GB567882A - Improvements in salt-bridge tubes - Google Patents

Abstract

567,882. Glass manufacture. KENT, Ltd., G. March 17, 1943, No. 4334. Convention date, April 1, 1942. [Class 56] [Also in Group XXXVI] A salt-bridge tube for the reference electrode of an ion-concentration cell is formed with a minute crack to provide for diffusion of the salt-bridge solution into the solution surrounding the salt-bridge tube. This is effected by making a small hole in the salt-bridge tube while it is at a temperature sufficiently high to render it tacky and sealing this hole with a plug of material, such as another glass, having a different coefficient of expansion so that a crack is formed on cooling. Apparatus for determining pH values in which such a saltbridge tube is employed, is shown in Fig. 1. The solution under test is passed through a chamber 2, containing a glass electrode 3, and a chamber 4 in which is disposed a reference electrode 5. Between the reference electrode 5 and the solution S is a salt-bridge solution 6, such as potassium chloride, contained in a tube 7 which has a diffusion opening 8 affording a conducting path between the reference electrode 5 and the solution S. A movable contact 9 alternately engages contacts 10, 11 and thereby subjects a condenser 12 alternately to the voltage developed by the ion-concentration cell and to the voltage tapped off by a slider 13 on a potentiometer 15. If these voltages are equal, the charge on the condenser is unchanged and the potential of the grid 16 of a valve V remains constant so that there is no change in the current traversing a resistance 17 in the anode circuit. If the fixed contacts 10, 11 are not at the same potential, the voltage drop across the resistance 17 is changed and is recorded by a recording galvanometer 21, the deflection of which also effects a balancing adjustment of the potentiometer 15. The oscillation of the contact 9 between the contacts 10, 11 is effected by a cam 23 driven at constant speed by mechanism 24. The aperture 8 is formed by heating the 'tube 7 until it is soft, inserting a pointed hook and forcing it against the side to make a small opening 25, Fig. 2, and filling this with a molten or tacky globule of another glass having a different coefficient of expansion. If this coefficient is higher than that of the tube, a crack is formed partly surrounding the plug. If the coefficient of expansion of the plug is lower than that of the tube, radiating cracks form in the tube. For a tube of small diameter, the end of the tube is sealed with a globule of glass of a different coefficient of expansion and the end blown into shape. Specifications 483,072, [Group XXXVI], and 560,212 and U.S.A. Specifications 1,935,732 and 2,108,294 are referred to.