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Electrochemical and spectroscopic studies of the lanthanides in the AlCl3+1-n-butylpyridinium chloride melt at 40°C
J. Electroanal. Chem., 145 (1983) 139-146 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
139
E L E C T R O C H E M I C A L A N D S P E C T R O S C O P I C S T U D I E S OF THE L A N T H A N I D E S IN THE AICI 3 + I - n - B U T Y L P Y R I D I N I U M C H L O R I D E M E L T AT 40°C PART II. T H E T m ( l l l - l I ) , E u ( l l l - l l ) S Y S T E M S , APPLICATION OF N U G E N T ' S LINEARIZATION M E T H O D
J.P. SCHOEBRECHTS *, B.P. G I L B E R T ** and G. D U Y C K A E R T S
Laboratory of Analytical Chemistry and Radiochemistry, University of Likge, B - 4000 Sart Tilman-Liege (Belgium) (Received 5th October 1981; in revised form 26th October 1982)
ABSTRACT The possibility of obtaining stable solutions of Eu(ll) and especially Tm(ll) in the acidic side of 1-n-butylpyridinium chloride-A1Cl 3 melts at 40°C is presented. It is based on the Refined Electron Spin Pairing Energy Theory proposed by Jorgensen and improved by Nugent. The behaviour of Tm(III-II) and Eu(III-ll) has been qualitatively investigated by voltammetry and spectrophotometry, in order to confirm the predictions of Nugent's theory. Tm(lll) and Eu(llI) have been reversibly reduced to the divalent state in very acidic melts. The E l l 2 potentials measured with respect to the potential of an A1 electrode in a 57 mol % of A1C13 melt are respectively +0.020 V(Tm) and + 1.855 V (Eu). From the experimental E l l 2 values of T m and Eu and those previously reported for Yb and Sm, it has been possible to obtain the two parameters of Nugent's equation in room temperature melt and to compare their values with those reported in several other solvents. This comparison suggests that one of them could be related to the solvating power of the solvent.
INTRODUCTION
In previous papers [1,2], on the basis that inorganic Or organic chloroaluminate melts are appropriate solvents for the stabilization of low cationic oxidation states, the behaviour of lanthanides (Yb, Sm, Eu) in NaC1 + A1C13 mixtures at 175°C [1] and Yb, Sm in various 1-n-butylpyridinium chloride + A1CI 3 melts at 40°C [2] has been described. In each case, the trivalent lanthanides can be reversibly reduced to the divalent state and stable solutions are obtained. In near room temperature melt, the solvated cations are present under different chlorocomplex forms depending on
* Chercheur at the Institut Interuniversitaire des Siences Nuclraires, Brussels. ** To whom correspondence should be addressed. 0022-0278/83/0000-0000/$03.00
© 1983 Elsevier Sequoia S.A.
140
the pC1 value, the oxidation state and the atomic number. These electrochemical conclusions have been partially confirmed from UV-visible spectra modifications of trivalent lanthanides, as a function of the acidity of the melt. In this paper, we discuss, from considerations based on the Refined Electron Spin Pairing Energy Theory proposed by Jorgensen and improved by Nugent [3,4], the possibility of obtaining, m the same melt, stable solutions of Eu(II) and especially Tm(II), and we confirm it from experiments in electrochemistry and spectroscopy. The application of Jorgensen's theory on lanthanides enables us also to make a quantitative comparison on the relative solvating power of various solvents. EXPERIMENTAL
The experimental procedures and instruments are fully described in part I of this study [2]. Except when mentioned, all potentials are reported vs. the potential of an A1 electrode in a melt containing 57 mol% of A1C13. RESULTS A N D DISCUSSION
(I) Predictions on the half-wave potentials of the lanthanides
In the acidic solutions of sodium tetrachloroaluminate at 175°C, Eu(III) is a strong oxidant (its half-wave potential lies near the oxidation limit of the melt) and Tm(II) was found to be too reducing to be observed [1]. In order to estimate the reduction potentials of Eu(III) and Tm(III) in the room tenperature melt. the method of potentials linearization developed by Nugent et al. [4] was. as a first approximation, applied to the experimental El~ 2 values of Y b ( I I I - I I ) and Sm(III II). Nugent has indeed proposed that the standard oxidation potentials E ° of the ( I I I - I I ) or ( I V - I l l ) redox systems of lanthanides and actinides in the same aqueous solvent, are related to q (the number of f electrons in the oxidized form) following the equation E ° = W'+ (E-A')q-
F(q)
(1)
The values of F(q), non-linear with respect to the number of f electrons, and only dependent on q (and not on the oxidation state or the chemical bonding of the ion), have been determined from atomic emission spectroscopic data on free atoms, with a standard deviation of +0.25 V. Furthermore, Duyckaerts and Gilbert [5] have shown that for several solvents, the corrected experimental values ( E ° + F(q)) plotted vs. q, fall in each case on straight lines of identical slopes ( E A'). It was then proposed that a good approximation of the standard potential of all lanthanide and actinide couples can be obtained in a given solvent from only two experimental data. This extrapolation method is however only valid if the structure of the lanthanides (or actinides) remains constant in a given solvent. In the very acidic room temperature melt, it was shown that the standard potential of samarium (identical to the measured E~/2 if the diffusion coefficients of
