I. ALTUGAND M. L. HAIR
4260
Cation Exchange in Porous Glass
by I. Altug and M. L. Hair Research and Development Laboratories, Corning Glass Works, Corning, NEWYork
(Received April 10, 1967)
A study of ion exchange in porous glass by the pH titration method has indicated that the surface contains two types of exchange sites which differ in acid strength. The pK, value of the stronger acid site has been determined to be 5.1,while the weaker acid groups have been identified as the surface silanol site with a pK, value of about 7. A small ion equilibrium selectivity is exhibited by the porous glass and is in the order of K+ > Na+ > Li+. The exchange capacity at pH 8.45 is about 0.07mequiv/g.
Introduction The surface propert,ies of “dry” porous glass have been studied extensively and have been reviewed recently.’ However, little attention has been paid to the porous glass-solution interface. Recently, it has been shown that this material shows ion-exclusion properties2 and can be utilized as a calcium ion sensing e l e ~ t r o d e . ~In view of this, the nature of the glass surface in solution and its ion-exchange properties are of some interest. In this paper the ion-exchange properties of a porous glass are reported, and the results are compared with similar data obtained with a highpurity silica gel. The experimental method selected involved the pH titration of the glass with lithium, sodium, and potassium hydroxide solutions. Both the porous glass and silica surfaces are known to contain large numbers of silanol groups which can undergo ion exchange of the type +Si-0-H+
+ Na+
+Si-O-Na+
+ H+
(1)
Thus, when these materials are titrated with a base such as NaOH, ion exchange takes place, Na+ ions in the solution displacing the H + ions of the glass which in return combine with the OH- ions of the base forming H20. For an exchanger with weakly dissociated sites, the exchange reaction remains incomplete throughout a certain pH range and, therefore, the titration curve indicates a slow rise in pH with the added amount of base. When ion exchange approaches completion, a sharp rise in pH is observed and this point gives the number of exchangeable sites. The dissociation constant of these sites cannot readily be obtained from the titration curves because The Journal of Physical Chemistry
the pH of the external solution is not directly related to the pK, value of the dissociated sites. A derivation of such a relationship, which is restricted to weakly dissociated sites, has been given by Hemerich.‘ If K, is the dissociation constant of the surface site and represents the undissociated site, then
[m]
(s
The pH in the exchanger, related to pK. by the equation
= -log
[ g + ] )is (3)
where CY is the degree of dissociation. At Byo conversion of the exchanger from the H + form to the Na+ form, the final term in eq 3 vanishes and thus the relationship pK, = pR is obtained. If we consider the ion-exchange relationship given as eq 1, then, if is the total concentration of ionized sites, a t 5oy0 conversion, the derivation takes the final form pK, = pH
+ log [Na+] - log [*2I
(4)
(1) M. L. Hair, “Infrared Spectroscopy in Surface Chemistry,” Marcel Dekker, Inc., New York, N . Y., 1967. (2) K. A. Kraus, A. E. Marcinkowsky, J. 9.Johnson, and A. J. Shor, Science, 151, 194 (1966). (3) N. C. Hebert and A. Altug, Second International Biophysics Conference, Vienna, Sept 1966. (4) F. Helfferich, “Ion Exchange,” McGraw-Hill Book Go., Inc.. New York, N . Y., 1962, pp 81-88.
