A GAS-DENSITY BALANCE FOR STUDENT USE A. E. WERNER University of British Columbia, Vancouver, Canada GAS-density measurements have been used in many fundamental investigations. Ramsay determined the atomic weight of radon by measuring its density. Aston used the gas-density balance to study the isotopes of neon. An extensive bibliography of the applications of gasdensity determination has been compiled by Partington (1). A school of English chemists headed by WhytlawGray developed the instrument as a microbalance and used it to determine the atomic weights of carbon and nitrogen with an accuracy comparable to that obtained by the mass spectrograph (B). I n Germany, A. Stock and his co-workers studied the errors inherent in the instrument's design and then devised a form of gasdensity balance which is easy t o operate and has found use in research and industrial laboratories (3). Continued interest in this research tool is reflected by a number of recent publications, some of which describe gas-density balances for student use ( 4 , 5 ) . The majority of these papers deal with fragile all-glass apparatus unsuited to the use of undergraduates. A
balance has been developed in this laboratory which has proven itself to be sufficiently robust. DESCRIPTION OF APPARATUS
The main components of the apparatus are an ordinary analytical balance and a bell jar assembly consisting of a metal base plate and a thick-walled glass bell covering it. Accessory equipment includes reducing valves for the gas cylinders, a motordriven vacuum pump, a manometer, and a distributing line with two stopcocks (Figure 1). The balance column is fastened to the metal base of the bell jar by four screws. A rubber ring of about 2-mm. width and l-mm. thickness forms the gas-tight seal. A metal bellows is soldered to the moving central rod and t o the column (Figure 2). The pans of the balance are removed but the stirrups are retained to carry the suspension wires for the float and counterpoise. These suspensions consist of #28 Chrome1 A wire. The float and counterpoise are made of Pyrex glass and their weights are adjusted to
1. 1'. Gas cylinders: 2, 2', needle valves; 3. 3'. glass capillaries; 4, t w o - m u stoocock; 5 , two-way stopcook: 6, drying tube; 7, vaouum oump oonneotion; 8. manometer; 9, constriction; 10. meter stick; 11,mercury trap: 12, gas inlet tube: 13, counteryoise; 14. hook and weight; 15, balance beam; 16, float; 17, thermometer; 18, spirit level: 19. mit ti on of metal bellows.
JOURNAL OF CHEMICAL EDUCATION
394
balance in air a t atmospheric pressure. The dimensions of the float are 125.7 mm. X 25.4 mm., while those of the counterpoise are 80 mm. X 25.4 mm. The balance zero can be changed by hanging weights in the form of wire hooks on the vacant hook of the stirrup carrying the counterpoise. The weights are made from Chromel A wire and weigh from 10 to 60 mg. The gas inlet tube (at least in. in diameter) is screwed into the base plate. It is prolonged about 3 cm. inside the bell jar and has a closed end. The gas escapes through several 4-mm. holes drilled so that the escaping gas does not strike Fig"- 2. Bel- the balance mechanism. A thermometer lows Seal (0-5O0C., 0.1" divisions) and a spirit level Arrow shows are also provided inside the bell jar. direction of The manometer is a mercury-filled motion. Pyrex U-tube about 8 mm. in diameter and 1 m. in length which is mounted over a meter stick with one mm. divisions. It has a constriction to reduce the effect of surges and a trap with stopcock to protect the balance from the spilling of mercury. This trap connects the manometer to the manifold which has a calcium chloride tube attached to its open end. The vacuum pump, the gas cylinders, and the gas-density balance are connected to the manifold by thick-walled mbber tubing. Apiece of glass capillary tubing 1 mm. bore, 100 mm. long is inserted in each of the lines from the gas cylinders to reduce surges. The cylinders themselves are provided with needle valves.
base. When the jar has been evacuated, the pump is shut off and the motor stopped to prevent vibration. The balance beam is released, and the standard gas let in until the balance trips over. The gas supply is then stopped a t the needle valve and a t the manifold, the pump started again, and the pressure inside the jar reduced slowly until the balance is a t zero. The pressure and temperature inside the jar are now recorded and the gas pumped out again. The whole procedure is repeated with the unknown gas. It is advisable to rinse with each gas before taking the final reading because gases tend to linger under the bell jar. The recorded pressure p is reduced to 0°C. by the formula ppo = (p X 273)/T. The molecular weight of the unknown gas can then be calculated from the relation:
TYPICAL RESULTS
A- few ranresentative results are shown in Table 1. -~~These values are the averages of five independent trials. The molecular weights of a variety of technical-yade gases were determined. The results are given in Table 2. The composition of binary mixtures of nonreacting gases could also be determined provided that the molecular weights of the components differed sufficiently and that the components were present in more than trace amounts. The performance of this gasdensity balauce shows that the ideas embodied in its construction are sound. These ideas are (1) . . to increase the u ~ t h r u s oft the float by increasing its volume and thus obtain readily measMETHOD OF OPERATION urable forces, (2) to use the mechanism of an ordinary The appropriate weight is placed in the hook of the analytical balance to compare these forces, and (3) and the bell jar putin position and sealed to use a metal bellows for introducing the motion reright with Apiezon grease N around the outside of the ground wired for the beam-arrest mechanism. In this way a desirable ruggedness of the equipment is attained without sacrifice of accuracy. As can be seen TABLE 1 from the results tabulated above, the limitation lies in Determination of M.W. of NS,Using O1 es Standard the manometric technique and not in the performance M . W . of N . of the balance. Limiting density experiments or deassuming 0. = p,," of 0, in p," of N 1in mm. mm. 88.00 termination of compressibility coefficients could he done by using a more sensitive manometer such as a 28,06 + 0.07 330.0 zt 0 . 3 377.1 + 0 . 7 173.9 + 0 . 2 198.5 zt 0 . 5 28.05 + 0.07 McLeod gage. 113.0 + 0 . 2 129.0 + 0 . 2 28.07 + 0.11 This apparatus is subject to the restrictions that it 28'05 14.3 + 0 . 6 16.3 + 0 . 1 will handle neither small quantities of gas nor corrosive - A ~
~~
~
*
TABLE 2 Determination of M.W. of Technical-grade Gases
.- . Nitr,?gen C " ; P dio$de
.vooof. u n k m am i n mm. 557.5 557.5 362.3 362.3
Argon Ethylene Methyl Chloride Oxveen
389.9 557.3 314.5 494.6
M.W. found
M. W. theoretical
40.10 28.55 50.70 32.50
39.99 28.05 50.49 32.00
VOLUME 33, NO. 8, AUGUST, 1956
Eases. For the elementary physical chemistry labora.. . tory these limitations are not serious becausea fair selection of non-corrosive gases is available in compressed form and a lecture bottle of the gas is sufficientfor many experiments. LITERATURE CITED (1) PARTINGTON, J. R., "An Advanced Treatise on Physical
395 Chemistry," Longmans, Green & Co., London, 1940, Vol. I, p. 753. (2) LEADBEATER AND WHYTLAW-GRAY, Quadedy Rwiewa of the Chemical Society, N , 170 (1950). (3) SCHURMACHER, MOLLET, A N D CI.USIUS, He6. Chim. Aefa., 33, 2117 (1950). (4) DANIELS,MATHEWS, AND WILLIAMS, "Experimental Physical Chemistry," 4th ed., MoGraw-Hill Book Co., Ino., New York, 1949, p. 7. (5) ESEREIARD, W.H.,J. CHEM.EDUC.,27, 248 (1950).
