Properties of Strongly Basic Anion xchange Resins R. M. WHEATON A N D W. C. BAUMAN The Dow Chemical Co., Midland, Mich.
A
SERIES of new strongly basic anion exchange resins has been developed by the Dow Chemical Co. as a counterpart t o the highly acidic cation exchanger, Dowex 50 (marketed as Nalcite HCR, by National Aluminate Corp.). Each member of this series is a quaternary ammonium salt ( RR'RttR"W+.A-), in which one of the R's is derived from polystyrene, which in turn has been cross linked with divinylbenxene (DVB) for maximum exchange capacity and t o render the resin insoluble in aqueous and nonaqueous media. The resultant products are highly dissociated organic bases in which the large cationic body is limited in movement and the relatively small and mobile anion is free to exchange in a wide p H range. These strongly basic resins are capable of removing from solution such weak acids as carbonic acid, silicic acid, amino acids, hydrogen sulfide, and phenol. I n addition, neutral salt splitting may remove undesirable anions from solution or pick up desirable ones, depending on the point of view, and may be the first step in so-called reverse demineralization. Reverse demineralization is of particular advantage where low solution pH must be avoided at all times. As strong but insoluble bases, these resins may be used as catalysts in some reactions and then conveniently removed by filtration. The resins discussed in this paper are the commercially available Dowex 1 and Dowex 2 (marketed as Salcite S.4R, by National Aluminate Corp.). The standard products are essentially spherical materials of 20 to 50 mesh, cream to pale yellow in color, and containing about 40% water. They aIe cross linked with 7.5% divinylbenzene. Total exchange capacity of each of these resin- is approximately 1.2 equivalents per liter wet volume, or 1.2 normal. Expressed in usual water treatment terms, this is equivalent to about 26,000 grains of calcium carbonate per cubic foot. The bulk density of each of the resins, including voids, is about 45 pounds per cubic foot, while its real density, as determined by suspension in salt solutions of known density, approximates 1.15. RELATIVE BASICITY
These highly basic resins are of the quaternary amine type. I n the case of Dowex 1, three of the R groups in the general quaternary salt structure are methyl groups, whereas in Dowex 2 one of the methyl groups is replaced by hydroxy ethyl. The chief difference between Dowex 1 and Dowex 2, but an important one, results from the fact that Dou-ex 1 is somewhat more basic in nature than Dowex 2. This basicity is shown by titration curves (Figure 1) of the hydroxide form of the resins. For the purpose of these basicity determinations the authors have used fine-mesh (200- to 400-mesh) variations of the resins for direct titration with 0.1 N hydrochloric acid on the glass electrode in the absence of salt. The coarser materials are unsatisfactory, inasmuch as contact with the electrodes is insufficiently good t o record a true p H value, the hydroxide anion being tied to the bead structure. With the much finer material, contact is greatly increased. When stirring is stopped and the fine beads settle out, the p H approaches 7 , as very little of the hydroxide ion remains in the neighborhood of the electrode. I n the presence of salt the observed p H is considerably higher, of the same order as for sodium hydroxide. I n the curves, the break a t end point is not as sharp as might be expected because of the presence of small amounts of weakly
T h e development of strongly basic anion exchange resins has necessitated the evaluation of some of the more important properties of these materials. Dowex 1 and Dowex 2 (Nalcite SAR) are two commercial resins chosen for these studies. Important properties of these materials have been checked and reported, making considerable use of graphical presentation. These include such physical properties as physical stability, volume changes relative to ionic form, breakage, and screen analyses, and such chemical properties as chemical stability, degree of basicity, composition, capacity, and equilibria properties. Many of these data deal with resins of various degrees of cross linkage, showing the effect of these changes on water-holding capacity, total exchange capacity, and equilibrium constants. Data of the type presented in this paper will serve to acquaint interested parties with the fundamental makeup of the strongly basic anion exchange resins. By use of this information, possible applications of this type of product may come to light. Ion exchange resins will become an extremely important separating tool and this strongly basic anion type has been one of the most important missing links in the over-all picture.
basic groups. The third curve on Figure 1 is a standard titration of carbonate-free sodium hydroxide, which may be used as a comparison of the relative degrees of basicity. T h e most important effect of this difference in basicity is in the relative ease of regeneration of the resins into the hydroxide form, as will be shown in the selectivity pattern of the resins for various anions. CHEMICAL STABILITY
There is very little indication of caparbity drop in continued operat,ions with these resins a t room temperature even in the hydroxide cycle, although the hydroxide form of the resin 18 known to be the least stable form of thi3 resin type. Dourex 1 is somewhat more stable than Doweu 2. At elevated temperatures, Dowex 2 is markedly less stable, the stability curve breaking sharply above 150" C. (Figure 2). Dowex 50 (Nalcite H C R ) , the cation exchange resin prepared likewise from a styrene-divinylbenzene base, exhibits the same breakdown a t these temperatures (Z), which might indicate an actual breakdown of the polymeric structure. Some indication of the decreased stability of the hydroxide form is indicated here, though the difference is not great for these 16-hour runs, The stability of Dowex 2 in hydroxide, bicarbonate, and chloride forms has been checked for a period of days a t 50 O and 95' C. and is shown in Figure 3. The marked decrease in stability in the hydroxide form is very evident a t 95" C., in which case the resin capacity dropped over 50% in a month, while the chloride form exhibited no detectable change. Inasmuch as the hydroxide form is least stable, it was tried only a t 50" C. and only the slightest capacity drop is shown in more than 30 days. Dowex 1 and Dowex 2 are particularly resistant t o oxidation both because of their salt structure and because of the styrenedivinylbenzene base, particularly when compared with a resin of phenolic base which lends itself t o ready oxidative decomposition. Reports from the field on 300 cycles of operation with Dowex 1 at 60' C. in the presence of chlorine show no noticeable decrease in capacity. 1088
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1951
1089 I
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3 Figure 1.
