Tormod Førland

Membrane transference numbers from a new emf method

Abstract A new method for determination of the transference numbers of two ions in ion exchange membrances has been established. The method uses emf measurements in a cell with a membrane stack, and it avoids problems of concentration-polarization, diffusion and water transport. Stack thickness is about 10 mm. The method needs less time and is more precise than a corresponding Hittorf method. The method has been developed for a cation exchange membrane, with cation sites M−, in equilibrium with aqueous solutions of two electrolytes, first HCl and KCl, next HCl and NaCl. Transference numbers for both systems are reported with a precision of ±1%. The results imply that the ratios of the ionic mobilities, ui/uH+, for i=K+ and Na+ in the membrane are constant as the mole fractions vary.

Entropy production by heat, mass, charge transfer and specific chemical reactions

Abstract By using only well-defined measurable thermodynamic quantities, the entropy production by heat, mass and charge transfer in a discontinuous and a continuous system is derived. The derivation follows the principle that thermodynamic derivations should in general be carried out as far as possible without the introduction of any kind of model concerning structural units (ions)[1–3]. This is in contrast to the conventional thermodynamic treatment of electrolytes. The present description has fluxes and forces that depend on the choice of electrodes. It is, however, shown that the local dissipated energy does not have this dependency. The components of the system are in accordance with the Gibbs phase rule. Corrections in electric forces due to the use of different electrodes at different temperature or pressure are discussed. Chemical reactions with zero affinity are treated, and the complete coupling between mass transport and a chemical reaction is discussed.

Application of the activity concept in the physical chemistry of slags

In metallurgical literature, single ion activity coefficients are often used erroneously. The present analytical study deals with the introduction of the activity concept in the treatment of slag equilibria. It leads to the conclusion that the use of single ion activity coefficients is confusing. They may be used only in an analogous manner as in dilute aqueous solutions of electrolytes,i.e. when there is a large excess of one component which can be considered as the solvent. In this case, by convention, the activity coefficients are expressed in terms of the ideal dilute solution as standard state (Henrian activities). By studying systems (concentrated solutions) with all components having as standard states their pure liquid compounds (Raoultian activities), the use of single ion activities and single ion activity coefficients lead to obvious contradictions and should be avoided.

Irreversible thermodynamic treatment of frost heave

Abstract The coupled transport of mass and heat which occurs in frost heave, is described by irreversible thermodynamics. The approach is macroscopic phenomenological and thus it correlated macroscopic measurable parameters. It avoids as far as possible details about mechanism and is therefore complementary to derivations by, e.g., Everett (1961) based on microscopic models. A new derivation is given for the coupling between the transport of liquid water and the transport of heat from 0°C to the ice lens. Assumption of local equilibrium in this region is not necessary. The temperature difference over the region causes a water flux, which in turn causes a build-up of pressure gradients.

CATION EXCHANGE MEMBRANES AS SOLID SOLUTIONS

Abstract The ion exchange equilibrium between KCl and SrCl2 solutions and the cation selective membrane CR61 AZL386 has been examined. Equilibrium isotherms are measured for membrane—electrolyte equilibria for two total concentrations of Cl−in the electrolyte solution and at three different temperatures. The results show that Sr2+ is strongly preferred to K+ in the membrane. Selectivity increases with temperature in the range 10°C to 40°C. It decreases with increasing total concentration from 0.01 N to 0.03 N. Electrolyte absorption in the membrane was not detected for the experimental conditions used. Contents of water in the membrane is constant for external salt concentrations between 3 x 10−8 N and 3 x 10−2 N. The difference in water content between the pure K form and the pure Sr form is approximately 5%. A thermodynamic treatment of ion exchange is presented. The thermodynamic equilibrium constant for the exchange reaction 2 KR + SrCl2 = SrR2+ 2 KCl has been determined by two different integration procedures, R being an ion exchange site in the membrane. The variations of the thermodynamic functions ΔGmix, ΔHmix and ΔSmix are calculated from experimental data for an assumed mixing process of membranes in the pure forms. Activity coefficients for the membrane components are calculated from experimental data. Results are compared to two lattice models. A model with a random distribution of K+, Sr2+ and a vacant site seems preferred at low concentration of Sr2+. At high concentrations, Sr2+ and the vacant site associate.

Small contributions to emf from changes in electrostatic energy potentials

The emf of an electrochemical cell is interpreted by use of measurable local changes in Gibbs energy across the cell. It is shown by calculation that the contribution to emf from changes in electrostatic potentials is negligible for most conditions. The contribution is in principle measurable. This way of explaining contributions to emf is compared to the traditional interpretation in terms of sums of electrostatic potentials.