Determination of the External Volume of Ion Exchange Resin Particles HARRY P. GREGOR, K. SI. HELD, AND JUDITH BELLIKI Polytechnic I n s t i t u t e of Brooklyn, Brooklyn, .V. Y .
.A number of techniques are described for the precise determination of the external volumes of ion exchange resin particles. Techniques that involve freeing the resin particles of surface liquid to determine an accurate wet weight include the use of suction, blotting, and centrifugation. The use of a microcomparator for spherical particles is described. The accuracy of these measurements is determined
by their comparison w-ith extrapolated values from a wet weight-relative humidity curve. The best method (centrifugation) gives accurate results, precise to *0.2Yob. Because swelling or shrinking of an ion exchange system usually indicates not only the direction of the exchange process but also the state of equilibrium reached, volume measurement is important.
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HE determination of the external volume of ion exchange resin particles is an important measurement in ion exchange chemistry. The free energy changes involved in swelling or deswelling the resin matrix usually represent a significant and often a critical part of t,he total free energy change in ion exchange reactions, as shown by Gregor (3,4). Calculat,ions of ion exchange equilibrium constants, st,andard free energies, selectivity coefficients, and hydrated ionic volumes all require this measurement. The external volume of ion exchange particles is also important; in many cases it is the only means of characterizing various resins. For example, coinparison of the degree of cross linking of the sulfonated styrenedivinylbenzene resins is difficult, because of uncertainty in the composition of the divinylbenzene used. The resin volume, measured under standard conditions, is a better index of the degree of cross linking than is the percentage of divinylbenzene present in the nionomer mixtures. When resins have been equilibrated with various solutions, it is often very useful t o be able to calculate the exact composition of the resin phase with respect t o all its components, particularly the solvent. Gregor ( 4 ) has shown that the mole fraction of the solvent is an important term in expressions for exchange equilibria. This “wet weight” determination, which is a part, of the volume determination in most instances, requires the exact separation of the resin phase from the solution phase.
change states, it is convenient to calculate all resin properties as referred to 1 gram of resin in a standard state. For cation exchange resins, the standard state is the dry hydrogen resin; for anion eschange resins, it, is the dry hydroxide resin. Because the resins are acids and bases, respectively, there is no hydrolysis from these standard states when the resins are rinsed with distilled water aft,er regeneration (Jvith an acid or with a base). The resins are dried to the standard state over phosphorus pentoside to constant weight. The term “specific” refers to properties of a resin system, based upon the quantity of 1 gram of resin in the standard st,ate. Thus the specific wet weight and the specific volume of a cation exchange resin in a particular state are det,erniined b y weighing out 1.000 gram of dry hydrogen resin, converting it t o the state in question, and then determining its wet weight and volume. Ion exchange resin particles of the size customarily used rmge from 0.2 to 2 mni. in diameter, and are usually resistant to nioderate chemical att,ack (except in the case of strong pxidizing agents) and to mechanical attrition. They are usually, however, heterogeneous in structure and properties; this necessitates the tlevelopment of procedures which can be applied to 0.5- to 1.0-gram portions of resin. It must also be possible to determine the volume of resin particles in equilibrium with various aqueous and nonaqueous solutions by xhich they are, as a rule, strong]>-retted. Because the resins are highly cross-linked and thus cowtitute “stiff springs,” the volume changes that result, from their reactions may be very small, necessitating a highly precisc arid reproducible determinat,ion. An approximate coniparison of the relative volumes for the same resin in different exchange conditions can be obtained by observing the “sett,led volume.” Four methods have been used. The first consists in agitating a resin suspension in a graduate, then allowing the particles to settle, and taking the first, settled volume. The second is the standard A.S.T.11. backwash and drain volumes, obtained b y backwashing a column of resin to expand it by 50%, then allowing it to drain rapidly, and taking the first, settled volume. The third consists in tapping the settled columns until no further reduction in column height is obtained. The fourth consists in adding the resin to a known volume of a nonsivelling solvent such as hexane, and measuring the total volume of the system (9). None of these methods yields reproducible results. The backwash and drain volume is the most reproducible of the first three, but only t o +l’%a t best. The shape of the resin particles and the density of the equilibrating solution affect the results strongly. Where the liquid is drained and hexane is substituted to obtain a resin particle volume rahher than a bed volume, the reproducibility is also poor. Obviously, the column settled
DEFINITION OF TERMS
