Determination of the stoichiometry of some metal chlorocomplex ions in basic ambient temperature molten salts

413

J. Electroanal. Chem., 284 (1990) 413-429 Elsevier Sequoia S.A., Lausarme - Printed in The Netherlands

Determination of the stoichiometry of some metal chlorocomplex ions in basic ambient temperature molten salts Marc A.M. Noel and Robert A. Osteryoung Department (Received

of Chemistry

State University of New York at Buffalo, Buffalo, NY 14214 (U.S.A.)

3 October 1989; in revised form 4 January 1990)

ABSTRACT Oxidation waves of chloride at platinum microdisk electrodes in ambient temperature ionic liquid mixtures of 1-ethyl-3-methylimidazolium chloride (ImCl) and aluminum chloride were examined by normal pulse voltammetry to obtain information about the stoichiometry of several metal chlorocomplexes. It was found that FeCl;, GaCl;, CdCls-, and SmClz- are the only species present in basic melts, those containing excess ImCl. In solutions of Nd(III), an equilibrium between NdCli-, NdCl:and Cl- exists, for which the dissociation constant is evaluated as 3.7 mol/n?. An ill defined wave that could be the oxidation of chloride from CdCl:-, NdCland SmClz- is also observed. The StokesEinstein coefficient for chloride, using n = 1, was found to be: D q/T = 2.53 X lo-l5 kg m/s2 K.

INTRODUCTION

Room temperature ionic liquids composed of AlCl, and 1-ethyl-3-methylimidazolium chloride have been extensively employed as solvents during the past few years [l-8]. These mixtures exhibit a tunable chlorobasicity and a large, and variable, electrochemical window [l]. Practical incentives behind this research include the realization of room temperature molten salt batteries and synthesis of molecules difficult to achieve in other media. These molten salts are referred to as basic, acidic or neutral when the molar ratio AlCl,/ImCl is either less than, greater than or equal to one. The potential of an aluminum wire in a 1.5 : 1 melt (containing 1.5 mol AlCl, per mol ImCl) is quite stable [9] and has been used as a reference electrode in chloroaluminate melts. When added to a basic melt, metal chlorides are complexed according to the reaction: MCI, +&I-=

MCl,P,,

withp=land/or2and/or3... 0022-0728/90/$03.50

0 1990 - Elsevier Sequoia S.A.

(1)

414

Methods that have been used to determine p include mass spectrometry [lO,ll], Raman spectroscopy [12], H-NMR of the C-2 proton on the Im+ ring [13], voltammetry and/or potentiometry of the metal chlorocomplex [14-211, UV-visible spectroscopy [14,15,17] and chloride voltammetry [22,23]. Unfortunately, few of these work for all metal chlorides. For example, all metal chlorocomplexes do not absorb within the UV-visible window of the melts, all metal chlorocomplexes are not electroactive within the melt window(s), any paramagnetic chlorocomplex will render H-NMR measurements difficult, etc. However, careful monitoring of the chloride concentration as a function of metal chloride addition is a direct measurement of the complexation reaction. One may measure the chloride content of a slightly basic melt by voltammetry and infer the value of p provided the complex is not oxidized at a less anodic potential than the chloride ion [22,23]. Although still subject to experimental problems [22], the chloride oxidation appears to be a one-electron process [24]; i.e., Cl-+

e-+

l/2 Cl,

(2)

We will use n = 1 in all our computations. It has been shown that many ionic species in the melt obey the Stokes Einstein model of diffusion [16,19,20] in which the diffusion of species is described by a constant: K S-E

=

h/T

(3)

Ionic interactions in the basic melts have been studied extensively [7,8,25,26]. A model of these interactions showed the dependence of viscosity on chloride concentration and temperature [7,13]. As a metal chloride is added to a basic melt, the chloride concentration changes, eq. (l), thus resulting in changes of viscosity. At the time of previous studies [22,23], it was assumed that these changes were negligible. However, changing the chloride concentration from 200 to 30 mol/m3 decreases the viscosity by about 10% at room temperature. Microdisk electrodes, shown to be useful for a wide variety of problems, have been employed in this study [27]. A solution for chronoamperometric diffusion limited currents at microdisks electrodes has been given [28]: Ilim = 4nFrDcf( P)

