lolnExchange Processes in Liquid Ammonia Alan M. Phippsl and David N. Hume Department of Chemistry and Laboratory for Nuclear Science, Massachussetts Institute of Technology, Cambridge, Mass. The feasibility of utilizing conventional strongly acidic and basic polystyrene-divinylbenzene ion exchange resins in liquid ammonia systems at -74' C has been demonstrated. The swelling characteristics of both cation and anion exchangers are similar to those observed in water. Selectivity coefficients of L i t , Na+, K+, Rb+,Cs+, Ag+, TI+, Ca+Z, Srf2, Ba+2, Cu+2 and Ni+2 with reference to NH,+ have been determined and the effects of cross-linking, temperature, mole fraction in resin phase, nature of anion, and water content of the solvent studied. Characteristically the selectivity coefficients are much larger than the corresponding values in aqueous systems, particularly for polyvalent ions. Selectivity coefficients of CI-, Br-, I-, NOa-, SCN-, and Clod- have been determined. The practicality of a column separation of N a + and K f has been demonstrated.
THEINFLUENCE of the solvent on ion exchange processes has been found to be considerable and varied. Many studies have been made using a variety of mixed aqueous-nonaqueous systems, Since there is often a strong possibility of a superimposed liquid-liquid partition in these cases, it is felt that investigations of strictly nonaqueous systems may provide more useful information in regard to the effect of the solvent on the various factors which determine exchange behavior. There have been few such studies because of the limited resin swelling in most solvents and the very slow rates of exchange usually encountered. Liquid ammonia, having a moderately high dielectric constant, low viscosity, and considerable solvating and complexing ability, suggests itself as a useful medium in which to study ion exchange processes. Keenan and McDowell in a note (1) describe the removal of potassium ion from the reaction mixture after the reduction of BF3NH3with elemental potassium in liquid ammonia. An investigation of both cation and anion exchange behavior in liquid ammonia was therefore undertaken with sulfonated styrene-divinylbenzene copolymer type resins after establishing that these resins are stable to treatment with this solvent. EXPERIMENTAL
Reagents. Dowex 50W cation exchange resin in several bead sizes and percentages of divinylbenzene cross-linkage, 200-400-mesh Dowex 1-X8 anion exchange resin and the low capacity cation exchange resin DOW ET-561, 50-100 mesh, were washed with ethanol and conditioned by alternate treatment with 1M solutions of hydrochloric acid and sodium hydroxide. The various ionic forms of the resins were prepared by exhaustive washing on a column with aqueous solutions of the appropriate electrolyte, followed by washing with deionized water, until negative chemical or flame tests were obtained. The cation exchange resins and the reagent grade chemicals used were dried for 16 hours at 100" C in a
vacuum oven over P206,a procedure which reduces the moisture content of this resin below the level detected by Karl Fischer titration (2). The Dowex 1 resins were dried in a vacuum oven over P205at 60" C for 1 week. In the case of the nitrates of copper and nickel, which are not readily obtained as anhydrous salts, the ammoniates were prepared : Cupric tetrammine nitrate by the method of Horn (3) and nickel hexammine nitrate by the method of King, Cruse, and Angell (4). The ammoniates were dried in a vacuum desiccator over P20~. Apparatus. Swelling measurements and batch equilibrations were carried out in graduated centrifuge tubes immersed in an appropriate cooling bath, generally acetone-dry ice. Commercial anhydrous ammonia was condensed on the resin in these tubes after passing through a drying column composed of glass wool impregnated with metallic sodium. Chromatographic separations were performed using an 11- x 0.9-cm jacketed borosilicate glass condenser as a column, Ethanol was circulated by means of a centrifugal pump through the condenser jacket and through a copper coil immersed in an acetone-dry ice bath. A temperature of -65 O C could be maintained inside the column in this way. Procedure. An indication of the degree of swelling undergone by the resins was obtained from a comparison of the volume occupied by a 3- to 4-grams column of dried resin in a narrow tube and the volume occupied by this column after being equilibrated in liquid ammonia. Ammonia was condensed into these tubes to a level a few centimeters above the resin column and the tube was stoppered and swirled