Patent GB-567722-A (GB)
Title: Improvements in electrodes for ph measurement
IPC: G01N27/333
Filed: 1943-06-10
Abstract: 567,722. Determining physical qualities of materials voltage measurements ; potentiometers ; electrolysis. KENT, Ltd., G. June 10, 1943, No. 9370. Convention date, June 10, 1942. [Classes 37 and 41] A cell for measuring the pH of a solution essentially free of molecular hydrogen and of variable oxidation-reduction systems comprises in -contact with the solution a measuring electrode uncontaminated by occluded. hydrogen and of a metal of the group iridium, rhodium, osmium, and ruthenium. The iridium measuring electrode shown in Fig. 2 comprises a brass rod 6 covered at its lower end with a cap 8 of gold, silver, copper, lead. or other metal on which iridium may be deposited by electroplating, sputtering, or evaporation. A cork 5 supports the electrode with its lower end in contact with the test solution. In another construction, Fig. 3, a wire 9 of gold, silver, copper, lead, &c. is coiled about the lower end of a glass or other insulating tube'or rod 10, its ends being .anchored by fused beads 11, 12. Iridium is then deposited on the wire 9 by electroplating, sputtering, or evaporation. A lead-in conductor 13 passes through-the lower sealed end of the tube 10 for attachment to the lower end of the coil 9. Instead of the wire coil 9, a thin plate may be attached to the lower end of the lead-in conductor 13 and coated with iridium. The electrode shown in Fig. 5 comprises a brass rod 16 covered at its lower end by a cap 17 of gold, silver, copper, lead, &c. Moulded or cast about the rod 16 is a rod 18 of polymethyl methacrylate or other plastic. The lower exposed face of the cap 17 is then coated with iridium. A process for electro-chemically depositing iridium on the gold or other base surface of the electrodes consists in cleaning and burnishing the surface and then further cleaning by cathodically polarizing it in a hot solution of trisodium phosphate. The plating bath may be a solution of chloro-iridous acid or a sulphuric acid solution of iridium chloride, a carbon anode which has been soaked in aqua regia and washed in distilled water being used. The iridium coating is preferably formed by a series of electroplating operations, each producing a thin deposit which is cleaned and burnished before the next operation, in order to prevent porosity. Alternatively, the iridium may be deposited by an evaporation process by disposing the base electrode in an exhausted' chamber in which is also disposed a. tungsten wire coated with iridium and heated to a temperature high enough to vaporize the iridium but below the vaporization temperature of tungsten. Alternatively, a sputtering process may be employed by placing two electrodes, one having an iridium surface, in an evacuated chamber and applying a high unidirectional voltage. The base member for the measuring electrode is placed between these electrodes and the degree of vacuum is varied until the base member is just outside the cathode dark space extending from that high-voltage electrode which serves as the source of iridium. Measurement of the potential of the iridium electrode should not require a current density of more than 10-<7> amperes per square centimetre of active electrode surface, in order to avoid polarization errors. A high impedance detector may be connected directly to the electrodes of the measuring cell, or a high resistance may be connected in series with a low impedance detector. In the measuring circuit shown in Fig. 9, a high resistance 23 is connected in series with the iridium electrode I and reference electrode R of the ion-concentration cell 24. A two-way switch 25 is arranged so that when engaging the contact 26 a condenser C is charged to the voltage of the cell 24. The switch 25 is then thrown into engagement with the contact 27 and the condenser C is discharged through the ballistic galvanometer G, the scale of which may be calibrated in pH values. A high impedance detector circuit suitable for direct connection to the cell electrodes is shown in Fig. 10. A valve V has its grid g connected to the reference electrode R of the cell 24 and its cathode h connected. to the iridium electrode I through a biassing potentiometer B, switch 28, and balancing potentiometer P. With the switch 28 on contact 30, so as to substitute a resistance 31 for the cell 24 and potentiometer P, the biassing voltage is adjusted by moving the contact 32 on the slidewire 33 of the potentiometer B until there is no change in the anode current of the valve V on opening or closing a switch 29. The switch 28 is then engaged with contact 28a, whereupon zero change of anode current on opening and closing the switch 29 indicates that the voltage of the cell 24 is balanced by the opposed voltage of the potentiometer P. The scale 37 may be calibrated in pH value. For covering a wide range the connection from the cell 24 to the potentiometer P is to a tapping 38 which is adjusted by using a solution of known pH value in the cell 24 and setting the contact 35 to the corresponding graduation on the scale 37. The tap 38 is then adjusted for null deflection of the galvanometer on opening and closing the switch 29. In a modification, Fig. 11, the electrode I is connected to the slider 35 of the potentiometer P, the slidewire 36 of which is connected in series with resistances 40 ... 45 across a source of D.C. voltage. To check the current in the slidewire 36 a switch 46 is depressed so that its movable contacts 47, 48, connected to a galvanometer G, engage the contacts 49, 50 of the standardizing circuit including a standard cell SC. The resistance 45 is then adjusted until the voltage drop across the resistance 41 is equal to the voltage of the standard cell SC. The switch 29 is closed to apply zero voltage to the electrodes g, h of the valve V and with a desired setting of the contact, 51 on the resistance 52 and with the contacts 47, 48 of the switch 46 in their upper position, the contact 53 is adjusted along the slidewire 54, in parallel with the resistance 43, until there is null deflection on the galvanometer G. The switch 29 is then opened and the contact 35 of the potentiometer P is adjusted until there is null deflection of the galvanometer G. The ion-concentration is then read directly from the scale 37. Specification 526,771, [Group XXXV], and U.S.A. Specifications 1,474,151, 1,846,354 and 2,109,284 are referred to.