The Maintenance of an Inert Atmosphere

A whole laboratory can be readily served by one purification system if the purified gas is fed into a manifold with several outlets. Copper or aluminum tubing with Swagelok fittings and needle valves are convenient for this purpose. 

Where possible, gas connections should be made with metal or glass tubing. Where short flexible connections are necessary, PVC (Tygon) tubing is recommended. Most plastic materials have an appreciable permeability, a 1-m length of l-in.-ID polyvinyl chloride tubing with 13-in wall thickness will allow the diffusion of about 0.8 ppm of oxygen at a flow rate of 150 mL min-1. 

A common practice is to saturate the purified, oxygen-fiee gas with solvent in a gas-dispersion cylinder that is filled with a solution identical to the test solution. This avoids the evaporation of the solvent or the differential evaporation of a volatile constituent of the test solution that would otherwise accompany the continuous purging of the system. Such a practice is particularly important for volatile solvents like acetonitrile or for solutions that contain volatile constituents like ammonia. 

The rate of circulation in the purification train is important a throughput not less than one box-volume per minute is recommended. The time required to achieve a specific reduction in the level of oxygen is approximated by the relation 

E = efficiency (fraction of 02 removed on passage through the purification train) 

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