Precipitation Membranes. I. The “Conditioned” state

Membranes which are generated from two ions of opposite charges by diffusion-controlled precipitation and which can act as selective ion barriers, are called “Precipitation Membranes.” The “conditioned state” of the membrane is responsible for the barrier effect. A prerequisite for the existence of this state is that certain minimum concentrations of the two ions, from which the precipitate is formed, be present in the adjoining solutions. The ion barrier is an electrical barrier formed by a positive and a negative equipotential plane within the membrane which are caused by adsorbed surplus ions on two precipitate layers. Conditioning experiments are carried out with BaSO4-cellophane membranes, the system studied particularly in this and the preceding papers. By studying the time evolution of the membrane potential, one gains insight into the process of “conditioning.” A rather sudden rise of the curve indicates the spontaneous creation of the ion barrier. Obviously, this event is a cooperative phenomenon of a statistical character. The counter diffusion of the two reagents involved plays an essential role

Strom-Spannungs-Messungen an der BaSO4-cellophan-Membran

Strom-Spannungs-Messungen sind sehr geeignet für das Studium der Permeabilität der BaSO4-Membran für Ionen. Mit verschiedenen Kombinationen von Lösungen erhält man charakteristische i/E-Linien. In geeigneter Versuchsanordnung setzt die BaSO4-Membran Konzentrations-energie (vorzugsweise von H+- und OH−-Ionen) direkt in elektrische Energie um. Diese Energietransformation ist, im Prinzip, thermodynamisch reversibel. Man kann mit Hilfe der Strom-Spannungs-Messungen die Permeation einer einzelnen Ionenart messen. Die Permeabilität der Membran für eine Ionenart (z.B. K+) kann weitgehend modifiziert werden. Ihre Permeabilitäten für verschiedene Ionenarten sind sehr verschieden

Precipitation membranes: III. Reversible changes of membrane properties induced by alterations in ionic concentrations

The conditioned state of a precipitation membrane with its particular properties exists within a limited range of membrane potentials and requires certain minimum concentrations, C lim, of the generating ions in the adjoining solutions. We investigated these quantities for the BaSO4 cellophane membrane and found C lim to be 10×10−5N (0.5×10−4M), equally for Ba++ and SO−−4. Beyond these limits, the membrane becomes deconditioned. This transformation is a reversible process provided the limits have not been surpassed too far. The capability for de- and reconditioning is a characteristic and unique property of precipitation membranes, not found in other membrane systems. The phenomenon is explained by the adsorption theory for precipitation membranes. It allows wide modifications and quick variations of the electrical properties and permeability of the membrane in an easy and reversible manner

Precipitation membrane effects in biologic membranes: The role of calcium

Biologic membranes display rectification of electrical current, as well as other properties, in many respects, similar to precipitation membranes. The experiments reported here, performed in frog skin, show that these characteristics are dependent upon the presence of calcium. Upon elimination of calcium from the bathing solution, the property of electrical rectification is lost, the current-voltage relation assuming a linear form. Read-ministration of calcium brings about complete recovery of the rectification pattern. This behavior is analogous to chemical deconditioning of precipitation membranes. Our findings support the assumption that the binding of calcium in biologic membranes produces electrical effects characteristic of precipitation membranes

Precipitation membranes. II. Experiments on the electrochemical deconditioning of BaSO₄ membranes

Precipitation membranes, like biological membranes, are specifically impermeable to certain ions. This property can easily be removed (deconditioning of the membrane) and restored again (reconditioning). The assumption is that special properties of the membrane are due to ion charges, adsorbed on precipitate layers. In this paper we study the deconditioning of the BaSO4 precipitation membrane by an imposed electrical field. To achieve this electrical deconditioning, a threshold potential across the membrane has to be surpassed during a certain minimum of time. If this is done, the membrane potential of the conditioned precipitation membrane is lowered to the sum of the liquid junction potentials in the system in absence of an ion barrier. The rectification action of the membrane is lost, too. After switching off the imposed field, a spontaneous reconditioning takes place. The readjustment of both the membrane potential and the rectifying properties were followed. The first phase of reconditioning is obviously diffusion-controlled. All the results reported confirm the assumption stated above. The phenomena described can easily be explained as caused by the removal and the readjustment of adsorbed ions

Discrimination between the permeation of H⁺ and OH⁻ ions in precipitation membranes

The unique property of precipitation membranes, which allows determination of permeation rates of single ion species by measurement of current—voltage relationships, has been used to measure the transport rates of H+ and Oil ions. The permeation of OH− was found to be approximately 4-5 times higher than that of H+ in both BaSO4 and calcium oxalate precipitation membranes

Acid-Base-Equilibria at Semipermeable Membranes, their Influence on pH-Triggered Permeabilities

The theory of Donnan-equilibria is extended to systems in which chemical equilibria are involved. Equal temperatures of the outer phases, as well as ideal solutions are assumed. The membranes shall be impermeable for at least one ionic species. Comparison of the theory with experimental data at precipitation membranes shows a conspicuous dependence of the permeabilities on the pH-values of the fluid phases. A hypothetical model to explain this effect is discussed