Electrochemical studies on Dowex-50 membrane using sodium chloride and urea solutions having variable composition

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1 Indian Journal of Chemistry Vol. 41A, March 2002, pp Electrochemical studies on Dowex-50 membrane using sodium chloride and urea solutions having variable composition Kehar Singh*, A K Tiwari & Chandra Shekhar Dwivedi Chemistry Department, DDU Gorakhpur University, Gorakhpur , Indi a Received 21 November 2000; revised 16 October 2001 Membrane potential and membrane conductance measurements have been carried out using Dowex-50 - Kynar membrane, sodium chloride and urea mixtures of variable compositions to examine the effect of non-electrolytic constituent of the mixture on electrochemical character of the membrane. A significant change in membrane permselectivity is observed in the presence of urea due to changed co-ion exclusion because of the accompanying alteration in the swollen state of the membrane matrix. Information in regard to the nature and extent of permselectivity, a membrane is endowed with, can be had on the basis of membrane potential measurements l -4. Bi-ionic membrane studies in some situations can also be used with advantage for electrochemical characterization of the membrane Alteration in membrane permselectivity with nature and concentration of electrolytes can be understood, in substantial measure, in terms of swelling/deswelling of the membrane network as a result of which coion exclusion is affected. The possibility of characterization of membranes when electrolyte mixtures are used has recently been looked int0 7. Effect of the presence of a nonelectrolyte alongwith an electrolyte on the permselective nature of an ion exchange membrane is also of interest. We have carried out electrochemical characterization of a cation selective Dowex-SO (sulphonated divinyl styrene benzene) membrane on the basis of membrane potential and membrane conductance measurements using sodium chloride and urea solutions of variable compositions. These studies show that the presence of urea indeed alters the electrochemical character of the membrane. Attempt has been made to explain this observed difference in electrochemical activity of the membrane when sodium chloride solutions are used with and without urea. Materials and Methods Membrane preparation was carried out as reported earlier 8. Dowex-SO (J T Baker Chemical Co., Phillipsburg, NJ) having ion exchange capacity of 6.S meq/gm was dispersed in solution of Kynar (polyvinylidene fluoride) in N,N-dimethylacetamide. The slurry thus formed was spread on a clean dried glass plate; and the glass plate was kept in an electric oven at C for half an hour to remove the solvent. It was then immersed in distilled water to detach the membrane. A piece of the prepared membrane was fixed in a glass cell and equilibrated with 1M solution of sodium chloride and kept in the experimental solution of sodium chloride overnight. Solutions were renewed before measurements were started. Similar procedure was adopted with the mixed solutions having different compositions with respect to sodium chloride and urea. The set-up used for the membrane potential measurements may be represented as, I SCE NaCI(C,) NaCI(C,) SCE Urea (C) Urea (C) S8 M S8 SB, M and SCE represents KCl salt bridge, Dowex-SO membrane and saturated calomel electrode respectively. Membrane potential developed across the membrane was measured by a digital multi meter (Phillips, PM 2S18). Conductance measurements were carried out by applying potential difference across the membrane using saturated calomel electrodes. Currents corresponding to the applied potential were measured with a digital multi meter (HIL 2161) using silver-silver chloride electrodes. Conductance values were derived from current-potential plots.

