Stopcocks design

A simple, basic two-way glass stopcock (shown in Fig. 3.17) will have a single hole drilled through a solid plug. The arms are straight and placed 180° from each other. The plug is typically held in place by a rubber washer or metal clip at the small end. This stopcock design is easy to make and is inexpensive. The usual problems that arise from this type of stopcock are as follows ... 

All of the above designs are called two-way stopcocks because there are two arms, or connections, to the stopcock. Two-way stopcocks typically are open/ close valves with no proper orientation. The exception for this statement are the stopcock designs shown in Figures 3.18, 3.19, and 3.20, which require a specific... 

Following this procedure, the funnel is positioned in a padded ring in a ring stand and left undisturbed for a period of time to allow the two immiscible layers to once again separate. The purpose of the specific design of the separatory funnel is mostly to provide for easy separation of the two immiscible liquid layers after the extraction takes place. All one needs to do is remove the stopper, open the stopcock, allow the bottom layer to drain, and then close the stopcock when the interface between the two layers disappears from sight in the stopcock. The denser of the two liquids is the bottom layer and will be drained through the stopcock first. The entire process may need to be repeated several times, since the... 

No. Besides the convenient use of the separatory funnel for the actual extraction, they are also designed for easy separation of two immiscible liquids after the extraction through the stopcock. It would not be easy to separate a liquid from a solid through the stopcock. 

Fig. 4. Schematic vacuum system for metal atom reactions. X represents the stopcock or Teflon-in-glass valve. Satisfactory components (for a general discussion of vacuum line design see References 1 and 4) forepump, 25 L/min free air capacity diffusion pump, 2 L/sec main trap is removable and measures about 300 mm deep main manifold has a diameter of about 25 mm, stopcock or valve in manifold should be at least 10 mm substrate container is removable container with 1-2 mm Teflon-in-glass needle valve connected to bottom of container. Connection between this needle valve and the reactor may be 1/8 in. od. Teflon tubing is used. Alternatively, the substrate may be added as shown in Fig. 3.

If visual inspection of the most suspect joints or stopcocks fails to reveal the leak, a systematic isolation of parts of the vacuum system is in order. For a vacuum line of conventional design (e.g., a line approximating that in Fig. 5.2), it is generally best to turn off all stopcocks which interconnect the various parts of the vacuum system. As a result, the high-vacuum manifold is isolated from the pumps and the rest of the line, and it is checked by determining if there is a steady pressure rise in that section. If this section appears to be intact, but it... 

C. Stopcocks for Pressure and Vacuum. In a typical Schlenk system, the apparatus is repeatedly exposed to a small positive pressure and to vacuum. This causes stopcocks to become dislodged therefore, satisfactory performance requires good stopcock retainers. An excellent retainer design is illustrated in Fig. 8.8. This particular retainer requires the use of the stopcock produced by the same manufacturer.6... 

An excellent stopcock and retainer design tor use with Schlenk systems is available from Kontes Glass Co. (address in footnote 1). 

Fig. 9.2. Constant-volume tensimeter. The sample tube (enlarged view, upper left) is loaded with solid in a dry box or by sublimation from the vacuum line. The lube is evacuated, sealed, weighed, then glassblown to the tensimeter. After evacuating and flame-drying the tensimeter, the mercury level is raised and the break-seal cracked. Since the mercury serves as the cutoff to the vacuum manifold, the sample is not exposed to grease, stopcocks, or joints. This design is desirable when very long equilibration times are necessary. The mercury level is adjusted at the volume-calibrated reference point, such as A on (he diagram, if it is important to know the gas volume in the apparatus.

Grignard reagents where traces of oxygen and stopcock grease may lead to erroneous results.60 61 As illustrated in Fig. 9.34, the apparatus is designed with a specific sequence of operations in mind. 

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