Patent GB-1481509-A (GB)
Title: Ion selective electrodes and in methods of measuring the concentrations of ions
IPC: G01N27/416
Filed: 1973-07-18
Abstract: 1481509 Ion selective electrode NATIONAL RESEARCH DEVELOPMENT CORP 10 July 1974 [18 July 1973] 34132/73 Heading G1N An ion selective electrode comprises a metal or alloy which forms a hydride and means for supplying hydrogen to or controlling the hydrogen concentration in the metal within a predetermined range. This electrode measures pH and may be partially coated with a mixture of an acid and a salt of that acid for measuring the metal present in the salt. The metal is preferably one of palladium, yttrium, zirconium titanium and vanadium or an alloy between two or more of these elements. The acid is preferably steoric acid and the salt, the steorate of calcium, magnesium, strontium, barium, yttrium or lanthanum. The hydrogen concentration is maintained constant by supplying hydrogen which may be generated electrolytically at the electrode or remotely or may be supplied under pressure. Circuitry. For pH or other ion measurement the potential difference between the ion selective electrode 10 and a reference electrode 14 is measured. Measurement cycles alternate with cycles which test and regenerate the electrode. The hydrogen content may be determined by measurement of potential relative to another electrode 38, Fig. 4 separated from the measurement electrode 10 by electrolyte. It is preferably determined by measuring the resistance of the electrode material which resistance increases with hydrogen content. Threshold circuits respond to a voltage from the resistance measuring circuits and provide signals which (a) indicate adequate hydrogen content, (b) start the hydrogen supply and (c) as in (b) but signal that the hydrogen content is too low for use. The hydrogen is preferably generated in situ between the reference electrode and a platinum electrode 12. The rate of generation is controlled by differentiation of the resistance proportional voltage. This can only be done in the solution being measured when it is flowing, as the reading would otherwise be affected, but may take place in electrolyte contained in the electrode body or remotely. The electrode may be cleaned by pulsing it positive relative to electrode 12. Electrode Constructions. As shown in Fig. 2, a palladium wire 10 wound in threads 16 of an insert body 17 which contains reference electrode 14, electrolyte 19 and leak 18. The electrode  requires an external electrode for hydrogen generation. In a similarly shaped electrode Fig. 3 (not shown) the palladium wire is continued inside the electrode and a second electrode therein is used for hydrogen generation. An external reference is required. In the electrode of Fig. 4, hydrogen under pressure is supplied via valve 27 to the centre 34 of electrode 10. The gas may also be supplied to a similar electrode from an electrolysis cell. In a further embodiment the electrode extends from a teated pipette, along a hydrogen supply tube to an electrolysis cell. The metal ion selective electrode are preferably made by coating screw threads a PTFE body with the acid/salt material, winding the electrode wire in the screw threads and back diagonally across the threads and partially coating the wires at the intersections.