periodically until full swelling was attained. Swelling of the same resin was measured in water using the same tubes. The dry volume was taken to be the volume occupied by the resin column in n-heptane. Batch equilibrations of 5- to 6-gram samples of resin in liquid ammonia solutions were carried out in graduated 50-ml centrifuge tubes. After weighed samples of dry resin and metal ammonium nitrate mixtures were added to these tubes they were heated at 105 " C and attached to the ammonia supply. After cooling, liquid ammonia was condensed into the tube to a volume of 20 to 30 ml and the contents were mixed by swirling periodically for about 4 hours. After the equilibration, a sample of the external solution was obtained by forcing it with dry nitrogen through a filter stick into a 15-ml centrifuge tube. The short portion of the filter stick outside the tube was cooled with dry ice before the transfer. This sample (generally 10-15 ml) was then transferred to a 30-ml crucible or beaker and allowed to evaporate at room temperature. The observed total volume of solution plus resin was corrected for the volume occupied by the resin by means of a displacement factor calculated from volume swelling data. Alkalis and alkaline earths in the batch equilibrations were all determined by the same basic procedure. The residue remaining after evaporation of the sample was twice taken up in concentrated hydrochloric acid and evaporated to dryness on the steam bath. After heating just below red hot to remove ammonium salts, the cooled residue was dissolved in
Present address, Department of Chemistry, Boston College, Chestnut Hill, Mass. 02167.
(1) C. W. Keenan and W. J. McDowell, J . Am. Chem. SOC.,75, 6348 (1953).
(2) R. W. Gable and H. A. Strobel, J. Phys. Chem., 60, 513 (1956). (3) D. W. Horn, Am. Chem. J . , 37,620 (1907). (4) H. J. S . King, A. W. Cruse, and F. G. Angell, J. Chem. Soc., 1932, p. 2928. VOL. 39, NO. 14, DECEMBER 1967
1755
deionized water and passed through a hydrogen form Dowex 50W-XS 50-100-mesh resin column, and the liberated hydrogen ions were titrated with standard sodium hydroxide, The validity of this technique was checked by weighing out mixtures of ammonium and metal nitrates and analyzing them by the same procedure. Analyses for the total electrolyte concentration in the external solution were done in the same way, taking up the sample in deionized water and passing it through the column directly after evaporation of the ammonia. Other cations and most of the anions in the evaporated residues from equilibrated solutions were determined by standard analytical methods. Perchlorate in the presence of nitrate was weighed as silver chloride after igniting the sample residue with sodium hydroxide. Bromide in the presence of chloride was determined by a modified Doeringvan der Meulen method. Thiocyanate in the presence of bromide was determined by the method of Schulek. Iodide in the presence of bromide was determined iodometrically after oxidation to iodate with bromine water and removal of excess bromine with phenol. In all cases the validity of the analytical procedure was checked with standard mixtures. The weight capacities of the various resins were determined by standard procedures before and after treatment with liquid ammonia solutions. For the chromatographic separations, a weighed amount of dried 100-200-mesh Dowex 50W-X8 ammonium form resin was equilibrated with liquid ammonia in the 50-ml centrifuge tube-filter stick arrangement used for batch equilibrations and the mixture was poured into the column. The resin was held in place by a wad of glass wool inserted into the base of the column. The eluted fractions were collected in 15-ml centrifuge tubes immersed in an acetone-dry ice bath. A drying tube was attached to the top of the column when additions were not being made. Sodium and potassium nitrate mixtures were dissolved in the minimum amount of liquid ammonia, added to the column, and eluted with various concentrations of ammonium nitrate. The eluent was collected in 3-5-ml fractions which were evaporated, taken up in 250 ml of deionized water and the sodium and potassium determined by flame photometry. RESULTS AND DISCUSSION
