0. D. BONNER
318
Vol. 58
A SELECTIVITY SCALE FOR SOME MONOVALENT CATIONS ON DOWEX 50 BY 0. D. BONNERI Department of Chemistry of the University of South Carolina, Columbia, S. C . Received October 19, 1966
Equilibrium studies*involving lithium, hydrogen, sodium, ammonium, potassium and silver ions on Dowex 50 resins of approximately 4 and 16% divinyl benzene content have been made while maintaining a constant ionic strength of approximately 0.1 molar. These results are compared with those reported previously for an approximately 8% divinyl benzene resin, and a quantitative selectivity scale has been established for these resins.
Introduction Although heterogeneous cation-exchange equilibria may be formulated in terms of the Langmuir adsorption mechanisma or of the Donnan membrane equilibrium,a perhaps the formulation most susceptible to simple thermodynamic treatment is that based upon the application of the lam of chemical equilibrium to the exchange, regarded as a simple metathetical reaction. Thus when both of the cations are univalent, let us consider the reaction to be that represented by the equation (A+), (B+)i = (A+)i (B+)o where A+ and B+ are the cations involved in the exchange and the subscripts i and o represent the resin phase and the outside solution, respectively. From a purely thermodynamic viewpoint, then, the following equation should be applicable
+
+
where U(A+)o and u ( B + ) ~are the activities of the respective ions in the solution and U(A+)j and u ( B + ) ~the activities of the two ions in the resin phase when equilibrium has been attained, and K , the activity quotient, must be constant at a fixed temperature. If the aqueous solution is sufficiently
dilute so that the ratio of the activity coefficients of the ions is approximately unity, the above relationship may be expressed by the equation
where y represents the activity coefficient of the ion in the resin phase and k is the experimental selectivity coefficient or equilibrium quotient. At this point one must choose a standard reference state for the ions in the resin phase. Perhaps the preferable reference state from a theoretical viewpoint would be the infinitely dilute aqueous solution. For this choice, K would necessarily be unity, since a t equilibrium U(At)i = U(At)o and U(B+)i = u(B+)o. The selectivity coefficient, k , would then be equal t o the activity coefficient ratio Y(B+~/?(A+)~. From a practical viewpoint, however, it is preferable to choose as a standard reference state for the resin, the resin anion with so that the activity only one associated ~ation,~vb of each pure resin is unity. For this choice of standard states it is found that the equilibrium constant may be calculated from the equation4 log K =
L1
logk dN
(3)
where N represents the molar fraction of the resin associated with A+. Equation 3 was used for
81%D V B
3.0
2.5 4
3
4
. I
B
Y
. I
% 2.0
8
.e
E
.g
2 2.0
2
. I
1
w"
ws
1.5
. I
1 .o 1.0
0
20 40 60 80 Mole % hydrogen resin. Fig. 1.-Hydrogen-lithium exchange.
100
(1) These results were developed under a project sponsored by the United States Atomic Energy Commission. (2) G. E. Boyd, J. Schubert and A. W. Adamson, J . A m . Chem. SOC., 69,2818 (1947). (3) W.C. Bauman and J. Eichhorn, ibid., 69, 2830 (1947).
0.5
0
20 40 60 80 Mole yo sodium resin Fig. 2.-Sodium-hydrogen eschange.
100
(4) 0. D. Bonner, W. J . Argersinger and A. W. Ditvidson, ibid., 74, 1044 (1952). (5) E. Ekedithl, E. Hogfeldt and L. G . Sill&, Acta. Chem. Scand. 4, 556 (1950).
April, 1954
SELECTIVITY SCALEFOR MONOVALENT CATIONS ON DOWEX50
3 19
'7.0 4
8 .* Y o 5.0
$ 3.0
%
al
.m
U
$ 2 .* 3.0
gm
.d
.d
w
2
.C c
I
1.o
g 2.0
0 1.0
0
20 40 60 80 Mole % ammonium resin. Fig. 3.-Ammonium-hydrogen exchange.
