Patent GB-560212-A (GB)
Title: Improvements in glass cell electrodes and measuring apparatus for use therewith
IPC: G01N27/30
Filed: 1942-10-07
Abstract: 560,212. Glass manufacture. KENT, Ltd., G. Oct. 7, 1942, No. 14091. Convention date, Feb. 21, 1942. [Class 56] [Also in Group XXXVI] An electrode comprised in a half-cell used for example for pH measurement is thermally isolated from the test solution by the interposition of a barrier or plug of low thermal conductivity between a salt-bridge or buffer solution in heat transfer relation to the electrode and a salt-bridge or buffer solution in heat transfer relation to the test solution, thereby enabling measurements to be made of test solutions while at temperatures above 50‹ C. within a range including or extending beyond 100‹ C. The measuring half-cell MC, Fig. 1, comprises a glass tube 1 provided with a thin glass membrane 2 constituting an electrode surface in contact with the test solution T. The lower part of the tube 1 contains a buffer solution 3 of selected hydrogen-ion concentration, e.g. a solution of potassium chloride and acid potassium phthalate. The difference in potential of the opposite faces of the glass membrane 2 is a function of the difference between the known pH value of the buffer solution 3 and the unknown pH value of the test solution T. Electrical conduction between the solution 7 within the electrode RI and the solution 8, Fig. 1, of the same composition external thereto is afforded by the threads of the cap 4 when it is screwed on to a member 11 and by a notch 12, Fig. 1, in the upper edge of the cap 4. A platinum wire 13 dips into the mercury 5 and is cemented into the member 11 where it is connected to a copper conductor 14 for external connection. A plug 15 of low thermal conductivity separates the bodies of solution 3, 8, and may comprise a hollow glass plug loosely filled with glass wool. Other suitable materials for the plug are cork, balsa, packed asbestos, heat-resistant wax, refractory brick, rubber, or heat-resistant plastic such as phenol formaldehyde, phenol furfural, urea melamine formaldehyde, vinylidene chloride, or methyl methacrylate. For pH measurement the glass electrode MC may be used with a reference half-cell such as a mercury-mercury ion electrode or a silver-silver chloride electrode or, as shown in Fig. 1, an electrode RX similar in construction to the internal reference electrode RI. The electrode RX is thermally isolated from the test solution T by a plug 151 similar to the plug 15. The bodies 103, 108 of liquid on opposite sides of the plug 151 are of salt-bridge solution such as potassium chloride. The lower tubular part 16 of the half-cell RC is provided with a small passage or hole 17 through which the bridge solution 103 slowly diffuses, this loss being replaced by flow of solution from the upper body 108 of bridge solution through the restricted path through or about the plug 151. Alternatively the upper part of the vessel may have a T-connection to receive a rubber hose from a separate reservoir of potassium chloride solution. The diffusion opening 17 may be made by heating the tube 16 to soften it, punching a hole in the hot tube and filling it with a plug of lead glass having a co-efficient of expansion greater than that of the glass comprising the tube 16. This insert fuses into the wall of the tube 16, but as the tube cools a small crack is formed extending through the wall of the tube for a part of the circumference of the plug. Specification 477,849, [Group XXXVI], is referred to.