CATIONEXCHANGE IN POROUS GLASS
4261
Thus, if the cation uptake is measured as a function of pH, eq 4 can be used to determine the pK. values of weakly dissociated ~ i t e s . ~ f j
Experimental Section The porous glass sample used in these experiments was similar in composition to sample B described by The base glass was subjected to a Hebert and A l t ~ g . ~ heat treatment at 580" for 3 hr and leached in 1.0 N HIYOs. The surface area and pore diameter of the resultant glass were determined by the nitrogen adsorption method to be 110 m2/g and 32 A, respectively. The high-purity silica gel was prepared by dehydrating silicic acid solutions and had a surface area of 479 m2/g and a pore diameter of 57 A. The samples of porous glass and silica gel were converted to their H + form by treatment in 1.0 N HC1. The samples were then washed with water and centrifuged until no C1- was present in the solution. A progressive titration technique was applied because reactions require several weeks to reach equilibrium. Samples of 0.250 g each were weighed into separate volumetric flasks (25 ml) and distilled water was added, together with different amounts of base, into each flask. After the equilibrium was established, the pH measurements were made with a CORNINGB Model 12 pH meter and Type 47602 electrode. The K+, Xa+, and Li+ ion concentrations in the solution were determined with a flame photometer after the equilibrium was established. The cation uptake was calculated by taking the difference of the ion concentrations in solution before and after the exchange reaction. Results and Discussion a. Types of Exchange Site. The pH titration curves for silica gel and the porous glass are shown in Figure 1. Both curves show the characteristics of a weak acid titration curve, the pH of the external solution rising slowly with the added amount of KOH. The titration of silica gel results in a curve which is linear throughout the pH range covered, indicating that the sites participating in the exchange reaction are all similar in nature. The exchange behavior of the porous glass, however, is diff'erent from that of silica gel. In this case, the pH rises more rapidly in the first stages of the titration, a break occurring in the titration curve at about pH 8. This is attributed to the presence of two types of site OIL the glass surface which differ in acid strength. Below pH 8, the rise in the titration curve is steeper for the glass than for the silica gel. In this lower pH region, more sites for ion exchange must exist on the glass surface and this implies that the surface has acid
PH
jl
7
-
IO2 n#9
mH/a
I
I
SI
I
Figure 1. pH titration of porous glass and silica gel: x, porous glass; 0, silica gel.
sites which are stronger than the silanol groupings on the silica. The pK, value of these sites is determined by use of eq 4. The Na+ ion uptake of the porous glass as a function of pH is shown in Figure 2. The titration curve indicates a sharp rise in pH at the point which corresponds to the complete exchange. For example, the total number of exchanged sites is found to be 0.14 mequiv/g, and at the point of halfconversion the exchange capacity is 0.07 mequiv/g. The external pH corresponding to this point (50% conversion) is 8.45, and the external concentration of Na+ ions (determined analytically) at this pH level is 1.8 X N . The concentration of ion-exchanged sites is determined by considering the pore volume of the sample. The glass used in these experiments had a surface area of 110 m2/g and a pore diameter of 32 A and, by assuming cylindrical pores, the pore volume is calculated to be 0.0875 ml/g. Thus, the concentration of exchanged sites at the point of 50% conversion (X/2 in eq 4) is 0.80 mequiv/ml. By inserting these values into eq 4 the pK, value is calculated to be 5.1. As Figure 1 indicates, the porous glass surface contains two types of sites which differ in acid strength. Consequently, it should be possible t o calculate two pK. values. In order to do this, however, the titration curve used in determining the pK, values must indicate two distinct steps so that the points of half(5) Reference 4,pp 152-156. (6) H.T i Tien, J. Phys. Chem., 68, 1021 (1964).
Volume 71, Number 13 December 1967
4262
I. ALTUGAND 31. L. HAIR
6dNa ION UPT.., mq/g Figure 2. Na+ uptake of porous glass as a function of pH.
conversion can be obtained. The curve obtained from the experimental data (Figure 2), defines the exchange behavior of only one type of site. Even though the data plotted in Figure 1 indicates the presence of silanol groups, which constitute the weaker acid sites in porous glass, the titration curve does not extend far enough to give the point of complete 3SiOH dissociation. Thus the determined pK, value of 5.1 pertains to the stronger acid sites of porous glass. The 3SiOH groups are known to be weakly ionized sites with higher pK, values. Maatman and coworkers’ have reported pK. values of about 7 for the silanol groups of silica gel. In view of the superimposition of the porous glass and silica gel titration curves a t the higher pH values, it is reasonable to expect that the pK. value of the weaker sites on porous glass is around 7 . In summary, porous glass behaves as a polyfunctional ion exchanger, which possesses two types of acid sites. The weaker acid site is identified as the 3-SiOH, the silanol site with a pK, value of about 7 . Information concerning the exact nature of the stronger acid site is not known at this time, but the pK, value of this group has been determined to be 5.1. It is almost certainly connected with the boron atoms known to exist on the surface of the glass and which will be discussed later. The existence of sites of varying acid strength in some clay minerals has been previously reported. In his studies of clay membranes, Marshalls obtained titration curves similar to those of weak acids by titrating beidellite-water suspensions with KOH. Garrels,S using Marshall’s data, has interpreted these T h e Journal of Physical Chemistry