(4)

with P = 4Dt,/r2

(5)

For the experimental conditions in this manuscript, f(P)

= ( a/4P)“2

+ a/4 + o.094P”2

P -c 0.82 and (6)

Thus the current is always larger than the current given by the Cottrell equation [29]: Ilim = nFAD”2c/(

vrt,)1’2

(7)

415

For a given process, the total amount of charge passed is roughly proportional to the electrode area. The amount of material reacting at a microdisk will be small and the product of the reaction will also diffuse into the bulk of solution relatively faster because of the important non-planar diffusion effects [27]. These are two useful properties we take advantage of for the chloride oxidation. In this study we employ platinum microdisk electrodes to oxidize chloride reproducibly and obtain information on the stoichiometry of metal chlorocomplexes and chloride ion diffusion. EXPERIMENTAL

The principle of our experiment is that of an amperometric titration in which we measure the decrease of the chloride ion limiting current as a function of the addition of a species that reacts quantitatively with it to form a chlorocomplex. Melts of known chloride concentration were prepared and partitioned into five cells for three of the metal chlorides studied. To each cell, weighed amounts of metal chloride were added and the mixture was stirred and heated to 50 o C. The highest concentrations of metal chloride used are listed in Table 1; no attempt was made to evaluate the saturation values. To test for a possible systematic bias, the Cd(I1) and Sm(II1) experiments were performed so that, before the metal chloride additions, each cell contained a melt of a different composition. Normal pulse voltammetry was performed with a PARC 273 potentiostat controlled by a DEC PDP-8/e computer [30]. The ratio of pulse time to waiting time was l/40, thus eliminating the need for stirring between pulses; pulse times of 2 to 500 ms were used. The potential was varied between +0.6 and + 2.0 V vs. Al(III)/Al in a 1.5 : 1 melt. During the waiting time, the potential imposed at the working electrode was + 0.6 V. The limiting currents were evaluated with the “ three lines” method [31]. The counter electrode was a BAS platinum disk (1.6 mm diameter) and the reference consisted of an aluminum wire in a 1.5 : 1 melt separated from the solution by Vycor “thirsty” glass. All electrodes were wiped and cleaned in acetonitrile between each experiment. The working microelectrode (Pt, 13.93 pm radius) was

TABLE 1 Highest metal chloride concentrations prepared MCI,

c/m01 mm3

T/K

CdCl 2 FeCI, GaCl, NdCl 3 SmCl,

200 170 174 60.1 183

301 299 299 305 303

416

calibrated both optically and electrochemically [32]. The cell was placed in a grounded Faraday cage. The lead wires and the electrometer were also shielded and similarly grounded. All experiments were performed in a Vacuum Atmosphere Drybox equipped with an HE 93 Drytrain. The temperature for the experiment was at ambient, rather than controlled, because of the large noise induced by use of a typical furnace. The temperature varied between 26 and 32” C. Sturdy platinum and tungsten microdisk electrodes of radii down to 2.5 pm, capable of being handled in a drybox, were manufactured by threading the wire into a 20 cm long thick walled 2 mm inner diameter Pyrex tube. Good glass to metal seals were obtained under vacuum in a specially designed furnace (1.5 cm inner diameter, 2.5 cm hot spot) at 800°C. The electrodes were polished to a 0.05 pm finish with alumina Buehler polishing suspensions. Aluminium chloride and 1-ethyl3-methyl imidazolium chloride were prepared and used as described previously [33,34]. CdCl, (anhydrous 99.995% Alfa products), SmCl, (anhydrous 99.9% REO, Morton Thiokol), GaCl, (anhydrous Spectrapure, Johnson Mathey&Co., Lim.), NdCl, (anhydrous 99.9% Aldrich) and FeCl, (anhydrous 98%, Aldrich) were used as received. Pt (Aesar) microwires were used to build the working electrodes. All statistical calculations are given with 95% confidence intervals. Computations were carried out on an IBM PC computer with Quick-Basic source programs. RESULTS AND DISCUSSION

Figure 1 shows a typical normal pulse voltammogram for the oxidation of chloride on a Pt microdisk. The currents are very reproducible with standard deviations less than 1% of the average measured value (Table 2). Also included in Table 2 are the values of Z&j(P); these are seen to be constant, within experimen-

0.21 -0.L

.

