Platinum Black Electrodes Research Articles

An Electrodeless System for Measuring Electric Logging Parameters on Cores and Mud Samples

Introduction A recently developed system for measuring electrical resistivity of liquids without use of electrodes offers some interesting possibilities in electric logging technology. The equipment as supplied by the manufacturer is satisfactory for continuous mud logging on a drilling rig or for measuring mud or filtrate resistivity in the laboratory. A simple modification of the commercially available instrument makes it suitable for measuring resistivity of core samples in the laboratory. The continuous measurement of mud resistivity on a drilling rig is a convenient means for detecting mixing of formation water with the drilling mud. Such information is useful to the geologist, the mud engineer and the logging engineer. However, continuous mud resistivity logging by conventional electrode-type resistivity cells is beset with difficulties. The mud, sand and rock chips abrade the electrodes, thereby changing the cell constant and eventually destroying the cell. Also, additives and crude oil in the mud may poison the electrodes by coating them with a nonconductive material. An electrode-type resistivity cell, therefore, may give erroneous readings under certain conditions. Electric logging companies circumvent the electrode poisoning problem by using a four-electrode resistivity cell for measurement of mud resistivity. In this cell, change in electrode area does not change the cell constant. However, the four-electrode cell is difficult to adapt for continuous reading and does not solve completely the problem of electrode abrasion by the sand and cuttings in the mud. The measurement of electric logging parameters on core samples in the laboratory encounters some of the same problems discussed in connection with mud logging. Ideally, the electrical resistivity of a core sample should be measured by placing platinum black electrodes in direct contact with the plane ends of a cylindrical or rectangular sample. Platinum black electrodes however, are much too fragile and easily abraded to be brought in contact with a rock sample. Also, oil or other constituents in the fluid contained in the sample will poison platinum black. In practice, gold-plated brass electrodes, in an AC bridge circuit operating at about 1,000 cps, are used for routine core analysis. For more precise work in research studies, a four-electrode scheme is used. Preparation of the samples for the four-electrode method is much too involved for routine core analysis.

Electrometric acid-base titrations by means of the quinhydrone electrode and its application under physiological conditions.

It has recently been shown that an acid solution containing equimolecular proportions of benzoquinone and hydroquinone, a condition which is most easily obtained by dissolving the crystalline addition product, benzoquinhydrone, in the medium under examination, promptly produces a stable and reproducible oxidation-reduction potential on a properly prepared inert electrode, preferably of gold. In acid solutions the observed potential has been shown to be a linear function of the PH and may, therefore, serve as a very simple means of determining hydrogen ion concentrations. For measurements on this system and references to the literature see. Using this method, Biilmann has shown that the potential is stable in a phosphate buffer at PH 6.81, but unstable in an alkaline borate solution at PH 9.24, and further that the method possesses the distinct advantage that it may be used where the platinum black hydrogen electrode is inapplicable, e.g., in the presence of certain mild oxidizing agents as 0.1 M nitric acid or unsaturated compounds of the acryhic acid type. We have made a study of the quinhydrone electrode in order to determine more definitely the range of its applicability under physiological conditions. The validity of the method depends upon maintaining the ratio of the concentrations or more correctly of the activities of the quinone and hydroquinone strictly equal to unity or to some other constant and known value. The factors which may upset this relationship are: (A) Deviations from the simple oxidation-reduction equation of Peters due to hydroquinone acting as a weak dibasic acid. (13) Changes in the activities of the dissolved quinone or hydroquinone molecules due to the presence of salts. (C) The presence of other oxidizing or reducing substances which react with the quinone substances with measurable velocity.