Swelling. The extent to which an ion exchange resin is swelled by the solvent in which it is placed reflects to a considerable degree the exchange activity which the resin will exhibit. Indeed, Boyd (5, 6) has shown that under suitable conditions, if the ratios of activity coefficients both in the resin and solution phases remain constant, the logarithm of the selectivity coefficient is linearly related to the weight swelling of the resin. In general, solvents having dielectric constants lower than that of water tend to swell a given cation exchange resin to a lesser degree. The extent of swelling, however, is impossible to predict on the basis of dielectric constant alone. While the electrostatic repulsion of the fixed ionic groups would be increased in a solvent with a dielectric constant lower than that of water, the solvation tendency of these groups and of the counterions would in general be reduced. A low dielectric constant also favors ion association in the resin phase which would both nullify the repulsion of fixed ionic groups and reduce the osmotic activity of the counterions. The ratios of the swollen volume to the dry volume determined in liquid ammonia and water for various ionic forms of Dowex 50W-X8, and Dowex 1-X8 200-400-mesh resins ( 5 ) G. E. Myers and G. E. Boyd, J. Phys. Chem., 60, 521 (1956). ( 6 ) G. E. Boyd, S. Lindenbaurn, and G. E. Myers, Ibid., 65, 577
1:8
0, I
.-+ 1.7 0
0 [r (Ln
2 -
1.6
E v, 1.5
1.4
1.3
ANALYTICAL CHEMISTRY
1.4
1.5
1.6 1.7 1.8 Swe!ling Ratio NH3
1.9
Figure 1. Correlation between swelling ratio of Dowex SOW-X8 in water at 25OC and liquid ammonia at -74°C for various cations
are given in Table I. These ratios were reproducible to within a range of 2%. No attempt was made to correct for the interstitial (void) volume. The aqueous swelling ratios agree closely with the values obtained by Gable and Strobe1 (2), who used this technique for the estimation of swelling in absolute methanol, and are 5-8 % lower than valiles reported for this resin from pycnometric data. Swelling equilibrium was reached in a maximum of 1 hour. Larger resin beads required considerably more time. Rates of equilibration three orders of magnitude slower than in water are not uncommon in nonaqueous solvents (7). The rather striking parallelism between swelling of Dowex 50 in liquid ammonia and in water for many cations is shown in Figure 1. The swelling in liquid ammonia tends to be somewhat less than in water but much greater than in other nonaqueous solvents. Comparison with swelling in absolute methanol is of special interest because the dielectric constants of the two solvents are quite close (approximately 29 and 32 for ammonia and methanol, respectively, at the temperature at which the swelling was determined). Only the hydrogen form (1.70) and the ammonium form (1.43) swell to any marked extent in methanol. In comparison, the sodium form swells only slightly, 1 .I2 6s. 1.83 in liquid ammonia. Among the alkalis, only sodium and lithium are highly hydrated in aqueous solution and give evidence of ammine formation in the presence of water. The corresponding resins exhibit high swelling ratios in both ammonia and water, while those of potassium and cesium show lower swelling ratios in both solvents. The silver form of this resin swells to a greater extent in liquid ammonia than in water, Ag+ being the only univalent cation in Table I which forms an ammine complex in preference to an aquo complex. Thallium(1) in contrast shows negligible complexing tendency with ammonia. (7) T. Vermeulen and E. H. Huffrnan, Ind. Eng. Chem., 45, 1658
(1953).
(1961).
1756
I.9
The extent of swelling exhibited by anion exchange resins is generally considerably less than for cation exchange resins with the same styrene-divinylbenzene matrix, They swell to a limited extent in a variety of solvents (8,9). The identical aqueous and ammonia swelling ratios may be compared to the ratio of 1.34 observed for cesium form Dowex 5OW-X8 in liquid ammonia, and could be regarded simply as the swelling of the matrix alone, fixed and counterions being unsolvated. In anion exchange generally, swelling pressure and ionic size seem to have less importance than in cation exchange, probably because the weaker solvation and greater polarizability of the anions favor specific interactions. Selectivity of Cations. The experimentally determined value used in this study to characterize exchange behavior is a mass law concentration product ratio for an assumed exchange reaction of the type:
RB
+ M + a R M + B+
This parameter is defined as :
Table I.
Swelling of Dowex Resins in Ammonia and Water. Liquid ammonia, Water, vol. (swollen)/ vol. (swollen)/ Ionic form vol. (dry) vol. (dry) H+ .*. 1.94 Lif 1.86 1.99 Na+ 1.83 1.92 KT 1.44 1.68 Cs+ 1.34 1.63 NHI” 1.83 1.86 Ag+ 1.80 1.68 Tlf 1.68 1.47 Caf2 1.67 1.74 Ba+Z 1.58 1.64 Cut2 1.76 1.84 Nif2 1.51 1.54 Br1.30 1.30 NO*1.33 1.33 Cation resins Dowex 50W-X8 200-400 mesh. Anion resins Dowex 1-X8, 20&400 mesh.
