Stopcocks leakage

Because of difficulties in adjusting an ordinary glass stopcock to avoid leakage and to maintain a drop rate that would add the un-saturated cyanoester solution over a 40-hour period, the checkers recommend the use of a funnel equipped with a Teflon stopcock. 

Leakage A burette must be tested for any sort of leakage before putting it into operation. Teflon stopcocks are usually adjusted by a knurled nut for perfect use. Glass stopcocks may require a small quantity of a special type of grease or lubricant to allow both ease of operation and to check leakage. 

The long-term storage of liquids and gases is best accomplished in an ampule or bulb fitted with a break-seal. Once the sample is sealed off in such a container, leakage is no problem as it would be if a stopcock or valve were used. Furthermore, this practice is economically sound, since a break-seal is much less expensive than a stopcock or valve. 

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. 

After the distillation, the flask containing the bromine is attached to the reactor at the ball joint S. Stopcock Rt is kept closed. The bromine is cooled to —40°C. (alcohol bath ), and the system is evacuated through stopcock Ri. After evacuation, Ri is closed, the condenser C is cooled to — 40°C., and flask V is warmed to 25°C. Bromine vapor is introduced into the reactor through stopcock R . The rate of bromine addition should be adjusted so that the product condensing in C is colorless. R3 should be closed until the rate of bromine addition is properly adjusted. (Br2 attacks Hg rapidly.) If the pressure in the reactor system rises much above 100 mm. Hg because of gas leakage, gases can be pumped out periodically through R3. 

Assuming that the joint or stopcock selected is proper for the application and there is evidence of leakage, you should look for potential human errors ... 

Procedure Carefully add the Sample Preparation to the prepared column. Open the stopcock, and adjust the flow rate to about 2 mL/min, discarding the eluate. Rinse the sample beaker with 5 mL of chloroform, and add the rinsing to the column when the level drops to 2 cm above the silica gel. Never allow the column to become dry on top, and maintain a flow rate of 2 mL/min throughout the elution. Avoid interruptions during elution as they may cause pressure buildup and result in leakage through the stopcock or cracks in the silica gel packing. 

Vacuum stopcocks are more expensive than those intended for use at ordinary pressures. The ground surfaces are more precise and the makers test their products for vacuum tightness before offering them for sale. One type of vacuum stopcock, shown in Fig. 11.11, has an oblique bore, which reduces the possibility of leakage through channels in the grease layer. The T-type and L-type, also shown in Fig. 11.11, are excellent for vacuum work. The keys of these types are held in place by atmospheric pressure when the apparatus is evacuated, and are therefore unlikely to be disturbed from their seatings when rotated. 

The lubricant in a joint or stopcock serves to protect the surface from wear, to prevent leakage, and to facilitate the separation and rotation of the two members. Petroleum jelly is used for many assemblies but is unsuitable for equipment operating at high temperature or under vacuum. Some laboratories prepare special greases for use under conditions that do not permit hydrocarbons to be used. 

For high-vacuum work, the type of lubricant used for stopcocks, standard taper joints, and ball-and-socket joints is of great importance. With joints and stopcocks which are rarely opened, it is desirable to use a low-melting wax which vidll seal the joint and prevent leakage. When... 

Use of the buret. Check the stopcock (if ground glass) of your buret for proper lubrication, and be sure the bore and tip are clear. Fill with distilled water and place in the buret clamp the water should be at room temperature. Displace any air bubbles from the tip of the buret, and adjust to near the zero mark. Allow to stand for a few minutes to check for leakage and read the initial volume to the nearest 0.01 mL. Use a meniscus reader, and be careful to avoid parallax. 

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