curves and, by assuming an ion-exchange reaction, he has shown that there are two types of acid sites which give rise to ion exchange. The exchange constants were calculated to be about 10-2.5for the stronger acid site and about 10” for the weaker acid site. Pommerlo has interpreted titration of montmorillonite with NaOH and has shown that these clay-water suspensions behave as weak dibasic acids having pK. values of 6.4 and 8.3. Despite its high silica content (96%), the surface of porous glass is known to be excessively heterogeneous in nature. Infrared studies by several attribute t,his heterogeneity to the presence of exposed boron atoms on the surface, and Hair and ChapmanI4 have estimated that the surface B:Si ratio may be as high as 1:3. The surface excess of boron is assumed to be caused by a reprecipitation process during the leaching of the glass wherein a borosilicate gel, similar to an aluminosilicate gel, is formed within the porous glass body. The similarity between such a gel and the clay surfaces discussed above lends credence to the dualsite interpretation that is proposed. In the present case, the total number of strong acid sites was calculated to be 0.14 mequiv/g or one site per 120 A2 of surface. b. Ion Selectivity. Equilibrium selectivity coeficients, KH + M f , of the porous glass have been calculated for each of the Li+, Na+, and K + exchange reactions by the use of the equation
H~~
=
[mal[H
I [RH I [Na+I +
A plot of these values against pH shows that the selectivity coefficients are not constants but vary with the concentration of ions. The magnitude of the order of selectivity exhibited by porous glass is insignificant. For example, the selectivity coefficients calculated a t a constant pH level of 7.8 have been found to be 5.80 X lo+, 6.58 X and 6.65 X respectively, for Li+, Na+, and K + ion exchange, the order being K + > Na+ > Li+. This order of selectivity is indicated more clearly in ~~~~
~
(7) R. W.Maatman, et al., J . Phys. Chem., 68, 757 (1964). (8) C. E.Marshall and W. E. Bergman, ibid., 46, 52 (1942). (9) R.M. Garrels and C. L. Christ, Am. J . Sci., 254, 372 (1956). (10) A. M.Pommer and D. Carroll, Nature, 185, 595 (1960). (11) I. D.Chapman and M . L. Hair, Trans. Faraday SOC.,61, 1507 (1965). (12) N. W.Cant and L. H. Little, Can. J . Chem., 42, 802 (1964). (13) M. J. D.Low and N. Ramasubramanian, J . Phys. Chem., 70, 2740 (1966). (14) M. L. Hair and I. D. Chapman, J . Am. Ceram. SOC.,49, 651 (1966).
CATIONEXCHANGE IN POROUS GLASS
4263
of equivalent radius r-, is related to its acid dissociation constant. Highly dissociated sites, such as (A1OSi)- groups (pK, -8)) show preference for the alkali cations over H + (the order being K + > Na+ > Li+) and possess lower field strength. A weakly dissociated site having a pK, of about 9.8 (i.e., high field strength) is expected to be highly H + selective and to exhibit a selectivity in the order of Li+ > Na+ > K+. Thus, as the field strength of sites increases, the pattern of cation selectivity is reversed. Within the vicinity of the point of reversal, the magnitude of selectivity is generally insignificant. The relationship between the equivalent radius rof a site and its dissociation constant is given by Eisenman as
-
5L I, IO
1
6'
cAnoN UPTAKE Figure 3. Uptake of Li+, Na+, and K + as a function of pH showing slight selectivity: 0, Li+; X, N a + ; . , K+.
Figure 3, where the cation uptake is plotted as a function of pH. At a given pH, the Li+ uptake is smaller than either the Na+ or K + uptake, and the selectivity order is K + > Na+ > Li+. However, the magnitude of this selectivity is small, and a quantitative comparison is subject to experimental errors. In his discussion of the origin of the glass electrode potential, Eisenman16 has proposed that the pattern of cation selectivity exhibited by glass electrode membranes depends on the anionic field st,rength of the sites. The field strength of a site, expressed in terms
r- = 0.044pK,
+ 1.5
By the use of eq 5 and the pK value of 5.1 determined for the stronger acid sites of porous glass, an r- value of 1.27 has been calculated. A figure given by Eisenman indicates that a hydrated site having r- values of 1.31.6 in a 0.1 m surrounding would exhibit cation selectivities of very small magnitude. (Concentration of pore liquid is around 0.1 N in porous glass.) On considering the pK, value determined for porous glass and the corresponding anionic field strength, a high magnitude of selectivity is not expected. The calculated r- value of 1.27 is therefore found to be in good agreement with Eisenman's theory of ion specificity. (15) G . Eisenman, Biophys.
J., 2, 286 (1982).
Volume 71, Number 13 December 1867
