (the ean value of the oxidation and reduction peak potentials) were perfored for each Ce concentration. In order to deterine the stoichioetry of the Ce() coplex, a second set of experients was ade as follows : [Ce] t 2 was kept constants but ph and [ CO ] were varied siultaneously by stepwise acidification of the initial (P M Ce) test solution by eans of CO 2 (Air Products, quality 4.). The gas was previously passed through a NaClO 4 solution of sae olar ionic strength as that of P1. At each step of acidification, the test solution was left for stabilisation with an argon cover at the surface until constant ph (variation <. ph unit during 1 inutes) was obtained. The voltaogra of the solution was then recorded and the ph was checked again. Five deterinations of the E 1/2 potential were perfored for each ph step. For both sets of experients (with test solutions of general initial coposition c M Na 2 CO, (I-c ) M NaHCO ), the junction potential arising between the test solution and the REF electrode was easured by eans of the cell REF.2 M NaCl, c M Na 2 CO, (I-.2-c ) M NaHCO REF, where REF = Ag/AgCl.2 M NaCl, (I-.2) M NaClO 4, REF is an Ag/AgCl wire, denotes a liquid-liquid junction (capillary extension) and denotes liquid-solid contact. The variations of the junction potential as a function of ph were investigated by stepwise acidification of an initial.2 M NaCl, c M Na 2 CO, (I-.2-c ) M NaHCO solution with CO 2 gas, and easuring the electrootive force of the cell rapidly after stabilisation (to avoid the daage to the REF electrode in direct contact with carbonate ions). After each easureent, the REF electrode was controlled by eans of the cell REF.2 M NaCl, (I-.2) M NaClO 4 REF, where REF is an electrode of sae type as REF. Differences higher than.2 V were never observed. A linear variation of the junction potentials versus ph (with a slope of about 1 V/pH unit) was found in the ph range under study. The easured E 1/2 potentials of the Ce(/) couple in carbonate edia were corrected for the junction potentials calculated fro the experiental regression straight line at the ph of the corresponding test solutions. The ph was easured with a Radioeter XC11 cobined glass electrode connected to a Tacussel ISIS 2 ph-eter. The Ag/AgCl reference electrode copartent was filled with a.2 M NaCl, (I-.2) M

3 Electocheical Characterisation of the Ce() Carbonate Liiting Coplex. Ch. Riglet-Martial, P. Vitorge, V. Calon /1 NaClO 4 solution of sae olar ionic strength I as that of the initial NaHCO /Na 2 CO test solution. As the acidification by CO 2 gas involved a gradual variation of the ionic strength of the test solution, the glass electrode was standardised in activity units (ph = -log 1 a H + and not -log 1 [H + ]) by eans of two NaHCO /Na 2 CO buffer solutions of sae Na + concentration (. M) as that of the initial NaHCO /Na 2 CO test solution. The theoretical ph of these buffer solutions were calculated on the basis on their cheical copositions, deterined by acid-base titration, and by using the equilibriu constants and SIT coefficients fro table 1. The precision of the ph easureent for the buffer solutions was about. ph unit. In addition, the accuracy of the ph electrode response was controlled by eans of a standardisation between ph 7 to 12. The ph range fro 8.2 to 12 was checked by a titration of a 1 M NaHCO solution with a 1 M NaOH solution, while the ph range under 8.2 was controlled by easuring the ph of a 1 M NaHCO solution in equilibriu with CO 2 gas. The slope of the glass electrode (figure 1) was found to be the theoretical one (8. V/pH unit at 19. C) within a precision of. ph units in the ph range between 7. to 1. Above ph=1, a systeatic error varying between. to. ph unit was observed. Because of the liited solubility of NaHCO (about 1 M), the control of the accuracy of the ph electrode could not be carried out with concentrated carbonate solutions (about 1. M) siilar to that used for the electrocheical study of ceriu. Treatent of the data Preliinary experients showed that the interval between the oxidation and reduction peaks of Ce(/) increased with the potential scanning rate, which was an indication of a quasi-reversible redox syste. For reversible or quasi-reversible systes, the cyclic voltaetry curves are theoretically syetrical around the (constant) half-wave potential E 1/2, whatever the potential scanning rate. This feature was observed for scanning rates varying fro.2 to.8 V.s -1 (at higher scanning rates, the shift of the oxidation peak towards the oxidation wall of ercury was such that the oxidation peak potential could not be easured any ore). In addition, the current intensity of the oxidation and reduction peaks varied linearly as a function of the square root of the scanning rate, with siilar absolute values of the slope during oxidation (+.1) and in reduction (-.11). We then assued that the Ce() and Ce() diffusion coefficients were siilar and that the experientally easured E 1/2 potential could be used as an estiate of the foral redox potential E / of the Ce(/) couple. 2 The speciation at varying ph and [ CO ] was calculated fro the experiental conditions ( c Na CO 2, c NaHCO, ph) using equations (1 to 8) and the definitions and nuerical values listed in table 1. The Na 2 CO and NaHCO initial (olal) total concentrations are denoted and Na 2 CO NaHCO. Since H could always be neglected, the approxiation H = 1 was used in the ass balance equation (4) and in the definition of the ionic strength I (Table 1). log 1 p q Na 2 CO 2 + q CO NaHCO HCO (1) log 1 a HO r 2 Na 2 CO 2 + r CO NaHCO HCO (2) Na = 1 (2 2 HCO H 2O / Na 2 CO NaHCO ) 1 NaHCO 2 H2O / () Na + H = 2 2 CO + HCO + OH (4) K 1 = K 1 / K e = HCO CO OH D(I ) =. I 1 1. I () log 1 K e (I ) = log 1 K e () + D(I ) - (Na +,OH - ) Na + log 1 a HO 2 (7) log 1 K 1 (I ) = log 1 K 1 () + D(I ) - ([(Na +,CO 2 ) - (Na +,HCO )]) Na (8) 2 Previous studies of the Ce( )CO H 2 O syste indicate that the liiting carbonate coplex of Ce(), of stoichioetry Ce( CO) 4, is the ajor soluble species in edia of [ CO 2 ] > 1. M [1]. The electrocheical equilibriu between Ce() and Ce(), and its equilibriu potential are : Ce( CO) 4 + ( j n - 4) CO 2 + i n H 2O 1 Ce ( OH) ( CO) n 4ni n i j () + i n H+ + e (9)

4 Electocheical Characterisation of the Ce() Carbonate Liiting Coplex. Ch. Riglet-Martial, P. Vitorge, V. Calon 4/1 E = E / 1/ n in / a a H + A log 1 jn / 4 in / (1) a a a Introducing the expression of activity coefficient (Table 1) into equation (1) : 1/ n E = E / + A log 1 CO 2 HO 2 where E / = E / (I) + A [ i n log 1 H - ( j n - 4) log 1 2 CO ] (12) and E / (I) = E / (11) 1/ n in / H () + A log 1 jn / 4 in / (1) a In cyclic voltaetry, the half-wave potential E 1/2 corresponds to equal concentrations of the two oxidation states at the working electrode. As the difference between the diffusion coefficients is usually negligible (as we shall verify that the Ce() coplex is ononuclear, this approxiation is certainly valid), E 1/2 is assued to be equal to E / (equation 11). Siilarly, if Ce polynuclear species are