Low-Resistance Glass Electrodes H. MOUQUIN AND R. L. GARMAN, Washington Square College, New York University, New York, N. Y.
the outside of the electrodes may be given a light coat of paraffin wax to sign, the universal use of speprevent creeping of solutions. cia1 glass, and the increased simObviously, the method of formplicity and reliability of electronic ing the glass membrane is the esseninstruments, all have contributed tial feature, and other constructo the present-day popularity of the tional details lend themselves t o glass electrode. many variations. For example, if The greatest obstacle in successthe initial bulbs are blown in the fully designing such an electrode center of one or both tubes, two or is the inherent high resistance of the more side tubes can be kept on glass membrane, as well as its esseneither side or both sides of the memtially fragile nature. Any successB brane. These membranes may also ful model must be a compromise bebe used in making an all-glass comtween these two practical factors. pensated manometer. I n order to Most of the electrodes now commake this instrument capable of mercially available have sufficient standing considerable abuse and yet mechanical strength, but their elecretaining its sensitivity near zero, trical resistance is of the order of 10' three membranes intersecting in a ohms and there is no practical model common line may be formed by witharesistance below onemegohm. blowing simultaneoudly on three This relatirely high r e s i s t a n c e Pyrex glass bulbs. creates inconveniences, the most The details of setting up the elecobvious of which are (1) the imA trode for actual use can also be perative use of a sensitive elecvaried considerably. I n this laboratrical measuring instrument such FIGURE1 tory, the stopper used in the original as a quadrant e l e c t r o m e t e r o r mounting is kept on the finished vacuum-tube voltmeter, ( 2 ) t h e necessity of shielding, and (3) the need of special precautions electrode in order to facilitate subsequent damping; then by in insulation. Although these difficulties are not insuperable, means of a capillary pipet the intact hemisphere is filled with a the errors caused by deviation films on the glass electrode itself reference solution; finally, the whole electrode is placed in a are not always so easy to control (1). stoppered wide-mouthed bottle containing enough of the same The type of glass electrode described here is sturdy enough reference solution to cover the membrane on the side of the pierced bulb, When required for use, two saturated calomel €or all practical purposes and yet has an electrical resistance half-cells are introduced into the circuit and the asymmetry ranging from lo4 to lo5 ohms. This relatively low resistance potential of the electrode system is determined. The calomel overcomes most of the inconveniences inherent in the older models. half-cells and glass electrode are easily mounted on a single stand by means of three buret clamps, and the unknowns This new type of electrode can be made in quantities, substituted for the reference solution on the pierced side by easily and rapidly; and no special glass-blowing skill is replacing the liquids in a small beaker which can be raised or quired. Although a t first it is best to practice with a few lowered around the glass electrode. A short piece of tubing pieces of ordinary glass, the low-resistance electrode must be made from special low-resistance glass tubing (Corning KO. drawn to a capillary on one end can be attached to one of the calomel cells in order to make contact with the solution on the 015,5- to 10-mm. bore, is convenient). reference side of the electrode. Starting with two pieces of tubing about 10 cm. long, a small A precision better than one millivolt or 0.02 pH unit is sturdy bulb is blown at one end of each tube (bulbs should be easily obtainable with electrodes having a resistance of lo5 about twice the diameter of the tube bore). The two tubes are ohms used in conjunction with a Leeds & Korthrup enclosed inserted in a two-hole stop er, which is mounted on a blowing lamp and scale portable galvanometer. In the range usually tube as shown in Figure 1, making it easy t o blow equally into both tubes. The two bulbs are then heated as uniformly as posconsidered valid for glass electrodes, repeated experiments sible, allowing them to touch and fuse together at one point. A have shown that the accuracy is a t least as good as this. final blow will cause the point of contact to expand into a thin Some of these electrodes have been used successfully in this plane membrane (Figure 1, B)-so thin that a few faintly colored laboratory for a period of over 6 months. interference rings should appear near the center of the membrane. If a gray or black spot appears, the blowing has been too prolonged; on the other hand, if the exterior hemispheres expand Summary excessively,the pressure has been applied too suddenly; in either case the electrode will be excessively fragile. Procedure for making a low-resistance glass electrode is A small hole near the bottom of one of the hemispheres (not given. Electrodes made in this manner are fully as satistoo near the membrane) is necessary for drainage (Figure 1, C). factory as the traditional types, with the additional advantage It is made while the glass is still hot and before removing from that the measurements may be made with the ordinary potenthe mounting, by using a narrow pointed flame and blowing gently 8 s the glass softens in the flame. tiometer and a portable galvanometer of medium sensitivity. Several other precautions may prove useful. Since the No. 015 glass tends to crystallize, only fresh tubing should be used Literature Cited and the number of reheatings kept to a minimum. If, on cooling, the electrodes tend to crack into two halves, the tubes should be (1) Kahler, H.,and DeEds, F., J. Am. Chem. SOC., 53,2998 (1931). mounted in the blower at a greater angle to each other. This will place the tubes farther away from the membrane. Finally, RECEIVED March 4, 1937.
T
HE many improvements in de-
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