141 the trivalent and divalent and cations are similar) corresponds to the solvated Sm 3+ Sm 2+ couple [2]. For ytterbium, only the standard potential E °' of YbC12+ YbC1 + could be measured [2]. Relation (1) can thus not be rigorously applied. However, the E °' potential is shifted with respect to the E ° ( Y b 3 + - Yb 2 ÷) in a way depending only on the relative magnitude (the ratio) of the u n k n o w n equilibrium dissociation constants of YbC12÷ and YbC1 ÷. The difference between the F ( q ) terms for Sm and Yb is also very large and, therefore, we have calculated as a first approximation the standard potentials of T m ( I I I - I I ) and E u ( I I I - I I ) by applying the method explained in ref. [4], using our Yb and Sm experimental standard potentials and neglecting the complexation effects. The calculated values are respectively - 0 . 0 1 5 V (Tm I I I - I I ) and + 1.945 V (Eu I I I - I I ) . Eu(III) should then be stable in a very acidic (66 mol% A1CI3) melt whose oxidation limit occurs at = + 2.1 V. Furthermore, there is some hope of reducing T m ( I I I ) into Tm(II) in less acidic melts if it is remenbered that: (1) the average deviation on the F ( q ) terms and consequently on the predicted El~ 2 values is rather large ( + 0.25 V). (2) the melt reduction limit (A1 deposition) shifts to more negative potentials when the pC1 is decreased. (3) the A1 reduction on a glassy carbon electrode occurs with an overvoltage of = 100 mV [6].
(II) Experimental results The Tm(III)- Tm(II) system A solution of T m ( I I I ) in a 61 mol% A1C13 melt exhibits a clearly defined reduction wave on a glassy carbon electrode (Fig 1). The E 1/2 reduction potential of T m ( I I I ) occurs at 4-0.02 V whereas the AI deposition begins at 0.05 V. W h e n it is remembered that the equilibrium potential of an A1 electrode in a melt containing 61 mol% of A1C13 is +0.05 V, the Tm reduction is then observed only because of the TABLE 1 Absorbances (A) and wavelength (k) of the maxima of the UV-visible absorption bands of Tm(II} in an acidic ( > 60 mol% of AICI3) A1C13-BPC melt. ITm(lI)J= 2.10 -3 mol kg-t; cell path length 1 mm /nm
565 495 425 395 380 350 330 320 285
A
0.054 0.028 0.082 0.12 0.14 0.134 0.124 0.13 0.72
142
0E/V
o15
10.02
I o-.'s
o.lI
~OE/V
o .o o
.D
c)
2 pAl
O'.s
300
? b E/v
/~C)O
500
660 k/nm 700
i0.02 b)
360
~0
s60
660 ~/.m 760
Fig. 1. Reduction wave observed on a pure melt and a 1.9.10 -3 mol kg-l solution of Tm(Ill) at 40°C. Compostion: 61 mol% of A1CI3. Reference potential: potential of an A1 electrode in a 57 mol% of A1C13 melt. (a) Cylic voltammogram--background of the pure melt. Scan rate = 100 mV s- l;T = 40°C; glassy carbon electrode area = 0.07 cmZ; (b) Cyclic voltammogram: Tm(III) solution. Conditions as in (a); (c) Differential pulse polarogram: Tm(III) solution. Scan rate = 5 mV s 1; AE = 25 mV. Other conditions as in (a). Fig. 2. UV-visible spectra of 2.10 -3 mol kg t acidic solutions of Tm(II) (a) and Tm(IIl) (b).
a l u m i n i u m d e p o s i t i o n overvoltage on the glassy c a r b o n electrode ( - - 100 mV). T h e possibility of T m ( I I I ) r e d u c t i o n into T m ( I I ) in acidic melts has also been c o n f i r m e d from UV-visible a b s o r p t i o n spectra. T m ( I I ) solutions have been easily o b t a i n e d b y c o n t r o l l e d p o t e n t i a l electrolysis of a T m ( I I I ) solution in a glassy c a r b o n crucible. A s it can b e seen from Fig. 2, the s p e c t r u m of the solution changes d r a s t i c a l l y when passing from the trivalent to the divalent o x i d a t i o n state. A T m ( I I I ) solution exhibits only one a b s o r p t i o n b a n d at 285 n m which can b e assigned p r o b a b l y to the TmC12+ charge transfer b a n d as for Y b ( I I I ) acidic solutions. Solutions of T m ( I I ) exhibit a m o r e c o m p l i c a t e d s p e c t r u m with b r o a d a n d intense a b s o r p t i o n b a n d s (Table 1). T h e m o s t energetic b a n d , very intense c o m p a r e d to others, suggests that the T m ( I I ) could also be c o m p l e x e d as was p r o p o s e d for Y b ( I I ) in acidic melts. A similar b e h a v i o u r b e t w e e n T m a n d Y b solutions is also o b s e r v e d f r o m electrochemical experiments. However, b e c a u s e of the p r o x i m i t y of the A1 reduction, o n l y some T m ( I I I ) solutions of variable acidity have been investigated. F r o m 66 to
143 60 tool% A1C13 solutions, the behaviour of the reduction wave is independent of the acidity. A further decrease of pC1 causes first a decrease of the reduction wave due to precipitation and secondly a cathodic shift of the half-wave potential. If the same equilibria as for ytterbium are involved, the p K c value of the equilibrium (2) is found equal to 11.2 + 0.07 (in the mol k g - 1 scale). TmCI~- ~ TmC12÷ + C1-
(2)
The solubility products ( - log Ksp ) of the mono- and dichlorocomplex were found equal to (29 + 0.5) and (16.2 + 0.5) respectively. The orders of magnitude of these results are close to those found for Yb(III) showing again that owing to their similar sizes and charge densities, both lanthanides exhibit a similar behaviour. The acidity independent half-wave potential measured on very acidic solutions must then also be regarded as an apparent potential.
