M. Liquornik, B. Ale, and J. A. A. Ketelaar
1398
-
higher states. The situation that f1 > f 2 a t higher veloci2 excitaties is then due to the increased efficiency of 0 1. These results on the variation of f n with tions over 0 the collision velocity is similar to those reported by Robinson for the multiple Coulomb excitation of vibrational nuclei.7
-
Conclusion
-
A general expression for the 0 n vibrational transition probability is obtained for the perturbation energy which includes linear and quadratic terms in the oscillator coordinate. The linear term is responsible for stepwise excitation process, whereas the quadratic term leads to the contribution of the direct two-quantum excitation process to
the overall transition. The leading term of the final expression Pon simply takes the form of the Poisson distribution, but the interference between the two processes leads to the appearance of an important factor in the expression. This factor always exceeds unity so the inclusion of the direct two-quantum process leads to the constructive interference. For Hz-Hz and 12-12, the interference is important at 2 substantially high velocities. Particularly, in the 0 probability the nonadjacent transition makes a n important contribution to the overall process even at moderate collision velocities: This demonstrates that the conventional approach to calculate vibrational transition probabilities in terms of the linear forcing potential alone is inadequate a t such collision velocities.
-
Ion Exchange in Molten Salts. V I . The Occluded Sodium Nitrate in Zeolite A as an Anion Exchanger. The CI --NO3 Exchange in Molten Na(N03,CI) Mixtures M. Liquornik, B. Ale, and J. A. A. Ketelaar" Laboratory tor Electrochemistry of the University of Amsterdam, Amsterdam, The Netherlands (Received November 16, 1972) Publication costs assisted by the Laboratory for Electrochemistry
Anion exchange of chloride and nitrate ions has been observed between a Na(N03,Cl) melt and a sodium zeolite A with occluded NaN03: Na22(AlSi0&2(NO&o a t 375 and 450". The replacement exchange can be described by an ideal exchange isotherm with a maximum exchange capacity of 1.81 and 1.67 C1- ion per zeolite unit a t 375 and 450", respectively. The standard free enthalpy of exchange was found to be . ~ G o . T=, -7.7 kcal/mol a t 375".
Introduction In a previous paper1 one of the authors showed that the synthetic zeolite Linde A (subsequently called NaA), in contact with molten NaN03, loses its water and occludes ten Na+-N03- ion pairs.2 It appears that there is no way of differentiating, by chemical means, between the occluded and the structural sodium cations and, therefore, the formula Na22.(A102)12.(Si02)12.(N03)10 for the unit cell of the so-called occluded zeolite was proposed. The question arises whether the NO3- ions are also exchangeable for another anion or, in other words, whether the occluded zeolite can be considered a bifunctional ion exchanger. This would be the first instance that anion exchange has been observed in a molten salt media. Experimental Section (a) Materials. NaA (20 g) was stirred with 250 ml of water, poured into a cylinder, and allowed to stand for 2-3 hr. The precipitate was practically free of gel and fines, and was collected, washed with distilled water, and dried at 110". The product was kept open to the air to ensure equilibrium with the atmosphere and therefore constant weight was achieved. The water content was determined by the weight loss on heating to 900". Reagent grade The Journal of Physical Chemistry, Vol. 77, No. 1 1, 7973
chemicals were used without further purification beyond drying a t 105" for a t least 24 hr. ( b ) Methods. Air-dried NaA (1.2 g), 30.0 g of sodium nitrate, and varying amounts of sodium chloride were weighed into a 50-ml round-bottom centrifuge tube, introduced in the oven a t 375" and kept there for 24 hr. After equilibration, the content of the tubes was transferred to a porcelain dish and allowed to cool. The mixture was dispersed in distilled water and the zeolite washed until it was free of chlorides, and then dissolved in dilute nitric acid. The equilibrium concentration of the chloride ion in was determined by precipitating it with the zeolite, excess silver nitrate, and potentiometrically back-titrating the excess silver with sodium chloride. The equilibrium concentration of chloride in the melt, Ccl, was calculated from the difference between the initial amount of chloride and eel, In cases where it was found advisable to determine the concentration in the melt directly, this was done by the same method. Sodium and silica were determined by spectrophotometric methods. Concentrations are expressed as molali-
e,,,
(1) M. Liquornikand Y. Marcus, J. Phys. Chem., 7 2 2 8 8 5 (1968). (2) Attempts have also been made to extend this work to the zeolite Linde X, but they were unsuccessful due to the difficulties in isolating a fully occluded Na-X.
