UNCLASSIFIED
ORNL




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902

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ORNL-P-902
ITIES
/-/ رس ہے اک سازی
ANHYDROUS ION - EXCHANGE STUDIES OF
ALKALI METAL IONS IN ETHYLENEDI AMINE
D. A. Lee and J. L. Pauley**
Chemistry Division, Oak Ridge National Laboratory
Oak Ridge, Tennessee
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Abstract
SECTION
Of all the work done in ion exchange, only a small percentage
has been done in nonaqueous systems and most of it has been in
aqueous-organic mixtures. Helferrich has devoted one chapter in
his book on this subject and summarizes the work through 1961.
Since that time others have reported in this field. The nature of
the ion exchange process limi:s the use of solvents other than
water. In order for a solvent to be usable, it must be able to
dissolve and at least partially dissociate the solutes. For
completely anhydrous systems, the selections of solvents is there-
fore restricted to those that have fairly high dielectric constants
and even many of those are eliminated because of the slow ion
exchange kinetics.
Although there are only a few solvents which meet ion exchange
requirements, they are worth investigating because selectivity
coefficients are often larger in nonaqueous systems. Considering
these aspects of the problem, selectivity and diffusion coefficients
* Research sponsored by the U. S. Atomic Energy Commission under
contract with the Union Carbide Corporation.
*Department of Chemistry, Kansas State College of Pittsburg,
Pittsburg, Kansas.
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have been measured for alkali metal ions in anhydrous organic
solvents. This work describes the experiments performed in
ethylenediamine.
Ethylenediamine has many interesting properties which may be
beneficial to ion exchange selectivity. The relatively high hont
of vaporization (11.2 kcal per mole at 20°c) indicates considerable
association in the liquid state. Many salts are soluble in ethylene-
diamine and its ability to form coordination complexes may induce
bidentate character of ethylenediamine could cause complexes to be
formed which are sterically hindered in the resin phase. Ethylene-
diamine has a dielectric constant of 14.2 at 20°C which is relatively
high for organic solvents, still it is much lower than water and may
cause different selectivities due to ion associations. Ethylene-
diamine is a basic solvent and, consequently, it rapidly absorbs
carbon dioxide and water from the atmosphere. Its basicity will
also influence the affinity it has for the fixed sulfonate groups
of the resin, which in turn may contribute to variation in the ion
exchange properties.
The chief limitation of ethylenediamine as a practical ion
exchange solvent could conceivably be that the ion exchange kinetics
is too slow for particular ions. An essential study, then, is to
determine the diffusion coefficients for cations in this medium.
With organic solvents particle diffusion is probably the rate-
controlling mechanism. However, the situation is much more
complicated than with aqueous solutions. Film diffusion and the
chemical exchange reaction may contribute markedly to the rate.
Because the mechanism is not well defined, the diffusion coefficients
should be considered only qualitatively.
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The measured diffusion and selectivity coefficients for alkali
metal ions in ethylenediamine ion exchange systems are summarized
in Table 1. For each pair of ions, experiments were made on xl, x8,
and x16 Dowex 50 resins. In every case the selectivity increased
and the diffusion coefficient decreased as the crosslinkage increased.
For the Lit-cst and Nat-cst pairs, the diffusion coefficients were
about two orders of magnitude faster than the K+-Cst and Cst-Cs+
pairs. This may be due to the fact that resins loaded with Li or
Na ions will sweil in ethylenediamine while K+ and Cst loaded resins
do not swell appreciably. Of course, diffusion is more strongly
obstructed in a matrix that has less solvent. Also, as the matrix
is swollen more co-ions can invade the resin phase. The presence
of co-ions enhances the diffusion rate according to the theory of
Schlogel. The free anions in the resin phase act as carriers for
the cations and thereby increase the diffusion rate, Even though
the resin is swollen, the Li+-Nat exchange is slow because both
ions are solvated by the large ethylenediamine molecules. Cst and
k which are not solvated appreciably by ethylenediamine have a
strong affinity for the fixed anionic groups in the resin. This
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association may contribute to the slow exchange as well as the
lack of solvent in the resin beads.
The selectivity coefficients are much larger for L1*-Cst and
Na*-Cs* in ethylenediamine than in water. This may be attributed
to solvation differences; unlike Cs+, Lit and Nat are strongly
solvated by ethylenediamine. Further, the lower dielectric constant
of the medium facilitates Annoniation of the fixed mulfonata groupo
and Cst. The similarity in size of the K and Cs ions accounts for
the magnitude of their selectivity coefficients. The coefficients
for Na* and Lit in water and ethylenediamine are similar because
both liquids solvate these ions in a similar manner. For x16 resin
the Knis rather large and this may be due to considerable
desolvation of Na* in the resin phase.
The general models suggested for ion exchange in aqueous
systems seem to apply as well for ethylenediamine systems. The
larger ethylenediamine molecule being basic and having a lower
dielectric constant makes the differences in selectivities and
rates of exchange more outstanding. The separation of Na* from
Cs* and Lit from Cs* can be accomplished very readily at rates
which are nearly comparable with aqueous systems. Much more work
needs to be done to understand nonaqueous ion exchange, neverthe-
less, variation of the solvent offers many interesting possibilities
for practical application in separation processes.
Table 1. Diffusion and Selectivity Coefficients in Ethylenediamine
System
Dz
kTracer*
M +
134
cm2/000
3.6 x 10-7
2.7 x 10-7
2.x 10-/
2.1 x 10'
1.6 x 10'
8,6 x 1008
1.2 x 109
5.0 x 10-10
1.5 x 10-10
2.7 x 20-9
1.3 x 10-9
4.3 X 10-10
3.4 x 10-8
5.8 x 10-9
5.2 x 10-10
Dowex-50-xl-L11.134Cs vs. 0.IN LISCN-EDA
Dowex-50-X8-Li+.19408 vs. 0.1N LiSCN-EDA
Dowex-50-X16-L1'..-34Cs vs. 0. IN LISCN-EDA
Dowex-50-xl-Na+_+S4C8 vs. 0. IN Na SCN-EDA
Dowex-50-X8-Na*,1540s vs. 0.IN NASCN-EDA
Dowex-50-816-Na*_134C8 ys. 0.1N NaSCN-EDA
Dowex-50-xl-K*.134cs vs. 0. IN KSCN-EDA
Dowex-50-X8-K*_134Cs vs. 0. IN KSCN-EDA
Dowex-50-x16-K*.***Cs vs. 0. IN KSCN-EDA
Dowe.:-50-xl-Cs*_134 cs vs. 0. IN CSSCN-EDA
Dowex-50-X8-Cs*.154Cs vs. 0.1N CSSCN-EDA
Dowex-50--816-Cs+-+*C8 vs. 0. IN CSSCN-EDA
Dowex-50-xl-L1+.Na vs. 0. IN LISCN-EDA
Dowex-50-X8-L11-22 Na vs. 0. IN LISCN-EDA
Dowex-50-816-L1+cNa vs. 0.IN LISCN-EDA
16.1
58.5
500
11.0
27.9
200
1.4
2.5
3.5
+ 134
+ 134
1.6
9
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DATE FILMED
41/ 6 / 65




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