The Eu(111)-Eu(11) system In a 66 mol% AIC13 melt, the trivalent Eu can be reduced and the El~ 2 of the voltammetric wave equals + 1.855 V. If the pC1 is decreased by a BPC addition, the colour of the solution changes (from pale yellow to greenish) and the rest potential of the indicator glassy carbon electrode shifts from 1.92 V (66 mol% A1C13 solution) to + 1.66 V (57.5 mol% solution). A voltammogram obtained from the last solution exhibits no reduction wave but a well defined oxidation wave at the same potential (E~/2 = 1.855 V). The change in colour of the melt and the disappearance of the reduction wave suggest a chemical oxidation of the melt by Eu(III) probably due to the cathodic shift of the melt oxidation limit as the pC1 decreases. The occurrence of the voltammetric oxidation wave Eu(II)--* Eu(III) at the same potential can be explained only on the basis of a slow chemical reaction of Eu(III) with the melt and must involve an overvoltage in the electrochemical oxidation of the melt at the glassy carbon electrode.
(111) D&cussion on the bas& of Nugent's equation As expected from the good correspondance between "predicted" and experimental potential values, the four experimental standard potentials, corrected for the F(q) terms, obey Nugent's equation, and a plot of E ° + F ( q ) vs. q gives a straight line (slope = 0.885 + 0.008, intercept = - 3.05 + 0.008). Considering the relatively large experimental errors in the F(q) terms, the normal potentials of the Y b ( I I I - I I ) and T m ( I I I - I I ) can then be considered as close to the apparent standard potentials. In a previous publication [5], from a comparison of the various values of ( E - A') and I4" in several solvents, we have observed that the slope ( E - A') is not seriously affected by the solvation of the ions even when considering solvents as different as molecular liquids and ionic melts. The average value of the slope was found equal to 0.865 + 0.008 V. This conclusion can also be applied to the room temperature chloroaluminate melt, where the experimental slope ( E - A') equals 0.885 + 0.008 V.
144 T h e solvation effects m u s t then be i n c l u d e d essentially in the W ' term of N u g e n t ' s equation. In o r d e r to investigate if that term can i n d e e d be related to the solvation, the e x p e r i m e n t a l s t a n d a r d p o t e n t i a l s o b t a i n e d in different solvents have b e e n recalculated, when possible, with respect to a single reference, n a m e l y the c o b a l t o c e n e - c o b a l t i c i n i u m couple. It has i n d e e d b e e n p r o p o s e d that some large m o l e c u l a r r e d o x systems of the charge t y p e 0 / + 1 (such as the chosen reference s y s t e m ) h a v e a s t a n d a r d r e d o x p o t e n t i a l a p p r o x i m a t e l y i n d e p e n d e n t of the solvent [7] a n d so only the solvation differences of the l a n t h a n i d e ions w o u l d then influence the W ' values. T h e c o r r e s p o n d i n g W ' values in various solvents are p r e s e n t e d in T a b l e 2 along with the D o n o r N u m b e r which accounts for the relative solvating powers, as defined b y G u t m a n n [8]. A s a first qualitative c o m p a r i s o n , it can b e seen that a higher solvation (high D.N.) c o r r e s p o n d s to a m o r e negative value of W'. Such a c o m p a r i s o n is, however, very limited, because the D . N . refers to the h e a t of r e a c t i o n of SbC15 with various solvents a n d then expresses the basic c h a r a c t e r of the solvent respect to a common acid. O n the o t h e r hand, IV' accounts for the relative v a r i a t i o n in the solvation of t w o different o x i d a t i o n state ions. T h a t W ' is nevertheless q u a n t i t a t i v e l y r e l a t e d to the solvation will n o w b e described as follows. It is well established that the free e n t h a l p y of transfer AGt°S~ represents the solvation change when a solute is transferred f r o m one solvent S~ to a n o t h e r S2 [10]i By a p p l i c a t i o n of this concept to a l a n t h a n i d e redox couple, transferred at a c o n s t a n t t e m p e r a t u r e , it follows [10] that: E s,) ° -- AGt°S~(ln(III)) - A G t S ~ ( l n ( I I ) )
F(E~or
(3)
= 2 . 3 R T ln[ ys2 ( l n ( I I I ) ) / y s , 2 ( l n ( I I ) ) ]
(4)
where E°s~ = the s t a n d a r d p o t e n t i a l of the L n couple in solvent S i versus the c o l b a l t o c e n e - c o b a l t i c i n i u m couple in the same solvent, ysS2(ln(III)) = the transfer activity coefficient of ln(III) f r o m S~ to S 2.
