Electrochemistry of lanthanides in molten chloroaluminates

J. Electroanal. Chem., 89 (1978) 123--133

123

© Elsevier Sequoia S.A., L a u s a n n e - Printed in The Netherlands

E L E C T R O C H E M I S T R Y O F LANTHANIDES IN MOLTEN CHLOROALUMINATES P A R T I. THE ELECTROCHEMICAL B E H A V I O U R OF Eu(III), Yb(III) A N D Sm(III)

B. GILBERT, V. DEMARTEAU and G. DUYCKAERTS

Analytical Chemistry c~nd Radiochemistry, University of Lidge, B-4000 Sart TiIman (Belgium) (Received 5th May 1977; in revised form 15th July 1977)

ABSTRACT The electrochemical behaviour of Eu(III), Yb(III,II) and Sm(III,II) has been investigated in NaCl--AlCl3 mixtures, in a temperature range between 150 and 250°C. Principal investigation methods are cyclic voltammetry, pulse polarography and chronoamperometry. The three tervalent lanthanides can be reduced reversibly and solutions of divalent oxidation states are stable. Ell 2 potentials measured with respect to an A1 electrode in a saturated melt are located respectively at 2.295 V (Eu), 1.630 V (Yb) and 1.080 V (Sin). In acidic chloroaluminates, the three investigated lanthanides are more oxidizing than in other more complexing solvents. If the acidity of the melt is decreased, precipitation of the trichloride (or the dichloride) occurs and the solubility product of the different species is directly related to the cationic size.

INTRODUCTION

The existence of the divalent oxidation state in aqueous solution is limited to the Eu 2+ and Yb 2+ ions. It has indeed been shown that only those two ions are sufficiently resistant to oxidation to persist for appreciable periods and they can be obtained in aqueous media b y polarographic reduction of the terpositive species [1,2]. Owing to the strong reactivity of Ln 2+ cations with water, various organic solvents have been used in order to stabilize cations other than Eu 2+ and Yb 2+ [3--10]. To our knowle~lge, only Sm 2÷ and possibly T m 2÷ [9,10] have been detected in these media, in addition to the previously mentioned cations. If the standard potential E ° of the EuS+]Eu 2+ couple, for instance, is studied as a function of the organi c solvent nature [6] it can be concluded that the +2 oxidation state is stabilized in solvents less complexing than water, such as benzonitrile or acetonitrile and is unstabilized in solvents more complexing such as dimethylsulfoxide. By using different mixtures of DMSO (complexing) and PC (propylene carbonate, non complexing), it is indeed possible to shift the E1/2 potential of E u 3 + / E u 2+ almost 1 V, depending on the amount of DMSO present in the PC solution [8]. However b y using organic solvents, one has to realize that the results are often complicated by water, present as impurity in the solvent itself or coming from

124

poorly dehydrated solute, specially in non complexing solvents [7,9,10]. In molten chloride solvents such as LiC1--KC1, it has been possible to overcome the problem due to water or other impurities, but with considerable difficulties. From chronopotentiometric data, Campbell [ 11 ] postulated disproportionation of Eu(II), Yb(II) and Sm(II) in order to explain the observed decrease at each cycle. On the contrary, Johnson and Mackenzie [12] found no evidence of disproportionation but scrupulous precautions were needed to keep the solvent free from impurities which readily react with the highly reducing ions, specially Sm(II). Standard potentials measured vs. 1 mol kg -1 Pt(II--0) for Eu(III--II), Yb(III--II) and Sm(III--II) were respectively --0.554, --1.375 and --1.729 V. Stability of Sm(II) in molten LiC1--KC1 has also been confirmed more recently by Lebedev et al. [13]. An even greater stability of the divalent oxidation state is expected in molten chloroaluminates (AICls--MC1 mixtures, where M÷ is an alkali cation). It has indeed been shown that the acidic nature of chloroaluminate melts makes them particularly good solvents for studies of lower oxidation states [14]. Cations such as Cd 2+, Pb +, Sn~ [15], Bi+, Bi5a÷ [16,17], Hga2+ [ l S ] , We42+ [19], We2+ [20], Te2z+ [21], Se~ ÷, Sea~+ [22] and various low oxidation states of Nb [23] have been identified especially in acidic (A1Cla rich) mixtures which contain essentially large anions, A1CI~- and A12CI~-, of low solvating power. The purpose of this work was first to investigate in molten chloroaluminates on one hand the electrochemical behaviour of lanthanides which may be expected to be reduced as +2 cations, and on the other hand the influence of the melt composition on the stabilization of the divalent oxidation state. EXPERIMENTAL

