AN IMPROVED DEMONSTRATION EXPERIMENT ON GAS ADSORPTION1 FORREST BLANKENSHIP and PAUL DONALDSON University of Oklahoma, Norman, Oklahoma
LEcTum demonstration experiments on gas adsorption are often not entirely satisfactory. For example, the demonstration which depends on the replacement of adsorbed gas by mercuryZ is tedious and difficult to observe in a large class. In another experiment, adsorption is illustrated by passing benzene-saturated natural gas over a bed of activated charcoal before burning the gas;3 without adsorption, the flame is yellow, whereas when the benzene is adsorbed, the flame is colorless. This result is not particularly impressive nor clear-cut because the change from a yellow to a colorless flame can also be effected by merely changing the air adjustment on the hnrnnr. .~ ~ - - - - A marked improvement has been found to lie in the substitution of methyl borate for the benzene in this latter type of demonstration. The brilliant green flame resulting from the presence of the volatile borate ester is quite spectacular and demonstrates the presence or absence of adsorption in a pronounced fashion. A convenient arrangement of the apparatus, employing two burners for comparison of the flames, is presented in Fig. 1. Probably because of the more complicated procedures usually followed in preparing boric esters, it is not commonly recognized that merely moistening boric acid crystals with anhydrous methanol produces sufficientester to give an intense green coloration to an otherwise colorless flame. The natural gas supply is passed through or over an alcoholic solntion of boric acid and thence to a Y connection. One fork of the Y leads directly to a laboratory burner while the other fork passes the gas through a small bed of adsorbent and thence to a burner. On lighting the two burners, whichhave been adjusted to give colorless flames, the green color where no adsorption has taken place, offers a marked contrast to the colorless flame. If the flames are allowed to bum for several minutes, the adsorbent bed may become saturated, and the colorless flame turns green. When this occurs, substitution of pure methanol for the boric acid solution mill reverse the colors of the ~
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flames as the alcohol vapor strips the ester from the adsorbent. Natural gas alone will strip the adsorbent much less efficiently. A white deposit, presumably boric oxide, can be obtained by holding a cool surface a t the tip of the green flame. Regarding the reaction in the methyl borate flask, some observations made by Maurice Arquet in 1936
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1 Presented before the Southwest Regional Meeting of the American Chemical Society, Houston, Texas, December 12-13,
1947. 2 ART-,
P., "Lecture Dcmonstrstiorls in General Chemistry," McGraw-Hill Book Company, Ino., New York, 1939, p. 291. "AUSBR, E. A,, AND J. E. LYNN,"Experiments in Colloid Chemistry," McGraw-Hill Book Company, Inc., New York, 1940, p. 61.
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to give an intense green by the addition of a small amount of concentrated sulfuric acid. Qualitative tests with anhydrous methanol were run with a view to learning whether kinetic measurements could be made on the esterification a t room temperature in the absence of mineral acids. The pressure in a flask containing methanol and boric acid cyrstals rose rapidly as long as the boric acid dissolved, and thereafter increased by an insignificant amount, indicating that the rate of solution was the controlling factor. Also conductance measurements were tried in a saturated solution of boric acid in methanol. A flask which was kept stoppered showed no appreciable change in conductance with time. However, the conductance in an open flask gradually increased. This may have been due to an increase in the amount of water produced as esterification proceeded and the ester escaped in the air. This adsorption experiment has been tried with success using activated charcoal, activated alumina, and silica gel. The length of time for saturation varies with the rate of gas Ho\v, the amount and nature of a(lsor11ent used. and whether or not the uus is bubbled tllroueh the ester solution, or merely passed over it. For example, a trial with 28-200 mesh silica gel ( E i e r and Amend) forming a 4-in. bed in a 6-in. drying tube required 45 minutes for saturation. 4 A R Q ~ T M., , Bull. Soc. Chim., 3, 1 4 2 2 4 (1936), through Attempts to detect a change in the ultraviolet Chem. Abstrs., 30 (19361, S O W . HORSLEY, L. H., Ind. and En$ Chem., Anal. Ed., 19, 508 fluorescence of the adsorbentson saturation gave uniformly negative results. (1947).
may be of interest4. He found that 68% of a quantity of H3BOa dissolved in 99.5% methyl alcohol was lost by spontaneous evaporation overnight. Using absolute ethyl alcohol, 30% of the boric acid volatilized. He concluded that the esterification took place a t room temperature without the addition of mineral acids. Arquet further noted that the trimethyl ester of H,BOs boiled a t 56%. Moisture of the air decomposed the ester. On burning the vapors, ester from 99.5y0 methyl alcohol yielded an intensely green flame. In the case of absolute ethanol a green flame was ohtained; 96% ethanol gave a slightly green flame; and 90% ethanol gave a very slight green flame. Isopropyl, isobutyl, and butyl alcoholic gave colorless flames with boric acid. Using 1 part of methanol to 9 parts acetone, a coIorIess flame was obtained, but with 2 parts methanol to 8 parts acetone a slightly colored flame resulted. In general, the results of Arquet regarding color intensity of the flame have been confirmed in our laboratory. However, in reporting the boiling point of the ester as 56OC., he has evidently confused a minimum boiling azeotrope (32% methanol) with pure trimethyl borate (b.p. 68.7"C.)." Moreover, 95% ethanol denatured with acetone, which does not give an intense green flame, can be made
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