Table 11. Selectivity Coefficients from Batch Equilibration Ks (M+,”&+I
where 2 = ionic fraction in the resin phase X = ionic fraction in the external solution Z = charge of the counterion The model chosen in the interests of providing some visualizable physical significance to the process of ion exchange consists of an inert, elastic matrix with fixed ionic groups which absorbs solvent to give an internal solution of fixed anions and mobile cations in a Donnan-type equilibrium with an external electrolyte solution. The selectivity coefficient as defined above was determined for several cations through the equilibration of approximately 0.1M nitrate solutions with Dowex 5OW-X8 resin. Ammonium ion was employed as ion B+ in all cases. The quantity actually determined by the chemical analyses is the number of equivalents of cation M+ in the external solution phase. The amount of ammonium ion present is obtained by difference since the total number of equivalents originally present (weighed out as metal nitrate plusammonium nitrate) is known and equivalent exchange was obtained. Absorption of nitrate by the resin is assumed to be negligible since determinations of the total analytical amounts of electrolyte in the external solution after equilibrating agree with the values calculable from equivalent exchange in the range 0.5-2.0 for these exchanges. The Ks values obtained for the alkali metal, silver, and thallous ions are given in Table 11. The values reported for the specific are felt to be reliable to the order of experimental conditions given. The averaged K, values of Bonner and Smith (IO) for the corresponding aqueous exchanges are listed for comparison. In each case results obtained by starting with the resin in the ammonium form were checked with experiments starting with the resin in the metal form. Metal form resins required more than 4 hours to reach equilibrium for resins of bead size larger than 100-200 mesh.
+5z
(8) G. W. Bodamer and R. Kunin, Ind. Eng, Chem., 45, 2577 (1953). (9) F. S . Chance, Jr., G. E. Boyd, andH. J. Garber, Ibid., p. 1671. (10) 0. D. Bonner and L. L. Smith, J. Phys. Chem., 61,326 (1957).
M+
Li+ Nay K+ Rb+ Cs+
Ag+
Ti+
R 11
(“a)
3.0 1.7 17.0 27.0 35.0 2.0 17.0
0.32-0.65 0.26-0,64 0.54-0.80 0 614.76 0.53-0.70 0.34-0.43 0.70-0.74
K,(M +/Hi) (HzO) (101 0.80
I
1.5 2.1 2.3 2.3 6.8 9.9
The aqueous selectivities observed for the alkali metal cations have often been viewed as a function of the solvated ionic size. Since these ions are solvated by water in preference to ammonia, the swelling of the resins reflecting this, the aqueous K, sequence might also be expected in ammonia but with slightly higher values. This is seen to be the case for sodium and lithium ions only. In absolute methanol solution where the solvation number for Na+ would appear to be essentially zero on the basis of swelling behavior, the K , value for the Na+/HC exchange for this resin is 3.4 (2). This is not much greater than the K, for Rb+ and Cs+ the “unhydrated” ions in aqueous solution where the swelling is greater, possibly from solvation of the fixed ionic groups. It seems certain, therefore, that the large values of Ks observed in ammonia for cesium, rubidium, and, to a somewhat lesser extent, potassium are the result of factors other than the size of the solvated ion. Ion association between the counterion and specific fixed ionic groups in the resin phase is the explanation most often invoked to explain very large values of Ks. The relatively large selectivity coefficients observed in aqueous solution for the Ag+ and T1+ exchanges with H+ are usually attributed to this effect on the basis of the high polarizability of these ions. In absolute methanol the K, for the AgT/H- exchange is approximately four times the aqueous value. The marked decrease in the selectivity of Ag+ in liquid ammonia suggests some influence from the strong ammine complex formed by this ion. Addition of ammonia to aqueous solutions of silver ion, however, does not significantly affect the exchange behavior. This is generally observed to be the case for neutral ligands, with the inference that there is a replacement of a solvent molecule with the ligand, the size and charge remaining the same. VOL. 39, NO. 14, DECEMBER 1967
e
1757
Thallous ion shows only a very slight tendency to form an ammine complex in the presence of water; the T l i form resin, however, does swell more in ammonia than in water where the swelling is particularly low. The observed K, for the Tl+/NH*+exchange in liquid ammonia is of the same order as the aqueous value (which varies between 10 and 24 in the literature). It would appear that the solvation is low and the ion association in the resin phase relatively high in either solvent. The exchange behavior of several representative divalent cations was investigated. Ions were chosen from among those which form strong ammine complexes and those which do not. Ca+2, Sr+2, Ba+2, Cu-2 and Ni+2 were equilibrated as nitrates. In aqueous solution the exchanges of these ions with hydrogen ion exhibit K, values ranging from 3.0 for copper to 9.0 for barium. In liquid