fored, we have : ( ) 1/2 = n ( ) 1/2 = Ce / 2 (14) where Ce is the Ce total olal concentration. The introduction of equation (14) into the Nernst relation (11) gives : E 1/2 = E / + A[(1-1 n ) log n log 1 n] - (1-1 n ) A log 1 Ce (1) The plot of the experiental data E 1/2 as a function of log 1 Ce gives (equation 1) a straight line with slope (1-1/n) which we shall use for the deterination of the stoichioetric coefficient n of the Ce() coplex. For n = 1, E 1/2 = E / (equation 1). When n is known, equation 12 and the data (E /, CO 2, ph) give the stoichioetric coefficients i and j, and the foral potential E / (I) at ionic strength I. Results and discussion As [Ce] t had no influence on E 1/2 (figure 2), the Ce() coplex is ononuclear (that is n = 1). The experiental data were then interpreted with the siplified equation (12a) : E / = E / (I) + A (i log 1 H - (j - 4) log 1 2 CO ) (12a) The effects of the ionic strength variations during the experients were estiated to be relatively sall : aong all the cheical conditions investigated during the redox easureents, the Debye-Hückel ter D(I ) varied fro.2 to.29 and the Na + olal concentration varied fro. to.1 ol/kg. This induced a axiu variation of log 1( CO 2 ) of.1 (fro to -1.2) which corresponds to a variation of the potential of less than 1V (which is negligible). In addition, all the attepts to fit the experiental data according to equations (12) and (1) by taking into account the variations of the activity coefficients (by using the SIT forula) proved to be unsuccessful because the ionic strength changes were not large enough. Hence, all activity coefficients and the E / (I) potential, could be considered as constant. The sall ionic strength variations were neglected in the qualitative slope analysis (figures and 4) but not for the following calculations. All the experiental data could be interpreted with the siplified equation (12a) where the i and (j - 4) coefficients had clearly to be set to and 2 respectively (figures and 4). Nevertheless, other sets of (i, j) coefficients were tried e.g. the Ce( CO) species which gave poorer standard deviation and soe systeatic deviations, while species such CeHCO( CO ) 7 4, CeHCO( CO) 7, CeOH( CO) or CeOH( CO) 9 gave theoretical curves copletely erroneous when copared with experiental data. Finally the following interpretation is proposed : Ce( CO ) + 2 CO 2 Ce( CO ) 8 + e (9a) 4 and E / = E / (I) - A 2 log 1 2 CO A value of E / (I) =.1.2 V/SHE (where the uncertainty is 1.9 ties the standard deviation) was fitted. Assuing partial dissociation of the Ce() liiting coplex did not significantly iprove the fitting, which gave log 1 k CO 2 HO 2 (12b) =.49 and E / (I) =.11.1 V/SHE. In order to ake E / (I)

5 Electocheical Characterisation of the Ce() Carbonate Liiting Coplex. Ch. Riglet-Martial, P. Vitorge, V. Calon /1 consistent with the first estiation, the uncertainty was increased and k was considered as a axiu possible value (table 1). The Ce(/) redox potential shift between carbonate and non coplexing edia is deterined by the ratio of the Ce() and Ce() coplexing constants. To avoid ionic strength correction we used the 4 ( I ) and ( I ) definitions given in table 1. For these constants, as for E / (I ), the nuerical values are valid for a given ionic strength and a given Na + concentration (here. ol.kg -1 ), whatever the ajor anion in the aqueous solution. The therodynaical cycle is calculated in the following way using the definitions given in table 1. E = E / + A log 1 (11a) = E Ce 4 Ce () + A