Magnetic break-seals

In order to ascertain the nature of the propagating species in the polymerisation of styrene catalysed by perchloric acid in methylene dichloride we have investigated the behaviour of this system by (a) calorimetric, (b) spectroscopic, and (c) conductimetric techniques. All experiments were carried out in high-vacuum apparatus, with highly purified and dried reagents which were mixed by breaking phials or break-seals magnetically. 

Fig. 2.8. Break-seal ladder the different parts are not drawn to the same scale. Several portions of known volume of a liquid A are to be distilled into a container or apparatus B. Procedure One of the break-seals C is broken with the steel ball D and the required volume of A is distilled into a hanging burette (not shown) beyond E. The broken break-seal is sealed off by fusing at its seal-off point F then this first portion is distilled from the burette into B. For the next operation, D is transferred by a magnet into the next rung of the ladder. When all the break-seals have been used up, a second ladder can be attached at G and H without breaking the vacuum.

An alternative to the use of small spherical phials, which often has advantages in terms of the flexibility of the apparatus, is the use of calibrated vessels or ampoules with break-seals. They consist usually of a tube 3-6 mm i.d. and 20-70 mm long with a calibration mark and carrying a fragile glass membrane which can be broken when desired, so as to admit the contents of the ampoule to a mixing vessel or reaction vessel. The construction of different types of magnetic break-seals has been described in Section 2.2.4.4. 

A magnetic breaker is inserted in the side arm B, and the vessel is attached to the vacuum line and evacuated. The lower portion of the apparatus is cooled with liquid nitrogen and the break seal is opened. Because the sudden release of hydrogen pressure into the vacuum line can sometimes propel the breaker upward with considerable speed, it is advisable to cushion the upper surface of the inlet port with glass wool to avoid possible breakage. 

Pyrex tube (closed at one end) with side-arm B terminating in an inner ball joint for loading the vessel. A 13-mm. break seal, C, and an inner ball joint are used for removal of the organic materials after reaction. A small Teflon-coated magnetic stirring bar is placed inside the vessel to aid in thorough mixing of the reactants. 

Fig. 33. Break-seal valves magnetically operated hammer bar.

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