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Thermodynamic Equilibrium Constants of Alkali Metal Ion-Hydrogen Ion

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Thermodynamic Equilibrium Constants of Alkali Metal Ion-Hydrogen Ion Indian Journal of Chemistry Vol. 31A,June 1992, pp. 317-322 Thermodynamic equilibrium constants of alkali metal ion-hydrogen ion exchanges and related swelling free energies in perfluorosulphonate ionomer membrane (Nafion-117) in aqueous medium Sita T Iyer, Deoki Nandan & R M Iyer" Chemistry Division, Bhabha Atomic Research Centre, Trombav. Bombay 400 085 Li + IH + , Na + IH t , K + IH + , Rb + IH + and Cs + IH + exchange equilibria on a perfluorosulphonate ex- changer (Nafion-1l7) have been investigated at 0.1 M ionic strength in aqueous medium and thermody- namic equilibrium constants of 0.78 to 12.6 (Li + IH t to Cs + IH + ) have been evaluated and the alkali me- tal selectivity sequence has been found to be Cs + > Rh + > K + > Na + > Li ". The various ionic forms of Nafion-1l7 involved have also been subjected to isopiestic water vapour sorption investigations and the hydration numbers and swelling free energies determined. The exchange selectivities have been found to be consistent with the sequence of ionic hydration and swelling free energies, the exchange free energies showing a linear relationship with the difference in free energies of hydration of the exchanging ions as per Eisenman's model. The expanded selectivity as well as swelling free energy ranges observed in Nafion- 117 compared to Dowex SOW type of resins have been interpreted in terms of hydrative and osmotic swelling behaviours of the two exchangers. Existence of a solvent shared ion pair in Nafion-1l7. i.e. - SO; (H20)Cs + observed in an earlier study has been supported by present selectivity data. Due to its exceptional chemical inertness and fa- have, however, emphasized that compared to PSS- vourable electrical properties, Nafion l-4 (a novel DYB exchangers, Nafion-120 exhibits greater selec- perfluorinated ion exchanger) membrane has been tivity spread for alkali metal ions which could be at- exploited for a number of chemical and electro- tributed to the cluster morphology of Nafion and chemical applications" including its use as a separa- the lower charge density on - SO; compared to tor in chlor-alkali cells. This ionomer of fluorocar- that in PSS-DYB. The authors supported these ob- bon backbone (structure I) has pendant side chains servations through satisfactory column chromatog- terminating with - S03 - H + exchange groups (m is raphic separation of alkali metals using powdered nearly unity and n varies between 5 to 11 thus gen- Nafion-120. erating an equivalent weight EW of 1000-1500). Al- In the recent past, a study from this laboratory' on. though, conventional polystyrene-DVB sulphonate the isopiestic water sorption isotherms of H + , Li + (PSS-DYB) exchangers+ such as Dowex 50, Am- and CST forms of Nafion-117 (EW -1100) mem- bcrlite IR-120 etc., too possess the same exchange brane has revealed larger swelling free energy group and have been thoroughly studied for their changes and swelling pressures generated from Naf- ionic selectivity behaviour in aqueous medium, ion-117 -water interactions involving H + and Li + there is only one study reported! (on the alkali metal forms. The hydration numbers (n +) for H +, Li + and alkaline earth ions versus H+) for Nation-120 were also deduced to be larger (6.0, 5.5 respect- (EW - 1200) and even this study reports selectivity ively) compared to 3.8 and 3.0 for PSS-DYB resins coefficients (Kc) at 50% exchanger loading (also no (hydration number for - SO; group was consid- attempt was made to incorporate solution phase ac- ered to be same, 1.0)3.Again, contrary to PSS-DYB tivity coefficients to obtain equilibrium constants (Cs "), Nafion-117 (Cs ") revealed the presence of which represent the true selectivity). The -authors? solvent shared ion pair [- SO; (H20)Cs + ]. Another investigation relating to deuteriumlhydrogen frac- tionation effects" revealed lesser hydrogen bonded (CFZCF2)n CFZCF- structure of water in Nafion-117. These data relating I to Nafion-117 membrane may indeed support the (OCFZCF) OCF CF -SO-Hi" I m Z 2 3 possibility of larger selectivity spread for alkali me- tals (and particularly high selectivity for Cs + in Naf- CF3 (I) ion-117 which can be a very useful consideration) 31H INDIAN J CHEM, SEe. A, JUNE Ig92 but suffer from the following (i) no water sorption is- 1L otherms are available for Na +, K + and Rb " forms (ii) no selectivity data are available concerning alkali metal - H + exchanges on Nafion-117. As the total water sorption data sct obtained for H + and alkali ion forms of Nafion-117 is apparently