ANALYTICAL C H E M I S T R Y
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gradually applied until the gage records 12 to 14 inches of mercury "v&cuum," and the stopcock is closed (samples of cellophane tubing vary somewhat in bursting strength and a trial on the particular batch to be used should be run). The liquid level must he maintained well above the point of attachment of the sac a t all times.
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After the ahsence of obvious leaks has been ensured, the apparatus may be left unattended, except for occas i o n a l r e s t o r a t i o n of the vacuum should slow leakage CELLOPHANE SAC d e v e l o p . Bacterial action during long runs (24 to 48 rlLTER FLASK hours) may he reduced by placing the unit in the refrigerator, or complete sterility assured by autoclaving the flask assembly prior to use aftrr insertion of cottonplugs. The condensate collecting on the inner walls of the flask during refrigeration must ultimately be combined with the ultrafiltrate in precise andytical work. Although the pressure differential in this apparatus is not so great m in certain other methods ( f - 3 ) ,the large filtration surface permits rapid formation of protein-free ultrafiltrate. The siee of the flask and sac may be varied over wide limits to suit individual requirements, and the fluid may be introduced into the sac on a continuous-flow basis if it is of large volume. LIT1