Anhydrous lanthanide trichloride was obtained following the procedure given by Taylor and Carter [24]: it involves heating under vacuum, a molecularly dispersed mixture of h y d r a t e d lanthanide halide with six times more NH4C1 until the water and a m m o n i u m halide are expelled; it gives a pure anhydrous lanthanide halide. The lanthanide trichlorides prepared in this way are completely soluble in water and in acidic chloroaluminates and give clear solutions. Chemical analysis gives a correct chloride content within experimental errors. Melt preparation is carried out as has been reported previously [ 18] : recrystallized, dried NaC1 is mixed with sublimed A1Cla ("Iron free" from Fluka) and A1 metal (m4N, Fluka) and the mixture is digested for several days at 300°C, in a sealed ampoule. Only water clear melts have been used. All handling and preparation of the mixtures were carried out in an argon filled, h o m e made, dry box having a very low moisture level. The argon atmosphere is continuously recycled on molecular sieves and copper catalyst columns. The argon gas coming from the tank {rated at 5 ppm H20) is also purified by passing through a 2 m long molecular sieve column. The cleaned glass ware and the electrochemical cell are dried overnight at 130°C and introduced h o t in the prechamber which is evacuated to 5 × 10 - ~ Torr. In this way, the moisture level, continuously monitored by a coulometric cell ("Hygromite" from Beckman), can be maintained below 2 ppm. : The electrochemical cell, with an example of electrode, is shown in Fig. 1. It

125

C

A Fig. 1. Electrochemical cell (A and B), example of a Pt counter electrode (C). (a) Viton " O " ring, (b) teflon " O " ring, (c) platinum foil (1 cm 2 surface area), (d) stirrer, (e) tungsten wire, (f) platinum wire, welded to the tungsten wire and sealed in glass.

has been designed in order that a small a m o u n t of melt is needed for each experiment (melt preparation is indeed time consuming) and the electrodes can be removed in the course of an experiment; therefore the t o p of the cell includes several male and female unground sockets which can be provided with different types of electrodes or a thermocouple well to measure the melt temperature. Each electrode or socket is fitted with t w o packing rings (a Teflon and a viton " O " ring) which are grease-free, chemically inert and allow vacuum, even at 240°C. A longer tube, fitted on the t o p with a teflon stopcock allows vacuum or gas introduction when the cell is inside the furnace. The working electrodes used are made of platinum or tungsten wires, sealed in glass and t h e y are cleaned as previously described [18]. In all measurements, the reference electrode is an A1 wire in a NaC1 saturated chloroaluminate melt contained in a separate glass compartment. Owing to the high resistance of this (thin) pyrex membrane, a platinum quasi-reference or an aluminium reference electrode are dipped in the melt respectively when Ln 3+ or Ln 2* is the solute. Potential measurement with respect to the reference c o m p a r t m e n t is monitored continuously with a H.P. 3465 A digital voltmeter. For measurements involving coulometry, the reference

126 aluminium electrode and the (A1) counter electrode were separated from the bulk solution by a medium porosity frit. In some experiments, the melt has been purified inside the cell by electrolysis between two large aluminium electrodes which can be easily removed afterwards. Stirring of the melt is provided by a magnetic bar sealed in glass, driven by a rotating magnet placed under the furnace. The furnace consists of two concentric tubular cores (aluminium and stainless steel) provided with suitable insulation and water cooling; it has been built, leak proof, below the b o t t o m panel of the dry box and can be reached only from the inside of the box. The temperature is controlled within 2°C by means of a proportional temperature controller. Our experimental setup combines the advantages of each of the two systems described in the literature for molten chloroaluminate investigations [18,25]. Owing to the type of electrode fittings, the whole cell, vacuum tight, is placed inside the furnace and any loss of A1C13 by sublimation is avoided. On the other hand, if necessary each electrode can be replaced as all the experiments are performed inside the dry box. Cyclic voltammograms, pulse polarograms using normal, differential or derivative modes and coulometric experiments were performed with a P.A.R. Model 170. The X-ray diffraction powder patterns of the precipitates, sealed in vacuum in very thin silica capillaries, were obtained with a Philips unit, Model PW 1010. RESULTS AND DISCUSSION As is frequently observed in molten chloroaluminates, the results are essentially depending on the melt composition. The simplest behaviour occurs in very acidic mixtures, where NAtal3 (N = mole fraction) is higher than 0.52 and it will be described first.

