Internal Mobilities in the Molten Binary System (K, Rb) N03

Internal Mobilities in the Molten Binary System (K, R b )N 03
C hao-cheng Yang and Isao Okada
Department of Electronic Chemistry, Tokyo Institute o f Technology, Nagatsuta, Midori-ku,
Yokohama, Japan
Z. Naturforsch. 42a, 1017-1020 (1987); received July 7, 1987
Ratios of internal cation mobilities in the molten binary system (K, R b )N 0 3 have been measured with
the Klemm method in a wide range o f concentration (0.0537 < * Rb< 1.00; xRb denotes mole fraction of
RbNOj) and tem perature (603 K < 7"<783 K). From these and available data on the densities and
conductivities, the internal mobilities of the two cations, b K and b Rb, have been calculated. At high tem ­
perature and high ;cRb, the Chemla effect has been observed, whereas in previous similar experiments *
the Chemla effect had not been observed. Except for xRb> 0.9, bK can be expressed by
bK = [ A / ( V - Vq)] exp ( - E / R T), where V is the m olar volume; A , E and V0 are constants having almost
the same values as in the molten binary system (K, C s)N 0 3.
Introduction
W e have so far m easured ratios o f internal
m obilities o f tw o cations in various binary alkali
nitrate melts (see Table 1 in [1]) with K lem m ’s
countercurrent electrom igration m ethod [2]. In the
system (K, R b ) N 0 3 Ekhed and Lunden [3] m ea­
sured the internal m obilities o f the tw o cations
with the same m ethod. Their m ain aim was to
determ ine the isotop e effect o f these cations and
they did not cover a w ide range o f concentration
and temperature. Thus, they did not clearly
observe the C hem la effect [4, 5]. In all binary
alkali nitrates except this system the C hem la effect
has been observed and no plausible reason could
be found that only this system is excep tional. In
som e binary alkali nitrate m elts such as
(N a, K ) N 0 3 [6 ] and (R b, C s ) N 0 3 [1], in which the
relative differences in the radii o f the cations are
sm all, the C hem la effect occurs at high concentra­
tion o f larger cation at high tem perature. There­
fore, also in (K, R b ) N 0 3 the C hem la effect was
supposed to occur under such con d itions.
It has been found [6 , 7] that, w hen the C oulom bic attraction is expected to play a dom inant
role for the internal m obility, the interal m obility
o f cation 1 can be expressed by
b\ = [ A / { V - V q)] e x p ( - E / R T ) ,
(1)
* A. Ekhed and A. Lunden, Z. Naturforsch. 34a, 1207 (1979).
Reprint requests to Prof. Isao Okada, D epartm ent o f Electronic
Chemistry, Tokyo Institute o f Technology at Nagatsuta, Midoriku, Yokohama 227, Japan.
0932-0784 / 87 / 0900-1017 $ 01.30/0. -
where A , V0 and E are constants characteristic o f
cation 1 and nearly independent o f the co-cation
and tem perature. It was another aim to learn
w hether ( 1 ) also holds in the present case.
Experim ental
The chem icals K N 0 3 and R b N 0 3 used were
prepared by Soekaw a Chem . C o ., T okyo; the
form er was o f reagent grade and the purity o f the
latter was better than 99% . The salts were dried at
1 2 0 °C overnight, mixed in a chosen ratio and
m elted.
The electrom igration cell and experim ental pro­
cedure were quite similar to the previous ones [5].
In m ost o f the runs the tem perature could be kept
w ithin ± 2 ° C with a tem perature controller.
At x Rb= 1.00 (x Rb: m ole fraction o f R b N 0 3),
radioactive 4 2 K, w hose h alf life is 12.36 hr, was em ­
ployed; this was produced by irradiating ca. 2 0 mg
K N 0 3 with thermal neutrons at a Triga II type
nuclear reactor at A tom ic Energy Research
Laboratory, M usashi Institute o f T ech nology. The
irradiated K N 0 3 was poured into ca. 20 g o f
R b N 0 3 melt contained in a sm all quartz vessel; the
concentration o f K N 0 3, x K, was about 0.0015.
