DAVIDH. FREEMAN A N D GEORGE SCATCHARD
70
+ T N M is
> 104k46,so that the reaction of HOz apparently negligible in all our experiments.
Acknowledgment. We wish to thank Ed. Backstrom, whose careful Linac operation was essential for the precision attained in these results, Steve Petrek for const,ruction and maintenance of electronic equipment,
and Miss Lynn Bogur of Rockford College, who as a summer student aide gave valuable help in carrying out experiments and calculations. We also are greatly indebted to Dr. B. H. J . Bielski of Brookhaven National Laboratory for information on procedures for handling and purifying T N M and for unpublished data on the steady-state radiolysis of TNM.
Volumetric Studies of Ion-Exchange Resin Particles Using Microscopy
by David H. Freeman Department of Chemistry, Washington State University, Pullman, Washington
and George Scatchard Department of Chemistry and Laboratory of Nuclear Science, Massachusetts Institute of Technology, Cambridge, Massachusetts
(Received M a y d v 1964)
Volume measurements of ion-exchange resin particles are obtained from diameter measurements after establishing equilibrium in varied concentrations of aqueous HCl, LiCl, NaC1, and KC1. The volume of cation-exchange resins, equilibrated in water and then immersed in concentrated LiCl, passes through a minimum and slowly increases to reach equilibrium.
The volume swelling of ion-exchange resin is a property of fundamental importance to the understanding and application of ion-exchange processes. Classical methods of measuring resin volume give discordant results; this is described in the work of Gregor, Held, and Bellin' and is further discussed by Scatchard and Anderson. The latter workers investigated the errors of weighing centrifuged resin particles. Later, it was found3 that low cross-linked resin may lose internal water during centrifugation. Such considerations led to the present efforts to refine the precision and accuracy of microscopic measurements of ion-exchange resin particle size.4 The obvious advantage is that the measured particle is immersed in a given medium with no disturbance of the state of the system by measurement. We have investigated the accuracy of microT h e Journal of Physical Chemistry
scope measurements at low magnification (300X , or less) and find it possible to measure particle diameters rapidly, and with 0.1% relative error, or slightly less. In addition to the study of resin volume at equilibrium, the measuring microscope can be effectively applied to the study of resin swelling rates. The feasibility of precise particle size measurements depends upon the particle geometry and the quality of (1) H . P. Gregor, K. M. Held, and J. Bellin, Anal. Chem., 23, 620 (1951). (2) G . Scatchard and N . E. Anderson, J . Phys. Chem., 65, 1536 (1961). (3) A. T . Schwartz, Ph.D. Thesis, Department of Chemistry, hlassachusetts Institute of Technology, 1963. We have confirmed this. (4) Others have used this method. See, for example, C . Calmon, Anal. Chem., 24, 1456 (1952).
VOLUMETRIC STUDIES OF ION-EXCHANGE RESINPARTICLES USINGMICROSCOPY
the optical and mechanical components used in the measurements. Ion-exchange resin particles that are derived from suspension copolymerized styrene-divinylbenzene are almost, but not quite, perfect spheres. The accuracy of diameter measurements is asphericity limited, so that less than 0.1% errors in the average diameter would only be obtained with access to higher than the usual degree of particle sphericity. The measuring components meet available standards of high quality as furnished by a good apochromatic objective, achromatic substage condenser, and a microscope free from focusing backlash. The measurements further depend upon conditions of illumination, alignment, and mechanical precision that have been described elsewhere5 in connection with the accurate determination of microscope magnification and its variation with object size. The systematic errors are handled by an equation of measurement6 which relates the image size D to the particle size d
a=- D
+ 6 + e
M
where the magnification M is independently corrected for distortion and periodic screw errors, 6. The edge error E can be estimated from other works-8 which gives e = -0.2 k/(N.A.), with a calculated value of -0.4 p in the present work.g Instead of using this value, we determined t as an assumed constant for each observermicroscope combination to within 0.3 ~ron the basis of measured test particles furnished to us by the Bureau of Standards.lo The study of resin particles was confined to particle diameters of 300 p, or larger, so that the accuracy was within a maximum tolerance of 0.1% in the measured diameters. In the next section are described the experimental aspects of working with single resin particles, methods of establishing equilibrium, and studies of swelling in various aqueous electrolyte solutions.
Experimental Materials. Samples of anion-exchange and cationexchange resins of the styrenedivinylbenzene copolymer type, Dowex 1 and Dowex 50W, respectively, of various degrees of cross linking were obtained from the Dow Chemical Co. and thoroughly conditioned by washing with alternating base and acid, followed by ethanol and conductivity water. The ethanol wash was followed by vapor-phase transfer of water to the resin in a desiccator in order to prevent possible resin damage due to shock swelling. Reagent grade electrolyte solutions were prepared using conductivity water and were analyzed by potentiometric titration.