log a / 1 a 4 Ce Ce = E Ce 4 Ce () + A [log / 1 4 ( I ) - log 1 ( I ) - 2 log 1 2 CO + log 1 ] Coparing with equation (12b) the potential shift is E / (I ) - E Ce 4 Ce () = A [log / 4 ( I ) - log ( I ) ] (1) This last forula was used to estiate the foration constant of the Ce() liiting carbonate coplex (table 1). Cyclic voltaetry was successfully used at high ionic strength but soe difficulties arose at lower ionic strength, probably due to precipitation when the Ce() liiting carbonate coplex is dissociated. This work confirs that Ce() fors the Ce( CO) 8 coplex, while the U and transu liiting carbonate coplexes are rather An( CO) 2 in siilar cheical conditions (.7M< [ CO ] <2M. This result corroborates the conclusions of Salvatore et al. [1] obtained at higher ionic strength, but the interpretation of our data is ore straightforward since our ethod avoided any Na + activity coefficient correction. However, our ethodology did not allow the deterination of the Na + content of the Ce( CO) coplex, as suggested in reference [1]. The estiation of the Ce( CO) foration constant value (table 1) is one order of agnitude or slightly higher than for actinide() [1]. The cheical for, Ce( CO) 8, of the liiting carbonate coplex of Ce() is consistent with the known coordination cheistry of Ce() and Th(), e.g. the NO group, which is isoelectronic with the CO 2 2 group, gives rise to 12-coordinated nitrate coplexes like Ce( NO) and Th( NO ) 2 in which the bidentate groups define a nearly regular icosahedron [22,2] (In that respect, it sees that the size of the etal is not necessarily a doinant factor [22]). In addition, the sae icosahedral 8 coordination geoetry was found for the Th( CO) anion in the solid state structure of tuliokite Na BaTh(CO ).H 2 O [14]. Further inforation about the olecular and solid state structure of the liiting carbonate coplex of Ce() could be obtained by using XRD and EXAFS spectroscopy techniques, which have already been used successfully to clarify the structure of the liiting coplex Pu( CO) [9]. The ratio / 4 of 28. calculated in the present work at I = 4. ol.kg -1 sees consistent with the reported estiation of 2 (using very approxiate ionic strength corrections) at I = 12 ol.kg -1 [1] : the difference is in the order of agnitude of the activity coefficient (or ion-pairing, as proposed in [1]) corrections. As for Ce(), no evidence of any ixed OH - -CO 2 or polynuclear Ce() coplex was found. For quantitative 8 coparison with actinides(), further work could consist in studying the Ce( CO) dissociation into Ce( CO) (probably), and perforing a siilar investigation on the A()/A() syste [] with proper junction potential easureents and I corrections [7, 8]. Siilar electrocheical studies referring to the Np() [4] and Pu() [, 18] liiting carbonate coplexes would be ore tedious as those species are involved in irreversible redox couples only. References [1] Vitorge, P. : CEA-BIB-24 (199). [2] Ciavatta, L., Ferri, D., Grenthe, I., Salvatore, F., Spahiu, K. : Inorg. Che., 22 (198)