TABLE 2 Standard oxidation potentials E°(III-II) of lanthanides. Values of W' and (E - A') (a) ref. [5] (b) this work; D.N.: Donor Number of the solvent, as defined by Gutmann [8]; reference potential: E ° of the cobaltocene-cobalticinium system Solvent
(E - A')/V
W'/V
Ref.
D.N./kcal mol- 1
Dimethylsulfoxide (DMSO) Dimethylformamide (DMF) Dimethylacetamide (DMAC) Water Propylene Carbonate Benzonitrile AICI 3- BPC
0.867 0.87 0.87 0.872 0.867 0.85 0.885
-
(a) (a) (a) (a) (a) (a) (b)
29.8 26.6 27.8 18 15.1 11.9
4.83 4.37 4.23 420 3.70 3.41 2.17
145
From eqn. (1) and from the invariability with the solvent of ( E - A ' ) (experimental findings) and F(q), eqn. (3) can be rewritten as F( W,'$2 - W,~,) = AGt° s~ (ln(III)) - AGt°sS~(ln(II))
(5)
which shows that the difference in IV' between two given solvents is quantitatively related to the difference in free enthalpy of transfer for the two oxidation states of the lanthanides. The last equation cannot be rigorously applied to experimental results because the W' are obtained from measurements on a series of lanthanides and correspond then to an average for the whole series, whereas the AGt° can be obtained for a single lanthanide and may vary with the atomic number. Furthermore, the results of this study were obtained at 40°C and not at 25°C as with the other mentioned solvents. In spite of these restrictions, eqn. (5) is experimentally verified. For instance, if one considers the transfer of the Y b ( I I I - I I ) or E u ( I I I - I I ) couples from propylene carbonate (P.C.) to DMSO, the experimental differences between the AGt° of the two oxidation states are respectively ( - 92 + 4) kJ m o l - 1 and ( - 100 + 4) kJ m o l - 1 [9]. From the respective W' values in P.C. and DMSO (reported in Table 2), and by application of eqn. (3), the difference of AGt° can be evaluated as ( - 108 + 16) kJ m o l - l , a value close to the preceding ones. CONCLUSIONS
(1) This study is an additional confirmation of the validity of Nugent's method of potentials linearization applied to the L n ( I I I - I I ) systems. The agreement between theory and experiment is remarkably good, especially if one considers that the corrective terms F(q) are obtained from a completely different type of experiments. (2) The value of the experimental slope ( E - A ' ) in the investigated melt is also in agreement with our previous conclusions regarding the independency of the slope with the nature of the solvent. From the determination of only o n e E ° value, it is then possible to calculate an approximate value of the E ° of any other lanthanide or, in principle, actinide couple. (3) Finally, the W' parameter of Nugent's equation has been related to the solvating power. Differences of F(W') between two solvents represent the average difference of free enthalpy of transfer. In the acidic 1-n-butylpyridinium chloroaluminate, it can be concluded that the + 3 oxidation state of the lanthanides is much less solvated than in several other room temperature solvents (cf. Table 2), with respect to the + 2 oxidation state. ACKNOWLEDGMENT
We are indebted to the Institut Interuniversitaire des Sciences Nucl6aires for the financial aid given to our laboratory.
146 REFERENCES 1 2 3 4 5 6 7 8 9 10
B. Gilbert, V. Demarteau and G. Duyckaerts, J. Electroanal. Chem., 89 (1978) 123. J.P. Schoebrechts, B. Gilbert and G. Duyckaerts, J. Electroanal. Chem., 145 (1983) 127. L.J. Nugent, R.D. Baybarz, J.L. Burnett and J.L. Ryan, J. Inorg. Nucl. Chem., 33 (1971) 2503. L.J. Nugent, R.D. Baybarz, J.L. Burnett and J.L. Ryan, J. Phys. Chem., 77 (1973) 1528. G. Duyckaerts and B. Gilbert, Inorg. Nucl. Chem. Lett., 13 (1977) 537. J. Robinson and R.A. Osteryoung, J. Electrochem. Soc., 127 (1980) 122. H. Strehlow and H. Schneider, Pure Appl. Chem., 25 (1971) 327. V. Gutmann and E. Wychera, Inorg. Nucl. Chem. Lett., 2 (1966) 257. J. Massaux and G. Duyckaerts, Bull. Soc. Chim. Belges, 84 (1975) 519. B. Tr6millon, in Chemistry of Non-aqueous Solvents, Reidel, Dordrecht-Boston, 1974, pp. 218, 219.
