Determination of the Ion Exchange Capacity of Solid Ion Exchangers by Difference Weighing Kurt BunzI* and Bruno Sansoni' Gesellschaft fur Strahlen- und Umweltforschung GmbH Miinchen, lnstitut fur Strahlenschutz, Gruppe Radiochemie und Analytik, 8042 Neuherberg, FRG
T h e ion exchange capacity is one of the most fundamental quantities for the characterization of any ion exchange material. It is the amount of ions in a definite quantity of material, which is available for the ion exchange process under specified experimental conditions. An extensive review of the definitions of the different ions exchange capacities, various principles, methods, and sources of errors can be found in the first communication of this series (1).Although the pure ion exchange capacity of a solid ion exchanger can be determined in several ways, a gravimetric method by difference weighing has as yet to our knowledge not been applied. We will show, however, t h a t such a method offers for many ion exchangers the advantage of relatively high accuracy a n d very simple equipment requirements for only one difference weighing, without any analytical-chemical determination of ions.
PRINCIPLE Because, as long as no complications occur, the number of fixed ionic groups per g ion exchanger, i.e., the pure ion exchange capacity, is a constant, a difference in the dry weight of the ion exchanger when loaded first with counterions A of equivalent weight EA a n d subsequently with ions B of equivalent weight EBwill be observed. If we denote X = number of equivalents of counterions of the sample, M = weight of the dry ion exchanger matrix without counterions, WA, WB = weight of the dry ion exchanger when loaded with counterions A and B, respectively, the pure ion exchange capacity Q of the sample in the A form is defined as QA (mequivlg dry weight) = x.103/wA
(1)
WA and WB are given as
+ X-EA= M 4- Q A W A E A . ~ O - ~ wg = M + X*EB= M 4- Q A W A E B - ~ O - ~ WA = M
(2) (3)
Subtraction of Equation 3 - 2 finally yields the pure ion exchange capacity as: &A(mequiv/g dry weight) =
( EB-- EA wB
wA)
(4)
For reasons of comparison, the ion exchange capacity is usually given in mequivlg dry ion exchanger in the H+ form for a cation exchanger and in mequivlg dry ion exchanger in the C1- form for an anion exchanger. If Equation 4 is applied to a cation exchanger and the ion A is not already the H+ion, the ion exchange capacity QA can be converted to t h a t of the H+ form according (1)to QH(mequiv/g dry weight) =
QA
1 - QA(EA - 1.008).10-3
(5)
and for an anion exchanger Qcl(mequiv/g dry weight) =
QA
1- QA(EA- 35.453)-10-3
(6)
Present address, Kernforschungsanlage Julich GmbH, Zentralabteilung fur Chemische Analysen, D-517 Julich, FRG.
In order to demonstrate which weight differences will be observed by using either 1 g dry cation exchanger (QH = 5 mequivlg dry weight) or 1 g anion exchanger (Qc1 = 3 mequivlg dry weight), we calculated WB - WA according t o Equation 4 for different pairs of counterions A and B (Table I). T h e weight differences t o be expected can be easily determined accurately by analytical balances a n d will thus allow a simple gravimetric determination of the pure ion exchange capacity. Application of this method implies that: T h e ion exchanger can be dried completely when loaded with either ions A or B. There is no weight loss of the ion exchanger during the drying period arising from thermal degradation of the matrix. Equivalent amounts of ions are exchanged, when the ion exchanger is converted from the A to the B form. T h e ion exchanger can be saturated completely with ions A and B. T h e thermal stability of the sample is important, b u t the problem of degradation of the ion