Because a uniform terminology has not been established in this new field, several terms must be defined. When t,he resin is equilibrated with gases or vapors, its weight is determined readily. Where it is equilibrated with a liquid, the resin particles must be freed of adherent droplets t,o determine the “wet weight.” This wet weight is the weight of the bulk volume of the resin particles. It includes the weight of any liquid absorbed by the resin, contained in its capillaries, intermicellar spaces, etc. The resin volume is defined by the dimensions of the particles themselves. The determination of resin volumes as described in this paper involves a pycnometric technique. Where the resin has been equilibrated with a specific solution, and that solution serves as pycnometric liquid, the resin is not altered by the measurement. But where the pycnometric liquid is not the equilibrating solution, no interaction must take place. The pycnometric fluid must not enter the resin capillaries, nor cause it t o swell, nor dissolve in any equilibrating solution already present. Under these circumstances, the resin wet weight and resin volume are independent of the nature of the pycnometric liquid used, I n order to compare the properties of a resin in various es1
Present address. Montefiore Hospital, New York, S . Y
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V O L U M E 23, NO. 4, A P R I L 1 9 5 1 volume as determined cannot be compared directly with the actual volumes of resin particles. It was decided that the pycnonietric technique was the most suitable for the problem at, hand. Here the volume of resin particles in equilibrium with a solution ir: determined, using that same solution as pycnometric fluid. ;is the accuracy of py-cnometric measurements is high (I), that part of the cleterminat,ion which is subject t o the great,est error is the wet weight determination. Several techniques were investigated. The first involved drawing t,he excess moisture off the particles by suction, the second used a blotting technique, and the third involved centrifugation. The blotting t,echnique was shown to be suitable for the determination of the volume of various films, as described by Hitchcock'(8), \T'eech and hIichaelis ( I O ) , and Gregor and Sollner ( 7 ) . Preliminary experiments by Chaya ( 2 ) indicated that both the suction and blotting techniques might be satisfactory for approsimate results. \Tolume changes u-ere also determined directly, using a microcomparator. This could be accomplished readily with spherical :mi nearly spherical ion exchange particles which are the product, of "pearl" polymerizations and condensations. PREPARATION OF RESINS
Two cation exchange resins were used for this study: Duolite C-3 (Chemical Process Co., Redwood City, Calif.), a phenolformaldehyde-sulfonic acid resin, and Dowex 50 (Dow Chemical Co., Midland, Mich.), a sulfonated polystyrene. Both resins, Liftrr being screened to -20+30-mesh size, were conditioned and put in the hydrogen state as described by Gregor and Bregman (5). Air-dried resins were weighed out and used as such. The moisture content of the air-dried resin, determined over phos.. phorus pentoxide, varied from 10 to 50%) depending upon conditions of air drying. Oven drying of the hydrogen resin a t 105" C. was found to result in a slow but continuous decomposition of the material as shown by weight loss. This effect was considerably lees when the resin was in the alkali metal state, but was in some instances still observable on prolonged drying. The air-dried resin samples were equilibrated with various .elutions by shaking the resin with a hundredfold excess of the olution several times. After the final equilibration, the solution \vas analyzed to make sure that it was unchanged. \There resin dried to constant weight over phosphorus pentoside was used, it was very carefully protected from air, for it is extremely hygroscopic.
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trials. The particles are transferred rapidly to a covered weighing bottle and weighed. The probable error for a single determination using the blotting technique is approximately *0.2570. Centrifugation, The wet weight of resin particles was also determined by placing the wet resin in a small basket, made by heat-sealing a 60-mesh ;\Ionel screen onto the bottom of a 3-em. section of a polyst,yrene t.ube 1 cm. in diameter. This basket \vas placed in a 16-nil. centrifuge tube, and supported at the mitlpoint of the tube by "dimples" in the glass. A few milliliters of equilibrium solution were retained in the tube in order to niaintain the proper water vapor pressure. This entire procedure was performed in a constant temperature system The tube was then centrifuged a t 3000 r.p.m. for varying periods of time. In order to maintain constant tempcrature, the centrifuge was mounted in an air thermostat and the rotor head was brought to the ambient temperature by a steam of impinging water. \Vit,hin 2 minutes of centrifuging spherical particles of Dowex 50 attained a constant weight, which did not change nithin experimental error up to 10 minutes. Beyond that time a slight decrease in wet weight was found, which may have been due to the temperature rise of the centrifuge head-for example, the wet weight of a resin equilibrated with 0.01 M sodium chloride solution \vas determined using centrifugation periods varying from 2 to 10 niinut,es. -4fter each centrifugation and wet weight determination, the resin \vas re-equilibrated with the solution and the process was repeated. The wet weights in grams were: 1.878, 1.871, 1.876, 1.873, 1.874, 1.882, 1.872, 1.870, 1.875, and 1.877. Here the precision is somelvhat greater than that customarily obtained, because the resin was unusually uniform in size and free of all imperfections and cracks, and the same resin sample \vas used each time.
