1. Besbes Rafik 1,2 , Ouerfelli Noureddine3 , and Latrous Habib 3
Self Diffusion Coefficients of Trivalent Element Ions in Moderately Dilute Aqueous Solutions: Comparative Study between Lanthanum and f-elements the Effect of Hydrolysis and chlorure ion association.
1 Institut préparatoire aux études d’ingénieurs de Sfax- Tunisia
2 Unité de recherche physico-chimique moléculaire (IPEST) Tunis-Tunisia
3 Faculté des Sciences de Tunis, 2092 El-Manar, Tunisia
s: ;  ;
Session:
AC-TuA8
Abstract # 1046

2. INTRODUCTION
Several studies, several techniques have helped lead to the calculation of stability constants of hydrolysis or association of lanthanides and actinides. In this work we took the example of lanthanum to correct, if possible, his self-diffusion coefficient taking into account the phenomena of hydrolysis or association where appropriate.
La3+ + H2O → LaOH2+ + H+
Hydrolysis
La3+ + Cl- → LaCl2+
LaCl2+ + Cl- → LaCl2+
Association

3. Abstract:
We used Weingärthner .H, results for self-diffusion of LaCl3 in aqueous medium. there are two areas of self-diffusion coefficient of variation of trivalent lanthanum: a first zone of hydrolysis where the concentration is low and a second area of association between La3+ ions and Cl- which are paired and have triplets. We calculated self-diffusion coefficients D° limits at infinite dilution of La3+ , LaOH2+ , LaCl2+ and LaCl2+ based on the Onsager relation and given bibliographic stability constants of La(OH)2+ , LaCl2+ and LaCl2+ . Similarly we calculated the mean activity coefficient of LaCl3. Comparison with bibliographic results gives satisfaction .
D°LaCl2+ = cm2s , D°LaOH2+ = cm2s-1 ,
D°La3+ = cm2s , D°LaCl2+ = cm2s-1 .
We deduce the following entities:
La3+(H2O)9 , [La(H2O)9 Cl(H2O)4] and [LaCl2(H2O)6].
There seems to be a swing of between eight and nine structures coordination .

4. Some snapshots geometries of the La3+
first hydration shell
La(H2O) La(H2O) La(H2O)103+
M. Duvail et al. / Chemical Physics Letters 448 (2007) 41–45

5. Technical Introduction

6. Diffusion experiments
At constant temperature (25 ± 0.01°C), aqueous solutions were labeled with the tracer , where quartz capillaries (with length l ≈ 3 cm) were filled with an active solution. Radioactive solutions were kept in contact with inactive solution of the same composition. Activities of the gamma emitter could be counted directly in the glass tube by a Packard-Tricarb liquid scintillation spectrometer.

7. Non radioactive solution
T = 25 ± 0.01°C
Trace of radioactive solution of LaCl3
[LaCl3] = Co
[LaCl3] = Co
Quartz capillary
At constant temperature (T = 25 ± 0.01°C), aqueous solutions were labeled with the tracer , where quartz capillaries (with length l ≈ 3 cm) were filled with an active solution. Radioactive solutions were kept in contact with inactive solution of the same composition. Activities of the gamma emitter could be counted directly in the glass tube by a Packard-Tricarb liquid scintillation spectrometer.
Non radioactive solution
Selfdiffusion ≡ the same concentration

9. Open end capillary method OECM

10. Results and discussion
In first step, we compared self diffusion coefficient variation "D" for 4f / 5f ions trivalent in perchlorate medium at pH = 2.5 (no hydrolysis and no pairing phenomena).
4f: Eu3+, Gd3+, Tb3+
5f: Bk3+
with La3+ in LaCl3 aqueous solution. figure 1 (red curve) shows a strong diminution of DLa with ionic strength . It is due to pairs and triplets formation between La3+ and Cl- ions.

11. figure 1

12. Di = D°i [1 - 0.7816 zi2 (1 - √di) √I/((1 + BDH a° √I )(1+ √I/2))]
In second step, for compliance of our work, we choice too zones of concentrations (see figure 2) : in low concentrations we have hydrolysis phenomena. We deduct DºLa3+ and D°LaOH In hi concentration we have association of La3+ with Cl We deduct DºLaCl2+ and DºLaCl2+ .
Using Onsager Law:
Di = D°i [ zi2 (1 - √di) √I/((1 + BDH a° √I )(1+ √I/2))]
with
di = 1 / (2 I).Σj [ cj |zj| l°j) / (l°j / |zj| + l°i / |zi|)]
And ionic strenght I = 1 / 2 Σ ci.zi2
a° = R La R Cl- ≈ 5.8 Å BDH. a° ≈ 1.88 dm3/2mol-1/2
Marcus values

14. Calcul of hydration Layer radius
D°i = kB T l°i / (|zi| NAe2) = l°i /|zi|
According to the Nernst–Einstein relation, the limiting
diffusion coefficient of the ion, D◦ is given by
D° = kT/6πηrst
where rst is the stocks ionic radius and η : viscosity , with k the Boltzmann constant and T the absolute temperature (K).
We correct rst by “ R”
R= rst rst rst3

15. Imprecision is about ± 5%
Results
D°LaCl2+ = cm2s , D°LaOH2+ = cm2s-1 ,
D°La3+ = cm2s , D°LaCl2+ = cm2s-1 .
Imprecision is about ± 5%
We deduce the following entities:
La3+(H2O)9 , [La(H2O)9 Cl(H2O)4] and [LaCl2(H2O)6].
There seems to be a swing of between eight and nine structures coordination

16. Calcul of γ± of LaCl3 in different concentrations
To calculi mean activity coefficient of LaCl3 , we have used Nernt Hartley expression
We compared our results with Davies equation using real ionic strength of mixture , using stability constants of hydrolysis and association (bibliographic data Kh , β1 and β2 ).
and Frank Walsh measurement

19. Conclusion
We have seen that D°, the self-diffusion coefficients in ionic strength I = 0 may be adjusted taking into account the phenomenon of hydrolysis and formation of pairs or triplets
End