Selecting the Proper Equipment for Gravity Filtration
Video. Selecting a receiving container.
The Receiving Container
The receiving container should be much larger than the amount of liquid that it will ultimately contain. Add the volume of the mixture and the volume of any rinses or washes together to approximate the total volume that the flask will have to contain. Then, select a beaker or Erlenmeyer flask that is much larger than this total volume (if the approximation is 100 mL, select a 250 mL flask).
IMAGE. A glass funnel.
The Funnel Size
A funnel should be large enough to contain the solid as a thin layer after the filtration is complete. Smaller funnels will cause the filtration process to be slower and the risk of contamination is higher.
IMAGE. The filter paper should be slightly larger than the funnel.
The Filter Paper
Select a piece of filter paper that, when folded, will barely extend above the top of the funnel. In general the diameter of the unfolded filter paper circle should be about twice the diameter of the top of the funnel.
Paper is available with different separation properties. Coarse, medium, and fine grades are typically available. Coarse grades will separate relatively large particles from the liquid. Fine grades will separate all but the smallest particles from the liquid, and medium grades will perform somewhere in between. The finer the grade, the more time it will take for the filtration. Thus, you should select the coarsest grade that will completely separate the solid from the liquid (this selection comes with experience).
Note: If the filtrate contains solid after the filtration, check for leaks or channels in the filter. Barring a leak or channel, select a finer grade of paper or fritted glass for the next experiment.
