Filtration disaster

Another procedure which has adequate sensitivity for determining the glucose in 1 microliter of serum of filtrate, is the method which uses copper reduction, and subsequently determination of the cuprous ion with a suitable reagent (15) However, one must be careful that one has obtained complete precipitation, for, if uric acid or any other impurities remain, false high values will be obtained. This would result in disaster for the hypoglycemic infant. To uncover this condition is often one of the major reasons for doing this test. 

Every year I run a chem lab and when someone is doing a vacuum filtration, suddenly I ll hear a scream and a moan of anguish, as water backs up into someone s filtration system. Usually there s not much damage, since the filtrate in the suction flask is generally thrown out. For vacuum distillations, however, this suck-back is disaster. It happens whenever there s a pressure drop on the water line big enough to cause the flow to decrease so that there is a greater vacuum in the system than in the aspirator. Water, being water, flows into the system. Disaster. 

The key to successful gravity filtration is the fluted filter paper. A fluted filter paper decreases the area of contact between the filter paper and the funnel, thus allowing rapid filtration. If you use traditional cone-folded filter paper, note that all sides of the paper are touching the sides of the funnel and on half the filter paper the liquid has to pass through three thicknesses of paper, all of which slow the rate of filtration. Slow filtration can lead to disaster in hot filtration during recrystallization (p. 100). 

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