71
Sample Preparation. Particles that were free from apparent asphericity and internal flaws were selected under a survey microscope. A probe was used to isolate a selected particle, and transfer into a desired solution was accomplished by withdrawing the particle into a dropper pipet containing the same solution. When necessary, particles were drained and washed on a fine porous glass frit using suction. Temperature was controlled by air thermostating at 23 -f 0.5’. Individual particles were transferred to a cell made by cementing a glass annulus (A. H. Thomas Catalog No. 7050) to a standard microscope slide using epoxy cement (“Crystal Clear,” Epoxy Coatings Co., South San Francisco, Calif.). The cell was filled with solution, covered with a No. l l / z cover slip, and then sealed around the edge of the cover slip with vinyl cement (Carter’s Ink Co.). Fdr kinetic studies, a small air bubble was sealed into the cell and caused agitation during rotation of the cell a t 20 r.p.m. Microscope Measurements. Most of the visual measurements of particle size were made with a Cooke, Troughton, and Simms image splitting ocular attached to an E. Leitz Ortholux microscope equipped with a Berek condenser and a 12.5 X apochroinat objective. The light was passed through a heat filter followed by a 550-mp interference filter before entering the condenser. Each particle image was carefully centered and focused to determine the average focus level. Kohler illumination at constant aperture was precisely achieved. In most cases, the fullest aperture of the condenser was used. For 1% cross-linked resin, however, it was necessary to increase the image relief by stopping to six-tenths of the maximum aperture. The setting was determined by measuring the relative light intensity in the ocular position with a photometer. Then, four sets of four diameter measurements each were obtained every 4.5’ by rotating the ocular. A statistical analysis was made of several hundred particle measurements obtained with. the image splitting ocular. This confirmed the expectation that the (5) D. H. Freeman, A p p l . O p t . , 3 , 1005 (1964). (6) A. A. Michelson, J . Opt. SOC.Am., 8, 321 (1924). (7) R. P. Loveland, A S T M BuEl., 143, 94 (1952). (8) H. Schedling, Acta Phys. Austriaca, 2, 13 (1948); ibid., 3, 293 (1949). (9) There may be a small additional error upon varying the relative refractive indices of particle and medium, a s reported by Bishop, J . Res. Natl. B u r . Std., 12, 173 (1934). However, Loveland (ref. 7) points out t h e dependence of observer judgment upon image relief, or contrast, so t h a t refractive indices per se become of secondary significance. (10) T h e standard particles were four polystyrene balls, 0.4 t o 0.7 mm. in size, t h a t were measured by the National Bureau of Standards. Sizes were measured interferometrically a t varying deformations of 0.275, or less, under 2.7- t o 0.7-9. loads; the measurements were then extrapolated to zero load.
Volume 69, Number 1
January’ 1966
72
DAVIDH. FREEMAN AND GEORGE SCATCHARD
-
consistency of volume measurements of the same particle in varied swelling states deteriorated with increasing particle asphericity. Those particles with an oriented diameter that deviated by 0.5% or more froni the average were found likely to show volume deviations in excess of 0.3%; these were rejected from the study. It was also evident that the slightest internal crack or visible flaw would preclude precise measurements. When an occasional particle did inexplicably develop such a flaw, the entire set of measurements on that particle was rejected. A portion of the measurements to be reported was obtained by photoelectric comparator measurements of photographic plates that were exposed to the magnified particle images. The conditions of exposure, and particularly of plate development, had to be controlled precisely in order to attain precise measuring criteria. This approach is discussed elsewhere.59' The precision of photomicrographic plate measurements, a t least for a given diameter orientation, was tenfold better than that obtainable by visual measurements. However, its achievement required considerably more time, and sorile experience with precise photographic methods. These techniques were especially useful in the study of particle asphericity. After determining that the magnification did not vary detectably upon rotation of the axis of measurement, it was found that the particle images were not circular, but showed characteristic deviations of 0.1% from the average diameter. The deviations were irregular and apparently random, implying the absence of ellipsoidal and of oblate spheroidal geometry. During kinetic swelling measurements, however, distorted geometry usually developed and caused a loss of precision which improved upon approaching swelling equilibrium.
