Oct., 1959
EXCHANGE EQUILIBRIA IN A CARBOXYLIC RESIN
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EXCHANGE EQUILIBRIA I N A CARBOXYLIC RESIN AND I N ATTAPULGITE CLAY1V2 BY C. E. MARSHALL AND G. GARCIA Departments of Soils and Agricultural Chemistry, University of Missouri, Columbia, Missouri Receiued March 9, 1060
Cation-exchange e uilibria were determined for the carboxylic exchange resin IRC 50 and for Attapulgite clay using Na-K, Na-Rb, Na-85, Mg-Ca, Mg-Sr and Mg-Ba. Curves showing variations in selectivity number with composition of the solid phase were obtained at low ionic strength of the outer solutions. The resin showed small variations in K. with monovalent cations but very large variations with divalent. This is ascribed to partial blocking of sites by M OH+ ions. With attapulgite, very large variations in K, are found, suggesting a polyfunctional character with fixation of Rb and Cs by one mechanism, and of Mg by a different mechanism.
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Introduction Numerous electrochemical studies of clay minerals have, without exception, shown a polyfunctional character with respect to the dissociation of monovalent and divalent cation^.^ Cation-exchange studies with synthetic resins usually have been formulated as though the resin phase were monofunctional, and exchange reaction “constants” have been widely employed under such terms as “relative affinity coefficients,” ‘(selectivity coefficients,” “concentration equilibrium quotients,” “apparent equilibrium constants,” and “selectivity constants.” In recent publications and in technical application the terms “selectivity constant” and “selectivity coefficient” have come into widespread use. At the same time their variability has become more generally recognized. To avoid any false impression of constancy we shall here employ the term “selectivity number,” defined purely in terms of the analytical results. For cations of the same valency, the selectivity number KS = (C~/CZ)E/(Cl’/cZ’)S
where C1 and Cz refer to analytically determined concentrations in the external solution a t equilibrium, and C1’and Cz’ are similarly determined for the total cations completely displaced by suitable solutions from the substrate phase. The ratio C,’/ Cz’ is thus equal to M l ’ / M Z fthe 7 molar ratio of the cations a t equilibrium on the substrate. To determine the relative behavior of the substrate phase with respect to two exchange cations it is desirable to eliminate ionic interactions in the external solution. When the latter is sufficiently dilute and selected cations of the same valency are employed, the concentration ratio Cl/CZ equals the activity ratio al/azto a good approximation. We shall here assume this to be true for solutions of chlorides below M/100 of the pairs Na+/K+, Na+/ Rb+, Na/Cs and Mg++/Ca++, Mg++/Sr++ and Mg ++/Ba++. This brings a further advantage. Donnan theory indicates that when the effective cation concentration in the solution phase is much below that in the substrate phase the salt concentration in the latter will be negligible and the activity ratio al/az in the ( 1 ) Contribution from the Missouri Agricultural Experiment Station, Journal Series No. 1977. Approved by the Direator. (2) Experimental results from M.S. Thesis of G. Garcia, University of Missouri. January, 1959. (3) C. E. Marshall, bnd Nat. Coni. CEaus, NatE. Acad. Sci.. 827, 364 (1954).
external solution will equal the activity ratio of the two cations of the substrate phase. Thus (C~/CZ)E= (ul/a& = (ul’/azf).