(A) Electrochemical behaviour in AlCla--NaCl (0. 52--0.63 m.f.) mixtures Anhydrous lanthanide trichlorides are completely soluble (at least up to 5 × 10 - 2 M) in very acidic mixtures where NAme3 is above 0.52 but dissolution is always very slow. Dilute ( 1 0 - a - - 1 0 - 2 M) solutions of Eu 3+, Yb 3+ and Sm 3+ are almost colorless but solutions of Eu 2÷, Yb 2÷ and Sm 2÷ are respectively orange, pale yellow and dark red. An extensive study has been performed initially on Yb 3÷ solutions. A typical voltammetric wave obtained on a soluhon of YbC13 in a 0.61 m.f. A1C18 mixture is shown in Fig. 2A with the corresponding differential pulse polarogram (Fig. 2B). The reduction wave of Fig. 2A presents the following characteristics: (1) the ratio ipc/ipa (ip = peak intensity for the cathodic (c) and anodic (a) waves) is equal to 1. (2) the peak potentials Epc and Epa are independent of the scan rate (in the investigated range of 0,02 to 2.0 V s - l ) . ( E , ¢ - Epa ) equals 85 mV which is expected for a one electron reversible transfer at 155°C. This conclusion is also confirmed by: (1) the width at half height of the differential wave (AEex~. = 130 mV, AEth = 129 mV) (see Fig. 2B).

127

i0f Lima [I

A

I

i

i

LImA

Sm3,

t3

.O.5t-

2.01i./mA

1.9

""

1.7

-

yb3* e : . . rn.t)

1.5 E/V 1.3

2.2

1.8

1.4

1.0 E/V

Fig. 2. R e d u c t i o n wave o b t a i n e d o n a 4 . 8 2 x 1 0 - - 2 M s o l u t i o n o f Y b 3+. NA1CI3 = 0.61 m.f. ( A ) Cyclic v o l t a m m o g r a m . Scan rate = 0.1 V s - 1 , T = 1 5 5 ° C . W e l e c t r o d e , S area = 0 . 0 9 c m 2. (B) Differential pulse p o l a r o g r a m . A E = 10 m V , scan rate = 5 m V s - 1 . O t h e r c o n d i t i o n s identical as in A. Fig. 3. Cyclic v o l t a m m o g r a m a n d d i f f e r e n t i a l pulse p o l a r o g r a m o b t a i n e d o n a s o l u t i o n o f Eu 3+, Y b 3+ a n d S m 3+. [ E u 3+ ] = 8.6 X 10 - 3 M; [ Y b 3+] = 8.4 × 10 - 3 M; [Sin 3+ ] = 1 . 0 4 X 10 - 2 M. NAICl3 = 0.63 m.f.

(2) the slope 5 of a plot of log[Lid -- Off] versus E constructed from the normal pulse polarogram (Sexp. = 0.0848 _+ 0.0002; ~th = 0.085). Finally, quantitative controlled potential electrolysis concludes to a reduction involving one electron (nexp. = 0.98 + 0.02). Further experiments have shown that ip is proportional to the concentration so that the diffusing species does not undergo any change in the range 10-3--5 × 10 - 2 M. Ip is also proportional to v 1/2 (scan rate) and the reduction wave is consequently diffusion controlled. E1/2 potential measurements (defined as Eo --(RT/nF) ln(fR/fO) --(RT/nF) ln(Do/DR )1/2, where f and D are respectively the activity and diffusion coefficients [ 26], have been obtained from cyclic voltammetry (by using the relationship E1:2 = Ep + 1.1 RT/nF [26]), normal, derivative and differential pulse data. E1/2 potentials measured by differential pulse polarography have been corrected for the pulse height A E by the a m o u n t ~ AE [27]. After corrections, the results obtained by the different methods agree each other within 5 mV. For the Yb3+--Yb 2+ couple, the EI/2 potential was calculated as 1.630 V with respect to the AI(III--0) reference in a NaC1 saturated melt. As is shown in Table 1, E1/2 is not affected, within experimental errors, by the melt composition and consequently by a pCl change. It can be concluded that in this range of melt composition, the diffusing species is probably the Yb 3÷ cation and not a

128 TABLE 1 Electrochemical behaviour of Yb(III--II), Sm(III--II) and Eu(III) in acidic melts (NAlCl3 from 0.73 to 0.52 m.f.)

