T H E ADSORPTION O F K A T E R AND ETHYL ACETATE VAPORS BY SILICA GELS BY ALBERT L . ELDER AXD 0 . L . B R A S D E S
Recently, numerous experiments on the adsorption of various substances by silica gels have been reported. A comparative study of the relative efficiencies of Holmes’ chalky silica gel and of Patrick’s commercial gel, for adsorbing water from an air stream containing the vapor at various partial pressures, has not’ been reported. The object of this investigation was to study the adsorbing powers of these two gels under similar conditions. In some of the experiments, an air stream containing water vapor at saturation values ranging from z j - I O Oper cent was used as the material to be adsorbed. In later experiments, ethyl acetate was substituted for water in order that comparisons between organic polar compounds and water could be made. Preparation of the Gels The Holmes’ chalky gel used in this work was prepared according to the directions given by Holmes.’ 0 . 8 6 j o liters of sodium silicate of density 1 . 3 7 j were diluted up to 17.3 liters with tap water. 1.16 liters of z Pi. ferric chloride were added slowly to the water-glass solution. The contents of the vessel were stirred during the addition of the ferric chloride and for a few minutes after the last of the ferric chloride had been added. The precipitate which formed slowly was allowed to stand for j o hours and then filtered on fine cheese cloth. Five days later, the gel was cut into large lumps and placed on a drying rack. When a moisture content of 5 4 . 2 per cent had been attained, the gel was placed in a glass-stoppered bottle and allowed t o “sweat” or synerize. During this process a better gel structure resulted. One week later, the iron-gel was treated with steam for one hour. The iron-oxide was then removed with 9 N. sulfuric acid at a temperature of 8o°C. Following the removal of the soluble salts by thorough washing with distilled water, the gel was air-dried and bottled. As needed, samples of the chalky silica gel were removed, reduced to the required size, activated, and used in the adsorption measurements. The Patrick gel used was the ordinary commercial silica gel. Apparatus and Experimental Procedure The apparatus used was essentially that employed by Holmes and Elder2 in their recent work on silica gels. Briefly, it consisted of two carefully calibrated flowmeters, a drying train, a train of saturators, a vapor mixing tube
* “Laboratory Manual of Colloid Chemistry,” 193 (1928). * J. Phys. Chem., 35, 82-92 (1931).
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and a gel adsorption tube. The saturators, mixing tube, and the adsorption tube were placed in a constant-temperature air-bath. Adsorption measurements were made by placing a weighed sample of the activated gel of size 10-20 mesh in the adsorption tube and permitting a stream of air containing water or ethyl acetate vapor a t a known partial pressure to pass over the gel. At intervals, the gel tube was removed from the thermostat, reweighed, and the gain in weight recorded as the per cent saturation of the gel under the conditions as specified by this experimental procedure. This process was continued until no more vapor was adsorbed by the gel.
FIG.2
FIG.I Adsorption of a a t e r by silica gels under different partial pressures.
Adsorption of ethyl acetate by silica gels under different partial pressures.
Activation of the gel was accomplished in all cases by placing the gel in a fused quartz tube surrounded by a n electric furnace heated to zoo°C. A stream of dry air was passed over the gel in the quartz tube at a rate of 300 cc per minute for two hours. Any desired per cent saturation of air stream with either water or ethyl acetate could be obtained by mixing together known amounts of saturated and dry air. The rate of flow of air and water vapor through the apparatus was 300 cc per minute. The air and ethyl acetate mixtures were passed through at I O O cc per minute. All determinations were made at 3oOC.
Results The results of the experiments showing the adsorption of water vapor from a n air stream are shown in Fig. I. The saturation values of the air
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stream for these experiments were 100,9 0 , 80, i j , j o and 2 5 per cent. B similar series of experiments are shown in Fig. z for the adsorption of ethyl acetate from an air stream at, the saturation values of I O O > j, j o and 2 j per cent. The per cent saturation of the air stream with water or ethyl acetate has been plotted in Figs. I and 2 against the per cent adsorption by the gels of water and ethyl acetate a t the known partial pressures. By per cent adsorption is meant the ratio of the grams of adsorbed material per gram of gel.
Summary The data plotted in Figs. I and 2 show that at relatively low partial pressures Patrick's gel is more efficient than Holmes' chalky gel for adsorbing either water or ethyl acetate from an air stream. At higher partial pressures Holmes' chalky gel has the larger adsorption capacity of the two gels for adsorbing either water or ethyl acetate. Department
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