EXCHANGE The Adsorption of Ammonia by Silica Gel DISSERTATION SUBMITTED TO THE BOARD OF UNIVERSITY STUDIES OF THE JOHNS HOPKINS UNIVERSITY IN CONFORMITY WITH THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY BY LEVI YORGEY DAVIDHEISER BALTIMORE June, 1922 EASTON, PA. : ESCHENBACH PRINTING COMPANY 1922 The Adsorption of Ammonia by Silica Gel DISSERTATION SUBMITTED TO THE BOARD OF UNIVERSITY STUDIES OF THE JOHNS HOPKINS UNIVERSITY IN CONFORMITY WITH THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY BY LEVI YORGKY DAVIDHEISER BALTIMORE June, 1922 EASTON, PA.: ESCHENBACH PRINTING COMPANY 1922 CONTENTS Acknowledgment 4 Introduction 5 Apparatus 6 Material 6 Procedure 6 Table I 7 Figure I 8 Table II 9 Figure II 9 Figure III 10 Figure IV 11 Biography 13 043931 ACKNOWLEDGMENT The author wishes to express his thanks and appreciation to Dr. W. A. Patrick under whose instruction this work was carried on for the aid and suggestions received. I also wish to take this opportunity to extend my sincere thanks to Drs. Frazer, Reid, Lovelace and Thornton for their kind assistance in the laboratory and lecture room. THE ADSORPTION OF AMMONIA BY SILICA GEL. This investigation is a continuation of the studies of Patrick of adsorp- tion of gases and vapors by silica gel. In Gottingen he studied the adsorption of ammonia, sulfur dioxide and carbon dioxide. Since then many improvements in experimental technique ha\e been introduced, as well as radical modifications of the theoretical treatment of the experi- mental results. The experimental method and an outline of the theoretical views ha\e been given in a paper dealing with the adsorption of sulfur dioxide. 1 The adsorption of ammonia was studied for a number of reasons. In the first place, this gas, in the measurements made in Gottingen, was found to be more strongly adsorbed than sulfur dioxide although the latter exhibits a higher critical temperature. In the second place, our previous measurements indicated that the behavior of ammonia was anomalous from the standpoint of the rate at which equilibrium was reached. Again, ammonia is extremely soluble in water and it was there- fore hoped that by a careful study of its adsorption by silica gel, light would be thrown on the question as to the nature of the small amount of water that is always associated with the gel. Furthermore, in am- 1 MacGavack and Patrick, /. Am. Chem. Soc., 42, 946 (1920). monia we have a gas that has been the subject of numerous careful in- vestigations, and as a result the physical constants, such as vapor pres- sure, density and surface tension, are very accurately known. This latter point is especially important inasmuch as a knowledge of the above con- stants is necessary for the testing of our theoretical views as to the course of the adsorption. In the following measurements of the adsorption of ammonia by silica gel, it will be shown that this gas is not anomalous in its behavior, either from the standpoint of the extent or the rate of the adsorption, the earlier discrepancies being due to the uncertain water and acid content of the gel. Especial study was made of the influence of the water content of the gel on the adsorption, and it was found that small differences produced large variations in the adsorptive ability of the material. Apparatus. The experimental method has been described in detail by MacGavack and Patrick. 1 To give an idea of the precision attained, it may be stated that the volume of ammonia was measured to within 0.005 cc., the pressure to within 0.03 mm., and the temperature of the thermostat regulated so as not to vary more than 0.05. Material. The ammonia used was purified liquid ammonia. Com- mercial ammonia was treated with metallic sodium for one week, and the accumulated gas allowed to escape at intervals of 12 hours. The purified sample was tested by allowing a small stream of the gas to bubble into sulfuric acid contained in a gas buret for 10 minutes; as no residue was shown by this procedure it was assumed that the ammonia was free from permanent gases. The silica gel was an ordinary commercial sample that was further purified by treatment with nitric acid and a thorough washing with dis- tilled water. When dried in a vacuum of 1 to 5 mm. at a temperature of 300 for 3 hours the gel still contained 5.21% of water, and further- more an appreciable amount of nitric acid, to which reference will be made later. Procedure. The adsorption apparatus was evacuated by means of a rotary oil pump and a Gaede mercury pump, connected in series. Before any gel was put into the adsorption bulb, the apparatus was thoroughly evacuated and then swept out with ammonia and evacuated again until the MacLeod gage showed no pressure after standing under a vacuum for 4S hours. The gel was weighed in the adsorption bulb, which was then directly attached to the apparatus by means of a ground joint, and sealed by mercury. The pumps were always started before the stopcock between tin adsorption bulb and the main apparatus was opened, so that the air in tin- bulb would have- k-ss rhauce to become adsorbed on the walls of the apparatus. The heating of tin- adsorption bulb was started at the same time, care briiijj taken not to heat the gel to a higher temperature than that at which it was prepared, so that the water content might not be disturbed. The evacuation was continued until the MacLeod gage showed no pressure, usually from 3 to 8 hours, at a temperature from 290 to 300. In the beginning it was found almost impossible to make two determ- inations that would agree. After a considerable number of measure- ments had been made at 30, a few were found to check fairly well. How- ever, a number of measurements made at 40 showed that the gel exhibited greater adsorptive ability at this temperature than at- 30. Such in- consistent and disturbing results led us to stop and thorougly inspect our apparatus and method of procedure. The apparatus was first exam- ined for leaks. It was thoroughly evacuated and allowed to stand for one week, at the end of which the MacLeod gage showed no increase of pressure. The ammonia was again examined and no foreign gases found. Another series of measurements was made at 30 in which great care was taken that all manipulations should be as nearly identical as possible. The electric furnace, in which the adsorption bulb was heated during evacuation, was kept constant and evacuation continued for exactly the same length of time. Under these conditions checks could be made with but little variation. This proved that the gel did not remain constant TABLE I EXPERIMENTS AT 0, 30, 40 AND 100 Expt. XX Temp., 30; H 2 O content, 4.93%; Wt. of gel., 0.5739 g.; D, 0.5939; a, 18.03; I/TV, 0.2103; #,57.30; P , 874.90. P X/M v 0.07 26.82 0.034 1.009 68.05 0.087 37.726 126.66 0.161 Expt. XXIII Temp., 40; H 2 O content, 4.93%; Wt. of gel, .5730 g.; D, .5769;