139
E L E C T R O C H E M I C A L A N D S P E C T R O S C O P I C S T U D I E S OF THE L A N T H A N I D E S IN THE AICI 3 + I - n - B U T Y L P Y R I D I N I U M C H L O R I D E M E L T AT 40°C PART II. T H E T m ( l l l - l I ) , E u ( l l l - l l ) S Y S T E M S , APPLICATION OF N U G E N T ' S LINEARIZATION M E T H O D
J.P. SCHOEBRECHTS *, B.P. G I L B E R T ** and G. D U Y C K A E R T S
Laboratory of Analytical Chemistry and Radiochemistry, University of Likge, B - 4000 Sart Tilman-Liege (Belgium) (Received 5th October 1981; in revised form 26th October 1982)
ABSTRACT The possibility of obtaining stable solutions of Eu(ll) and especially Tm(ll) in the acidic side of 1-n-butylpyridinium chloride-A1Cl 3 melts at 40°C is presented. It is based on the Refined Electron Spin Pairing Energy Theory proposed by Jorgensen and improved by Nugent. The behaviour of Tm(III-II) and Eu(III-ll) has been qualitatively investigated by voltammetry and spectrophotometry, in order to confirm the predictions of Nugent's theory. Tm(lll) and Eu(llI) have been reversibly reduced to the divalent state in very acidic melts. The E l l 2 potentials measured with respect to the potential of an A1 electrode in a 57 mol % of A1C13 melt are respectively +0.020 V(Tm) and + 1.855 V (Eu). From the experimental E l l 2 values of T m and Eu and those previously reported for Yb and Sm, it has been possible to obtain the two parameters of Nugent's equation in room temperature melt and to compare their values with those reported in several other solvents. This comparison suggests that one of them could be related to the solvating power of the solvent.
INTRODUCTION
In previous papers [1,2], on the basis that inorganic Or organic chloroaluminate melts are appropriate solvents for the stabilization of low cationic oxidation states, the behaviour of lanthanides (Yb, Sm, Eu) in NaC1 + A1C13 mixtures at 175°C [1] and Yb, Sm in various 1-n-butylpyridinium chloride + A1CI 3 melts at 40°C [2] has been described. In each case, the trivalent lanthanides can be reversibly reduced to the divalent state and stable solutions are obtained. In near room temperature melt, the solvated cations are present under different chlorocomplex forms depending on
* Chercheur at the Institut Interuniversitaire des Siences Nuclraires, Brussels. ** To whom correspondence should be addressed. 0022-0278/83/0000-0000/$03.00
© 1983 Elsevier Sequoia S.A.
140
the pC1 value, the oxidation state and the atomic number. These electrochemical conclusions have been partially confirmed from UV-visible spectra modifications of trivalent lanthanides, as a function of the acidity of the melt. In this paper, we discuss, from considerations based on the Refined Electron Spin Pairing Energy Theory proposed by Jorgensen and improved by Nugent [3,4], the possibility of obtaining, m the same melt, stable solutions of Eu(II) and especially Tm(II), and we confirm it from experiments in electrochemistry and spectroscopy. The application of Jorgensen's theory on lanthanides enables us also to make a quantitative comparison on the relative solvating power of various solvents. EXPERIMENTAL
The experimental procedures and instruments are fully described in part I of this study [2]. Except when mentioned, all potentials are reported vs. the potential of an A1 electrode in a melt containing 57 mol% of A1C13. RESULTS A N D DISCUSSION
(I) Predictions on the half-wave potentials of the lanthanides
In the acidic solutions of sodium tetrachloroaluminate at 175°C, Eu(III) is a strong oxidant (its half-wave potential lies near the oxidation limit of the melt) and Tm(II) was found to be too reducing to be observed [1]. In order to estimate the reduction potentials of Eu(III) and Tm(III) in the room tenperature melt. the method of potentials linearization developed by Nugent et al. [4] was. as a first approximation, applied to the experimental El~ 2 values of Y b ( I I I - I I ) and Sm(III II). Nugent has indeed proposed that the standard oxidation potentials E ° of the ( I I I - I I ) or ( I V - I l l ) redox systems of lanthanides and actinides in the same aqueous solvent, are related to q (the number of f electrons in the oxidized form) following the equation E ° = W'+ (E-A')q-
F(q)
(1)
The values of F(q), non-linear with respect to the number of f electrons, and only dependent on q (and not on the oxidation state or the chemical bonding of the ion), have been determined from atomic emission spectroscopic data on free atoms, with a standard deviation of +0.25 V. Furthermore, Duyckaerts and Gilbert [5] have shown that for several solvents, the corrected experimental values ( E ° + F(q)) plotted vs. q, fall in each case on straight lines of identical slopes ( E A'). It was then proposed that a good approximation of the standard potential of all lanthanide and actinide couples can be obtained in a given solvent from only two experimental data. This extrapolation method is however only valid if the structure of the lanthanides (or actinides) remains constant in a given solvent. In the very acidic room temperature melt, it was shown that the standard potential of samarium (identical to the measured E~/2 if the diffusion coefficients of