6 Electocheical Characterisation of the Ce() Carbonate Liiting Coplex. Ch. Riglet-Martial, P. Vitorge, V. Calon /1 [] Grenthe, I., Fuger, J., Konings, R.J.M., Leire, R.J., Muller, A.B., Nguyen-Trung, C., Wanner, H. : NEA- TDB Cheical therodynaics of uraniu. Elsevier Science Publishers B.V., 1992, 71p. [4] Delau, L.H., Vitorge, P., Capdevila, H. : CEA-N-287 (199). [] Capdevila, H., Thèse de l Université de Paris-Sud (//1992). [] Bourges, J.Y., Guillaue, B., Koehly, G., Hobart, D.E., Peterson, J.R. : Inorg. Che., 22 (198) [7] Robouch, P. : Thèse de l Université Louis Pasteur, Strasbourg (1/11/1987). [8] Silva, R., Bidoglio, G., Rand, M., Robouch, P., Wanner, H., Puigdoenech, I. : NEA-TDB Cheical therodynaics of aericiu. Elsevier Science Publishers B.V., 199, 74p. [9] Clark, D.L., Conradson, S.D., Keogh, D.W., Neu, M.P., Paler, P.D., Rogers, R.D., Runde, W., Scott, B.L., Tait, C.D. : counication T-A1 and Neu, M.P., Clark, D.L., Conradson, S.D., Scott, B.L., Paler, P.D., Reilly, S.D., Runde, W.H., Tait, C.D. : counication T-B, Actinides 97 International Conference, Baden-Baden, Gerany, Septeber 21-2, [1] Dervin, J. : Thèse de l Université de Reis, France (197). [11] Dervin, J., Faucherre, J. : Bull. soc. chi. France n (197) [12] Dervin, J., Faucherre, J. : Bull. soc. chi. France n 4-11 (197) [1] Haltier, E., Fourest, B., David, F. : Radiochi. Acta 1 (199) [14] Clark, D. L., Hobart, D. E., Neu, M. P. : Che. Rev. 9, (199) [1] Salvatore, F., Vasca, E. : J. Coord. Che., 199, Vol.21, [1] Ferri, D., Grenthe, I., Hietanen, S., Salvatore, F. : Act. Che. Scand., A7 (198) 9- [17] Grenthe, I., Plyasunov, A.V., Spahiu, K. : Modelling in Aquatic Cheisty. pages 2-42 edited by Grenthe, I. and Puigdoenech, I. OECD-NEA Paris (1997). [18] Capdevila, H., Vitorge, P. : Radiochi. Acta 8, 1-2 (199). [19] Pitzer, K.S. : Activity Coefficients in Electrolyte Solutions, 2 nd edition, edited by Pitzer K.S., pages 7-1 (1991). [2] Bard, A.J., Parsons, R., Jordan J. : Standard Potential in Aqueous Solutions, Dekker M. Inc., New York (198), NY 11, p. 4. [21] Sillén, L.G., Martell, A.E. : Stability constants of etal ion coplexes. Special Publ. No.17, 194, 74p and Suppl. No. 1, Special Publ. No. 2, 1971, 8p. The Cheical Society, London ; Högfeldt, E. : Stability constants of etal ion coplexes, part A : inorganic ligands. IUPAC cheical data series (21), 1982, 1p, Oxford. [22] Greenwood, N.N., Earnshaw, A. : Cheistry of the Eleents, Pergaon Press, [2] Cotton, S., Lanthanides and Actinides, Macillan Physical Science Series, 1991.

7 Electocheical Characterisation of the Ce() Carbonate Liiting Coplex. Ch. Riglet-Martial, P. Vitorge, V. Calon 11/21 Table 1. Physical and cheical paraeters used or deterined in this study. For a pure electrolyte : log 1 a HO r, p = / c and log 2 1 p q ; where c and denote the olar (ol.l -1 or M) and olal (ol.kg -1 ) concentrations of the salt in solution. When the electrolyte is a ixture of two salts (Na 2 CO, NaHCO ), the approxiations log 1 p q q 2 2 and log 1 a H 2 O r r 2 2 are used, where q x and r x are the q and r paraeters for the pure salt x, and x is the olal concentration of the salt x in the ixture. p values (l.kg -1 ) are taken fro [8]. The q and r values are calculated in this work. The log 1 a H 2 O values (to obtain r) are calculated fro Pitzer paraeters [19]. c i, i, a i and i denote respectively, the olar (ol.l -1 or M) concentration, the olal (ol.kg -1 ) concentration, the olal activity and the activity coefficient of the species i : a i = i i and p = i / c i. The coefficients i. are calculated by using the SIT forula [, 8, 1]. The SIT coefficients ( z M, z A A ) (kg.ol -1 ) are taken fro [8] ; the uncertainty is increased for the SIT coefficients which are estiated by analogy. Electrolyte X q X r X (i,j) j = ClO 4 j = OH - j = CO 2 j = HCO j = Cl - j = Ce( CO ) 4 NaHCO i = Na a Na 2 CO i = Ce b M H2O =.18 kg.ol. -1 i = Ce c a Estiated by analogy with UCO ( ) 4 4 and UCO ( ). b estiated fro [19] by analogy with La and Pr,.47. is tabulated in [, 8] c Estiated by analogy with U 4+. 4