0.0

.

0.2

.

0.4

E/V(vs

*

0.s

.

0.e

.

I.0

I.&?

I..

.

1

,.e

.

,.a

.

L.0l.l

Al/Al WI1 I” 1.5 1 melt)

Fig. 1. Normal pulse voltammogram of chloride oxidation at platinum; Pt electrode radius 13.93 pm, Ei +0.6 V, E, = + 2.0 V, tP = 5 ms, 1, = 200 m.s and [Cl- ] = 212 mol/m3.

417 TABLE 2 Reproducibility of measurements. Reproducibility of chloride oxidation waves: [Cl-] = 210 mol/m3, T = 303 % melt 0.955 : 1, Platinum microdisk, r = 13.93 pm. Limiting currents f standard deviation, and average limiting currents divided by f(P). D = 4.37 X 10-l c&/s

t,/m

ZliJd

((4i,) * w/n‘4

(him) f(fT’/d

10 10 10

533.8 532.0 532.1

533

52.6

20 20 20

405.0 404.3 403.8

404.4kO.6

54.6

40 40 40

306.4 302.7 303.3

304

*2

55.6

100 100 100

212.3 213 214.5

213

*l

56.6

200 200 200

161.4 162.9 162.4

162.2 f 0.8

*1

55.7

tal error. As the pulse width is varied from 10 to 200 ms, the non-planar current contribution varies from ca. 10 to 33%. Because of the very low values of D, quite a long time would be required to obtain steady state currents at the electrode employed. Upon addition of a metal chloride to a basic melt, some of the initial chloride, [Cl-Ii, is complexed by the metal ion and, neglecting the very small volume changes, one may rewrite eqn. (1) as:

[cl- IexGss = [Cl-Ii-P [MCl,PYp]

(8)

Since the current, Ifi,, is proportional to the concentration of “free” chloride, i.e., [C1-1,,,,, we consider ways to find p. The most direct procedure corresponds to a typical amperometric titration, i.e., a plot of Iti vs. [MCl,P,,] is made. Such plots are shown in Figs. 2 and 3 for additions of GaCl, and NdCl,. Note that such a plot is valid only if all increments of metal chloride are added to the same solution containing excess [Cl-Ii; this means that neither the cadmium nor samarium results should be interpreted this way (see Experimental section). The value of the intercept should correspond to the concentration of metal chloride needed to complex all chloride ion initially present. In both cases, [Cl-Ii = 212 mol/m3, and, from the intercept, GaCl, is complexed by one chloride while NdCl, is complexed by three chlorides. We compute the average value of p with the

418

GA(CI),

/

mol I-’

Fig. 2. Chloride oxidation limiting current at platinum (13.93 pm radius) at various pulse widths for five top to bottom = 10, 20, additions of GaCl, to a melt composition of [C1-]i,,,ual = 212 mol/m3. r,/ms, 50,100,200 and 500.

intercepts at six different pulse widths (Table 3) and the initial chloride concentration according to p = [Cl-Ii/average

of intercepts

(9)

and we obtain p = 1.024 (+0.015, - 0.012) for Ga(III), p = 2.89 (+0.045, -0.04) for Nd(II1) and, similarly, p = 1.025 (+0.016, - 0.015) for Fe(II1). This latter value is in agreement with previous results [20,22,23]. Table 3 shows the slopes of I, vs. [MCl,] at six pulse widths. The slopes should equal -p -4nFrDfP). From Table 3, we see that the slopes corresponding to the additions of Ga(II1) and Fe(II1) are almost identical. This, however, is not the case if we compare the slopes for Nd(II1) additions with the slopes obtained for the other

400

0 0.01

0.02

0.03 0.04 0.05 [ NdC131 / mot I-’

Fig. 3. Same as Fig. 2 for Nd(III)

0.06

additions.