141 the trivalent and divalent and cations are similar) corresponds to the solvated Sm 3+ Sm 2+ couple [2]. For ytterbium, only the standard potential E °' of YbC12+ YbC1 + could be measured [2]. Relation (1) can thus not be rigorously applied. However, the E °' potential is shifted with respect to the E ° ( Y b 3 + - Yb 2 ÷) in a way depending only on the relative magnitude (the ratio) of the u n k n o w n equilibrium dissociation constants of YbC12÷ and YbC1 ÷. The difference between the F ( q ) terms for Sm and Yb is also very large and, therefore, we have calculated as a first approximation the standard potentials of T m ( I I I - I I ) and E u ( I I I - I I ) by applying the method explained in ref. [4], using our Yb and Sm experimental standard potentials and neglecting the complexation effects. The calculated values are respectively - 0 . 0 1 5 V (Tm I I I - I I ) and + 1.945 V (Eu I I I - I I ) . Eu(III) should then be stable in a very acidic (66 mol% A1CI3) melt whose oxidation limit occurs at = + 2.1 V. Furthermore, there is some hope of reducing T m ( I I I ) into Tm(II) in less acidic melts if it is remenbered that: (1) the average deviation on the F ( q ) terms and consequently on the predicted El~ 2 values is rather large ( + 0.25 V). (2) the melt reduction limit (A1 deposition) shifts to more negative potentials when the pC1 is decreased. (3) the A1 reduction on a glassy carbon electrode occurs with an overvoltage of = 100 mV [6].
(II) Experimental results The Tm(III)- Tm(II) system A solution of T m ( I I I ) in a 61 mol% A1C13 melt exhibits a clearly defined reduction wave on a glassy carbon electrode (Fig 1). The E 1/2 reduction potential of T m ( I I I ) occurs at 4-0.02 V whereas the AI deposition begins at 0.05 V. W h e n it is remembered that the equilibrium potential of an A1 electrode in a melt containing 61 mol% of A1C13 is +0.05 V, the Tm reduction is then observed only because of the TABLE 1 Absorbances (A) and wavelength (k) of the maxima of the UV-visible absorption bands of Tm(II} in an acidic ( > 60 mol% of AICI3) A1C13-BPC melt. ITm(lI)J= 2.10 -3 mol kg-t; cell path length 1 mm /nm
565 495 425 395 380 350 330 320 285
A
0.054 0.028 0.082 0.12 0.14 0.134 0.124 0.13 0.72
142
0E/V
o15
10.02
I o-.'s
o.lI
~OE/V
o .o o
.D
c)
2 pAl
O'.s
300
? b E/v
/~C)O
500
660 k/nm 700
i0.02 b)
360
~0
s60
660 ~/.m 760
Fig. 1. Reduction wave observed on a pure melt and a 1.9.10 -3 mol kg-l solution of Tm(Ill) at 40°C. Compostion: 61 mol% of A1CI3. Reference potential: potential of an A1 electrode in a 57 mol% of A1C13 melt. (a) Cylic voltammogram--background of the pure melt. Scan rate = 100 mV s- l;T = 40°C; glassy carbon electrode area = 0.07 cmZ; (b) Cyclic voltammogram: Tm(III) solution. Conditions as in (a); (c) Differential pulse polarogram: Tm(III) solution. Scan rate = 5 mV s 1; AE = 25 mV. Other conditions as in (a). Fig. 2. UV-visible spectra of 2.10 -3 mol kg t acidic solutions of Tm(II) (a) and Tm(IIl) (b).
a l u m i n i u m d e p o s i t i o n overvoltage on the glassy c a r b o n electrode ( - - 100 mV). T h e possibility of T m ( I I I ) r e d u c t i o n into T m ( I I ) in acidic melts has also been c o n f i r m e d from UV-visible a b s o r p t i o n spectra. T m ( I I ) solutions have been easily o b t a i n e d b y c o n t r o l l e d p o t e n t i a l electrolysis of a T m ( I I I ) solution in a glassy c a r b o n crucible. A s it can b e seen from Fig. 2, the s p e c t r u m of the solution changes d r a s t i c a l l y when passing from the trivalent to the divalent o x i d a t i o n state. A T m ( I I I ) solution exhibits only one a b s o r p t i o n b a n d at 285 n m which can b e assigned p r o b a b l y to the TmC12+ charge transfer b a n d as for Y b ( I I I ) acidic solutions. Solutions of T m ( I I ) exhibit a m o r e c o m p l i c a t e d s p e c t r u m with b r o a d a n d intense a b s o r p t i o n b a n d s (Table 1). T h e m o s t energetic b a n d , very intense c o m p a r e d to others, suggests that the T m ( I I ) could also be c o m p l e x e d as was p r o p o s e d for Y b ( I I ) in acidic melts. A similar b e h a v i o u r b e t w e e n T m a n d Y b solutions is also o b s e r v e d f r o m electrochemical experiments. However, b e c a u s e of the p r o x i m i t y of the A1 reduction, o n l y some T m ( I I I ) solutions of variable acidity have been investigated. F r o m 66 to
143 60 tool% A1C13 solutions, the behaviour of the reduction wave is independent of the acidity. A further decrease of pC1 causes first a decrease of the reduction wave due to precipitation and secondly a cathodic shift of the half-wave potential. If the same equilibria as for ytterbium are involved, the p K c value of the equilibrium (2) is found equal to 11.2 + 0.07 (in the mol k g - 1 scale). TmCI~- ~ TmC12÷ + C1-
(2)
The solubility products ( - log Ksp ) of the mono- and dichlorocomplex were found equal to (29 + 0.5) and (16.2 + 0.5) respectively. The orders of magnitude of these results are close to those found for Yb(III) showing again that owing to their similar sizes and charge densities, both lanthanides exhibit a similar behaviour. The acidity independent half-wave potential measured on very acidic solutions must then also be regarded as an apparent potential.
