Fasih A. Siddiqi

Studies of membrane phenomena: I. Effect of temperature on diffusion of electrolytes through a parchment-supported silver iodide membrane

Summary The diffusion of LiCl, NaCl and KCl through parchment-supported silver iodide membrane has been studied at various temperatures. The diffusion rate is in the order: KCl>NaCl>LiCl. The membrane resistance Rm and the membrane potential Em are found to decrease with increase in temperature. The membrane resistance is in the order: Rm(LiCl)>Rm(NaCl)>Rm(KCl) whereas the membrane potential is in the order: Em(KCl)>Em(NaCl)>Em(LiCl). The results have been discussed in the light of TMS theory and the views of Sollner, Gregor and Eisenman. The diffusion rate and the enthalpy of activation of diffusion have been related to the number of hydration, heats of hydration and other ionic quantities.

Studies with inorganic precipitate membrane: evolution of thermodynamically effective fixed charge density and test of the most recently developed theory of membrane potential based on the principles of non-equilibrium thermodynamics

Abstract Electrical potentials developed across nickel, manganese chromate and cupric iodide membranes using various 1:1 electrolytes are reported. Thermodynamically effective fixed charge density, which is an important parameter governing the membrane phenomena, has been evaluated by the recently developed theory of Nagasawa et al. Most recently developed theories of Toyoshima and Nozaki based on the principles of the irreversible thermodynamics has been examined to predict the bi-ionic potentials developed across the membranes. Theoretical predictions were borne out quite satisfactorily by our experimental results.

Studies with inorganic precipitative membranes: XI. Membrane potential response, characterization and evaluation of effective fixed charge density

Abstract Membrane potentials arising across parchment supported nickel and cobalt phosphate membranes when they separete 1:1 electrolyte solutions of concentration c 1 and c 2 such that c 1 =10 c 2 , have been measured. The membranes in contactwith dilute solutions have been found to carry a negative charge whereas the charge reversal was observed when the membrane was separating concentrated solutions. The membrane potential data have been used according to the procedure prescribed by Teorell-Meyer-Sievers theory (the TMS theory) to derive the value of effective fixed charge density of membranes. The electrical double layer at the membrane-solution interface has been suggested to control the over all rate of diffusion.Abstract Membrane potentials arising across parchment supported nickel and cobalt phosphate membranes when they separete 1:1 electrolyte solutions of concentration c 1 and c 2 such that c 1 =10 c 2 , have been measured. The membranes in contactwith dilute solutions have been found to carry a negative charge whereas the charge reversal was observed when the membrane was separating concentrated solutions. The membrane potential data have been used according to the procedure prescribed by Teorell-Meyer-Sievers theory (the TMS theory) to derive the value of effective fixed charge density of membranes. The electrical double layer at the membrane-solution interface has been suggested to control the over all rate of diffusion.

Studies in the membrane permeability of chromic ferro and ferricyanides

Abstract The permeability of various electrolytes (with a common cation) through parchment-supported chromic ferro- and ferricyanide membranes was investigated, employing the constant flow method. The permeability order was found to be Cl− > Br > NO3− > CNS− > CH3COO− > SO4= > FeCy6=− > FeCy6== in both cases. A considerable decrease in these values was observed when potassium ferro- and ferricyanide treated membranes were used. Adsorption and membrane potential studies revealed an order that was just the reverse of that for permeability. The energy of activation for the diffusion process for various electrolytes was found to lie between 5000 and 6500 calories per mole, values which are higher than those determined for the free diffusion of electrolytes. The role of Hofmeister series has been considered in relation to the peptizing of the membrane material during the diffusion process and such peptizing appears to be one of the factors controlling the permeability phenomenon.

Studies of membrane phenomena: II. Determination of membrane potentials and evaluation of the membrane fixed charge density and permselectivity of parchment-supported silver iodide membrane

Summary Membrane potentials across parchment-supported silver iodide membrane in NH4Cl,KCl,NaCl and LiCl solutions of various concentrations have been determined to evaluate fixed charge density (ω) and permselectivity of the membrane; ω of silver iodide membrane has been found to be −0.01 N. The effect of concentration on transport numbers, apparent anion mobility and permselectivity has been studied. The results have been discussed in the light of the TMS theory.