Ion Exchange in Molten Salts
1399
TABLE I: Experimental Results of the Distribution of Chloride between NaA and a Molten (NO3,CI)Na Solution at 375' CCl
O.lD056 f 0.004 0.0144 f 0.0006 0.032 f 0.001 0.0550 f 0.0002 0.077 f 0.001 0.1473 f 0.003 0.220 0.002 0.468 f 0.002 0.7162 f 0.0003 0.965 f 0.001 1.1636 f 0.0003
*
e,,
DCI
llCla
0.1 7 f 0.03 0.32 f 0.03 0.57 f 0.03
32 f 2 22 f 2 18f2
0.63 f 0.03 0.71 f 0.02 0.81 f 0.02 0.88 f 0.02 0.99 f 0.05 1.020 f 0.002 1.02 f 0.02 1.06 f 0.01
11 1 1 9.1 f 0.4 5.5 f 0.1 4.03 f 0.05 2.1 f 0.1 1.425 f 0.004 1.05 f 0.03 0.91 f 0.01
0.29 0.54 0.97 1.07 1.21 1.38
f 0.04 f 0.02
f 0.05 f 0.05 f 0.04 f 0.03 1.50 f 0.03 1.69 f 0.1 1.740 f 0.004 1.74 z!z 0.04 1.81 f 0.02
Reversibility Check 0.106 f 0.003
0.105 f 0.001 a
0.734 f 0.02 0.80 f 0.04
1.251 f 0.002 1.36 f 0.06
6.9 f 0.1 7.6 f 0.4
Number of moles of CI- /unit cell of NaA.
TABLE II: Experimental Results of the Distribution of Chloride between NaA and a Molten (N03,CI)Na Solution at 450" CCl
CCl
D
VCl
0.073 f 0.001 0.22 1.18 f 0.02 1.3493 f 0.0000 4.14 f 0.5
0.86 f 0.03 0.94 0.9554 f 0.0001 0.9768 f 0.0004 0.98 f 0.01
11.8 f 0.5 4.3 0.81 f 0.1 0.72 f 0.01 0.24 f 0.03
1.47 f 0.05 1.60 1.6287 i= 0.0003 1.6652 f 0.0006 1.67 f 0.02
ties, uiz., mol/kg of solvent ( e . g . , NaN03) or mol/kg of NaA anhydrous. The experiments were performed at 375 and 450". The temperature of the oven was controlled by an Eurotherm temperature controller and the variations in temperature over a 24-hr period were not more than f1.5".
Results Eight exchange experiments made with the same initial Ccl, but at different times, gave the following average results: Ccl = 1.01 f 0.007, Ccl = 0.97 f 0.01, and D = 1.04 f 0.01, were D, the distribution coefficient, equals €a/ Ccl. At lower chloride concentration a somewhat higher error is expected. No significant change in the distribution coefficient as a function of time was found over a period of 24-216 hr. The variations in the distribution were random and less than 1%( D = 1.05 =k 0.01). Samples of zeolite from the above experiments were subsequently used to check the reversibility of the exchange reactions. They were equilibrated with pure sodium nitrate a t 375" and both phases were analyzed for their chloride content. The results are given in Table I. The concentrations of the chloride ion in the molten sodium nitrate solution a t 375" varied from 0.005 to 1.16 m; the limiting factor being analytical for the lower values and the solubility of sodium chloride at the higher values. The solubility was determined experimentally and found to be about 1.5 m. At 450°, the highest concentration, Ccl = 4.14 m, was reached by using a melt saturated with solid sodium chloride. The equilibrium concentration of the solution was determined in a decanted and clear sample of the melt. The results for the distribution of chloride from NaCl in NaN03 obtained at 375 and 450" are summarized in Table
I and 11, respectively. Each result is the average of two experiments and the variation given corresponds to the difference between duplicates. The tables also contain the values of the distribution coefficient D = Ccl/Ccl and of the number R of occluded chloride ions per unit cell of zeolite A. Table I11 gives the corresponding results for the distribution from an eutectic mixture of sodium-potassium nitrate at 375". Five samples of the exchanged zeolite were analyzed for sodium and silica. The Na/Si ratio proved to be within 2% of the theoretical value of 22/12 for the occluded zeolite with the formula mentioned above. We are dealing, therefore, with a selective anion exchange reaction with the occluded salt and not one of sodium chloride being additionally absorbed; the latter would have led to an increase of this ratio. Discussion Lumsden3 has shown that the (N03,Cl) Na system can be considered in first approximation as an ideal solution. The very small positive deviations from ideality can kie neglected and, therefore, the effects observed, viz., the strong dependence of the distribution coefficient on the concentration, must be ascribed to the solid phase onby. For NaN03 as a solvent the very high value of the distribution coefficient D at low concentrations points to a high activity coefficient for the chloride ion in the zeolite with respect to the occluded nitrate ion, or in other words to a strongly negative free enthalpy for the exchange of nitrate by chloride in the zeolite. On the other hand, a t high concentrations a maximum value for the number of occluded chloride ions is observed, corresponding to a steep decrease of the distribution coefficient, even less than 1. The Journal of Physicai Chemistry, Vol. 77, No. 1 7 , 1973