The Eu(111)-Eu(11) system In a 66 mol% AIC13 melt, the trivalent Eu can be reduced and the El~ 2 of the voltammetric wave equals + 1.855 V. If the pC1 is decreased by a BPC addition, the colour of the solution changes (from pale yellow to greenish) and the rest potential of the indicator glassy carbon electrode shifts from 1.92 V (66 mol% A1C13 solution) to + 1.66 V (57.5 mol% solution). A voltammogram obtained from the last solution exhibits no reduction wave but a well defined oxidation wave at the same potential (E~/2 = 1.855 V). The change in colour of the melt and the disappearance of the reduction wave suggest a chemical oxidation of the melt by Eu(III) probably due to the cathodic shift of the melt oxidation limit as the pC1 decreases. The occurrence of the voltammetric oxidation wave Eu(II)--* Eu(III) at the same potential can be explained only on the basis of a slow chemical reaction of Eu(III) with the melt and must involve an overvoltage in the electrochemical oxidation of the melt at the glassy carbon electrode.
(111) D&cussion on the bas& of Nugent's equation As expected from the good correspondance between "predicted" and experimental potential values, the four experimental standard potentials, corrected for the F(q) terms, obey Nugent's equation, and a plot of E ° + F ( q ) vs. q gives a straight line (slope = 0.885 + 0.008, intercept = - 3.05 + 0.008). Considering the relatively large experimental errors in the F(q) terms, the normal potentials of the Y b ( I I I - I I ) and T m ( I I I - I I ) can then be considered as close to the apparent standard potentials. In a previous publication [5], from a comparison of the various values of ( E - A') and I4" in several solvents, we have observed that the slope ( E - A') is not seriously affected by the solvation of the ions even when considering solvents as different as molecular liquids and ionic melts. The average value of the slope was found equal to 0.865 + 0.008 V. This conclusion can also be applied to the room temperature chloroaluminate melt, where the experimental slope ( E - A') equals 0.885 + 0.008 V.
144 T h e solvation effects m u s t then be i n c l u d e d essentially in the W ' term of N u g e n t ' s equation. In o r d e r to investigate if that term can i n d e e d be related to the solvation, the e x p e r i m e n t a l s t a n d a r d p o t e n t i a l s o b t a i n e d in different solvents have b e e n recalculated, when possible, with respect to a single reference, n a m e l y the c o b a l t o c e n e - c o b a l t i c i n i u m couple. It has i n d e e d b e e n p r o p o s e d that some large m o l e c u l a r r e d o x systems of the charge t y p e 0 / + 1 (such as the chosen reference s y s t e m ) h a v e a s t a n d a r d r e d o x p o t e n t i a l a p p r o x i m a t e l y i n d e p e n d e n t of the solvent [7] a n d so only the solvation differences of the l a n t h a n i d e ions w o u l d then influence the W ' values. T h e c o r r e s p o n d i n g W ' values in various solvents are p r e s e n t e d in T a b l e 2 along with the D o n o r N u m b e r which accounts for the relative solvating powers, as defined b y G u t m a n n [8]. A s a first qualitative c o m p a r i s o n , it can b e seen that a higher solvation (high D.N.) c o r r e s p o n d s to a m o r e negative value of W'. Such a c o m p a r i s o n is, however, very limited, because the D . N . refers to the h e a t of r e a c t i o n of SbC15 with various solvents a n d then expresses the basic c h a r a c t e r of the solvent respect to a common acid. O n the o t h e r hand, IV' accounts for the relative v a r i a t i o n in the solvation of t w o different o x i d a t i o n state ions. T h a t W ' is nevertheless q u a n t i t a t i v e l y r e l a t e d to the solvation will n o w b e described as follows. It is well established that the free e n t h a l p y of transfer AGt°S~ represents the solvation change when a solute is transferred f r o m one solvent S~ to a n o t h e r S2 [10]i By a p p l i c a t i o n of this concept to a l a n t h a n i d e redox couple, transferred at a c o n s t a n t t e m p e r a t u r e , it follows [10] that: E s,) ° -- AGt°S~(ln(III)) - A G t S ~ ( l n ( I I ) )
F(E~or
(3)
= 2 . 3 R T ln[ ys2 ( l n ( I I I ) ) / y s , 2 ( l n ( I I ) ) ]
(4)
where E°s~ = the s t a n d a r d p o t e n t i a l of the L n couple in solvent S i versus the c o l b a l t o c e n e - c o b a l t i c i n i u m couple in the same solvent, ysS2(ln(III)) = the transfer activity coefficient of ln(III) f r o m S~ to S 2.
