Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

102 EXPERIMENTAL METHODS FOR SLOW REACTIONSH6Thermometer0 10 20 30 crnI I IFig. 71. Gas density balance. From ref. 22c.Further details are given in ref. 22c. The instrument has long been superseded forconventional analysis by the mass spectrometer. However, it remains as an absolutecontrol for gas chromatographic analysis.4.5.8 Quartz3bre manometerThe damping of the oscillation of a quartz fibre depends on the pressure (p)and molecular weight (M) of the gas present. The time for the amplitude to decreaseto one half of its original value t+ is given by equation (V')A and B are constants (B being characteristic of the gas present). A simple modelis shown in Fig. 72. A gentle tap is required to set the fibre in vibration, the amplitudeof which may be observed with a microscope carrying a scale. As the equationimplies, the gauge may be used to measure pressure or molecular weight.It might be used, for example, to determine the extent of dimerisation of NO,.4.5.9 Inteferometry and refructometryGreen and Robinson334 have estimated small amounts of germane in hydrogenby interferometry. A parallel beam is split in two by a double slit and passedthrough two identical tubes, containing the gases to be compared, and then throughtwo identical glass plates. The emergent beams are combined to form interference