Determination of diffusion coefficients of mixed electrolytes in BDH- Cation and anion exchange membranes

Abstract

Interdiffusion coefficients D-ij of various mixed electrolytes in BDH cation and anion exchange membranes under zero current and at constant pressure and temperature under zero current and at constant pressure and temperature were determined. The composition of external mixed electrolyte under study was varied from 0.02M to 0.1 M with respect to one ion. The total counter ion and co ion concentration in compartment 1 and 11 was kept at 0.1M. The study was repeated with electrolyte system with co ion and counter ion concentration of 0.5M.

The systems studied were MX, NH4X/CM/NH4X and MX2, MgX2/CM/MgX2 I II I II

Where M = Na+, K+, Ca2+ X =Cl- CM = cation membrane

For the anion exchange membrane the systems studied were, MCL, MNO3/AM/MNO3 I II

and MCL, M2SO4/AM/M2SO4 where M = Na+, K+ I II And AM is anion exchange membrane.

The principles of irreversible thermodynamics was employed on the treatment of the transport processes to derive an expression for D-ij. Flame photometry was used to determine Na+, and K+ ion concentrations in compartment II. Tritrimetry was used to determine Ca2+ and Cl- ions concentrations in the compartment.

D-ij increased with stirring and co-ion size, it decreased with counter-ion size and decreased with increase in concentration of external electrolyte solution. The observations are discussed in terms of the effects of the Nernst - diffusion layer, electrostatic interactions of the ions with the membrane matrix and the shrinking properties of the membrane. D-ij was found to be directly proportional to the reciprocal of the external concentration in the range 0.02M to 0.1M.

The mobility of Na+, K+ and Ca2+ ions in the membrane at 0.1M total counter-ion concentration in compartment I and II at equilibrium constant Km =1.03 was found to be 6.48 x 10-4cm S-1, 5.96 x 10-4cm S-1 and 3.66 x 10-4cm S-1 respectively. At 0.5M total counter-ion concentration in compartment I and II mobilities were 9.60 x 10-4cm S-1, 7.62 x 10-4cm S-1 and 0.96 x 10-4cm S-1 respectively. The mobility of Cl-1 ion in the membranes at 0.1M total counter-ion and co-ion concentration in compartment I and II was found to be 7.84 x 10-4cm S-1, 8.29 x 10-4cm S-1 and 7.68 x 10-4cm S-1 taking the equilibrium constant (Km) as 2.5.

At 0.5M total counter-ion and co-ion concentration in compartment I and II the mobility of Cl-1 ion in the membrane was 2.65 x 10-4cm S-1, 3.30 x 10-4cm S-1 and 5.70 x 10-4cm S-1 respectively.

The high mobility of the ions in the membrane than the ionic mobility in aqueous solution has been discussed in terms of the equilibrium constant KM. The equilibrium constant KM was taken as 1.03 for cation exchange membrane. For the anion exchange membrane it was taken as 2.5. The low ionic mobility in the membrane have been discussed as due to electrostatic interaction of the ions with the membrane matrix. For cation exchange membrane there was much retention of the slow ion by the membrane at 0.1 moles 1-1 total counter-ion concentration range. At higher concentration range (0.5M) the retention of the slow ion in the membrane was less. But for the anion exchange membrane the retention of the slow ion in the membrane was almost the same for the two concentration ranges that were studied.