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pulled up, the lower check valve opens, allowing fresh absorbent to flow into the spray-head supply tube. Figure 4 shows the usual Orsat gas analysis apparatus with the substitution of three automatic pipets for the conventional pipets and the addition of a push-pull air pump and a control panel which supports the mercury seals, air cocks, and a pump motor switch.
Operation The performance of an analysis with the apparatus employing the simplified automatic pipet follows the same procedure as that employed with other automatic pipets and differs from that used with the older types of apparatus in that instead of the tedious, exacting operation of running the sample into the pipet and drawing it back into the buret several times, the sample is transferred to a pipet in which it remains until absorption is complete. The absorbent is pumped through and around the gas for a predetermined length of time. During this time no watching is necessary in comparison to the ever-alert attention required with the older method to check and reverse the flow in order to avoid sending the buret confining liquid into the pipet or drawing the absorbent into the buret. While the pumping proceeds other work can be done or another gas analysis apparatus can be operated. The rate of absorption in the simplified automatic pipet is rapid. The oxygen in 100 cc. of air can be removed socompletely in 2 minutes, using alkaline pyrogallol as the absorb-
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ent, that it is unnecessary to check the reading. Because of the increased speed of absorption and also because no attention is required during absorption, reagents which heretofore have been too slow-acting for. practicable use can be employed (1). The dependability of the apparatus may be attested to by the fact that equipment is now in use with which more than 10,000 analyses have been performed. The original pump and most of the original pipets are still in use.
Acknowledgment The authors are indebted to E. I. Thomas for the design and construction of the air pump.
Literature Cited (1) Bonney and Huff, IND. ENQ.CHEM.,Anal. Ed., 9, 157 (1937). (2) Egerton, A. C., and Pidgeon, L. M., J. Sci. Instruments, 8, 234 (1931). (3) Egerton, A. C., and Smith, F. L., Ibid., 11,28 (1934). (4) Huff, W. J., U. S. Patent 2,094,357 (Sept. 28, 1937). (5) Kleiber, M., J . B i d . Chem., 101, 583-94 (1933). (6) Lunkenheimer Co., Cincinnati, Ohio, Catalog 58 (condensed ed.), March, 1933, Figure 981. (7) Weydana, G., Arch. Wdmnewirt., 17, 242 (1936). (8) Winchester, C. F., Rev. Sci. Instruments, 9, 134-8 (1938). (9) Winchester, C. F., Science, 78, 607-8 (1933). RECEIVED October 10, 1938.
Use of Silica Cotton in Filter Crucibles W. WALKER RUSSELL
I
AND
J. H. A. HARLEY, JR., Metcalf Laboratory, Brown University, Providence, R. I.
N SOME recent work on the determination of lead as sul-
fate it was found that silica cotton (obtained from the Owens-Illinois Glass Company, Toledo, Ohio) is an ideal material for the filtering mats in Gooch crucibles. The lead sulfate precipitate is readily retained by the silica cotton, is easily and quickly washed, and upon ignition a t 600" C. no change occurs in the silica cotton. Glass cottons sinter and become friable a t this ignition temperature, whereas silica cotton does not sinter below 800" C. Observations, were also made upon the relative merits of silica cotton, glass cottons, and asbestos with respect to chemical inertness, retentivity, and hygroscopicity. Silica is more chemically inert than the so-called resistance glasses, while asbestos may be appreciably dissolved by acids, alkalies, or alkaline solutions of phosphates. Thus in the present work it was found that a Gooch crucible plus its silica cotton mat returned to the same weight (within less than 0.05 mg.) after filtering such precipitates as lead sulfate, silver chloride, or nickel dimethylglyoxime, if the crucible was treated with the appropriate s o l v e n t i . e., 10 per cent ammonium acetate, 6 N ammonium hydroxide, or 6 N hydrochloric acid and then 95 per cent alcohol-and was faally heated again. Silica cotton possesses good,retentivity for precipitates, so that relatively thin filtering mats, weighing only about 50 mg., are required to retain any of the above-mentioned precipitates as welI as silver bromide, or cuprous thiocyanate. This appears due, a t least in part, to the fineness of the silica mm. fibers which have diameters of the order of 3 X The glass cottons examined had fiber diameters about twice this size. The very low hygroscopicity of silica cotton is a further advantage. I n this respect it proved slightly superior to glass cotton and may be much better than asbestos. I n comparative tests, similar to those of Huttig ( I ) , in which Gooch crucibles were heated to 200" C. for one hour, cooled
in a desiccator, and weighed after various intervals of atmospheric exposure, no change in weight (less than 0.03 mg.) wa8 detected in the crucible containing the silica cotton mat during 2.5 hours' exposure, while the crucible having a resistanceglass cotton mat gained 0.1 mg. in one hour, and the crucible with a mat of highly purified asbestos gained 0.1 mg. in 30 minutes. When the temperature of heating was increased to 600" C. the crucible containing the silica cotton still showed no detectable gain in weight, while the crucible containing the asbestos gained 0.1 mg. in 10 minutes' exposure. That asbestos may be much more hygroscopic is familiar to all analysts, and is recorded in Huttig's work (1) in which gains in weight of 0.1, 0.6, 1.2, 2.4, and 3.4 mg. were found for a Gooch crucible containing dried asbestos exposed to the atmosphere for 2, 5, 10, 30, and 60 minutes, respectively. A Gooch crucible is prepared with silica cotton simply by winding a narrow ribbon of the cotton into a flat spiral about 3 mm. thick and of a proper diameter to fit the bottom of the crucible. After pressing into place the cotton felts together considerably and remains in position; however, the crucible should not be carelessly handled. The weight of silica cotton used in a 20-ml. crucible is about 50 mg. Although the silica cotton is more expensive than glass cotton, little is required, and a crucible once properly prepared and intelligently handled can be used repeatedly with negligible weight changes. On the basis of the authors' present experience it appears that a Gooch crucible prepared with silica cotton has most of the advantages of a sintered-glass filter crucible, and in addition can be used a t much higher temperatures.
Literature Cited (1) G . F. Hiittig, 2. angew. Chem., 37, 48 (1924).
RBCEIVED September 4, 1938.