20 40 60 80 Mole % potassium resin. Fig. 4.-Potassium-hydrogen exchange.
100
100
calculating the equilibrium constants presented in 50 this paper. Experimental and Discussion Equilibrium studies involving exchanges of six 2al 40 monovalent cations on Dowex 50 resins of apY proximately 4 and 16% divinyl benzene content 2e have been made a t 25'. A description of this synthetic cation-exchange resin has been given by ."$ 30 Bauman and Eichorn. The experimental pro."~a cedures have been described in detail p r e v i o ~ s l y . ~ *B The capacities of the 4 and 16% DVB resins in 20 the dry acid form were 5.13 and 4.72 meq./g., respectively. These figures may be compared wit,h 5.10 meq./g. for the 8% DVB resin previously reported.6 While it is unfortunate that the 10 capacity of the 16y0 DVB resin is low, probably due to incomplete sulfonation, it is believed that a significant comparison may be made between the 0 affinities of these ions for the three resins. 0 20 40 60 80 100 The data representing the maximum water hfole % silver resin. uptake of these resins in the various ionic forms and the selectivity of the resins for the ions relaFig. 5.-Silver-hydrogen exchange. tive to the lithium ion taken as unity are presented in Tables I and I1 (the values in Table I1 were gen and lithium ions being apparent exceptions in calculated from the equilibrium constants for the the case of the 4 and the 16% DVB resins. These various exchange reactions), The experimental exceptions are believed to be real, as these data selectivity curves (equilibrium quotient, IC, vs. were reproducible. It is also of interest to note resin composition) are presented in Figs. 1-5. In that the selectivity coefficient-resin composition each instance the corresponding data for the 8% curves for any pair of ions are similar, with the DVB resins are given for convenience of compari- curve for the highest DVB content resin showing son. The selectivity of the resin for any ion, rela- the greatest changes in slope. This fact results tive to lithium, is noted to increase with increasing in a distinct reversal of selectivity in the sodiumDVB content; L e . , decrease in maximum water hydrogen exchange on 16% DVB, the hydrogen uptake. The hydrogen ion on the 8% DVB is an ion being preferred by the resin when it is preapparent exception. The maximum water uptake dominantly in the sodium form. For all of these of each resin in any ionic form is noted to be in- exchanges except the silver-hydrogen and hydroversely related to its affinity for these ions, hydro- gen-lithium exchanges, the l G % DVB resin exhibits less selectivity than the lower cross linked (6) 0 D Bonner and Vicliers Rliett, THIS JOURKAL, 57, 25.4 (1958). resins when it is predominantly in the salt form. ( 7 ) 0 D. Bonner a n d Mi.H. Payne, i b i d , 58, 183 (1954). . I
c
s
320
VOl. 58
SHIGEHIKO KUROSAKI
TABLE I
TABLE I1
MAXIMUM WATERUPTAKEOF DOWEX 50 RESINSIN VARIOUS IONIC FORMS (G./EQUIV.)
SELECTIVITY SCALEFOR DOWEX50 RESINS
Li +
H+ Na+ NHd +
K+ Ag +
470 DVB
B%DVB
418 43 1 372 360 34 1 289
21 1 200 183 172 163 115
16% DVB
130 136 113 106 106 102
4% DVB
Li + H+ Na +
NH, + K+ Ag
+
1.00 1.30 1.49 1.75 2.09 4.00
8% DVB
1 .oo 1.26 1.88 2.22 2.63 7.36
16% DVB
1.00 1.45 2.23 3.07 4.15 19.4
for the sodium-hydrogen and silver-hydrogen exchanges, respectively. These results may be the capacity of the resin upon its selectivity. compared with the values of 1.54 and 13.4 for this I n a previous paper6 results were given for sodium- resin of lower exchange capacity. Although both hydrogen and silver-hydrogen exchanges on a resins were classified as nominal 16% DVB, it is nominal 16% DVB resin having a capacity in the possible that a part of the difference in equilibrium dry hydrogen form of 5.10 meq./g. The equilib- constants for the same exchange reactions may be rium constants were found to be 1.74 and 16.1 due to a slight difference in the cross-linkage.
