Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

3 APPARATUS 7Fig. 3. Ramsperger’s greaseless valve. From ref. 22c.cants, used for the isolation of gases. A problem associated with the use of lubricantsis their ready absorption of vapours and usually the inability to “bake” thetaps. The Echols type of tapz3 avoids these problems by using graphite as the lubricantand sealing with mercury. Apart from this, alternatives to the conventionaltaps are various greaseless valves2’. The simplest of these are mercury cut-offsshown in Fig. 2. There may be some difficulty if there is a large pressure differentialbetween either side of the U-tube. Polythene or teflon keys may be used with glasstubing to form a tapz4. However, they are subject to degassing and may have theirown vapour absorption problems. This also applies to taps using elastomer diaphragms.Rapid operation of the latter type, either manually or electro-magnetically,has been achieved by Verdin”. A modification of the Bodenstein all-glass tapz6is due to RamspergerZ7 (Fig. 3). The tap is closed by seating a plug of AgCl intoa Pyrex tube, of which it wets the surface. Seating is achieved by a screw attachedto silver bellows coated with AgCl. This is an excellent tap provided no chemicalreaction occurs between its components and the gases involved. A similar tap hasReferences pp. 108-111