Studies with model membranes. : XI. Evaluation of thermodynamic parameters for diffusion and fixed charge density by methods based on thermodynamics of irreversible processes

Abstract The various ionic processes occurring in membrane systems notably (i) ionic transport, (ii) membrane potential, (iii) electrical conductivity, and (iv) ionic distribution and the potentials within the membrane have been thoroughly studied with a parchment-supported silver phosphate membrane. By applying the theory of absolute reaction rates, various thermodynamic parameters, namely Δ H ≠ , Δ F ≠ and Δ S ≠ were evaluated. The Δ S ≠ values were found to be negative, indicating that diffusion takes place with partial immobilization in the membrane phase. The effective fixed charge density was also evaluated by methods based on the thermodynamics of irreversible processes.

Studies with inorganic precipitate membranes : XXV. Evaluation of membrane parameters and test of recently developed fixed charge theories for membrane potential

Abstract Effective fixed charge densities of parchment-supported cobalt and nickel phosphate membranes in contact with various 1: 1 electrolyte solutions have been evaluated from membrane potential measurements. The methods used for the estimation of charge densities were: (a) Teorell—Meyer—Sievers; (b) two different methods of Kobatake et al.; and (c) a more recently developed method of Nagasawa et al. The charge densities evaluated from different methods were of the same magnitude. The apparent transference number of coions and permselectivities of the membranes for electrolytes have also been calculated.

Studies with inorganic precipitate membranes: Part XXVI. Evaluation of membrane selectivity from electric potential and conductivity measurements

Biionic and multiionic potentials arising across parchment supported nickel and cobalt phosphate membranes using various combinations of 1:1 electrolytes at different concentrations have been measured. The intramembrane permeability ratio of cations has been derived using the plotting method. Conductivity of membranes in contact with simple 1:1 electrolytes has been experimentally determined in order to evaluate selectivity of membranes using the predetermined values of the intramembrane permeability ratio. The selectivity sequence of both the membranes has been found as K+>Na+>Li+, which on the basis of the Eisenman-Sherry model of membrane selectivity, points towards the weak field strength of the charge groups on the membrane matrix. Perfect Donnan exclusion of coions was realized in the dilute limit of the external solution. Potentiometric selectivity constants Kijpot have also been evaluated by the improved method of Buck et al. based on the integrated form of the Nernst-Planck flux equation.

Studies with inorganic precipitate membranes—XI: Evaluation of the thermodynamically effective fixed charge density by various methods

Abstract Thermodynamically effective fixed charge densities of parchment supported membranes were estimated by methods of (a) Teorell—Meyer—Sievers, (b) Altug and Hair, and (c) the most recent one of Kobatake based on the thermodynamics of irreversible processes. The two limiting forms of Kobatakes equation for dilute and concentrated ranges gave identical values of charge densities. It is interesting to note that these two values of limiting cases are closer to the Teorell—Meyer—Sievers and Altug and Hair values. The theoretical prediction for membrane potential by the Kobatakes equation were borne out quite satisfactorily by experimental results obtained with both the membranes.

Transport studies with model membranes

Abstract Recently developed irreversible thermodynamic theories are used for the evaluation of structural and transport properties of parchment-supported and polystyrene-based precipitate membranes which can serve as a model for biological membranes. The Toyoshima and Nozaki theory of bi-ionic potential, found to be applicable to these model systems, is compared with the latest theory of Tasaka et al. to get a relationship for the evaluation of theoretical values of bi-ionic potentials arising between two uni-univalent electrolyte solutions with different cations separated by a parchment-supported lead phosphate membrane. These theoretical values are found to be very close to the experimental data. The selectivity sequence K + > Na + > Li + , evaluated on the basis of bi-ionic and multi-ionic potential measurements, corresponds to low electrostatic field strength of the charged groups attached to the membrane matrix and indicates that the coulomb interactions are weak in comparison to ion dipole and ion-induced dipole interactions between counterion and fixed sites. The findings on selectivity are explained in terms of thermodynamic activation parameters calculated on the basis of the theory of absolute reaction rates.