Hence K. = ( u ~ ’ / a ~ ’ ) ~ / ( ~ ~ ~=’ /(flf/fzf)e Mz’l~
But M2‘/MI‘can be taken as equal to the corresponding ratio of cationic concentrations in the substrate phase. Then K s becomes the ratio of the two cationic activity coefficients fl’/fi’ in the substrate phase. Provided the external solution is sufficiently dilute Donnan theory indicates that the value of K s should be independent of actual concentration. The present work provides a comparison of two dissimilar exchangers in their behavior toward the same pairs of cations, namely, Na-K, Na-Rb, Na-Cs and Mg-Ca, Mg-Sr, Mg-Ba. The organic exchange resin chosen was a copolymer of methacrylic acid with divinylbenzene, 5-6% cross linked (Amberlite IRC 50, Rohm and Haas). Carboxyl groups occur on alternate carbon atoms of the chains except where the latter are cross linked by the benzenoid members. Hence the exchange capacity is high and chelation of cations is possible. The resin volume appeared to vary little with the nature and amount of the cations employed, although no exact measurements were made. The clay employed for comparison was the lathlike attapulgite, which bas a rigid structure with pores about 6.3 X 3.8 A. in cross section running parallel to the fiber axis. Its cation exchange capacity is caused by atomic proxying of Al for Si in the silica sheets and is commonly about 20-30 meq. per 100 g. This corresponds to one effective exchange site per 50 silicon atoms. Presumably, therefore, the sites are well separated. In comparison with other clays, attapulgite shows the greater dissociation of cations, both monovalent (potassium) and divalent (cal~iurn).~ Experimental Procedures (a) Exchange Resin IRC 5O.-The H form of the resin was thoroughly washed with hot doubly distilled water and air-dried. A 1 :1 suspension gave a pH of 4.8. Samples of 1 g. of air dry resin were weighed into calibrated plastic centrifuge tubes. Each was brought to neutrality wit,h4 ml. of 0.4728 N sodium hydroxide, i.e., 1.891 meq. This represents only about 20% of the exchange capacity of the resin as defined by the total carboxyl content. Normal sodium and potassium chloride solutions then were added in twelve different proportions so that the total cation content (4) 8. A. Barber and C. E. Marahall, Soil Sci., 72, 373 (1951).
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C. E. MARSHALL AND G. GARCIA
remained the same, namely, 100 meq. per tube. The systems were mixed and after 24 hours the clear supernatant liquid was carefully decanted. Doubly distilled water was then added to the mark, the Rystems were mixed and again decanted after 24 hours. This process was repeated. The equilibrium then attained was defined by analysis as follows. The liquid decanted was analyzed for chloride by the Mohr titration and for sodium and potassium by the flame photometer using air-acetylene. Working curves were obtained using standard solutions and no interference was found between sodium and potassium, rubidium or cesium. After removal of the outer solution the moist resin sample was treated with about 10 ml. of 1 N ammonium nitrate solution and transferred to a filter funnel. Further portions of 10-15 ml. ammonium nitrate solution were added until about 75 ml. of filtrate was collected. The latter was then made up to 100-ml. volume and analyzed for Na and K as described above. The chloride concentration was about 10-6 M , whereas in the equilibrium solution it was to 10-1 M . The complete series of 12 samples was run in duplicate and good agreement was obtained. The sodium-rubidium and sodium-cesium equilibria were conducted similarly except that 0.5-g. samples of resin were used, and correspondingly lower concentrations of aodium hydroxide, sodium chloride, rubidium chloride and cesium chloride. Single series only were employed. Under the experimental conditions used, concentrations measured by the flame photometer are believed reliable to 1p.p.m. In the divalent series 0.5-g. samples were initially titrated with 0.979 meq. sodium hydroxide; then 25 meq. of Mg++ C a + + as chlorides were added. With so large an excess of divalent ions relatively little sodium remained on the exchanger and it was reduced to entirely negligible amounts by the subsequent dilutions of the outer liquid. (Since the ratio(Monovalent)/1/Divalent tends, by Donnan theory, to remain constant upon dilution, it is necessary for the Ca/ Na ratio on the exchanger to increase). The same decantation procedure was used as before, 24 hours being allowed for the final equilibrium. In the external liquid, chloride was determined by mercuric nitrate titration and the sum of the divalent cations was obtained by Versenate titration as described by Metsons with the addition of a known amount of magnesium chloride solution which improves the end-point (Martell and Calvin6). The details were the same for the Mg/Ca and Mg/Sr systems but in order to obtain a good end-point with barium, Cook and Yardley’s modification was used. In this case an excess of standard magnesium chloride solution was added to the samples to be titrated. Calcium, strontium and barium were determined using the flame photometer. Suitable dilutions of the samples were made where necessary (notably at high Ba concentrations). The results appear reliable to 1 p.p.m. Displacement of divalent cations from the resin substrate followed the procedure for monovalent cationa, and subseuent analyses were as described above. Thus in all ca8es %e magnesium estimation was the difference between total divalent cations by Versenate titration and calcium, strontium or barium by flame photometer. All systems com rised twelve different proportions of divalent cations ancf all were carried through in duplicate. Good agreement was found. (b) Attapulgite Cla The sample used was A.P.I. sample H-43 supplied & W a r d s Natural Science Establishment. It contains 1-3% of quartz, as the major impurity. The chief exchangeable cations on the natural clay are calcium and magnesium. A 1:2 clay-water suspension of the sample gave a p H of 7.4. The crude clay was ground in a ball mill, then passed through a $@mesh sieve. The exchange capacity of attapulgite is much lower than that of IRC.50; around 0.30 meq. per gram; 25 meq. of Na plus K as chlorides were added first to 1-gram samples of the clay in centrifuge tubes. After 24 hours the supernatant liquid was decanted and the treatment was repeated. Again the supernatant liquid was removed and the tubes
Vol. 63
a,
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THE RESIN.
