Gravity filtration (Figure 1) is generally the simplest, fastest, and least expensive means of separating solids and liquids. It is also the least effective separation method since the solid and filtering medium usually retain residual traces of liquid. As such, gravity filtration is the method of choice for medium- and larger-scale separations in which the primary objective is to purify and isolate the liquid phase. It also may be employed when obtaining a dry solid is unimportant or when the solids are further purified in subsequent steps.
Figure 1. Gravity filtration with fluted filter paper.
The most common filtering medium is circular filter paper, which must be folded to fit in a funnel. A fluted fold provides faster filtration than a simple cone due to greater surface area for contact with the liquid. After seating the folded filter paper in the funnel, the mixture is slowly poured through, and the filtered liquid (filtrate) is collected in a clean vessel.
General Protocol
1. Place the fluted filter paper in a funnel.
2. Place the stem of the funnel in the receiving vessel and ensure that the funnel is appropriately supported, as necessary.
3. Pour the liquid/solid mixture into the funnel and allow the filtrate to drain completely into the receptacle.