M. Liquornik, B. Ale, and J. A. A. Ketelaar
1400
TABLE Ill: Experimental Results of the Distribution of Chloride between NaA and a Molten (N03,CI)(KNa)Solution at 375" D
CCl
CCl
0.0927f 0.0001 0.0950 f 0.0001 0.2338f 0,0001 0.2367f 0.0004 0.473f 0.001 0.49f 0.02 0.9517 f 0.004 0.9977f 0.005
0.132f 0.001 0.159 f 0.005 0.409f 0.001 0.4340f 0.0001 0.8457f 0.001 0.891 f 0.006 1.459 f 0.003 1.57 f 0.01
irCl
1.42f 0.01 1.68 f 0.05 1.752 f 0.005 1.833f 0.002 1.788f 0.003 1.8 f 0.1 1.533 0.004 1.58f 0.01
0.225 f 0.002 0.271 f 0.008 0.698 f 0.002 0.7398 f 0.0002 0.4417 f 0.0003 1.52 f 0.01 2.487 f 0.006 2.68 f 0.02
*
The molality Ccl is expressed by ~ / (-l
X )
= Cc1/11.76
as 1kg of sodium nitrate contains 1000/85.01 = 11.76 mol. Now 2 can be interpretated as the fraction occupied by C1- from the total number (per unit zeolite) of sites available for exchange by C1-. Thus 2 = R/Ro = C,,/CO, with Ccl as before the molality of chloride in the zeolite and CO the value corresponding to maximum of exchange.6 Equation 2 can be thus transformed into
co/ccl= 1 + 11.76/KCc, l/ccl= 1/e0+ 1 1 . 7 6 / K ~ o C c ,
or Figure 1. Reciprocal molalities 1/Ccl vs. l/& of the chloride in the molten liquid and in the zeolite A phase, respectively: temperature 375",line calculated for CO = 1.06 and K = 395.
Curiously enough the (Na,K)(N03) eutectic, instead of NaN03 as a solvent for the chloride ions, gives a completely different picture with a distribution coefficient independent of the concentration. Here it seems as if the zeolite behaves as a rather neutral "sponge," without preference for either ion and without a clear restricted exchange capacity. However, in this case of a Na,K mixed solvent, exchange of sodium for potassium in the original zeolite framework will have taken place to some degree,4 thus altering the anion exchange properties. It has been observed that potassium-zeolite A does not occlude K N 0 3 a t all.435 Because of this complication we have not gone into further detail about the combined anion-cation exchange in a mixed cation-anion solvent. The hyperbolic shape of the distribution curve for the in the zeolite us. the concenchloride concentration tration in the solution C suggest a dependence as given by an ion exchange equilibrium. Indeed if l / C is plotted as a function of 1/C a straight line is obtained over a large range (Figure 1). This empical distribution function with two constants a and b
e,,
l/c = a
t
b/C
(1)
can be derived from the expression for the ideal ion exchange equilibrium6
K = X ( l - 3~)/(1- 3C)x
Equation 3 is analogous to the empirical expression 1. From Figure 1 we deduce as best values from the intercept CO = 1.06 f 0.02 or = 1.81: f 0.035 and from the slope K = 395 f 7, all a t 375". From the few data a t 450" (Table 11) it follows that CO = 0.98 or RO = 1.67, slightly lower than a t 375". A relia'ble value for K can not be obtained though it appears to be higher a t 450" than a t 375". From these results the conclusion is drawn that only slightly less than two nitrate ions out of the ten present in the occluded zeolite can be exchanged for chloride ions. I t appears that this limit cannot be surpassed with NaN03 as a solvent. However, in the cation mixed solvent all ten nitrate ions appear to be exchangeable. The selectivity coefficient K can also be expressed as the standard free enthalpy of exchange
-RT In K =
AG0.6
At 375", AGo.5 = -7.7 kcal/mol. This standard free enthalpy is for the reaction (C1-)liq
+
( N O s - I o c c = (Cl-Iocc
+ (NO3-11iq
representing the substitution of an occluded nitrate ion by a chloride ion. The two standard states are for the occluded zeolite a t 50% of maximum exchange and for the liquid a t a composition with xcl = ~ N o 3= 0.5, respectively.
Acknowledgment. The investigations presented in this article have been carried out under the auspices of the Netherlands Foundation for Chemical Research (S.O.N.) and with the financial aid and a grant to one of the authors (M. L.) of the Netherlands Organisation for the Advancement of Pure Research (Z.W.O.).
(2)
K is the selectivity coefficient, x and 3 are the mole fractions of one component (Cl-) in the solvent and in the solid phase, respectively, and 1 - x and 1 - R the same for the other component ( NO3-). The Journal of Physicai Chemistry, Voi. 77, No. 11, 1973
(3)
(3) J. Lumsden, "Thermodynamics of Molten Salt Mixtures," London and New York, N. Y., 1966, p 124. (4) M. Liquornikand Y . Marcus, Isr. J. Chem., 6, 115 (1968). (5) M. Liquornik and Y. Marcus, J. Phys. Chem., 75, 2523 (1971). (6) R. Kunin, "Ion Exchange Resins," 2nd ed. New York, N. Y., 1958, pP 17-26.