A final point of iiiterest is the possible effect of
Y
THE DIELECTRIC BEHAVIOR OF SORBED WATER ON SILICA GEL BY SHIGEHIKO KUROSAKI Department of Applied Physics, Central Research Laboratory, Hitachi, Ltd., Tokyo, Japan Received October 88,1966
For the purpose of clarifying the state of sorbed water on silica gel, the sorption isotherms of water vapor on silica gel were obtained to determine the differential heat of sorption, as well as to measure the changes in the apparent dielectric constant of silica gel powder caused by an increase in water content and to calculate the specific polarization of the sorbed water. The apparent loss factor of silica gel having different moisture contents was also measured in the frequency range of 2 kc./sec.-1 Mc./sec. As a result, it has been ascertained that three different states exist for sorbed water on d i c a gel. In the first sorption stage (water content 94 mg./g.), generally present in trimolecular layers or more, is considered as capillary condensed water, showing a maximum loss factor in the vicinity of 10 kc./sec. The eVmax value can be explained semi-quantitatively by the theory of binary aggregates, and it is surmised that capillary condensed water on silica gel has a much more strongly developed hydrogen bonding (approaching that of ice) than liquid water.
Introduction I n determining the states of gases or vapors sorbed on solids, the measurement of apparent dielectric constant is a very helpful method that has been employed by several researchers to date.'-' Especially when water vapor is the adsorbate, dielectric measurements are believed t o be an effective means of determining the macroscopic or microscopic states of the water, because of its strong polarity. I n previous publications, 8-10 it was considered that water sorbed on solid polymers is similar in construction to water dissolved in organic solvents, and that interaction among the molecules of the sorbed water is not so strong. It has been established that the plots showing the relation between the dielectric constant and water content of vinyl (1) J. V. Zhilenkov, CoZEoid J. (b..S.,S'.R.), 4, 473 (1938). (2) I. Higuti, Science Rep. Tohoku bniv., [l] 33, 174 (1949). (3) Ibid., 33, 99 (1949). 66, 198 (1952). (4) M. Shimizu and I. Higuti, THISJOURNAL, (5) L. N. Kurbatov, Zhur. Fia. Khim., 24, 899 (1950). (6) R. McIntosh, E. K. Rideal and J. A. Snelgrove, Proc. Rou. SOC. (London), 8 2 0 8 , 292 (1951). (7) J. A. Snelgrove, H. Greenspan and R. McIntosh, Can. J . Chem., 81,72 (1953). (8) S. Kurosaki, J . Chem. Boc. Japan, Pure Chem. Sect., 71, 522 (1950). (9) Ibid.. '72,688 (1951). (10) Ibid., '78. 990 (1951).
polymers are nearly linear, but that in the case of sorbed water on paper alone, the water is present in two different states separated by a certain water content. The changes in the dielectric constant of silica gel due to differences in water content, already reported by McIntosh and other^,^^^ show that sudden variations in polarization of the sorbed water are observed before and after a certain moisture content, as in the case of water sorbed on paper. In this respect the results aredmilar to the facts observed by Higuti from the plots showing the apparent dielectric constant vs. moisture content relation of n-propyl alcohol adsorbed on titania gel, the only difference being that the polarization of the first stage sorbed water shows higher value than that of water sorbed after the first stage. As for the absorption of electromagnetic energy by silica gel with a moisture content, the results of McIntosh's measurements do not indicate a Debyetype dispersion, and are theref ore considered merely as showing dispersion caused by ion conduction a t low frequencies. I n this respect also further re-examination was desirable in connection with the restriction of the dipoles of sorbed water, and a detailed examination of the frequency characteristics of the loss factor has been made in this paper. The fact that the molecules of water sorbed on solids are firmly bound to the surface of the solid
.