O O l M NaCl
1.2
DETERMINATIONS
Suction Drying. The resin was equilibrated with a specific mpernatant, then introduced into a standard-type pycnometer and brought to temperature equilibrium a t 25.0" * 0.02" C. :lfter the assemblage was weighed, the resin was transferred to R special suction funnel, aspirated for a certain period of time, and then weighed. This constituted the determination of the \vet weight. -4glass funnel which had a platinum screen sealed between standard-taper glass joints was used. Aifterthe resin was dried by aspirat,ion, a fitted cap and base were put on and the whole \vas weighed. The results of various det,erminations with this latter technique are shown in Figure 1. Each point represents the average of five determinations. The rate of loss of weight decreases in about 3 minutes when the equilibrating solutions are 0.01 ill or greater in electrolyte content. I n the case of pure water, the rate of drying is: considerably slower. The precision of this measurement was tested by determining the wet weight of single samples ten times at the 3-minute period. The percentage error ranges from 1 0 . 1 to +0.5% for single determinations, and half that value for five determinations. Blotting. The equilibrated sample is removed from the pycnometer by inverting the latter over a piece of filter paper, allowing the particles to settle to the mouth of the vessel, and rapping sharply. The resin particles fall out almost free of adhering solvent, and are blotted gently between two sheets of filter aper. Blotting is continued with fresh paper until the particfes no longer adhere to one another, or to the paper, but are just "freeflowing." Thip critical point is readily established after a few
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O O l M HCI
001M N a C l $. 3 4 5
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6
TIME IN MINUTES
Figure 1.
Change in Wet Weight of Particles of Duolite C-3 (0) and Dowex 50 (A)
Equilihrated in various supernatant3 for different t i m e s o f aspirntion. Water-equilibrated samples were i n hydrogen state
For this det.ermination, a probable error of +0.2% is routinely obtained for spherical particles of different sizes, different degrees of uniformity, hardness, etc. This determination has been applied to various electrolytic solutions, in concent,rat,ionsranging from 0.001 to 1 M. I n all cases, the same precision is obtained. Wit,h nonspherical particles of resin, such as Duolite C-3, the centrifugation technique is not as precise, because droplets of solution become trapped between horizontal planes. The sample must, be shaken in the basket a t various int,ervals and then recentrifuged, to release trapped water droplets. Times of centrifugation up to 30 minutes are required and the reproducibility is not as high, the probable error being *0.5%. Extrapolation from Wet Weight-Relative Humidity Curves. The weight of resin particlcs which had been equilibrated through
ANALYTICAL CHEMISTRY
622 the vapor phase in a humidistat was measured at relative humidities approaching 100% by Gregor and Held ( 6 ) . No hysteresis effects were found. Data were obtained up to 96% relative humidity, and the curves extrapolated to 100% relative humidity. These data are shown in Figure 2. Measurements of this type are not rcliable a t 100% relative humidity, hecause of the condensation of water droplets. Although the accuracy and precision of the wet weight values obtained at relative humidities less than 96% are very high, the extrapolation obviously introduces an error of much greater magnitude.
I.340’
50
Figure 2.
I
60
70 80 RELATIVE HUMIDITY
90
100
centrifuging, for the same resin sample. However, most of the particles had deep cracks in them, usually reaching halfway across the sphere. DISCUSS105
The various techniques for the determination of the wet weight of resin particles are compared in Table I. The wet weight is that of 1.000 gram of a dry sulfonated polystyrene resin (Dowex 50) in the hydrogen state, which has been converted into the sodium state and equilibrated with 0.001 M sodium chloride. The n-et weight determination by extrapolation of the relative humidity curve may be taken to be the most accurate, because the procedure does not involve removal of an adherent liquid film nor depend upon uniformity of the resin particles. Results obtained using the centrifugation technique compare favorably with the extrapolation values. The blotting technique which leads to free-flowing particles obviously has removed some solution from the interior of the resin particles, for the resultant wet weights are lower than the centrifugation values. While it is possible to blot the particles to a less free flowing (wetter) end point, this is not well defined and the precision is poor. The suction technique gives the loivest values. The extrapolation technique is applicable only for resins equilibrated in dilute solutions ; here equilibration is through the vapor phase. I n dilute solutions centrifugation and extrapolation values agree, because the activity of the water approaches unity in both cases, and because Donnan effects almost completely exclude nonexchange electrolytes (ions not in exchange positions). These conditions do not obtain in concentrated solutions.