Ytterbium (A) Yb 3+ Melt composition NAICI3

T/° C

0.675 0.63 0.609 0.593 0.561 0.541 0.540

176 175 175 175 175 175 170

106 x D]cm 2 s- 1 7 5.3 5.4 6.35 6.45 5.6 5

E1/2/V 1.635 1.630 1.635 1.632 1.630 1.630 1.622

= 5.9

El/2 = 1.630

(B) Yb 2+ T/°C

106 X D/cm 2 s- 1

182 191 212 231 250

6 6.65 8.0 9.4 10.7

AH = 17 kJ tool- 1 (cyclic voltammetry) AH = 16 kJ tool- 1 (chronoamperometry)

Samarium

Sm 3+ Sm 2+

T[°C

106 X D/cm 2 s- 1

E1]2/V

175 175

3.5 4.0

1.075 1.080

T/°C

106 X Dicta 2 s---1

E1/2/V

175

4.0

2.295

Europium

Eu 3+

chloride c o m p l e x w h i c h w o u l d have s h o w n s o m e kind o f pC1 d e p e n d e n c e . This result c o n f i r m s t h e l o w c o m p l e x i n g p o w e r o f acidic m o l t e n c h l o r o a l u m i n a t e s . A d d i t i o n o f large a m o u n t s o f Na2COs (5 X 10 - 2 M) w h i c h i n t r o d u c e s o x i d e ions, d o e s n o t a f f e c t a n y o f t h e previous results and, c o n s e q u e n t l y , t h e o x i d e ions d o n o t play a n y role in the e l e c t r o c h e m i s t r y o f l a n t h a n i d e s in acidic melts, unless t h e initial solvent c o n t a i n s already a v e r y large a m o u n t o f o x i d e ( > 10 - 1 M) which is u n p r o b a b l e . All the c o n c l u s i o n s pertaining t o Yb s÷ solutions can be applied also to Eu 3+ and S m 3÷ solutions. A n e x a m p l e o f a cyclic v o l t a m m o g r a m and a differential pulse p o l a r o g r a m o b t a i n e d in a s o l u t i o n c o n t a i n i n g t o g e t h e r Eu 3+, Yb 3+ and Sm 3+ is s h o w n in Fig. 3. The t h r e e s y s t e m s u n d e r g o a reversible o n e e l e c t r o n

129 reduction. By controlled potential electrolysis, each lanthanide can be reduced quantitatively to the divalent oxidation state and a solution of Eu 2÷, Yb 2+ and Sm 2÷ is then easily obtained. Such solutions were found perfectly stable, unlike in organic and LiC1--KC1 solutions. Because of that stability, measurements were performed on solutions containing either Ln 3+ or Ln 2+ or both. Some of the results are summarized in Table 1. It can be seen that, for the same oxidation state, diffusion coefficients seem to increase at 175°C with decreasing of the size of the cation. However, no significant change in D was found for the same cation at different oxidation states, which is in contradiction with the previous result. More precise measurements will be necessary in order to decide whether or n o t the oxidation state changes the D values and, in this connection, we are intending to improve the temperature stability of our furnace. Activation energy for diffusion of Yb 2÷ has been calculated in the temperature range 175--250°C and found equal to 16.5 kJ mo1-1. Average E1]2 potentials of Eu3+/Eu 2+, Yb~+/Yb 2+ and Sm 8+/Sm 2+ have been obtained and correspond respectively to 2.295, 1.630 and 1.080 V +- 5 mV with respect to our aluminum reference. E1 [2 potentials measured in LiC1--KC1 at 450°C [12] can also be referred to the Al(III)--Al(0) system and the values in water, referred to the same system, are shown in Table 2. As can be seen, the couple Sm(III--II), for instance, is almost as reducing as A1 in LiC1--KC1 but much more oxidizing in chloroaluminate. The three investigated lanthanides behave then as strong oxidants in acidic chloroaluminates. Eu(III) is almost as oxidizing as C12 and its investigation is always complicated by the solvent oxidation. This drastic increase in oxidizing p o w e r has to be related to the different tendency of forming chlorocomplexes in the t w o molten media, specially regarding the higher oxidation state. Thermographic studies [28] of the KCI--SmC13--SmC12 and KC1--YbCI3--YbC12 systems postulated the existence of LnC142- and LnC13complex ions in the melt. Raman measurements have also shown the existence of LnC163- complex ions in different alkali halide melts [29]; these complexes are thus most likely to be found in LiCI--KC1 t y p e mixtures. On the contrary, the high p C l - of very acidic AIC13--NaC1 mixtures renders difficult a complex