8 Electocheical Characterisation of the Ce() Carbonate Liiting Coplex. Ch. Riglet-Martial, P. Vitorge, V. Calon 12/21 Table 2. Therodynaic equilibriu constants and redox potentials used or deterined in this work. The equilibriu constants are defined in olal units (as in the text, and when nothing else is stated for nuerical values). Subscript is used for olal units (only when needed in order to ake the difference with olar units). I = 1 2 z 2 i i is olal ionic strength. A = R T ln1 / F, typically.91 and.8 V at 2 and 19. C. and denote the Ce() and Ce() liiting carbonate coplexes. For each constant, the first value is usually the nuerical datu taken fro the corresponding reference, while values at other ionic strengths are calculated in this work by eans of the SIT forula [] by using the values listed in table 1. The E / (4.) (V/SHE) value was easured in this work in a.4 NaHCO 1.29 Na 2 CO aqueous solution at 19. C. ( 4. ) is calculated with equation (1) by using the necessary auxiliary data at about 2 C (the teperature corrections were assued to be less than uncertainty). Nae Definition Nuerical value Reference K w (I ) = OH - a H log 1 [K w ()] = (1) [17] page 71 K 1 (I ) = a E AgCl/ Ag (I ) E Ce 4 Ce 4 CO 2 HCO E AgCl/ Ag H log 1 [K 1 ()] = (1) [8] () - A log 1 a Cl - E AgCl/ Ag ().22 (1) at 19. C [2] 4 / (I) E 1/2 in acidic edia of ionic strength I E Ce Ce () I = [ Ce( CO) 4 ] [ Ce ][ CO ] 4 ( I ) = Ce( CO) 4 4 a Ce CO / (I) = (2) (1) at I = () I = () at I = (1) at I = 4 ( I ) = (4) at I = () at I = 4. E / (I ) E 1/2 + 2 A log 1 CO 2 E/ ( I) = (4) at I =. ( I ) = Ce( CO) 8 a 4 2 Ce CO k ( I ) = Ce( CO) 8 Ce( CO) CO 2 ( I ) = Ce( CO) 8 a 4 2 Ce CO Footnotes : lg ( ) lg k ( I ).11.8 () at I = 4. I = (4) at I = () at I = (4) at I =. lg ( )..19 () at I = 4. I 41.. (4) at I = () at I = 4. [21] [1] this work this work this work this work (1) Standard value (at ionic strength). (2) 1 M HClO 4 ; this value ay be in error since the hydrolysis correction does not see to be validated, this possible error would propagate on ( I ) and ( I ) values. Still our E value calculated at I= is consistent with the value selected in [1]. () M NaClO 4. (4) 2.7 M NaClO 4 which corresponds to Na + olality = I =. ol.kg -1. () Na 2 CO / NaHCO edia as used in the present work ; Na + olality =..4 ol.kg -1 and I = 4..7 ol.kg -1.

9 Electocheical Characterisation of the Ce() Carbonate Liiting Coplex. Ch. Riglet-Martial, P. Vitorge, V. Calon 1/21 Table : Ce(/) foral potential easured in concentrated carbonate/bicarbonate edia Fro the Na 2 CO and NaHCO initial concentrations (noted [Na 2 CO ] and [NaHCO ]) and the ph values easured while the acidic titration with CO 2 gas, the speciation is calculated by solving the syste of equations (1 to 8) with the paraeters listed in tables 1 and 2. [i] and i denote the olar (ol.l -1 or M) and olal (ol.kg -1 ) concentrations of the species i. The glass electrode is calibrated in activity unit, with buffers at the sae olar ionic strength as that of the initial test solutions. The potential E 1/2, ean value of the oxidation and reduction peaks, is easured on the voltaogras. It is then corrected for the junction potential E j (V), and the reference electrode potential E REF (V/SHE) (calculated with the paraeters of table 1 as indicated in the text) to give E / (V/SHE). Initial conditions Measureents Values calculated fro initial conditions [Na 2 CO ] [NaHCO ] [Ce] t E j ph I Na + HCO 2 OH - E REF E / CO E E E E E E E E E E E E E E E E E E E-4 to 1.9E E-4 to 1.9E E E E