Specific Wet Weight of Dowex 50-Na at Various Relative Humidities
Dotted curve and point are extrapolated values
Volume Determinations Using Nonaqueous Media. The volume of particles of sulfonic acid resins in the dry state can be determined using a nonswelling organic solvent. Several solvents were tested and hexane and octane \?-erefound to be most suitable. Particles of dry resin did not increase in weight after prolonged contact with t,hese solvents, followed by blotting or centrifugation. Direct observat,ion of the size of individual particles n-ith a microcomparator shoned no visible sv-elling. Thus the volumes of dry resins can be determined by the direct pycnometric method with corresponding precision and accuracy. For the volume det,erniiiiation of hydrated resins, octane can be used as a pycnometric fluid. The solubility of water in octane is too small to affect any dehydration of the sample. Here the precision and accuracy are those of the centrifugation technique. For example, a resin sample may be equilibrated with a specific solution and its volume determined n i t h the centrifugation technique using that solution as pycnometric fluid. Then if the resin is re-equilibrated, centrifuged, and placed in a pycnometer with octane as the fluid, the calculated volume of the resin sample is the same in both cases n-ithin experimental error ( * 0 . 2 % ) , Direct Volume Determination Using Microcomparator. A. sample of sulfonated pearl polymerized polystyrene (Dowex 50) was wet-screened to -14+16 mesh (U. S. Standard), and air dried, and the spherical particles were collected by rolling them down an inclined plane. These particles were almost perfectly spherical, as determined by observation with a microcomparator. The diameters of a number of particles (50) were determined directly using the microcomparator; the resin had been equilibrated with a 0.1 M lithium chloride solution. The probable error in each diameter measurement was *0.2q10. Then a 0.5-gram sample of the same resin (372 particles) was weighed in a pycnometer, using 0.1 Jf lithium chloride as the liquid. Using the calculated volume of the resin particles (from the comparator data) and assuming that the average particle volume for the 50 particles was the same as for the 372 particles, it was possible to calculate the specific wet weight of the resin. This wet aeight was 3.6y0higher than that determined by
Table I.
Comparison of Different Techniques for Wet Weight Determinations Wet Weight, Probable Error, Technique Grams % Suction 1,623 ~0.30 *0.25 1,693 Blotting t0.20 1.878 Centrifugation *0,5 1.870 Relative humiditv *O.?O >Iicrocomparatoi 1.945
The direct volume determination using the microcomparator is susceptible to error because the particles may tend toward the oblate spheroid shape and are cracked. This method applies only to nearly perfect spheres and to very small resin samples. These techniques apply equally well to other types of anion and cation resins. If an organic liquid is used as pycnometric fluid, it should be tested for the absence of swelling of dry particles. Certain less polar resins swell in some organic solvents. The centrifugation technique yields results which appear to be accurate, and n-hich have a suitable degree of precision ( t 0 . 2 7 4 ) ACKNOWLEDGMENT
The authors wish to thank the Office of Naval Research and the U. S. Signal Corps for support of parts of this study. LITERATURE CITED
(1) Bauer, N., “Techniques of Organic Chemistry,” Vol. I, New
Tork, Interscience Publishers, 1945. (2) Chaya, H., thesis, Polytechnic Institute of Brooklyn, June 1947. (3) Gregor, H. P., J . Am. Chem. Soc., 70, 1293 (1948). (4)Ibid., 73, 642 (1951). ( 5 ) Gregor, H. P., and Bregman, J. I., Ibid., 70,2380 (1948). (6) Gregor, H. P., and Held, K. M., in preparation. (7) Gregor, H. P., and Sollner, K., J . P h y s . Chem., 50, 53 (1946). (8) Hitohcock, D. I., J . Gen. Physiol., 9, 755 (1925). (9) Walton, H. F., J . Phys. Chem., 47, 371 (1943). (IO) Weech, 8.A , , and Michaelis, L., J . Gem Physiol., 12, 221 (1928). August RECEIVED
31, 1950.
