Stopcocks and Valves

Acetyl Peroxide-Carbonyl-I80. Acetyl peroxide-lsO was prepared by a modification of the usual procedure (16). An apparatus was devised to allow purification of the peroxide by remote manipulation of stopcocks and valves controlling nitrogen pressure. Recrystallization, filtration, and the addition of fresh solvent were accomplished without exposing the experimenter to the hazard of direct manipulation of crystalline acetyl peroxide. 

The clean, dry, calibrated bulb is attached to a vacuum system, such as site A on the line illustrated in Fig. 5.2, and evacuated to 10-3 torr or lower. Gas may be admitted to the calibrated bulb by isolating the working manifold from vacuum and opening the stopcocks and valves leading to one of the upper gas storage bulbs, C, while the pressure is monitored by manometer D. When the desired pressure is reached, the valve on the storage bulb is turned off, pressure and temperature measurements are made, and then the stopcock on the calibrated bulb is turned off. At this stage we know the pressure, temperature, and volume of the gas in the calibrated bulb, which permits the calculation of the number of moles via the ideal gas equation. 

Preliminary visual check of any greased stopcocks and valves. (Look for striations or channels in the grease.)... 

The difficulties created by stopcocks and valves can usually be minimized. However, it is occasionally necessary to completely eliminate these sources of leakage and contamination by the use of break-seals and vacuum seal-offs. Typical situations in which sealed tube techniques are widely used are quantitative hydrolysis and oxidation reactions which require elevated pressures and temperatures, precise physical measurements on highly reactive organometallic compounds, long-term storage of reactive samples, and nonaqueous reactions under high pressure (for example, SO2 or NH3 at room temperature). Each piece of apparatus must be constructed to meet a specific need, so it is not possible to outline an apparatus which is of general use. Nevertheless, several examples will be presented here which serve to indicate the approach. 

Attention also is directed to other chapters which are useful adjuncts to the present one Inert Gases and Their Purification (Chapter 3), Purification of Solvents and Reagents (Chapter 4), and Joints, Stopcocks, and Valves (Chapter 8). The solvent and chemical resistance of elastomers, which are used in O-ring joints and septa, are discussed in Chapter 8 and Appendix III. 

All Teflon stopcocks and valves must remain free of particulate contamination so the surface will not be scratched or damaged. Any abrasions, or distortions, of the central shaft are likely to be leak sources. In addition, do not place any filled container closed with a Teflon stopcock (or valve) in a refrigerator. Just as Teflon expands in heated environments, it contracts in cooled environments. Thus, what was a good seal at room temperature may leak in a cold environment. Do not try to compensate for the cold temperature by overtightening the shaft for two rea-... 

The reaction is very exothermic. If a laboratory has a methane leak and the gas finds an ignition source, the result will be an explosion. It is very important to make sure stopcocks and valves are turned off completely when finished using this flammable gas. 

The parent carbon hydride, on the other hand, is the fully covalent methane, CH4, the simplest of thousands of alkanes of general formula C 2n+2- Although others had prepared the corresponding silanes before, Alfred Stock, in 1916, was the first to carry out a systematic investigation of them with the aid of his vacuum hne and its mercury-activated stopcocks and valves. In the absence of air and water, he found that the metal silicides (as he had the borides, see p. 400) reacted with various acids, as represented in Equation (15.3) ... 

---