Fig. 1.-Selectivity numbers of I R C 50 in Na-K, Na-Rb and Na-Cs systems of varying composition.
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(5) A. J. Metson, “Methods of Chemical Analysis for Soil Survey Samples,” Govt. Printer, New Zealand, 1956. (6) A. E. Martell and 34. Calvin, “Chemistry of the Metal Chelate Compounds,” New York. N. Y . , 1963.
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i
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a,
Sr > Ca > Mg but i t is ON CUY. difficult to explain the tremendous diff ereiices from Fig. 4.-Selectivit numbers for attapulgite in Mg-Ca, Mg- calcium to barium on this basis alone. (b) The Sr and Mg-sa systems of varying composition. monovalent ion MgOH+, which from Bower and stancy from about 65% Na to 10% Na, but rising Truog’s works takes part in exchange reactions, might form a six-membered ring with a single carvalues of K , at both ends of the curve. Over the middle range the order was definitely boxyl group and the hydrogen atom of the carbon. ?j:K, > E:K, > P K , as might be expected from the Here magnesium would be more tightly held than properties of these ions in surface reactions gener- calcium, as shown by the low values of K , up to ally. The curves cross for small and more strik- 55% Mg. Sites occupied in this way by magneingly for large percentages of sodium. Except for sium would seriously interfere with the distributhe extreme value 2.7 in the K-Na system, the tion of (a) type sites, especially if, as seems likely, variations might well correspond to activity coef- the former occurred on alternate carboxyls. There ficient variations caused by interionic forces in the would be competition involving MgOH+ > CaOH interior of the resin, where concentrations are over (SrOH+ > BaOHf) with its very sharp decrease 1 molar. Kitchener curves7 showing activity co- in bonding from magnesium to calcium, and Ba++ efficient variations of acetates and p-toluenesulfo- > Sr++ > Ca++ > Mg++ which is the normal order. Hence what appears at first sight as an extreme nates indicate that the order depends on the anion. Thus acetates show YNaAc < YKAc < YRbAc < manifestation of polyfunctional character may Y C ~ Awhereas ~ for sulfonates YKS < YNaS < ‘)‘Lis. largely be explained on the basis of competing Glueckauf’s7 osmotic coefFicient curves for a weakly mechanisms. However, there is clear evidence cross-linked resin indicate Li > Na > K as the or- that some 5y0 of sites differ from the rest. der of dissociation, which agrees with the sulfonate (b) Attapulgite Clay.-The Na-K, Na-Rb and Na-Cs curves are shown in Fig. 3. The sodiumseries above and with the order found here. The distinct upward turn of K,for compositions potassium equilibria indicate that potassium is involving a high proportion of sodium shows itself normally more tightly held than sodium, but there even more strongly in the divalent cation curves (see exists a narrow region a t high potassium contents below) and strongly suggests that a small propor- where the reverse is true. The sodium-rubidium tion of the exchange sites falls into a separate cate- and sodium-cesium curves are complex. In both %
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(7) J. A. Kitobener, Ion Exchange Conference, Boe. Chem. Ind., 24 (1955).
(8) C. A. Bower and E. Truog, SoiE Sci. SOC.Amer. Proc., 5, 86 (1940).
E. L. TALBOT
1666
cases small amounts of sodium give selectivity numbers close to unity. Then a steep rise occurs, followed by a narrow zone in which the selectivity number is around 6 and changes little with increasing proportions of Na. From about 26% Na in the Na-Rb case and from about 14% Na in the Na-Cs curve another sharp rise occurs. Finally the NaR b system shows only slight changes between 45 and 75% Na whereas the Na-Cs system gives increasing values of the selectivity number. Cesium shows such high bonding that in the present series no system with less than 40% cesium resulted. Potentiometric titration curves of attapulgite with potassium hydroxide4 indicate that although this clay holds potassium somewhat less strongly than other clays, a certain proportion of the exchange sites have a much higher bonding energy than the rest. The polyfunctional character comes out even more strongly in the K , curves. The extremely high values for cesium indicate that special geometrical factors are important. Silicate exchangers with exposed silica sheets generally seem to have very high selectivity toward cesium, as shown by recent studies of the uptake of radioactive cesium by soils.g The mechanism is probably a fixation similar to that of potassium on micas. The Mg-Ca, Mg-Sr and Mg-Ba curves reproduced in Fig. 4 show other remarkable properties of attapulgite clay. In all cases the values of Ka are below unity up to about 70% saturation with magnesium. Hence magnesium is held more firmly than calcium, strontium or barium over a wide range of compositions. In the middle range the Ka values for Mg-Sr and Mg-Ba systems are close 9) H. Nishita. et aE., Soil Sci., 81, 317 (1956).