Results Kinetic Measurements. Swelling equilibrium was established upon attainment of constant particle diameter. Most of the equilibria studied with anionexchange resins were reached within a few hours, although agitation times were extended up to 2 days. A similar situation was found with cation-exchange resins. but the swelling rates in concentrated LiCl solutions were found to be exceptionally slow. When lithiuni form cation-exchange resin was transferred from water or dilute LiCl solution to 10 m (or higher) LiCl, the volume decreased very rapidly and then increased slowly until equilibrium was reached. ,4n example of this behavior is shown as the upper curve
The Journal of Physical Chemistry
1.0
0.8
i
d
2m 0.6
% B
.?. 0.4 c
L
e
d
Minimum time to reach
0.2
I
-. --
0 0
15
30 Time, hr.
45
60
75
Figure 1. External diameter and apparent diffusion boundary dimensions shown as upper and lower curve, respectively. Initial conditions: 500-p particle of lithium resinate, 8% divinylbenzene, in water. Zero time corresponds to immersion in 20 m LiCl. Compare this to Figure 5 of ref. 11.
minimum is explained by an initially rapid osmotic dehydration of the cation-exchange resin, followed by a slower inward diffusion of electrolyte and water. The first process impedes the second. After the first several minutes of the deswelling shown in Figure 1, an interference pattern was observed to be similar to that discussed by Gurney." Later, an infusion gradient became visible with ordinary illumination. The measurements of the apparent dimension of this shell are shown by the lower curve in Figure 1. These are not the actual dimensions because the resin behaves as a lens. However, the contraction and disappearance of the shell or the disappearance of the optical anisotropy of the particle provides sensitive criteria for the minimum time required to reach the state of equilibrium. The minimum agitation periods necessary for the achievement of swelling equilibrium with 1% crosslinked cation-exchange resin were approximately 20 and 40 hr. for a 500-p diameter particle in 15 and 20 m LiC1, respectively; for 16% divinylbenzene the corresponding times were approximately 50-fold larger. Swelling equilibrium was rapidly reached in water or dilute solution although an irreproducible size increase was usually observed as early as 30 niin. after immersion. In one instance, this artifact was found to have been caused by an accumulation of a penicillium fungus1*on the bead exterior. Equilibrium Measurements. The equilibrium swelling of various resins was studied in aqueous solutions of
VOLUMETRIC STIJDIES OF ION-EXCHANGE RESINPARTICLES USIXGMICROSCOPY
73
Table I : Swelling Properties of Ion-Exchange Resins in Aqueous Solution
Resin
C
9
Dowex 1-X1
5 00
5 23
Dowex 1-X2
4 95
4 13
Dowex 1-X7,5
4.38
1.65
Dowex SOW-XI Dowex 50W-X4 Dowex 50W-X8
6 89 7 26 5 71
6 68 2 95 1 97
Dowex 50W-X12 Dowex 5OW-X16
5 91 5.40
1 48 1 26
Electrolyte
HC1 LiCl XaC1 HC1 LiCl NaCl HC1 LiCl NaCl LiCl LiCl HC1 LiCl SaCl KC1 LiCl LiCl
-___-_-_ 0 2 m
0.89 0.875 0.878 0.916 0.864 0.870 0.991 0,991 0.990 0.995
0.991 0.991 0.987 0.990
Volume relative to t h a t in pure water (R) 15 2 0 4 0 6 0 m m m m
0 5 m
1 0 m
0.77 0.760 0.755 0.817 0.780 0.777 0.980 0.981 0.978 0.985
0.73 0.712 0.698 0.788 0,743 0.722 0,964 0.966 0.960 0.972
0.977 0.977 0.971 0.976
0.952 0.955 0.949 0.955
HC1, LiCl, NaCl, and KC1. Alternately high and low electrolyte concentrations were measured in succession in order to detect inconsistencies and demonstrate reproducibility. Single particles of ion-exchange resin were thus found to undergo cyclic variations of swollen volume with no evident irreproducibility, except for the rare instances noted earlier. Good agreement was also found in the swelling properties of different particles selected from the same resin sample. Such comparisons led to the initial conclusion that these materials were essentially homogeneous and apparent confirmation of this was r e p ~ r t e d . ’ ~In later work, small but real differences in the swelling properties of different particles were identified and a separate study of this is to be reported. The results of the swelling measurements were treated by finding the swelling ratio R for each particle: volume in electrolyte solution divided by the corresponding particle volume in water. The swelling ratios were then averaged a t each concentration among the three or more particles taken from each resin sample. The interpolated results are presented in Table I with the sodium or chloride dry exchange capacity, C (niequiv./nil.), rand wet to dry volume ratio, q. The consistency and accuracy is that of the experimental precision or 0.3% tolerance in the measured volume. The extent to which the averages reflect the bulk swelling properties clepends upon the control of sampling and homogeneity factors. The maximum error is estimated at approximately 0.5Yc for all but the 16Yc divinylbenzene resins.