Titration of Dowex 1 and Dowex 2 (Hydroxide Form) with 0.1 N Hydrochloric Acid
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CONTROL T E M P E R A T U R E (C.')
Chemical Stability of Dowex 2
Figure 2.
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1004
KCI CONCENTRATIONS
% I I I I I I I I I I / O
DRY VOLUME
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TIME AT CONTROL T E M P E R A T U R E ( D A Y S )
Figure 3.
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Volume changes of Dowex 2 in different salt concentrations and in different ionic forms are not large, which is a n added advantage of this resin, particularly for column operations where packing may become a serious problem. Figure 4 shows the volume change of Dowex 2 from the anhydrous form t o the water-swollen form followed by the shrinkage as salt concentration is increased, maintaining the chloride resin form throughout, Thc shrinkaje is secn to be slight. 1 h t a rhange in volumes of Dowex 2 for different ionic forms is shown in Table I. These differences are somewhat greater than above and compare in general with the selectivity pattern of Dowex 2 for various ions, which is discussed below. I n general, greater swelling of the resin means reduced selectivity as work i s required t o swell the elastic particles. The notable exceptions
Volume Changes of Dowex 2 in Various Ionic Forms .4nion Form
'
20
24
28
32
36
ltre hydroxide and phenate ions, each of which swells the re& more than would be predicted from t h a t selectivity pattern. The phenate ion actually swells the resin considerably, even though the resin is selective for t h a t ion, and this may be explained by the similar organic nature of the phenolic group and of the styrene nucleus of the resin. EQUILIBRIA PROPERTIES
For handling of these anion exchange resins it is of great assistance t o predict the operating and regenerant possibilities in various exchange cycles. To study the selectivity of Dowex 1 arid Dowex 2 for various anions, equilibrium experiments have been run, in general comparing the selectivity of the resin for each of these anions with chloride. T h e general equation is: R+C1-
Actual Volumea,
M1.
90.0 94.5 94.7 100.0 100.0 104.0 105.0 108.0 112.5 113.0 Divalent Sulfate 103.0 Carbonate 106.0 Trivalent Phosphate 106.0 Starting with 100 ml. of Dowex 2 in chloride form. Monovalent
16
Figure 4. Volume Change of Dowex 2 RESIN SWELLING
Table I.
12
% KCI
Chemical Stability of Dowex 2
Iodide Bromide Nitrate Nitrite Chloride Bicarbonate Phenate Hydroxide Acetate Fluoride
Volume Change,
+ C+A- + R f A + C+C1-
where R + is the large, polymeric, essentially immobile cationic body and C+A- is a highly dissociated acid, base, or salt. I n general, sodium salts were used in the studies. Equilibrium constants were determined by the equation:
%
-10.0 -5.5 -5.3 10.0
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+4.0 +6.0 +8.0 +12.0 +13.0 +3.0 +6.0 +6.0
in which Cl, and Cl, equal the molar concentratiom of chloride on the resin and in the solution, respectively, and [A,] and [ A i ] similarly represent molar concentration of other anion, on the resin and in the solution, all at equilibrium conditions. 1u = valence of anionic group, A-. A true equilibrium constanit, KO,in which molar concentrations are replaced b y ion activities in the various media (a product of molar concentration1 and activity coefficients) would be more accurate, b u t activity coefficients are not known for most of the anions studiedkinssolution
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Vol. 43, No. 5
INDUSTRIAL AND ENGINEERING CHEMISTRY 1.0
ionic ratios-ie., with varying proportions of one anion on the resin to another-and so this location is reported. Only by actually determining this K , value curve for each pair of ioiis could these values be reported a t one concentration. To the present, time such studies have been made on the chloridehydroxide and chloride-nitrate equilibria. However, only the numerical value and not the relative positions of the anions as shown in the charts would be changed. This chloride fraction on the resin is another indication of the resin's selectivity, as approximately equivalent amounts of anions were present in each of the solution and resin phasrs throughout the runs.
a
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Table 11. Selectivity of Dowex 1 for Monovalent Anions 0
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OH- F R A C T I O N IN SOLUTION (X,)
Figure 5.
Chloride-Hydroxide Equilibrium of Dowex 1 and Dowex 2
and 110 method is yet known for measuring these activity coefficients in the resin phase. Experimental. For this initial comparison of selectivities, 5 ml. (wet volume basis) of the resin, previously completely converted to the chloride form with hydrochloric acid or sodium chloride rinse, and 50 ml. of 0.1 N solution of the sodium salt of the desired anion were mixed for a minimum of 16 hours, although the authors have shown t,hat equilibrium is generally reached in less than 4 hours. The resultant solution and the resin xere then separated by filtration and each portion was titrated separately wit'h silver nitrate solution for chloride anion by use of the silver electrode to an end point of 0.24 volt (in the presence of sulfuric acid in the case of the resin). The other anion was t,hen determined by difference, after checks on the hydroxide-chloride cycle, in v,-hich hydroxide determinations were also made, showed determinations by difference to be accurate. Chloride has been selected as the standard a t 1.000 for three reasons: The resins as sold exist in the chloride form, chloride det'erminations are relatively simple and very accurate, and the chloride anion fits somewhere near the center of the over-all picture. From Tables I1 and I11 it may be seen that the anions fall into similar patterns on each of the resins, the notable esception being hydroxide ion. The values in column X R following each of the anion names indicate the ion fraction of the total capacity which is present' as chloride ion under the equilibrium conditions. I t has been found that these I