TABLE 2 Standard oxidation potentials of the Ln(III)/Ln(II) system for Eu, Yb, Sm in different solvents

Reference electrode: Al(O)/Al(III) Solvent

AIC13--NaC1 LiCI--KC1 (eutectic) Water

a This work.

Temperature~ ° C

175 450 25

Ref.

E1/2/V

Eu

Yb

Sm

2.295 1.21

1.630 0.39

1.080 0.03

1.31

0.51

0.11

a [12]

130

formation and this is confirmed, as discussed above, by the invariance of the properties when the pC1 is changed. This conclusion is, however, only valid for very acidic melts (0.54 < NA]C~3 < 0.70) where the pC1- (expressed in mol kg-1) is maintained between 5.8 and 7. For lower pC1- values, the behaviour of lanthanides changes as will be described afterwards.

(B) Electrochemical behaviour in mixtures (NAcz3

--

0.498--0.52 m.f.)

As soon as the melt acidity is decreased towards compositions containing less than 0.52 m.f. A1CI3, the reduction waves progressively disappear and a precipitate is observed. Figure 4 shows the decrease in the peak heights for a solution containing Yb s+ and Sm 3+ as a function of pC1-. From this Figure and from other experiments in which the pC1- has been lowered progressively in a known manner, the following results have been obtained: (1) Sm a+ precipitates at lower pC1- than Yb ~+, probably because of its greater size. (2) For the same reason, Ln 2+ is more soluble than Ln 3+ and precipitates at lower pC1 values. Therefore it is clear that a range of melt composition (thus of pC1-) exists where Yb 2+ is soluble and Yb 3+ is insoluble. By performing experi-

"m'l

yb3"{4,8.10-2M)

D'3tL/mA

0"2IpCI-=4.88

??

N~tct3 = 0,6114m.f.

I I

i

0.3~

l

NAICtffi0,5134m.t. pC1-=5,01

fN

~.4 0.2CI'=4.SS {}.I[ 0 -- I /

NAtCI3©0,5038 re.t.

pet-=4,44 ~.." -in

0.3 O. Cl-~4.27 0,1~ oi,

1'.8

1.3

.

/

I

/ ~

o.2~ pc''~'6 I

~

I

,

I

I

2"-~'.,

"

1'.6

I

'

'

Fig. 4. Differential pulse polarograms on solution of Yb 3+ and Sm s+ at different pCl-- values. [Yb 3+] = 4.8 × 10--2 M; [Sin 3+] = 1.33 × 10--2 M, Pt electrode, 0.045 cm 2. Fig. 5. Cyclic voltammograms obtained during Yb 2+ oxidation in the pC1-- range between 5 and 4 tool kg"1. T = 175°C, Pt electrode, 0.078 cm 2, scan rate = 0.1 V s- 1 , [Yb 2+ ] = 9 × 10 - 3 mol kg- 1 .

131

ments in this range (0.51--0.52 m.f. A1C13), it has been possible to investigate more thoroughly by cyclic voltammetry, the nature of the precipitate. Cyclic voltammograms obtained during oxidation of a Y b 2+ solution are indeed characteristic of an insoluble product formation and the peak potential (Ep) is a function of pC1- as shown in Fig. 5. In these experiments, the pC1- can be measured precisely by monitoring the potential of an A1 electrode in the melt with respect to the reference compartment. The potential is then related to the pC1- as explained by Torsi and Mamantov [30]. The precipitation produced by lowering the acidity of the melt, results probably from an increase in the Cl-, 0 2 - or A1CI~- content; however, as the precipitation occurs in a very narrow acidity range where the A1CI~ concentration changes slightly [30,31], the latter assumption can be ruled out. As a first hypothesis, we suppose that the precipitate has the formula YbClxY~3-x) where Y --- A1CI~-, the predominant species in this composition range. The precipitation (and oxidation) reaction will thus be the following: Yb 2+ + xC1- + (3 --x)A1C14 -~ YbClx(A1CI~-)3_x ~ + e Therefore the peak potential (Ep) is related to: Ep = E ° + Rn FT ln K's + ~ R T (0.924)2 __n_~ln[C1]x[y]¢3_x)_RTnF RT ln[yb2+]

