# 1.5A: Overview of Methods

## Overview of Filtering Methods

There are many methods used to separate a mixture containing a solid and liquid. If the solid settles well, the liquid can sometimes be poured off (decanted). If the solid has very small sized particles or forms a cloudy mixture, the mixture can sometimes be centrifuged or passed through a filter pipette (on the microscale, < $$5 \: \text{mL}$$).

The most common methods of solid-liquid separation in the organic lab are gravity and suction filtration. Gravity filtration refers to pouring a solid-liquid mixture through a funnel containing a filter paper, allowing the liquid to seep through while trapping the solid on the paper (Figure 1.66a). Suction filtration is a similar process with the difference being the application of a vacuum beneath the funnel in order to pull liquid through the filter paper with suction (Figure 1.66b).

Gravity and suction filtration have pros and cons, but what helps decide which method to use is generally whether the solid or filtrate is to be retained. The "filtrate" refers to the liquid that has passed through a filter paper (as indicated in Figure 1.66a). Gravity filtration is typically used when the filtrate is retained, while suction filtration is used when the solid is retained.

Gravity filtration is preferred when the filtrate is retained as suction has the potential of pulling small solid particles through the filter paper pores, potentially producing a filtrate contaminated with the solid compound. Suction filtration is preferred when the solid is retained as gravity filtration is much less efficient at removing residual liquid from the solid on the filter paper.