exchanger during drying is, of course, present for any capacity detrmination, where the corresponding value is given in mequivlg dry weight. Since, however, most ion exchanger manufacturers specify the thermal stability of their products, one can in general easily decide whether the gravimetric method can be applied. In order t o ensure equivalent ion exchange processes, it is advisable to select as counterions only monovalent ions. Several suitable ions, which should in general cause no complications, can be seen in Table I. 103- ions should be used with oxidation resistant ion exchangers only. NOS- ions can be applied without problems. T h e relatively small weight difference for a Cl-IN03- system can in most cases be overcome by using larger sample weights. For several inorganic ion exchangers as, e.g., clay minerals or zeolites, H+ions cannot, however, be used, since hydrogen clays are in reality hydrogen-aluminum systems. Cs+ ions, likewise, also cannot be applied in all cases, since-because of their large diameterthey cannot, e.g., in some zeolites replace all small counterions. Furthermore, it is advisable, to load aliquots only of the ion exchanger with the counterions A and B, to dry them simultaneously, and to discard them after weighing, rather then to
Table I. Calculated Weight Differences WB - WA for 1-g Dry Cation Exchanger ( QH = 5 mequiv/g dry weight) or Anion Exchanger ( Q c l = 3 mequiv/g d r y weight) Loaded with Counterions A or B Cation Exchanger Weight Counterion difference, A B (WB-WA)~
Anion Exchanger Weight Counterion difference, A B (Wg-Wdg
H+ H+ Li+ K+ Na+
C1ci-
K+ Cs+ Cs+ Na+ Ag+
0.190 0.659 0.630 0.080 0.424
NO3103-
0.0796 0.418
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
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Table 11. Pure Ion Exchange Capacity of Several Ion Exchangers, Determined by Titration a n d by the Gravimetric Method
Ion exchanger
Titration method Mean value (mequiv/g dry Re1 std devu H+ form) (%) weight-in weight-in quantity quantity 1.2g 0.1 g 1.2 g 0.1 g
Bio-Rad, AG 50W-X8,20-50 mesh Bio-Rex 70,20-50 mesh
4.96
4.80
0.82
4.29
10.47
10.54
0.49
2.90
Gravimetric method Mean value Rel. std. devu (mequiv/g dry H+ form) (%I weight-in quantity weight-in quantity 1.2 g 0.1 g 1.2 g 0.1 g 5.00 (H+/Cs+) 5.06 (H+/K+) 4.50 (H+/Na+) 10.55 (H+/Cs+) 4.23 (C1-/103-) 4.14 (Cl-/NOa-) 6.16 (Na+/Ag+)
4.81
...
... 10.34
0.31 1.92 16.2 0.97 1.21 5.92 0.83
Bio-Rad, AG 21 K, 20-50 mesh 4.30 4.23 0.93 1.33 4.17 Linde Molecular Sieve 13X, powder 6.16b 6.30b 2.65 3.24 5.71 Determined from 4 experiments each. Determination of the eluted Na+ ions by atomic absorption spectrometry.
convert the sample in the A form after drying into the B form.
EXPERIMENTAL Apparatus. An analytical balance and a vacuum oven were used. Reagents. All salt solutions required were made by dissolving weighed amounts of analytical grade reagents (p.a., Merck) in deionized water. Material. The ion-exchange resins used were: AG 50W-X8, 20-50 mesh, Bio-Rad Laboratories, a strongly sulfonic cation exchanger, composed of nuclear sulfonic acid groups attached to a styrene-divinylbenzene polymer lattice; BIO-REX 70, 20-50 mesh, Bio-Rad Laboratories, a weakly acid cation exchange resin, containing carboxylic acid exchange groups on an acrylic polymer lattice; Linde Molecular Sieve 13X, Union Carbide, fine powder, a zeolite type inorganic cation exchanger; and AG 21K, 20-50 mesh, Bio-Rad Laboratories, a strongly basic type anion exchange resin, composed of quaternary ammonium groups, attached to a styrene-divinylbenzene polymer lattice. All ion exchange resins were conditioned before use by several ion exchange cycles, using 1 N NaCl and CsCl solutions (2).