0.07

419 TABLE 3 summary of I,, vs. [Ma,] plots. Slopes, m, abscissa intercepts at Z = 0, b and coefficients of correlation, r, of Z versus [Ma,] plots for five Fe(III), Ga(III), and Nd(II1) chIoride additions to a 0.955 : 1 melt ([Cl-] = 212 mol/m3) for six pulse widths

r,/ms

MCI,

FeCl,

GaCl s

NdCl,

b/m01 m/nA r b/m01 m/nA

mW3 m3 mol-’ m-3 m3 mol-’

L/m01 m-3 m/nA m3 mol-’ r

10

20

50

100

200

500

211.7 - 2.214 - 0.9996 210 - 2.347 - 0.994 72 - 9.272 - 0.998

208.8 -1.644 - 0.9992 211 - 1.653 - 0.994 72.5 - 6.459 - 0.998

207.2 - 1.155 - 0.9996 207.7 - 1.192 - 0.998 74.2 - 3.910 - 0.997

205 -0.9152 - 0.9995 204.8 - 0.9498 - 0.997 75 - 2.834 - 0.998

203.7 - 0.7279 - 0.9997 204.5 - 0.7545 - 0.995 73.6 - 2.243 - 0.999

204.2 - 0.544 - 0.9997 205.6 - 0.5661 - 0.995 72.9 - 1.708 - 0.998

two metal chlorides. For example at t, = 0.01 s, there is a + 39% relative difference between the Nd and Fe experiments, but the discrepancy decreases with increasing pulse times. We have neglected, as a first approximation, any volume and viscosity changes that may be significant. Indeed, Table 4 shows viscosity changes due to variations of temperature and chloride concentration that correspond to the working conditions. The product D!(P) is clearly not constant and it may seem surprising to observe linear plots as shown in Figs. 2 and 3. However, the viscosity effect is not large enough to result in excessive curvature of these plots. It is clear, however, that the evaluation of p from such plots is not always adequate. Another approach that could lead to the determination of p consists in plotting Iti, vs. [c~-l,,,, together with corrections for changes in melt composition. There are three advantages to this procedure:

TABLE 4 Melt viscosity under working conditions. Viscosity variations as a function of composition and temperature [7]. The viscosity is given in centipoise (1 CP = lo-’ Pa s). The composition is expressed as chloride concentration. ?/“C

25 27 29 31

[Cl-]/m01

mm3

0

50

100

200

17.76 16.79 15.89 15.07

18.32 17.31 16.37 15.51

18.90 17.84 16.87 15.97

20.19 19.03 17.96 17.90

420

-0.1

0.0

{ICI1 -p [CdCl21}

0.1

0.2

0.3

I mol I-’

Fig. 4. Plots of chloride oxidation limiting current at platinum microdisk with tP = 100 ms, for five additions of Cd(H) to melts of various compositions, versus [Cl-],,,, see eqn. (8), with p = (a) 3, (b) 2 and (c) 1.

First, [Cl-Ii does not have to be constant so that the plotting procedure is more general. Second, all density changes are taken into account. We make, however, the approximation that small amounts of metal chlorocomplexes do not modify to any measurable extent the total magnitude of the ionic interactions in the melt providing one considers each metal chlorocomplex equivalent to p AlCl, molecules. This is not unreasonable since it has been shown that (a) the chloride-Im+ interaction is much stronger than the one between AlCl; and Im+ [6,7,13], (b) the concentration ratios of the metal chloride to the tetrachloroaluminate ion are always smaller than l/23, and (c) this approximation has been successfully utilized in a study that correlated the Im+ C-2-proton chemical shift with metal chloride additions to a basic melt [13]. Third, this procedure can indicate the presence of an equilibrium between chlorocomplexes which would result in a non-zero intercept for integer values of p. One may compute [Cl-],,, for several values of p and plot Ifi,.,, vs. [Cl-]__,. Such a plot with the correct value of p should approximate a straight line with zero intercept. Figure 4 shows such a plot for cadmium chloride additions to melts of different initial compositions with p = 1, 2, and 3; p = 2 is clearly the best value. Table 5 summarizes this procedure for all metal chlorocomplexes studied here. Interpretation of the slope may be inappropriate if we do not correct the currents for viscosity changes. To evaluate our procedure further, one should compare the ratios of the measured currents with the ratios of computed currents at different t,. This, however, requires knowledge of the chloride diffusion coefficients under all experimental conditions (see Appendix 2).

