O. Hello
Hobart Technical College Tasmania, Australia
Accurate Measurement of Gas Volumes
by a Soap-Film Method
T h e measurement of small gas volumes can be greatly improved by using a soap-film apparatus as shown in the diagram. It can be shown by injecting air into a closed system that a soap film will move under a pressure differential of 10-B atm and would clearly show the injection of 0.5 p1 of air into a 400 ml volume. This is about 1000 times more sensitive than a mercury filled gas buret, which would show only a noticeable movement of the mercury level after injecting 0.7 ml of air into the same volume. The apparatus can also be used as an accurate gas-flow meter. It consists of a 10/ 0.02 ml or 5/0.01 ml buret top or pipette A clipped borizontally on a board. Soap solution is added to the container C, from which the level in the T-piece and plastic junction B is controlled, until it is just below the tip of the measuring tube A . By gently blowing through the inlet tube D and lifting momentarily tube B one or more soap-films can be positioned in the measuring tube. The apparatus can be connected to a modified VictorMeyer apparatus as described by G. Wilson Drake,' which uses a micro-syringe injection method. As soon as the soap-film stops moving, temperature equilibrium
Apparatus for gar volume measurement.
568 / Journal of Chemical Education
has been established; the system is an accurate d i e r ential gas thermometer. A simplified Victor-Meyer apparatus is shown in the figure. The high-temperature septum F is enclosed by two ground glass tubes in the bung. The 10 pl sample is injected after the needle E has been pushed through the silicone rubber plug covering its tip and the septum F simultaneously. After injection, the plunger of the syringe is withdrawn to its original position and the needle is lifted out of the septum till it is just covered by the little plug again. This procedure will compensate for the extra volume of the injected sample and practically eliminate evaporation between weighings. A vacuum-flask outer container will give improved insulation. By connecting each end of the soap-film meter to a fixed-volume vessel, each provided with in- and outlet taps, a very accurate pressure or differential pressure meter for low pressures can be constructed.
-