0.75 0,702 0,681 0.780 0.732 0.697 0,962 0.955 0.946 0,959
0.928 0.933 0.931 0.935
0.75 0.700 0.672 0.782 0.726 0.686 0,964 0.947 0.933 0.949 0.587 0.755 0.906 0.912 0,915 0.916 0,940 0.958
0.76 0.719 0.655 0.834 0.735 0.677 0.981 0.928 0.902 0.924
0.832 0.834 0,872 0.855
0.78 0.719 0.637 0.854 0.750 0,680 0.993 0.919
0.788 0.775
--__ 100 m
150 m
--. 200 m
0.87 0.747
0.91
1.005 0.909
1.006 0.906
0.253 0.481 0.733 0.682
0.260 0.434 0.692 0.605
0.257 0,434
0.765 0.855
0.732 0.819
0.711 0.797
The results are consistent with those of Scatchard and Anderson2 on the weight of Dowex 50W-X8 and Dowex 1-X8 in HC1 and XaC1, but they are more extended in the range of electrolyte concentration, electrolyte type, and resin cross linking. The results also agree with the measurements of Gregor and co-workers’ although the present work is considerably more precise. This is especially evident in the study of swelling in LiCl solution with Dowex 50W-X8 where the method of centrifugal filtration appears to become fairly erratic as the viscosity increases. The most notable characteristic of the swelling measurements is the marked difference between anionexchange and cation-exchange resins. The volumes of the cation-exchange resins decrease steadily with increasing electrolyte concentration, though less rapidly a t high concentrations. Thevolumes of the anion-exchange resins start to decrease at the same or higher rate and then tend to pass through minima. This behavior becomes less markedin the seriesHC1, LiC1, SaCl, and KCl. The anion exchangers show substantially larger differences with different cations although they are all in the chloride form. In contrast, the cation exchangers are resinates of different cations in the different electrolyte solutions but exhibit substantially smaller differences among their relative volumes in the different solutions. Acknowledgments. This study was started at the
(13) W.D. Moseley, J r . , and D. H.Freeman, J . Phys. Chem, 67, 2225 (1963).
Volume 69, .\‘umber
1
January 1966
S. L. JOHNSON AND K. A. RUMON
74
Massachusetts Institute of Technology where it received the financial support of the U. S. Atomic Energy Comniissioti under Contract AT(30-1)-905. The work was completed at Washington State University under Contract iiT(45-1)-1544. Several stimulating discussions of the measurement problems were held with
Dr. Roger P. Loveland of the Eastman Kodak Research Laboratory and with Professor Arthur C. Hardy of the Massachusetts Institute of Technology. We are grateful to Miss Mary Hood, Miss Cathy Butts, and Mr. John Marvin, who helped with visual measurements of particle swelling.
Infrared Spectra of Solid 1:1 Pyridine-Benzoic Acid Complexes; the Nature of the Hydrogen Bond as a Function of the AcidBase Levels in the Complex'
by S. L. Johnson and K. A. Rumon Mellon Institute, Pittsburgh, Pennsylvania
16218 (Received M a y 6 , 1964)
The spectroscopic properties of 18 strongly hydrogen-bonded pyridine-benzoic acid adducts have been correlated with the ApK value of the complex. As the critical ApK for proton transfer is approached from either side of the critical region, increased broadening of the V-NHor v-OH and v-C=O bands is observed. Also, strong background absorption below 1200 em.-' occurs. At the critical ApK value, discontinuous changes take place in the v-C=O and v-C-0 regions of the spectrum, indicating that the adducts are either predominantly ionized or are predominantly un-ionized. In all of the complexes, either no band above 1700 em.-' could be ascribed to v-OH or v-NH, or two bands near 19002000 and 2500-2600 em.-' could be attributed to these modes. These two situations have been interpreted as corresponding to single-minimum- or near single-minimum-type and double-minimum-type hydrogen bonds, respectively. The single-minimum-type, hydrogen-bonded complex occurs only when its ApK is close to the critical ApK for proton transfer as determined from the v-C-0 and v-C=O bands.
Introduction A study of the benzoic acid-pyridine system in the liquid state led Hadzi2 to conclude that the strong hydrogen bond formed between the acid and the base was of the double-minimum type with a low potential barrier, giving rise to two OH stretching frequencies. The solid state spectra of nicotinic and isonicotinic acids gave similar results. In nonhydroxylic solvents, on the other hand, infrared3* and ~ l t r a v i o l e studies t ~ ~ indicate that pyridine and carboxylic acids exist in two Thc Journal of Physical Chemistry
tautomeric forms, that is, a double-minimum potential prevails, but the central barrier is high. The infrared study, however, dealt only with C=O stretching and ring vibrations, and not with v-OH or v-NH. ( 1 ) This investigation was supported in part by Public Health Service Grant GM 11834-01 from t h e National Institutes of Health. (2) (a) D. Hadzi, Vestn. Sloven. K e n . Drustaa, 1 , 21 (1958); (b) 2. Elektrochem., 62, 1157 (1958). (3) (a) G. M. Barrow, J . Am. Chem. soc., 78, 5803 (1956); (b) J . Nasielski and E. Vander Donckt, Spectrochim. Acta, 19, 1989 (1963).