421 TABLE 5 plots. Slopes, m, intercepts, b, and coefficients of correlation, r, of Zfi, summary of Z,b vs. [cl- I,_ for Ga(III), Fe(III), Cd(II), Sm(III), and Nd(II1) additions, with the “best” p and at versus [Cl- I,,, six pulse widths

Ga

Fe

Cd

Sm

1

m/nA m3 mol-’

1

b/nA r m/nA m3 mol-’

2

b/nA r m/nA m3 mol-’

3

b/nA r m/nA m3 mol-’ b/nA r

Nd

3

m/n4

m3 mol-’

2.239 -5.1 0.989 2.179 7.15 0.999 2.060 27.5 0.994 2.238 -5.8 0.998 3.096 13.5 0.998

1.654 - 2.7 0.993 1.617 -4.27 0.999 1.562 15.0 0.996 1.618 -2.7 0.997 2.157 12.6 0.998

1.192 -5.9 0.996 1.137 - 4.78 0.999 1.077 8.39 0.998 1.093 - 12.0 0.998 1.306 14.1 0.997

0.9495 - 7.4 0.995 0.9009 - 5.93 0.999 0.8568 0.96 0.999 0.8562 -3.3 0.997 0.9462 12.6 0.998

0.7549 -6.2 0.993 0.7164 - 5.6 0.999 0.6700 - 0.97 0.999 0.6871 - 5.0 0.995 0.7486 6.71 0.999

0.5664 - 4.0 0.993 0.5357 - 3.9 0.999 0.5057 - 1.36 0.999 0.5237 -4.5 0.995 0.5702 3.9 0.998

The measurement of chloride diffusion merits discussion. As chloride is oxidized at the electrode surface, the melt basic&y decreases to neutrality going from the bulk of solution to the electrode surface where [Cl-] = 0 mol/m3. In other words, the closer to the interface, the faster chloride moves (viscosity decreases if [Cl-] decreases). Thus, we have a complex diffusional behavior that includes the edge effects of a microelectrode and an effective concentration dependence of the diffusion coefficient. To our knowledge, there is no theoretical model available for this diffusion problem but we may still measure an average or effective diffusion coefficient, &r. The value of De, will be always higher than the true D in the bulk and lower than D in a neutral melt. By definition, we cannot measure D of chloride in a neutral melt ([Cl-] = 0 mol/dm3). Moreover, we expect De, to be dependent on chloride concentration so that De, decreases as [Cl-] increases. In fact, the opposite dependence was observed in a previous publication [24]; the explanation given for this non-intuitive result was a surface activation process. However, this cannot be totally responsible for the observed changes in measured chloride diffusion coefficients, especially in the light of the following: (1) it was necessary to compute DC,- from limiting current at long pulse times to limit the importance of surface processes, and (2) the same phenomenon was observed on platinum and aluminum electrodes. We favor an explanation that relies on the bulk physical properties of the melt. In these melts, the Schmidt numbers, N, (see appendix l), are usually 100 times greater than in water (in water N, = 2,000, in the melts N, = 300,000). This means that convection, when

422 TABLE 6 Summary of selected Zam vs. Z, plots. Slopes, intercepts, 95% confidence level intervals, standard deviations, Sy (nA), and coefficients of correlation, r, of Zlim vs. Z, plots for selected Ks_E. The number of data couples is 128; fStudcn,= 1.9788 1015 KS_,/ kg m SC* K-’

slope

2.40 2.41 2.42 2.43 2.50 2.52 2.53 2.54 2.59 2.60 2.70

1.028 f 1.026 f 1.023 f 1.021 f 1.006 f 1.002 f 1 .OOO f 0.998 f 0.988 f 0.986 f 0.966 f

0.023 0.023 0.023 0.023 0.022 0.022 0.022 0.022 0.022 0.022 0.021

intercept/ nA

S,/nA

r

-1.7*3.9 - 1.7* 3.9 - 1.8 f 3.9 -1.8k3.9 -2.Of3.9 -2.Ok3.9 - 2.1 f 3.9 -2.1*3.9 - 2.2 f 3.9 -2.31t3.9 - 2.5 f 3.8