2 SINGH e/ al. : ELECTROCHEMICAL STUDIES ON BOWEX-SO MEMBRANE 479 Results and Discussion To obtain information about permselective behaviour of a membrane, a knowledge of transport number of the counter ions in the membrane phase is required. Since permselectivity is a measure of the ease with which counter ion migration occurs through the membrane, it is defined as 9. strength of the electrolyte and A is a constant. Membrane potential data are summarized in Table 1 alongwith liquid junction potential values estimated using the relationship 1 1,... (4)... (1) in the case of a cation selective membrane 4 denotes transport number of the cation in the membrane phase and 4, is the transport number in the solution. For obtaining t+ values, we used the relationshiplo... (2) (t1q, )1=0 denotes experimentally measured membrane potential and al and a2 are activities of sodium chloride solutions which communicate through the, membrane. Activity coefficient values needed for the estimation of activities were derived using the relationship,... (3) where f = activity coefficient of the electrolyte; Z+ & Z. = ionic charges of the corresponding ions; I = ionic 4 is a measure of the fraction of the current carried by the cation and was estimated using the relationship l2,... (5) Ci, Ai and Zi are the concentration, equivalent conductance and charge of the ith ion respectively. A; values were obtained using the equation 13,... (6) A~ denotes equivalent conductance of the ion at infinite dilution and Cs is the concentration of the solution and Ai is a constant. Values of constant Ai for Na+ and cr are and If the membrane were non-selective, membrane potentials and liquid junction potentials should have been comparable. Conspicuous difference between the values of these potentials clearly shows that the membrane is indeed endowed with ion selectivity. In fact, the membrane is cation selective in nature. It Table I-Liquid junction potential and membrane potential values Electrolyte Composition [(t.q,),=oll (t.q,),=0 S. No. Side I Side II (my) (my) (A) S 18.2 (B) om S D S (C) D S (D) om S S S

3 480 INDIAN 1 CHEM, SEC A, MARCH 2002 may be emphasised here that a membrane prepared using a cation exchange material will not necessarily exibit cation selectivity. If the membrane is macroporous, it will not be able to differentiate between ions carrying opposite charges and both cations and anions will move with equal ease and will be endowed with transport numbers comparable to those of the ions in the solution. In the present case, membrane pore size is thus conducive for the membrane to exhibit ion selectivity. A representative plot demonse"ating the validity of Eg. (2) is shown in Fig. 1. The 4- values derived from the slopes of such plots are included in Table 2 alogwith permselectivity values obtained using Eq. (1). Permselectivity is seen to decrease with increase in concentration. This is in conformity with the expectation based on increased deswelling of the ion exchange membrane resulting in progressively lowered coion exclusion with increase in concentration. A distinct change in electrochemical character of the membrane occurs when urea is present alongwith sodium chloride in the solution. The experimental > 30 E o " data presented in Fig. 2 depicts variation of the membrane potential with concentration of urea when sodium chloride solution having identical difference of concentration are separated by the membrane. Mean concentration has also been kept fixed to obviate any alteration in the character of the membrane. The results clearly show that the electrochemical character of the membrane undergoes perceptible change in the presence of urea. While estimating permselectivities, same value of t+ has been used even when urea is present in the sodium chloride solution. This is justified in view of comparable values of specific conductance [4.99±O.03xlO- 3 ohm-' cm-' ] showing clearly that ionic mobilities are not affected in the concentration range of urea used in the present investigation. It may also be noted that even when viscosity of an electrolyte changes at relatively high concentration of non-electrolyte, transport numbers of Na+ and cr ions do not exhibit any change'4. Alteration in the electrochemical character of an ion-exchange membrane with change in concentration principally arises because of a definite possibility of change in coion exclusion due to expansion or contraction of the membrane matrix resulting in swelling or deswelling of the membrane. An ionexchange membrane undergoes sweiling because of the osmotic intake of the solvent by the membrane network. This osmotic action depends on the solute concentration; it decreases with increase in concentration'5 as a result of which solvent uptake by the membrane matrix decreases leading to des welling , > 30 E 0.!! 25 ~ "6- < " ' [Urea],104 Fig. 1-(6$)1=0 VS log~ plots for the varification of Eq. (2).(0) a M NaCI; (e) 0.05 M NaCI M Urea. Fig. 2-Variation of the membrane potentials with concentration of Urea. Mean concentration = 0.05 M Sodium Chloride in all cases. -e- ~C = 0.08 M; -0- ~C = 0.06 M; --~- ~C = 0.04 M; - x- ~C = 0.02 M.