Procedure. Approximately 2-3 g resin ion exchanger are loaded completely with counterions A in a small glass frit (4-cm @, 5-cm height) by conventional methods (2) with ca. 250 ml of a corresponding 1N salt solution. After carefully washing the sample with deionized water until the eluate is free of A ions, the sample is air dried in a Petri dish. From this sample G1gram (ca. 1g) is placed in weighing bottle I and G2 gram (ca. 1g) is loaded completely in the glass frit with counter-ion B by using ca. 150 ml of a corresponding 1N solution of this ion. After washing with deionized water, until no further B ions can be detected in the eluate, the sample is transfered completely from the glass frit to weighing bottle I1 and dried in the vacuum oven at 110 "C together with weighing bottle I for three days. The vacuum pump is connected to the oven only after the supernatant water in weighing bottle I1 has evaporated. If the dry weights thus obtained in weighing bottles I and I1 are Gl,dry and Gz,dry, respectively, the ion exchange capacity QAof the sample is according to Equation 4:
If counterion A is not already the H+ or C1- ion for a cation or anion exchanger, respectively, QH or Qcl can be calculated from QAaccording to Equation 5 or 6. In the case of the Linde Molecular Sieve 13X cation exchanger, conversion of the sample to the A or B form in the glass frit is not advisable, since it is difficult to recover the fine powder from the frit. Conversion of the sample was, therefore, performed by a repeated batch procedure in a 100-ml centrifuge tube using 3 times 80 ml 1N salt solutions each. After shaking for 6 h each, the suspension is centrifuged at 3000 rpm. Washing of the sample is achieved by adding 2280
3.5
... ... 1.73 2.57
...
5.00
5 times 50 ml deionized water to the sample in the centrifuge tube,
followed by centrifugation, and decantation. These samples were dried at 400 "C for 24 h. In order to compare the ion exchange capacities obtained by the gravimetric method with the corresponding values found by conventional methods, we determined the ion exchange capacities of the resins also by titration of the H30+ or OH- ions, eluted during an ion exchange process employing the cation and anion exchangers, saturated with H3O+ and OH- ions, respectively. Details of this standard procedure can be found in (2). Since the Linde Molecular Sieve cannot be loaded with H30+ ions, its ion exchange capacity was determined ih this case by converting a 1-g Na+-saturated sample in a centrifuge tube with 1N AgN03 solution to the Ag+ form as described above and determination of the displaced Na+ ions in the solution by atomic absorption spectrophotometry. Dry weight determination of the ion exchange resins and of the Linde Molecular Sieve was performed under the same conditions as described above for the gravimetric method. In order to compare the precision of the gravimetric method with that of the standard method, all ion exchange capacity determinations were carried out four times each and, besides using 1-g ion exchanger as weight-in quantity, all experiments described above were repeated using 0.1-g weight-in quantities.
RESULTS AND DISCUSSION The mean values of the pure ion exchange capacity of several ion exchangers obtained by the gravimetric and by t h e conventional titration method are listed in Table 11. T h e precision of each value is characterized by t h e relative standard deviation obtained from fourfold determinations of each value. In order t o compare the values of the ion exchange capacity as obtained by the gravimetric method with those of t h e titration method, we performed in all cases statistical t-tests for t h e comparison of means. With t h e exception of t h e experiment using t h e 0.1-g sample weight of t h e Linde Sieve 13X, we found in all cases t&sd < ti3.05,Le., no significant differences between the values of the ion exchange capacity determined by either method. The smaller value of the ion exchange capacity of the Linde Sieve 13X obtained by using 0.1 g sample weight is most probably due t o losses of the powdery material during loading and washing steps. This effect will become more pronounced, t h e smaller a sample weight is employed. The general loss of precision if one uses 0.1 instead of 1.2 g weight-in quantities can be seen clearly for both methods i n Table 11. In order to show the influence of the equivalent weights of the counterions A and B employed in the gravimetric method on the precision, we determined t h e ion exchange capacity for the AG 5 0 W - X 8 resin by using three different pairs of counterions, namely, H+/Cs+,H+/K+,and H+/Na+.As can be seen
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