13.4 13.4 13.4 13.4 13.3 13.3 13.3 13.3 13.3 13.3 13.2

0.9920 0.9920 0.9920 0.9921 0.9921 0.9921 0.9921 0.9921 0.9921 0.9922 0.9922

compared to diffusion, is ca. 100 times more important in the melts than it is in water. Therefore, even slight density changes, such as those created during chloride oxidation, may be sufficient to generate convective currents (for example, in a basic melt with [Cl-] = 263 mol/m3, the average density gradient across the diffusion layer after a 100 ms pulse is about 10 g/cm3 per cm!). The increasing D,, with increasing chloride concentration is thus due to the more important density variations, causing larger convective currents across the diffusion layer as [Cl-] increases. This is consistent with the positive density gradient across the diffusion layer, going from the bulk to the electrode surface, and the usual electrode setup, the electrode vertically immersed in solution and the interface horizontally oriented. Consequently, one should preferably measure D,rr at low rather than at high chloride concentrations. Therefore, we decided to reevaluate D,,, rather than use published data. Since Q.rr is the only accessible parameter and there is no theory to allow us to transform it to D, we will now omit the “eff’ subscript. To evaluate D from our data, a model was constructed for which the unknown is K S-E and the output is a computed diffusion limited current for chloride ions in the melts (Appendix 2). A plot of measured currents, I,im, vs. computed currents, I,, should have a slope and an intercept equal to 1 and 0, respectively. Table 6 lists slopes and intercepts for such plots for several KS_E values together with their intervals at the 95% confidence level. The confidence intervals for KS_. are, in turn, evaluated by finding the limiting values of KS-E that still give statistical slopes and intercepts of 0 and 1 respectively. In this way we find that KS_. = 2.53 ( + 0.06, -0.11)10-‘5 kg m/K s2. This value correspond to a D = 3.11 X lo-’ cm2/s in a 0.9 : 1 melt at 25 o C. We omitted all measurements involving Nd(II1) additions in the determination of KS_, (see discussion of Nd(II1) chlorocomplexes) and added measurements of chloride oxidations for basic melts containing no metal chloride.

423

600

0

100

4 200

400

300

500

60

Fig. 5. Plot of Z, versus I,, with KS_E = 2.53 X lo-l5 kg m K- ’ intercept = - 2.07 f 3.86 nA, coefficient of correlation, r = 0.992.

se2 128 data pairs, slope = 1.00~0.02,

Figure 5 is a plot of Iiim vs. 1, with KssE = 2.53 X lo-l5 kg m/K s2. The agreement between experimental data and the model is good. In order to assess further the validity of the model, measured and computed currents were ratioed and compared. Table 7 shows such ratios for Cd(I1) additions. The agreement is good. Our value of KS_ r is comparable to a previously reported value of 2.6 x lo-l5 kg m/K s2 (ref. 22 correcting for n = 1) at a rotating disk electrode. The range of chloride concentrations investigated was similar to this one.

TABLE 7 Comparison of current ratios. Calculated a and measured current ratios for Cd(H) additions. 0.1 s)/Z(r, = 0.2 s), b = Z(t,= 0.1 s)/Z(t, = 0.5 s) [Cl-],,_/mol

me3

178 z 109 ; 122 ; 65.3 6” 36.3 b” a The currents are computed

calculated

measured

1.29 1.75 1.29 1.74 1.29 1.74 1.29 1.73 1.29 1.73

1.28 1.72 1.33 1.78 1.33 1.76 1.32 1.83 1.32 1.77

according to the model described in Appendix

2.

n = Z(r, =

424

Fig. 6. Normal mol/m3. r,/ms

pulsevoltammograms

in the presence of Cd(H); [Cl-],, = 122 mol/m3, [Cd(II)] = 48.75 = (a) 2; (b) 5; (c) 10; (d) 20; (e) 50; (f) 100; (g) 200 and (h) 500.

