Reasons for using high vacuum techniques

High vacuum technique (h.v.t.) is one of several types of experimental technique which can be employed to obtain a controlled experimental environment. The most usual reason for wanting this is the necessity to exclude oxygen and/or water and, less commonly, carbon dioxide from the reaction being studied. Perhaps the most primitive example of creating a controlled environment is the use of a soda-lime tube to protect a store of sodium hydroxide from the ingress of carbon dioxide. 

Before considering the advantages and disadvantages of h.v.t. as compared with some of the alternatives, it is important to point out that, as 

Having decided that a controlled environment is needed to exclude certain normal atmospheric components, one must then decide on the levels to which concentrations of unwanted compounds must be reduced and below which they must be maintained. 

There is a sequence of experimental techniques ranging from the open beaker on the bench to the extremely rigorous h.v.t. used for studying the kinetics of radiation-induced cationic polymerisations. Each technique in the series has its characteristic level of extraneous materials, and as the level of cleanliness is raised, there is a concurrent loss of flexibility. 

If a controlled environment is necessary for the desired experiment, it is useful to examine some of the techniques alternative to h.v.t., to estimate the level of impurities involved and to assess their advantages and disadvantages as compared to h.v.t. 

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