External Electrolyte Research Articles

THERMODYNAMICS OF K–Ca ION EXCHANGE IN SOILS1

SummaryCation exchange characteristics of the K:Ca saturated forms of five soils were measured at 25°C and 50°C. The rates of isotopic exchange of 42K and 45Ca were too fast to be measured except that of 42K in the K:Ca Harwell soil at 25°C. The slower isotopic exchange of K in this soil was attributed to the presence of a zeolite, clinoptilolite. The intra‐particle diffusion coefficient, Di, of K in this soil increased with K‐saturation to a maximum at about 40 per cent K, probably because of the ‘blocking’ action of the larger hydrated Ca ions at small K‐saturations in clinoptilolite.The CEC, measured by isotopic exchange along the K:Ca adsorption isotherm, decreased with increasing temperature probably because some interlayer spaces collapsed.The standard free energy, enthalpy, and entropy changes were negative for the reaction Ca‐soil+2K+⇌ 2K‐soil+Ca++. These results seem to show that K is more strongly bound than Ca by the soil and that the Ca‐preference shown by the isotherm at small external electrolyte concentration is caused by entropy changes in solution.Calculated activity coefficients of the exchangeable ions changed with K‐saturation similarly at both temperatures but values at 50°C were smaller than at 25°C.

Studies of membrane phenomena. Part 3.—Electric resistance of membranes

A theory was developed for the electric resistance r of a charged membrane immersed in aqueous solutions of a uni-uni-valent electrolyte, on the basis of appropriate assumptions for the activities and mobilities of small ions in the membrane. Thus the “additivity” rule, known for dilute polyelectrolyte solutions containing an added electrolyte, was assumed for the single-ion activities. Adequate linear plots were derived by which to evaluate three basic parameters contained in the theory from experimental data for r and the equivalent conductivity of the external solution. Such data were obtained with oxidized collodion membranes of different charge densities in solutions of KCl, LiCl and KIO3 at 30°C, and the values of the three parameters for each pair of membrane and electrolyte were determined. From these results it was found that the thermodynamically “effective” concentration and the hydrodynamically “effective” one of the counterions dissociated from the polyelectrolyte constituting the membrane were nearly independent of electrolyte species for a fixed membrane and, moreover, their ratio was almost equal to unity for membranes of different charge densities. Comparison of the calculated and observed values of r showed that the present theory applies except at very low concentrations of the external electrolyte solution.

Interpretation of Electrolyte Invasion Data for Ion-exchange Polymers

IN recent years two quite independent theories have been developed to account for the observed electrolyte invasion properties of ion-exchange polymers. Scatchard and Freeman1 suggest that in the application of the Donnan law (1), deviations are best interpreted in terms of the mean activity coefficient of the electrolyte in the exchanger phase, ɣ±. where νc is the number of co-ions in a molecule of electrolyte; νg is the number of counter or gegen ions in a molecule of electrolyte, ν = νc + vg; M is the observed counter-ion concentration; m, the concentration of external electrolyte; m, the concentration of imbibed electrolyte; k, a constant; ɣ ± and γ ± are the mean internal and external activity coefficients of the electrolyte, respectively. In accordance with the theory of specific ion interactions2, an expression has been derived (2), in which a and b are constants, to account for variations in ɣ± for ion-exchangers based on styrene-divinylbenzene copolymers. This relationship has also been applied to polymethacrylic acid ion-exchangers3. It is assumed in this treatment that any deviations resulting from non-uniformity of the exchanger matrix are either negligible or can be eliminated by a small correction to the observed results.

Ion and water transference across “untreated” collodion membranes

Application of pressure to control membrane movement in an electrolysis experiment involving changes in volume of anode and cathode compartments is described. By this method, the transference number of water has been measured as a function of external electrolyte concentration. Ion transference also has been measured by the Hittorf's method as functions of both current density and external concentration. The results have been discussed from the standpoint of an existing membrane model.

Electrochemical properties of thin parlodion membranes

AbstractThin membranes (50‐800 A. thick) formed by spreading a solution of Parlodion in isoamyl acetate on the air‐water interface have been characterized. The electrical resistance and capacitance of these membranes prepared from each of the three Parlodion concentrations viz., 5, 10, and 20 mg./ml. solution, have been measured as functions of both membrane thickness and external electrolyte (KCI) concentration. Capacitance values corroborate the thickness inferred from the weight of material transferred to cover a fixed area. Membrane resistance drops with increase in thickness. Resistivity of a membrane 100 A. thick rises with decrease in external electrolyte concentration. Acid and alkaline solutions of KCI lower membrane resistance due to increased porosity of the membrane brought about possibly by the hydrolysis of the membrane material. Membrane potentials arising across these membranes also have been measured as functions of both membrane thickness and external electrolyte concentration. The bi‐ionic potentials confirmed the usual order of penetrability of alkali metal ions observed in artificial membranes. A study of the effect of pH on membrane potentials indicates that the electrochemical activity of the membrane may be attributed to the presence of odd and terminal carboxyl groups in the nitrocellulose matrix of the membrane.

Measurement of membrane potentials and test of theories

AbstractThe emf's of the system Ag, AgCl/NaCl (mI)/membrane/NaCl(mII)/AgCl, Ag containing phenol–formaldehyde sulfonate membrane have been measured at 30°C. The membrane phase has been analyzed for its electrolyte and water contents and intramembrane activity coefficients have been derived as a function of external electrolyte concentration. Transference numbers of Na+ counterion and water have been measured as functions of both current density and external concentration. The data have been used to test the principal theories of membrane potential. The fixed‐charge theory of Teorell and Meyer and Sievers did not give satisfactory agreement between observed and calculated emf's of membrane cells, whereas the Scatchard theory did.

Structural analysis of ion semi-permeable membranes by co-ion uptake and diffusion studies

Three different strongly basic exchangers with similar co-ion uptake behavior have very different co-ion diffusion characteristics. Not only are there differences in the order of magnitude of the co-ion permeabilities P at the same external electrolyte concentration c, but also marked differences in the shape of the P-c curves, which can be interpreted in terms of the internal micro- structure of the (inhomogeneous)membrand. Other structural details can be elucidated from the fact that polyvalent co-ions are more strongly excluded from local regions of high counterion concentration than are monovalent co-ions, and so lead to different P-c relations. Furthermore there was observed, especially for polyvalent ions, a great difference between the mechanisms of steady-state through-diffusion and of transient diffusion out of a saturated membrane, which also permits interpretation in terms of internal structure. The details obtained by the mathematical analysis of the diffusion data agree with, and go considerably beyond, what can be ascertained by electron microscopy. (auth)