We have also noticed the existence of a second oxidation process at potentials more positive than that at the chloride oxidation for the melts containing CdCl,, NdCl, and SmCl, (Fig. 6). A possible assignment of that process is presented for Cd(I1) only since it appears the general features of this ill defined wave are similar for Cd(II), Nd(III), and Sm(II1) containing melts. FeCl,

and GaCI,

In accordance with previous reports [10,20,22,23], we determined both as tetrachlorocomplexes (Table 5). This is not a surprising fact since both cations have almost identical ionic radii (64 and 62 ppm for Fe(II1) and Ga(II1) respectively) and the same charge. CdCI,

We found the stoichiometry of Cd(I1) in the melt to be a tetrachloro species. This is in accordance with a H-NMR study [13]. Further evidence was obtained through the composition independence and presence of only (a) one Raman shift at 262 cm-’ (Fig. 7) in agreement with a high temperature 2 : 1 CsCl : CdCl, melt [12], and of (b) one 39Cd NMR shift at 484.8 ppm vs. Cd(NO,), lo3 mol/m3 in D,O (Fig. 8). As mentioned previously, an additional oxidation wave at about +1.8 V was observed. This ill defined wave shows certain trends. Except at long pulse times, it is

I M

I

Y 1M

250

350

450

Wave Number/cm”

Fig. 7. Raman shift of CdCl$-;

[Cd(II)] = 220 mol/&.

425

485

485 I 0

A

Chemical

/ ppm

Shift

Fig. 8. 39Cd NMR shift vs. Cd(N03), [Cl- ]ititi/[Cd(II)] = 2.37.

in D,O at two compositions:

(A) [Cl- ]ititid/[Cd(II)]

= 12.58, (B)

a peak followed by a flat region whose height is proportional to the Cd(H) concentration (Fig. 9). Changing the initial potential to more negative values decreases the peak intensity. The wave/peak position shifts cathodically with increasing cadmium concentration (200 mV from 48 to 200 mol/m3) and pulse time (200 mV between 2 and 500 ms).

0.05 ICd(CI

0.10

0.15

0.20

)dl I mol I-’

Fig. 9. Plot of the second wave limiting current at a platinum microdisk

vs. [Cd(II)]. tp = 200 ms.

426

This oxidation process could be due to the oxidation of chloride from the metal chlorocomplex, i.e. Cd@-

+ CdCl; + e-f

l/2 Cl,

(IO)

The existence of CdCl; has been demonstrated in TlCl + CdCl, and RbCl + CdCl, melts [35,36] and since SmC12+ and Sm3’ have been found to exist in acidic ImCl + AlCl, melts [14], it is not too unreasonable to postulate the existence of SmCl:- and possibly NdClz- in a neutral melt (at potentials more positive than + 1 V, [Cl-] = 0 at the electrode surface and the melt is neutral). However, if we measure the limiting current of the second wave as a function of Cd(H) concentration, we compute the diffusion coefficient for CdCl:- to be 1.5 x lo-l2 m2/s in a neutral melt at 27” C. This value is considerably less than diffusion coefficients measured for other metal chlorocomplexes (D is about 3.5 x 1O-*1 m’/s). We attempted without success to measure the diffusion coefficient of CdC$ by reduction of Cd(I1) in a 0.75 : 1 melt on a large Pt disk electrode. The reduction of Cd(I1) has been found by others to be possible on tungsten at about - 1.2 V but only a rather ill-shaped stripping peak was observed as the result of electrolysis at the very limit of the melt window [37]. To summarize, we may say that we observe an oxidation process, the nature of which is not well understood, and which involves CdC12,-.