4 SINGH et al.: ELECfROCHEMICAL STUDIES ON BOWEX-50 MEMBRANE 481 of the membrane. Because of this the membrane openness increases accompanied by lowered exclusion of coions. Permselectivity of the membrane therefore decreases. Thus, even when electrolyte concentration remains unchanged, a reduction in membrane permselectivity is expected with increase in concentration of the solution with respect to urea. The results presented in Table 2 are indeed in conformity with this expectation. The membrane permselectivity is seen to decrease with increase in urea concentration even though electrolyte concentration is not changed. Membrane potential studies were also carried out using fixed urea concentration at 0.02 M and diffrent mean concentrations of sodium chloride solution, keeping same difference in concentration of the sodium chloride solutions. These values are included in Table 3 alongwith 4 values derived using Eq. (2). Increase in concentration results in decrease in the magnitude of the counter ion transport number as expected because of reduced swelling of the membrane matrix. Observed increase in membrane conductance with increase in concentration (Table 4) also support the above inference. The membrane conductance values derived on the basis of examination of the current voltage behaviour of the membrane are included in Fig. 3. The results presented in this study show that electrochemical activity of an ion-exchange membrane to a considerable extent depends on the nature and composition of the electrolytic environment in which the membrane is kept. The presence of even non-electrolytes alters the electrochemical character of the ion exchange Table 2-t + and P s values in the presence of Urea. S. No. Urea concentration P t+ s (A) (B) (C) (D) Mean concentration of sodium chloride = 0.05 M NaCI </>,mV Fig. 3--Current-voltage behaviour for conductance determination. (.) 0.02 M Urea M NaCl; (_) 0.02 M Urea M NaCl; (0) 0.02 M Urea M NaCI Table 3-Liquid junction and membrane potential values at different mean concentrations of NaCI and fixed concentration of urea. Electrolyte concentration [(~CP)I=olL (~CP)I=O S. No. Side I Side II (mv) (mv) Urea + NaCl (E) om (F) (G) om

5 482 INDIAN 1 CHEM. SEC A. MARCH 2002 Table 4-"i+ and P s values at different mean concentrations of NaCI S. Mean P t + s (I1L'1<p) X 10 5 No. concentration (mho) of NaCl (E) (F) (G) Concentration of Urea = 0.02 M membrane because of a distinct possibility of reduced coion exclusion as a result of the accompanying deswelling of the membrane matrix. Acknowledgement We are thankful to the Head, Chemistry Department, DDU Gorakhpur University, Gorakhpur (UP), India for allowing us laboratory facilities. Financial assistance from the UGC, New Delhi is also gratefully acknowledged. References I Helferich F, lon-exchange (McGraw Hill, New York) Noble R D & Stern S A. Membrane separations technology (Elsevier, New York) Lakshminarayanaiah N, Transport phenomenon in membranes (Academic Press, New York) Jensen J B, Sorensen T S, Malmgren-Hansen B & Sloth P, J Colloid IlIler Sci. 108 (1985) Singh K & Tiwari A K, Indian J Chem, 34A (1995) Singh K, Tiwari A K & Mishra N, J Indian chem Soc, 75 (1998) Singh K, Tiwari A K & Dwivedi C S. Indian J Chem, 39A (2000) Singh K & Tiwari A K. J. Colloid Inter Sci, 116 (1987) Helferich F, lon-exchange (McGraw Hill, New York) Glasstone S, An introduction to electrochemistry (Affiliated East-West Press, New Delhi) 1974, 122. II Singh K & Tiwari A K, J Memb Sci, 34 ( 1987) Glasstone S, An introduction to electrochemistry (Affiliated East West Press, New Delhi) 1971, Glasstone S, An introduction to electrochemistry (Affiliated East West Press. New Delhi) 1971, Robinson R A & Stokes R H, Electrolyte solutions (Butterworth Scientific Publications, London) 1959, Kesting R E, Synthetic polymeric membrane (McGraw Hill Book Company, New Delhi) Ch. 7, (1971) 183.