Electrochemical study of the pore structure of sandstone and similar media

Abstract

Three methods are discussed by which the internal geometry of a porous insulator may be investigated electrochemically. By adopting a simple model, the pore geometry is characterized by two parameters: the tortuosity and the porosity. In each method, a cylindrical sample of the porous medium is soaked with an aqueous electrolyte solution and its lower face is brought into contact with a mercury pool electrode. The other electrode is also a mercury pool, but in the first technique it is remote, whereas in the second and third methods it floods the upper surface of the cylindrical sample. The first technique employs a d.c. potential step, whereby the concentration of a probe ion is depleted at the lower electrode. The attempt of diffusion to replenish this surface concentration leads to a faradaic current whose time dependence is analyzed to provide estimates of the tortuosity and porosity parameters. The second method measures the frequency-dependent cell impedance. The pore structure parameters are estimated from the equivalent circuit of the cell. No faradaic process is involved in the third method, which measures the frequency- dependent conductance of the cell as a means of characterizing the pore structure. These methods are applied successfully to sandstone and fritted glass samples.