SMALL SAMPLES
A n EQUILIBRIUM
EXPERIMENT EARL H. BROWN
EARL H. BROWN
Antioch College, Yellow Springs, Ohio
Antioch College, Yellow Springs, Ohio
URING the past two years the author has had occasion to use smaU samples of sulfur trioxide. The followingmethod has been found satisfactory for the preparation, weighing, and handling of small quantities of the compound. The method is similar to that employed in the preparation of boron triethylimine.' A number of small bulbs of about 7 mm. diameter are blown. Capillary necks about 8 cm. long are left on the bulbs. They are carefully inspected and the weak ones discarded. They are then numbered, weighed, and
HE determination of the degree of dissociation of sulfur trioxide by the Dumas method is quite feasible for an experiment in a beginning course in physical chemistry. The following is a description of the technic developed for the experiment. A 500-cc. round-bottomed Pyrex flask, A (see illustration), with a neck a t least 10 cm. long is cleaned and dried. A bulb, B, of sulfur trioxide is placed in the flask and the neck drawn to a capillary. The capillary is left about 10 cm. long and bent a t right angles about 2 cm. from the end. The flask is then weirhed and placed in a suitable furnace, C, with the tip of the capillary down. The temperature of the furnace is then slowly raised to the desired value. The sulfur trioxide bulb breaks shortly after heating is started. It is necessary to flame that part of the capillary outside of the furnace to prevent condenL sation. The tip is pointed downward so that drops of sulfuric acid formed a t the end do not run back into the flask. The temoerature measurement is made by a chromel-alumel thermocouple, D. When the temperature is between 500 and 600°C., the tip of the capillary is sealed with an oxygen flame (exact temperature noted) and the flask removed from the furnace. When it has cooled to room temperature, i t is weighed, the volume is determined, and the necessary calculations are made. An undergraduate can perform the above-described experiment with ease provided he does not have to prepare the sulfur trioxide. This should be prepared by the instructor or assistant. A sufficient quantity of samples can be prepared a t one time to supply the class.
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inverted in tube C (see illustration). The capillaries should be well cleaned before the bulbs are placed in tube C. A brown color appears in the sulfur trioxide near those not cleaned after contact with the hands. A mixture of fuming sulfuric acid and phosphorus pentoxide is poured into flask A and phosphorus pentoxide into B. The system is then evacuated and the stopcock closed. Tube B is then immersed in a cooling bath of ice water and flask A gently heated. When a sufficient quantity of sulfur trioxide has condensed in tube B, the tube between A and B is closed with a flame. The cooling bath is then transferred from B to C, and the trioxide distilled into C. Dry air is then admitted to the system through the stopcock, forcing the liquid into the bulbs. Tube C is then opened, and the bulbs are removed, sealed, and weighed. It is well to cover the hands with vaseline while sealing the bulbs. This method of handling sulfur trioxide offers the following advantages: 1. Weighed samples are available for immediate use. 1 C. A. b u s AND E. H. BROWN, "Studies relating to boron. 11. Action of lithium on ethylammino boron trifluoride," 5. Am. Ckem. Soc.. 52.4414-8 (Nov.. 1930).
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2. A large number of samples can be made up a t one time. Twenty-five one-gram samples are easily prepared in one preparation. 3. Enclosed in the bulbs, the compound can be very safely handled.
