Patent GB-1091579-A (GB)
Title: Leak structure and method for producing same
IPC: G01N27/401
Filed: 1965-11-01
Abstract: 1,091,579. Measuring electrically. BECKMAN INSTRUMENTS Inc. Nov. 1, 1965 [Nov. 17, 1964], No.46115/65. Heading G1N. The leak structure providing the liquid junction of a salt bridge tube comprises a plug 11, Fig. 1, positioned in an opening at the bottom of a salt bridge tube 10 to provide a minute opening around the plug through which the liquid can flow. Both plug and tube are made of glass having the same coefficient of expansion so that the size of the opening stays constant over a wide range of temperatures. In one embodiment, the leak structure is formed by melting one end (22), see also Figs. 2-4 (not shown), of a glass rod (23) and forcing it into the opening (21) at the end of the tube 10. In this way two flanges 24, 25 are formed surrounding the opening, the former being produced by pressure inside the tube 10 or by flattening the molten glass with a carbon rod (26), Fig. 3 (not shown). To prevent fusion between the tube and plug, the end of the tube 10 may be coated with beeswax or silicon grease (27), Fig. 5 (not shown), or covered by a plastics sheet (28), Fig. 6 (not shown), which is carried with the glass rod through the opening. Traces of wax, grease or plastics are then removed by annealing the glass structure. This leak structure can be used in a reference electrode for a pH meter as in Fig. 1, where a silver-silver chloride half cell 13 is partially immersed in the salt solution 12 in the tube 10. The half cell solution communicates with the salt solution through an aperture 17 and the latter communicates with the solution under measurement by the aperture around the plug 11. In another form of leak structure the apertured end of a glass tube (30), Figs. 7-9 (not shown) is heated and a shaped glass rod (36) is forced through it. The end of the glass rod is covered by a plastics foil (33) and an annular groove (34) is provided on the rod in which the aperture edge (31) is engaged. In a further embodiment, the tube (44) Figs. 10- 12 (not shown) is provided with an annular groove (46) and an annular restriction (50). A rod (52), whose end (56) is molten, is then pushed into the plastics covered open mouth of the tube and takes up the shape of the tube's inner wall. This leak structure can be modified by using a hollow rod (64), Figs. 13-15 (not shown), whose end portion (68) is molten. Air blown into the rod then causes it to take up the shape of the tube (44). This latter modification can be used in a combination glass-reference electrode in which an ion sensitive bulb 80, Fig. 16, is cemented to the inside of the hollow plug 68. A glass rod 28, carrying a conductor 90, which terminates in a solution 94 in the bulb 80, is centrally mounted in the tube 44 with its end 84 extending through an opening 86 formed in the closed end 66 of the glass tube 68. This rod is sealed to the walls of the opening 86 so as to prevent flow of salt solution 88 into the bulb 80. A reference half cell 95 is immersed in the salt solution 88 to provide the reference electrode structure for the combination glass-reference electrode.