different from that for Nafion-120, the ionic selectivities in the two ionomers are also likely to be different. Whether a greater spread of selectivity also exists for Nafion- 117 thus remains to be explored. In view of above, ion exchange equilibria in aque- ous medium involving Li + IH+, Na + /H+, K +/H+, Rb ' IH+ and Cs ' IH+ exchanges on Nafion-117 have been investigated in the present work. With the objective of getting a deeper insight into selectivi- ties, isopiestic water sorption isotherms have also been determined for Na +, K + and Rb + forms while similar isotherms for H +,Li + and Cs + forms have been reobtained under identical conditions, extend- ing them to lower water activity (aw) regions. Materials and Methods Membranes- H +, Li + and Cs + forms were gen- erated as described earlier:'. Na ", K+ and Rb ' forms of Nafion-117 were generated from the H+ form using 0.5 M solutions (aq) of NaOH, KOH and RbCI following standard procedures. The mem- branes were stored in air dried (AD) forms. Ion ex- change capacities obtained on the totally dry basis are: H+ (0.94); Li+ (0.937); Na+ (0.915); K+ (0.900); Rb+ (0.870); Cs ' (0.845) meq/g. Fig. 1- Isopiestic water sorption isotherms for various ionic forms of Nafion-117 membrane (298 K) Isopiesticstudies- Water sorption as a function of water activity was investigated using two similar iso- piestic units fabricated earlier+? and aqueous elec- Table 1- Ionic hydration and swelling free energy data for trolyte solutions? of LiCI and H2S04, Sorption iso- Nafion-117 membranes (298 K) therms of various ionic forms of Nafion-117 were Ionic n -!!J.G,w obtained as n; (moles of water/equiv) versus a, ~ T n+ form (ala\\:= I) (integral) curves. Dry weights of the AD membranes for ob- kJ/mol taining n, were computed using another set of var- ious ionic forms which were heated in a vacuum ov- H+ 14.2 7,0 6.0 45,0 en at 413 K and the moisture contents determined. (3,8)* Ion exchange equilibria-Nearly 0.1 M solutions Li+ 13.6 6,5 5.5 36,1 of HCI, LiCI, NaCl, KCI, RbCl and CsCI were em- (3.0)* ployed, Batch method was used to investigate alkali 7.4 2.5 1.5 14.4 metal - H + exchange equilibria using approximate- Na+ ly 0.5 g each of membrane pieces (AD) and a total of (2.5)* 25 ml solution (HCI + MCI, M == alkali metal) in K+ 7.1 2.5 1.5 14.0 each erlenmeyer allowing 48 hrs for equilibrium at- (2.3)* tainment. Equilibrium concentrations of H + and Rb+ 4.1 1.3 0.3 7.3 M + ions in the outer solution were determined titri- Cs+ 3.2 1.0 0.0 6.1 metrically (NaOH, 0.05 M), and flame photometri- (2.0)* cally using Varian Techtron AN) model (Li, 670.8 nm; Na, 589.0 nm; K, 766.5 nm; Rb, 780.0 nm and *Values are for Dowex 50 resins (ref. 3,9). Cs, 852.1 nm) respectively. Quantities of H+ and IYER ('I III.: EXCHANGE EQUILIBRIA OF ALKALI IONS-HYDROGEN ION ON NAFION-117 31 » SO~----------------------------~ Results Water sorption isotherms for H + and alkali metal _-----H+ ionic forms of Nafion-l17 are shown in Fig. 1. As can be seen, the ionic sequence of water sorption at any water activity is H+ >Li+ >Na+ >K+ >Rb+ > Cs + (Na + and K + forms isotherms are identical- L, + 35 upto a, - 0.9). Slight differences in the isotherms of H+ , Li + and Cs + can also be observed on compar- )0 ison with those reported earlier" though their shapes remain essentially same. Differential swelling -0 E free energy (~Gsw) plots constructed from the data ~ are shown in Fig. 2, as computed using the relation- ship". 20 ~ VI 1'" <l 15 + + ~Gsw= -RTf'" nwdlnaw+nwRTlnaw ... (4) I 'l a •K aw = 0 10 FoUowing the arguments and methodology adv- anced earlier", total hydration numbers, nT (Table 1) derived from Fig. 2 yielded cationic hydration numbers (assuming hydration number for -- S03 group as 1.0p of 6.0, 5.5, 1.5, 1.5, 0.3 and 0.0 re- " spectively for H+, u-, Na+, K+, Rb+ and Cs+ ions. A comparison with literature values of similar Fig. 2-Differential swelling free energy (6 (;,w) versus n; plots hydration numbers in PSS-DVB exchangers+? for various ionic forms of Nafion-1l7 (298 K) shows (i) larger hydration of H + and Li + in Naf- ion-I17 (ii) lower hydration for remaining alkali me- M + in the membrane phase at equilibrium were tal ions in Nafion-l17. Thus, compared to a narrow then obtained from the total quantities added in the range of n + (3.H-2.0) for H + to Cs + in PSS-DVB. a erlenmeyer solution and the ion exchange capacities much larger range of 6.0-0.0 is obtained in Nafion- of membrane pieces. Selectivity coefficients (Kc) for 117.Also as can be seen, Cs + form exhibits n- value the exchange reaction (M + stands for alkali metal of unity thereby confirming the presence of solvent ion): separated ion pairs in Cs +-membrane.
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