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together; the curve for the Mg-Ca system lies considerably lower. The polyfunctional character of the clay is much less strikingly shown by the divalent series as compared with the monovalent. Nevertheless the downward course of the K , curves between 0 and 20% Mg, and their strongly upward turn between 85 and 100% Mg show a polyfunctional character. Structurally, the strongly preferential bonding of magnesium can be ascribed to the free access (by the fixed channels), to the octahedral layer, which is composed of magnesium and aluminum bonded to oxygen and hydroxyl. These octahedral units are in narrow strips which form two parallel walls of the roughly rectangular channels. Thus equilibrium between structural magnesium ions and exchangeable magnesium ions would be facilitated, the general effect being enhanced bonding energy for magnesium as compared with the larger ions calcium, strontium and barium which do not enter the octahedral structure. This fixation mechanism does not produce such exaggerated effects as does chelation in the carboxylic resin for reasons evident from the discussion. It is quite different also from fixation of Cs, Rb, K and NH4 at mica-like surfaces composed of Siz03sheets, in which the cation is accommodated in hexagonal rings of oxygen ions. An alternative explanation, namely, that small fixed channels in exchangers favor the bonding order for dehydrated ions rather than hydrated3 seems improbable, because in the monovalent series with attapulgite we have so clearly the order Cs > Rb > K > Na, characteristic of the hydrated series. I n the divalent case magnesium alone is exceptional. Calcium, strontium and barium fall in their normal order.
THE SURFACE TENSION OF PERFLUORO SULFONATES I N STRONG AND OXIDIZING ACID MEDIA BYE. L. TALBOT Contribution No. 154 from Minnesota Mining and Manufacturing Company, St. Paul, Minnesota Received March 6 , 1969
The surface tension of a saturated solution of n-CaFt,SOaK in water may be lowered from a vslue of 40 dynes to 17 dynes by the addition of a strong acid such as sulfuric, nitric or hydrochloric. The surface tension lowering is proportional to the M to 10 M . At concentrat,ons above 10 log of the increase in the hydrogen ion over the concentration range (H+) = M for sulfuric and nitric acid the surface tension increases again. Hydrochloric acid continues to lower the surface tension even above 10 M . Acetic acid also falls 011 the same curve when plotted according to the available hydrogen ion. Sodium chloride and potassium chloride a.lso decrease the surface tension of solutions of C ~ H H S O ~toK a lesser degree than do the strong acids.
Introduction The chemical stability of the perfluoro acids and salts under extreme reducing and oxidizing conditions has been established by several workers. Hydrocarbon surfactants have little utility in concentrated solutions of sulfuric and nitric acids because they are quickly decomposed, but the
perfluoro surfactants remain stable under these conditions and lower the surface tension of these acids in a very effective manner. Alkyl perfluorocarboxylic acids are relatively strong acids and at low concentrations exhibit a pH nearly equivalent to that of hydrochloric acid a t the same concentration^.^ In previous studies4p6
(1) E. A. Kauck and A. R. Diesslin, I n d . Eng. Chem., 43, 2332 (195 1). (2) J. 0. Hend..icks, i b i d . , 46, 99 (1953). (3) D. R. Husted and A. H. Ahlbrecht, “Chemistry of the Perfluoro Acids and Their Derivatives,” 122nd Meeting American Chemical Society, Atlant.io City, N. J., 1952, Abatracts, p. 29-K.
(4) E. L. Talbot, “The Effects of pH and Ionic Strength Upon the Surface Tension of Certain Perfluoro Alkyl Acids and Their Salts,” 124th meeting of American Chemical Society, Chicago, Ill., 1953, Abstracts 4-1. (5) S. R. B. Cooke and E. L. Talbot, Minino Enoineering, 1, 1149 (1965).
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