The stoichiometry was determined to be that of a hexachlorocomplex. As we observed for Cd(II), there is a second oxidation peak at about + 1.9V. The general behavior of that wave is identical to the cadmium case, i.e. it is not a well defined process. NdCI,

Chloride oxidation limiting currents are typically 20% higher for the neodymium containing melts than for any other at short pulse times (Tables 3 and 5). This point was reported, but not assessed, in a previous study of the neodymium chloride stoic~omet~ 1231.In this latter paper, the variation rather than the absolute values of the current were used to propose the hexachlorocomplex as the species in solution. However, when a basic melt containing 60 mol/m3 Nd(II1) was “neutralized” by addition of the amount of AlCl, corresponding to the “free chloride” concentration, assuming the initial formation of NdCli- , we were still able to measure a significant chloride oxidation wave. Knowing the diffusion coefficient of chloride, we used eqn. (4) and found the chloride concentration at different pulse widths to be 11.9 k 0.6 mol/m3 (Table 8). This suggests that there is an equilibrium between at least two chlorocomplexes of Nd(II1) and that the rate at which the hexachloro species loses one chloride is slow. From this we can evaluate the equilibrium constant of the reaction NdC13,- =: NdCl;-

+ Cl-

(11)

427 TABLE 8 [Cl-] in a neutralized melt containing Nd(II1). Free chloride concentration in a basic melt containing NdCli- that has been “neutralized”. The melt composition is: 1.1693 g of AlCl,, 1.3381 g ImCl and 0.0301 g NdCl, &&A

r,/ms

[Cl- ]/mol mm3 a

62.7 41.6 33.4 23.6 15.6 11.1 8.8

2 5 10 20 50 100 200

11.4 11.7 12.9 12.5 12.2 11.4 11.6

a [Cl-] average: 11.950.6 mol/m3.

as Kd = 3.7 mol/m3. If we use the value of Kd along with p = 3 to evaluate the computer currents, we are able to match experimental result within 10%. It is not clear why neodymium and not samarium exhibits such a behavior. As mentioned earlier, one can observe a second oxidation wave that is not well defined (see CdCl,). CONCLUSIONS

The oxidation of chloride at platinum microelectrodes can be a precise and useful method of investigation of some metal chloride complexes in basic molten salts. This is due to the low chloride concentration range investigated and the enhanced diffusion at microdisks. ACKNOWLEDGEMENTS

The authors wish to thank Dr. D. Sukumaran of SUNY at Buffalo and Dr. B. Gilbert of the University of Liege (Belgium) for their help with 39Cd NMR and Raman spectroscopy, respectively. This work was supported by the Air Force Office of Scientific Research. SYMBOLS

Electrode area (m*) Concentration (mol/m3) Concentration of X (mol/m3), equation number Diffusion coefficient (m*/s) Density (kg/m3) Potential (V) Dimensionless current function Faraday constant = 96, 487 C

428

I 17 n

&c v

P

P r %

L T

Current (A) Viscosity (Pa s) Number of electrons Schmidt number = n/S D Kinematic viscosity (m’/s) Stoichiometry coefficient Dimensionless parameter Radius of electrode (m), coefficient of correlation Pulse time (s) Waiting time (s) Temperature (K)

APPENDIX

1

The kinematic viscosity, v, is the ratio of absolute viscosity, q, to density, S: v=n/s

(12) It has dimensions of m*/s and expresses the ease with which large amounts of solvent, or solution, undergo motion; it is also referred to as the coefficient of momentum transport. The diffusion coefficient, D, has the same dimensions and is related to the motion of a solute in the solvent; it is the coefficient of mass transport. The ratio of these two quantities is the Schmidt number: N, = v/D

03) It is a dimensionless number that expresses the relative ease of bulk motion, momentum transport, to solute diffusion, mass transport [38]. APPENDIX

2

The melt composition is corrected for the amount of metal chloride added according to eqn. (8) assuming the metal chlorocomplexes show a behavior similar to that of AlCl; (see text), i.e. MCl, =p AlCl,

(14)

The melts are prepared by mixing weighed amounts of ImCl and AlCl, and the experimental temperature is obtained before each measurement; therefore, together with existing relationships [7], the melt physical characteristics are computed. The densities then permit the populations of [Cl-],,,,. The computed viscosities and the measured temperatures are used to evaluate D and P from a given estimate of K S_E (eqns. 3 and 4). Then, eqn. (5) is utilized to compute 1,. REFERENCES 1 R.A. Osteryoung in G. Mamantov and R. Marassi (Eds.), Molten Salts Electrochemistry, Dordrecht, 1987, p. 329-364.

Reidel,

429 2 3 4 5 6 7 8 9 10 11 12 13

14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

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