A fter electrom igration, the radioactivity in each
fraction ( 1 . 5 - 2 . 0 cm long) o f the separation tube
w as m easured with a well type N a l(T l) scintillation
counter, and then the radioactivity at a fixed time
was calculated by extrapolation.
The content o f K + and Rb + in each fraction was
determ ined with flam e spectrophotom etry; about
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1018
C. Yang and I. O kada • Internal Cation M obilities in the System (K, R b )N 0 3
1000 ppm o f C s + were added to each solution in
order to depress the easy ionization o f Rb.
Results
Table 1. Experim ental conditions and results.
•*Rb
7VK
//h r
Q /C
0.054 ±0.003
673
723
783
603
673
783
648
693
723
773
603
643
673
723
733
783
603
643
673
723
783
643
723
753
783
681
715
735
774
653
693
723
773
7.3
8.0
7.0
8.3
8.0
7.6
7.0
7.5
7.3
7.5
8.8
7.0
7.5
7.8
7.7
8.1
8.2
8.2
8.0
8.1
7.8
8.3
8.2
8.2
7.8
7.5
7.3
8.0
7.9
13.8
4.5
6.0
24.0
3668
3911
3961
3681
3603
3616
2476
2827
2550
2983
3140
2913
3302
2957
3649
3176
3635
3616
3566
3420
3350
3816
3869
3927
3467
3215
2874
3912
3265
2222
2250
1974
3302
The relative difference in internal cation m obili­
ties £( = { b K- b Rh) / b ; b = x Kb K + x Rbb Rh) was cal­
culated in the usual w ay [ 8 ]. In the case o f x Rb = 1,
the equation (2) o f [9] was used to calculate e.
From the £ value thus obtained and available
data [ 1 0 ] on the densities and the conductivities,
the internal m obilities b K and b Rb were calculated.
T he m ain experim ental conditions and the re­
sults are listed in T able 1.
In Fig. 1 the isotherm s o f b K and b Rb obtained in
the present study are shown together with those
obtained by Ekhed and Lunden [3].
0.257 ±0.002
D iscussion
0.968 ±0.003
Figure 1 show s that the present isotherm s agree
w ell w ith those estim ated from the data by Ekhed
and Lunden [3].
0.983 ±0.003
0.496 ±0.001
0.731 ±0.002
0.935 ±0.002
1.00
^Rb
Fig. 1. The isotherms of internal cation mobilities in the system
(K, R b )N 0 3. □ : bK [this work], L : bK [3], V : bRb [this work],
V : bRb [3],
£
0.0372 ± 0.0062
0.0301 ±0.0053
0.0250 ±0.0056
0.0291 ±0.0088
0.0270 ±0.0087
0.0182±0.0102
0.0030 ± 0.0096
0.0223 ±0.0098
0.0174 ±0.0083
0.0153 ±0.0071
0.0200 ± 0.0063
0.0156 ± 0.0044
0.0117 ±0.0033
0.0124 ±0.0033
0.0109 ±0.0030
0.0104 ±0.0043
0.0123 ± 0.0050
0.0114 ±0.0078
0.0104 + 0.0046
0.0077 ±0.0060
0.0057 ± 0.0040
-0 .0 0 1 0 ±0.0032
-0 .0 0 3 0 ±0.0016
-0 .0 0 4 9 ±0.0070
-0 .0 0 4 7 ±0.0050
-0.0051 ±0.0038
-0 .0 0 9 5 ± 0.0060
-0 .0 1 0 2 ± 0.0062
-0 .0 1 3 0 ± 0.0046
-0 .0 0 3 8 ±0.0018
-0.0051 ±0.0031
-0 .0 0 4 2 ±0.0052
-0 .0 0 3 0 + 0.0038
In the present isotherm s the C hem la effect oc­
curs and the crossing point shifts toward higher
con centration o f the sm aller cation with increasing
tem perature. This is a trend observed in all other
system s so far studied. Our interpretation has been
stated in previous papers [6 , 1 1 ].
