Sintered-Glass Filters and Bubblers of Pyrex d
HOSMER W. STONE AND LOUIS C. WEISS, University of California at Los Angeles, 405 Hilgard Avenue
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It has been used successfully on tubing up to 10 mm. in outside diameter and on capillary tubes by flaring the ends. The capillary type is described by Kirk (6) for use in quantitative microanalysis.
INTERED- or fritted-glass filters and gas distributors have come into general use during recent years (10). Their manufacture is covered by the patents of Schott and Gen., Jena, Germany, who offer an extensive line of frittedglass filters in the Jena glass (9). Because of the difficulty of sealing the sintered equipment of Jena glass with the borosilicate glass so widely used in this country, workers have made their own sintered Pyrex in various ways. With this situation in mind the authors call attention to the literature on the subject and describe an improvement in the existing methods of preparation, so simple that the average worker can make use of it. Certain of the more commonly used sintered devices are readily prepared without any special technique or equipment. These include the so-called “filter stick” for filtering liquids as they are removed from containers and gas-distribution tubes for dispersing gases in liquids for aeration, absorption, and washing purposes. There are two general techniques for the preparation of sintered-glass apparatus. One involves the preparation of a sintered-glass disk which is subsequently fused into the tube; the other sinters and fuses the powdered glass in place in a single operation.
Scra Pyrex glass is ground by any convenient method; both tge iron mortar and pestle and the power mill are satisfactory. The methods of cleaning and grading recommended by Bruce and Bent (9) are followed. The ground glass is heated with hydrochloric acid, washed with water, dried, and separated into 60 t o 80-, 80 to loo-, 100 t o 150-, and 150 to 200-mesh sizes for the preparation of filters of different porosities. A disk of light asbestos board is cut with cork borers to fit snugly inside the glass tube, supported by p wad of asbestos fiber as indicated in Figure 1. Powdered glass, of a size selected for the purpose in hand, is poured on top of the asbestos toform a layer about 5 mm. thick. The tube is then rotated in a needlepoint gas-air blast-lamp flame with the flame directed on the tube just outside the powdered glass. In a few seconds the exposed surface of the powdered glass is sintered suflicientlyt o permit inverting the tube. The flame is then played in the region of the asbestos until the latter acquires a brilliant flesh color. The heating is continued for about a minute at the maximum temperature of the air-gas flame. By this time the walls of the glass tube should have acquired a stippled appearance. A little experience will show how much heating is required. It is desirable t o test the sintering at this time by gently poking the surface with a wire. If it crumbles readily, it may be heated directly in the flame until it becomes firm. After suitable annealing and cooling, the asbestos is easily removed by alternately sucking air and water through the tube. In addition to filters in straight tubes this technique can be applied (8) to the construction of gas-distribution tubes (Figure 1, right). To obtain effective dispersion of a gas in a liquid, the gas must be delivered upward and the bubble size is governed not only by the pore size but also by the pressure drop across the filter and by the character of the liquid. It was expected that the smaller area of sintered glass necessitated by this method of preparation would make disks inferior to the gas-distribution disks of larger area. It appears, however, that a t moderate pressures only a few spots even on the larger disks deliver bubbles, the rest of the surface being ineffective unless the pressure is raised. Tubes prepared as described deliver gas a t ordinary pressures from a number of points at a rate comparable with the tubes of larger sintered surfaces.
FIGURE 1. FILTER STICKAND GASDISTRIBUTOR
Several writers (1, I, 7) have described methods of preparing the disks separately. The principle has also been used by Cool and Graham (3) for the preparation of sinteredglass aeration thimbles. The chief advantage of preparing the disks separately lies in the fact that larger and more uniform disks may be prepared. Its disadvantages are that more elaborate apparatus, greater skill, and a longer time are required. The second technique, in which the powdered glass is sintered and sealed in place in a single operation, has several modifications. Shatenshtein (11) holds the powdered glass in the tube with carbon rods and sinters in a blast-lamp flame. Kirk (6)places the powdered glass in a small bulb a t the end or side of the tube for sintering and then exposes the sintered surface by grinding. Furnstal and Johnson (4) make use of both techniques and recommend methods involving the use of a temperaturecontrolled muffle furnace. This is very satisfactory for those who have a large number of sintered-glass devices to prepare, but the equipment and technique are unnecessarily involved for the needs of the average chemist. The authors have developed a very simple method for preparing sintered-glass plugs in the ends of tubes which satisfy many of the sintered-glass laboratory requirements.
Summary Although sintered-glass filters have many applications, their use has been limited by the lack of a commercial supply in the borosilicate glass and of adequate directions and facilities for making them. This paper presents a technique for sintered-glass preparation, requiring a minimum of skill and equipment.
Literature Cited R., J. Chem. SOC.,13, 79-80 (1934). (2) Bruce, W.F.,and Bent, H. V., J. Am. Chem. SOC.,53, 990-2 (1931). (3) Cool, R. D., and Graham, J. D., IND. ENQ.CHEM.,Anal. Ed., 6, 470 (1934). (4) Furnstal, A. H., and Johnson, B., Plant Phwlsiol., 11, 189-94 (1936). (5) Kirk, P. L., IND. ENG.CHEM.,Anal. Ed., 7, 135-6 (1935). (6) Kirk, P. L,, Mikrochemie, 14, 1 (1933). (7) Kirk, P. L., Craig, R., and Rosenfels, R. S., IND.ENQ Can%, Anal. Ed., 6, 154-6 (1934). (8) Mattikow, M., Ibid., 7, 136 (1935). (9) Prausniti, P.H.,IND. ENC.CEEM.,16,370 (1924). (10) I b i d . , Anal. Ed., 4, 430 (1932). (11) Shatenshtein, W.E., J. Chem. Ind. (Moscow), 6,IS00 (1929). (1) Briscoe, H. V. A., and Lowe, A.
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