TABLE 2 Standard oxidation potentials E°(III-II) of lanthanides. Values of W' and (E - A') (a) ref. [5] (b) this work; D.N.: Donor Number of the solvent, as defined by Gutmann [8]; reference potential: E ° of the cobaltocene-cobalticinium system Solvent
(E - A')/V
W'/V
Ref.
D.N./kcal mol- 1
Dimethylsulfoxide (DMSO) Dimethylformamide (DMF) Dimethylacetamide (DMAC) Water Propylene Carbonate Benzonitrile AICI 3- BPC
0.867 0.87 0.87 0.872 0.867 0.85 0.885
-
(a) (a) (a) (a) (a) (a) (b)
29.8 26.6 27.8 18 15.1 11.9
4.83 4.37 4.23 420 3.70 3.41 2.17
145
From eqn. (1) and from the invariability with the solvent of ( E - A ' ) (experimental findings) and F(q), eqn. (3) can be rewritten as F( W,'$2 - W,~,) = AGt° s~ (ln(III)) - AGt°sS~(ln(II))
(5)
which shows that the difference in IV' between two given solvents is quantitatively related to the difference in free enthalpy of transfer for the two oxidation states of the lanthanides. The last equation cannot be rigorously applied to experimental results because the W' are obtained from measurements on a series of lanthanides and correspond then to an average for the whole series, whereas the AGt° can be obtained for a single lanthanide and may vary with the atomic number. Furthermore, the results of this study were obtained at 40°C and not at 25°C as with the other mentioned solvents. In spite of these restrictions, eqn. (5) is experimentally verified. For instance, if one considers the transfer of the Y b ( I I I - I I ) or E u ( I I I - I I ) couples from propylene carbonate (P.C.) to DMSO, the experimental differences between the AGt° of the two oxidation states are respectively ( - 92 + 4) kJ m o l - 1 and ( - 100 + 4) kJ m o l - 1 [9]. From the respective W' values in P.C. and DMSO (reported in Table 2), and by application of eqn. (3), the difference of AGt° can be evaluated as ( - 108 + 16) kJ m o l - l , a value close to the preceding ones. CONCLUSIONS
(1) This study is an additional confirmation of the validity of Nugent's method of potentials linearization applied to the L n ( I I I - I I ) systems. The agreement between theory and experiment is remarkably good, especially if one considers that the corrective terms F(q) are obtained from a completely different type of experiments. (2) The value of the experimental slope ( E - A ' ) in the investigated melt is also in agreement with our previous conclusions regarding the independency of the slope with the nature of the solvent. From the determination of only o n e E ° value, it is then possible to calculate an approximate value of the E ° of any other lanthanide or, in principle, actinide couple. (3) Finally, the W' parameter of Nugent's equation has been related to the solvating power. Differences of F(W') between two solvents represent the average difference of free enthalpy of transfer. In the acidic 1-n-butylpyridinium chloroaluminate, it can be concluded that the + 3 oxidation state of the lanthanides is much less solvated than in several other room temperature solvents (cf. Table 2), with respect to the + 2 oxidation state. ACKNOWLEDGMENT
We are indebted to the Institut Interuniversitaire des Sciences Nucl6aires for the financial aid given to our laboratory.
146 REFERENCES 1 2 3 4 5 6 7 8 9 10
B. Gilbert, V. Demarteau and G. Duyckaerts, J. Electroanal. Chem., 89 (1978) 123. J.P. Schoebrechts, B. Gilbert and G. Duyckaerts, J. Electroanal. Chem., 145 (1983) 127. L.J. Nugent, R.D. Baybarz, J.L. Burnett and J.L. Ryan, J. Inorg. Nucl. Chem., 33 (1971) 2503. L.J. Nugent, R.D. Baybarz, J.L. Burnett and J.L. Ryan, J. Phys. Chem., 77 (1973) 1528. G. Duyckaerts and B. Gilbert, Inorg. Nucl. Chem. Lett., 13 (1977) 537. J. Robinson and R.A. Osteryoung, J. Electrochem. Soc., 127 (1980) 122. H. Strehlow and H. Schneider, Pure Appl. Chem., 25 (1971) 327. V. Gutmann and E. Wychera, Inorg. Nucl. Chem. Lett., 2 (1966) 257. J. Massaux and G. Duyckaerts, Bull. Soc. Chim. Belges, 84 (1975) 519. B. Tr6millon, in Chemistry of Non-aqueous Solvents, Reidel, Dordrecht-Boston, 1974, pp. 218, 219.