Glass membranes

Introduction

A silicate glass used for glass membranes consists of an infinite three-dimensional structure of four negatively charged SiO4 groups in which each silicon is bonded to four oxygens and each oxygen to two silicons.

Within the network of this structure there are enough cations to neutralize the negative charge of the silicate group. Singly charged cations such as sodium and lithium move in the network and are responsible for the electrical conduction within the membrane.

Desiccation of glass membranes

The surface of glass membranes must be water-coated before it can function as a pH electrode. The amount of water involved is approximately 50 mg per cubic centimeter of glass. Desiccant glasses do not show any response to pH. Even desiccant glasses lose their pH sensitivity after dehydration by storage in a desiccator. However, this effect is reversible and the response can be restored by soaking the glass membranes in water.

The hydration of pH-sensitive glass membranes involves an ion exchange reaction between singly charged cations in the glass lattice and protons from the solution in which the electrode is immersed. The process involves only monovalent cations because divalent and trivalent cations are tightly held within the silicate structure and cannot exchange with ions in solution.

The equilibrium constant for this process is so large that the surface of hydrated glass membranes is usually composed entirely of silicic acid (H2O3). An exception to this is in strongly alkaline media where the hydrogen ion concentration is very low and the sodium ion concentration is very high. Here a significant fraction of the four negatively charged ShO4 sites are occupied by sodium ions.

Electrical Conduction Across Glass Membranes

For a glass membrane to function as a measuring electrode for cations, it must conduct electricity. Conduction across a water-filled membrane involves the movement of sodium and hydrogen ions. Sodium ions carry charge within the dry portion of the membrane, and protons are mobile in the gel layer. Conduction across the solution/gel interface occurs via the following reactions:

H + Gl↔ HGl

Glass1 Gass1      solution1

HGl↔ H + Gl

Glass2 solution2      Glass2

where subscript 1 refers to the interface between the glass and the analyte solution and subscript 2 refers to the interface between the internal solution and the glass. The positions of these two equilibria are determined by the concentrations of hydrogen ions in the solutions on both sides of the membrane. If these positions differ, the surface where more separation occurs will be more negative than the other surface. Thus, a boundary potential Eb is created across the membrane. The magnitude of the boundary potential depends on the ratio of the hydrogen ion concentrations of the two solutions. It is this potential difference that serves as the analytical parameter in potentiometric measurements of pH.

((Source: Fundamentals of Analytical Chemistry-Skoog-Westhaller))