with g~ = [Yb 3+] [C1-]~[Y-] ~8-~. A plot of Ep with respect to the pC1- is linear and exhibits a slope of 0.262 + 0.01 and the corresponding value of x equals 2.94; this means, in our hypothesis, that the precipitate is YbC13 and no A1CI~ anion has to be taken into account. From the intercept of the straight line, a solubility product of 10 -17"1 mol 4 kg- 4 can be calculated. It has also been shown that oxide ions behave as a tribase in acidic chloroaluminates following a reaction similar to [32--34] : O 2- + A1CI~- -* A1OC1 + 3 C1Consequently a slope of 4.5 for the variation of Ep with respect to the pC1is expected in the case of a Yb203 precipitation. Furthermore, an X-ray diffraction study of the precipitate shows clearly that it is indeed YbC13. In Table 3, we have summarized the main lines of the diffraction pattern, and compared it with the literature data for YbC13 [35] and NaA1C14 [36]. No trace of any lanthanide oxide or oxychloride was found. The precipitate being clearly YbC13, it is then possible to calculate the solubility products from the decrease of the reduction wave of YbCI3 and SmC13 with pC1-. Those determinations are less precise than the previous one because no aluminium electrode (which would reduce the Ln 3+) can be used in the melt and the pC1- has to be estimated from the amount of NaC1 added and the amount of C1- consumed by LnC13 precipitation; the corresponding composition of the melt is then calculated and the pC1- estimated from it [31]. Nevertheless, a value of Ks 10 -17.2 tool 4 kg-4 has been calculated for YbC13; it is in excellent agreement with the previous value. As expected, Ks = 10 -16 tool 4 kg- 4 for SmC13. Finally, preliminary experiments on a solution 5.4 × 10- 3 mol

132 TABLE 3 X-ray diffraction results (d-spacings/A) Precipitate

YbC13 a

YbC13 a

d

I

d

d

I

6.352 5.985 5.277 3.307 3.079 2.942 2.865 2.690 2.524 2.289 2.085 1.939 1.851

40 90 40 60 70 80 80 70 80 40 60 50 50

5.980

6.34 5.993

(40) (lOO)

3.307

3.302

(80)

2.689

1.939 1.864

2.686

1.935 1.846

NaAICI4 c d

I

5.32

50

3.09 2.948 2.880

75 lOO 100

2.541 2.294 2.100

75 50 45

(100)

(70) (60)

a From ref. 35. b Our results. c From ref. 36. kg-1 of Sm 2+ have shown t h a t SmC12 (red crystals) begins to precipitate at a pC1- of 3.75 which corresponds to a solubility product of 10 -9.8 mol 3 kg- 3 . The same solution of Sm 3+ would have begun to precipitate at a pC1- of 4.57 and this is in a ~ e e m e n t with the decrease of solubility corresponding to a decrease in cation size. Tentative experiments in order t0 redissolve the precipitates in NaC1 saturated melts by forming complex anions were unsuccessful in the temperature range between 175 and 200°C. . . . . No oxide precipitation has been observed as it could have been expected from the possibly high oxide content of those melts [33]. The oxide ions exhibit probably more affinity for aluminium than for the lanthanide and, consequently, the free oxide ion concentration is very low as further experiments have clearly shown [ 34]. The possibility of obtaining a Tm(II) solution in a chloroaluminate melt, the application o f the Nugent theory [ 37] and the prediction of the standard potentials for the other lanthanides and actinides in different molten salts, including chloroaluminates will be presented in a subsequent publication [ 38]. ACKNOWLEDGEMENTS We are indebted to the Fonds National de la Recherche Scientifique and to the Institut Interuniversitaire des Sciences Nucl~aires for the financial aid given to our laboratory. REFERENCES 1 H.A. Laitinen and W.A. Taebel, Ind. Eng. Chem, Anal. Ed., 13 (1941) 825. 2 H.A. Laltinen and E. Blodgett, J. Amer. Chem. Soc,, 71 (1949) 2260.

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