10 Electocheical Characterisation of the Ce() Carbonate Liiting Coplex. Ch. Riglet-Martial, P. Vitorge, V. Calon 14/ Measured ph ph = ph = - log 1 [a H+ ] Fig. 1. Glass electrode calibration. The calibration was checked by two titrations of a 1 M NaHCO solution with a 1 M NaOH solution at T = C. The theoretical ph (-log 1 a H and not -log 1 [H + ]) values were calculated by using the equilibriu constants and SIT coefficients listed in tables 1 and 2. The observed deviation at ph>1 was taken into account in the treatent of the experiental data of the Ce()/Ce() redox potential. No deviation (of the glass electrode slope) was observed at ph<8.2 where NaHCO /CO 2(g) buffers were used for the ph calibration n = 2 ph = 1. ph = n = (E1/2 / REF) / A -.1 n = n = log 1 ([Ce] t ) Fig. 2.Influence of the ceriu total concentration on the Ce()/Ce() redox potential in concentrated carbonate/bicarbonate edia. Two sets of experients were perfored: [Na + ] =. (or.1) ol.kg -1, [ CO 2 ] = 1.4 (or 1.) ol.kg -1, [HCO - ] =.1

11 Electocheical Characterisation of the Ce() Carbonate Liiting Coplex. Ch. Riglet-Martial, P. Vitorge, V. Calon 1/21 (or.8) ol.kg -1 and ph = 1. (or 9.8) respectively, T = C, A = 8. V/log 1 unit. This figure shows that the easured redox potential E / is independent of the Ce total concentration. Since in these conditions the Ce() ajor aqueous species is ononuclear [1], the Ce() species is also ononuclear (equation 1). Theoretical curves are plotted for n = 1 (solid line) and n = 2 (dotted line), where n is the Ce stoichioetric coefficient in the Ce() aqueous species (for n > 2 the deviation between the experiental data and the theoretical lines are even greater than for n = 2) Slope = -2 A ; = Ce(CO ) - Slope = -1 A ; = Ce(CO ) Partial dissociation of the 8- Ce(CO ) coplex E/ / SHE (V) log 1 ( CO2- ) Fig..Influence of the CO 2 concentration on the Ce(/) foral potential in concentrated carbonate/bicarbonate edia. [Na + ] =. ol.kg -1, 1. < [ CO 2 ] < 1.4 ol.kg -1, [Ce] t =.1 M, 9.2 < ph < 1., T = C, A = 8. V/log 1 unit. This figure shows that, within experiental uncertainties, the easured data falls into a single straight line (dotted line), with a slope corresponding to the theoretical one assuing that two CO 2 ions are exchanged during the redox reaction. Since in these conditions the Ce() ajor species is Ce( CO ) [1], the Ce() species is 8 i Ce( OH) i ( CO) ; As the ph influence plot (figure 4) confirs that i=, the ajor Ce() coplex is then Ce( CO) 8. That species is possibly partially dissociated into Ce( CO) (bolded curve) as proposed in this work (see text). 4

12 Electocheical Characterisation of the Ce() Carbonate Liiting Coplex. Ch. Riglet-Martial, P. Vitorge, V. Calon 1/21 E/ -E / (I) +2*A*log1 (CO2-) (V/SHE) Slope = ; = Ce(CO ) Slope = -1 A ; = Ce(CO ) (OH) 9- Slope = +1 A ; = Ce(CO ) (HCO ) ph = -log 1 a H+ Fig. 4. Fig. 4.pH influence on the Ce(/) foral potential in concentrated carbonate/bicarbonate edia. [Na + ] =. ol.kg -1, 1. < [ CO 2 ] < 1.4 ol.kg -1, [Ce] t =.1 M at T = C, A = 8. V/log 1 unit. The experiental data are the sae as in figure. This figure shows that the corrected redox potential E / - E/ () I + A 2 log 1 (equation 12a) is independent of ph. The Ce() and Ce() aqueous 2 CO species have then the sae OH - stoichioetric coefficient (i = ) : since the Ce() and Ce() ajor species are Ce( CO) 8i 4 [1] and Ce( OH) i ( CO) (Figure ), the forula of the the Ce() coplex is finally Ce( CO) 8, which is in agreeent with the interpretation of the figure.