ammonia, however, all exchanges of these divalent cations with ammonium ion exhibit very large values of K,, which can only be explained by essentially undissociated ion binding between the metal ion and the fixed ionic groups. Large values of K,, caused by this type of ion association invariably lead to proportional reductions in the rate of exchange. The exchange rate, much slower than for aqueous solutions in the case of univalent cations, was of critical importance for divalent ion exchanges, especially for equilibrations beginning with the resin in the metal ion form. A selectivity coefficient of 7.2 X l o 3 was obtained for the Ba+2/NH4+exchange with 200-400-mesh Dowex 50W-X8. Values above = 0.7 I-t 0.01 could not be achieved with NH4+ form resin regardless of the concentration of the external solution. This would indicate that the capacity of the resin for is only 73 % of the capacity for ammonium ion. In aqueous solution the capacity is the same in each case and the Ba+2form resin is swelled to the same extent in each solvent. An ion of the size of (CH3)IH+is required before there is a reduction in the capacity of this resin in aqueous solution. Values of K , for the Ca+2/NH4+ and Sr+*/NH4+exchanges were approximately the same as the value for the Baf9/NH4+ exchange within the experimental reliability which is not expected to be better than 10 %. A large K, is generally accompanied by an increased dependence of K , on Both the analytical method for the determination of and the arithmetic calculation of K, are quite sensitive to very small changes at large values of Ks (which means a small concentration of M-? in the external solution concentration). Therefore exchanges of calcium and copper were attempted in the interests of more numerically workable exchange data. Exchange rates were so slow that equilibrium was not apexchange reached proached after 6 hours. The CUTZ/NW~+ equilibrium more rapidly giving K , equal to 2.0 X lo3 for a range in x c U up to 0.85. A selectivity coefficient could not be determined for the Ni+?/NH4+exchange; after 6 hours equilibration, K, values differing by two orders of magnitude were obtained depending on which form of the resin was used. The difference in exchange rate exhibited by Nif2 and Cut2 can easily be observed in the discharge of color in the external solution during equilibration. In the absorption of ammonia gas by dry Nif2 and C U +form ~ resin one sees a similar difference in reactivity, The C U +form ~ resin becomes dark purple immediately on being treated with ammonia gas at 1 atm, and in one experiment absorbed 3.6 moles of NH3 in 15 minutes. The N P 2 form resin abper mole of CUT% sorbed only 1.5 moles NH3/Ni+2after 6 hours under the same conditions.
x~~
zhI.
1758
ANALYTICAL CHEMISTRY
Xm+
Figure 2. Isotherms for the exchange equilibria of Naf, Kf,Rb+, and Cs+ with NH4+, Dowex 50W-X8, liquid ammonia
There is evidently strong association between divalent cations and the resin anion groups. Even the considerable association of alkaline earth nitrates in liquid ammonia to form MN03+ and M(NO3h, shown by the conductance measurements of Franklin and Kraus ( I I ) , does not significantly reduce the high selectivity coefficients of calcium and barium. The Effect of ZLL on the Selectivity Coefficient. The family of isotherms observed for the alkali metal cations through rubidium is given in Figure 2. In this case the equivalent fraction of counterion M in the ion exchanger is plotted against the equivalent ionic fraction in the external solution. The diagonal dashed line represents a hypothetical resin showing no preference for one cation over another. In general, published data on the relationship of Ks to in aqueous solution for the exchanges listed in Table I1 indicate that K, is reasonably constant in the range X X from 0.3 to 0.75 (12). For the Ag+/H+and Tl+/H+exchangesthere is an increase and for Na+/H+ a decrease in K , at very high values of TAI.Semiquantitative determinations of the selectivity coefficients for the AgT/NH4+ and Na+/NH?+ exindicate that the two systems again dechanges at high viate in opposite directions, despite the similarity of their selectivity coefficients in this solvent. Some type of variation in K, with is to be anticipated for the model under consideration here since the activity coefficient ratio for ions M+ and B+ may be a different function of RAIin the internal “resin phase solution” than in the external solution. The extent of swelling exhibited by this resin in water or liquid ammonia implies that the concentration of this resin phase solution is on the order of 5M. The experimental K, values were randomly distributed about the ranges reported in Table 11. When this average over the -FIE range is rather small, comparisons with other solvents should be made at similar and total solution concentration. The Effect of Water on Cation Selectivity Coefficients. The effect which the presence of water in the liquid ammonia
xxL
xA1
xaI
xII
(11) E. C . Franklin and C . A. Kraus, Am. Chem. J., 23,292 (1900). (12) 0. D. Bonner and 3. C . Moorefield, J. Phys. Chem.,58, 555 (1954).