In order to see w hether (1) holds, we plotted the
reciprocals o f b K and b Rb against the m olar volum e
in Figs. 2 and 3, respectively.
In Fig. 2, the solid lines are drawn according to
(1) w ith the param eters A = 4.21 x 1 0 ~ 11 m 5 V ~ '
• s _ 1 m o r \ E = 16.74 k J m o P 1 and K0 = 1 0 .5
x l 0 ~ 6 m 3 m o l- 1 , w hich were previously
calculated from the system (K, C s ) N 0 3 [6 ].
Figure 2 show s that b K in the present system
deviates negatively from (1) at high .vRb. A negative
deviation has also been observed for &Nain the sys­
tem (N a , R b ) N 0 3 [12] and was explained as fo l­
lows: A t high concentration o f R b N O ? free space is
expected to be relatively great, and therefore an
1019
C. Yang and I. Okada • Internal Cation M obilities in the System (K, R b )N 0 3
V x \a 6/
m 3 m o l-1
Fig. 3. The reciprocal of bRb vs. molar volume in the molten
binary systems (M, R b )N 0 3; A: M = Li [5], O : Na [12], □ : K
(this work), V: pure R b N 0 3, <^: Cs [1].
V x to 6/ m
3m o| 1
N a + ion is attracted by the nearest neighbouring
N 0 3 ion with less interference from other neigh­
bouring N O f ions. In other w ords, neighbouring
N a + - N 0 3~ ion pairs are m ore structured and
therefore their separating m otion is retarded.
xVery strong interaction o f a sm all cation with
an ions at low density o f the anions has been sub­
stantiated also in the M D sim ulation o f m olten
(L i, K)C1 by Lantelm e and Turq [13], where the
L i - C l distance is 212 pm at x K = 0.90 w hile
221 pm in pure LiCl.
H ow ever, such a negative deviation has not been
observed for b Na in (N a, C s ) N 0 3 [7] nor for b K in
(K, C s ) N 0 3 [6 ]. Thus, this might be a p henom enon
peculiar to foreign alkali cations at high concentra­
tion o f R b N 0 3. A lso b u should be m easured under
such con d ition . T he reason for the negative devia­
tion o f £ Na and b K at high concentration o f R b N 0 3
rem ains to be investigated.
Figure 3 sh ow s that, even though b Rb in the
m olten system s (M , R b ) N 0 3 (M = Li, N a, K and
Cs) is well expressed in the form o f (1), the
parameters A , V0 and E are som ew hat different
from system to system . Figure 3 seem s to indicate
also that in the system s where M = Li, N a and K,
the agitation effect [6 ] by the M + ions on the inter­
nal m obilities o f Rb + ions m ight occur. A t a given
molar volu m e, i.e. num ber density o f N O f ions,
the agitation effect becom es greater in the order
L i+ < N a + < K + . This is presum ably because, at a
fixed m olar volu m e, the m ole fraction o f these
agitator ions is in this order. A t low m olar volum e
b Rb deviates negatively from ( 1 ), which is
attributable to the free space effect [6 ].
This work was financially supported by the
G rant-in-A id for Special Project Research N o.
61134043 from the M inistry o f E ducation, Japan.
O ne o f us (C . Y .) gratefully acknow ledges the Ishizaka Scholarship.
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Fig. 2. The reciprocal of bK vs. molar volume in the molten
binary systems (K, M ')N 0 3; A: M' = Li [14],
Li [15], o :
Na [6], €>: Na [16], <J: Na [17], ffl: pure K N 0 3 , V: Rb [this
work], () : Cs [6].
1020
C. Yang and I. Okada • Internal Cation M obilities in the System (K, R b )N 0 3
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