solutions has on selectivity was investigated for the Na+/NH4+ and K+/NH4+ exchanges. The values of K, for these exchanges in anhydrous ammonia are constant over a reasonably wide range of XM and different enough to suggest the influence of different factors. Batch equilibrations of the appropriate resins and nitrates were run as described above with measured amounts of water added by means of a syringe after 20-25 ml of ammonia had been condensed. The selectivity coefficients determined for these equilibrations are given in Table 111. Examination of the phase diagram for the ammonia-water system (13)indicates that it would be possible to extend the water content to = 0.7 at -74" C (the temperature of the approximately XHZO acetone-dry ice bath) before freezing takes place. However, at a water content greater than XH20 = 0.35 the solution became so viscous that attainment of equilibrium was unlikely under practical conditions. Increasing the temperature to - 50" C did not substantially affect the viscosity of the mixed solvent. Comparison of the behavior shown in Table I11 with that of methanol-water mixtures is of particular interest: Very small amounts of water greatly increase the selectivity coefficient for the Na+,"+ exchange in methanol ( 2 ) . At X H ~ O = 0.25 the K, value reaches a maximum value five times that observed in pure water. Gable and Strobe1 ( 2 ) found the ratio of the weight per cent of water in the resin to that in the external solutions to be on the order of 5 for 1% solutions of water in methanol. A distribution of about the same magnitude is reported for acetone-water and dioxane-water solutions (14). This would suggest that the K, values observed in methanol-water mixtures can be explained as a type of liquidlliquid partitioning due to a strong concentration of the water into the resin phase when only small quantities of water are present in the system. The solvent/solvent interaction between ammonia and water, apparent from the phase diagram, and a comparison of the swelling data available for these five solvents would lead one to expect that this partitioning effect would not be an important factor for ammonia-water mixtures. In the absence of a salting-out of the less polar solvent from the resin phase one might expect a simple decrease in K, between the pure ammonia and pure water values. An extrapolation of the data in Table I11 would indicate that the pure water value would be reached at approximately XH10 = 0.35 for the Na+/NH4+ exchange and for the K+/NH4' at somewhat beyond the maximum amount of water possible at this temperature. Effect of Temperature on Cation Selectivity Coefficients. In general, selectivity is the result of ion-solvent and ion-ion interactions in the solution and resin phases. These interactions tend to be diminished by a rise in temperature, but the observed effect on selectivity coefficients in aqueous systems is usually quite small, the number of these processes is too great and their effect too varied for much physical significance to be drawn from the results, regardless of model chosen. A temperature dependence similar in magnitude to that seen in water was found for the Na-/NH4+ and K-/ NH4+exchanges in liquid ammonia. K, values for these exchanges were determined in an acetonitrile-dry ice bath which maintained a temperature of -46" =k 1 ' C and in a 5 :1 cyclohexanone-acetone-dry ice bath which maintained a tempera-
(13) I. L. Clifford and E. Hunter, J . Phys. Chem., 37,101 (1933). (14) C . W. Davies and B. D. R. Owen, J . Chem. SOC.,1956,p. 1676.
Table 111. Effect of Mole Fraction of Water on Selectivity Coefficients K,(Na+/NH4+) Z
