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General Procedures for Filtration of a Precipitate

1. Figure 1: Technique for transferring a precipitate to a funnel

   Allow the precipitate formed to settle to the bottom of the container.
2. Wet the filter paper with a small amount of water or a liquid in which the precipitate is insoluble.
3. Decant (pour off) the bulk of the solution into the funnel using a stirring rod. This step, ensures the filtration will not be slowed by the precipitate clogging the filter paper prematurely since most of the solution will have already filtered through.
4. Mix the precipitate with the remaining solution and dump the resulting precipitate slurry into the funnel. Simultaneously, spray a stream of solution from a wash bottle to rinse out any slurry adhering to the walls of the container. (See Figure 1.) Remember: the liquid or solution used for washing the precipitate must be one in which the precipitate is insoluble.
• Wet a rubber policeman with wash liquid and use it to help transfer any precipitate still clinging to the container walls.
• Use a wash bottle to rinse out whatever precipitate remains.
5. Usually precipitates are washed or rinsed to remove impurities, unless the filtrate is the desired component.
• Typically 5 to 10 mL portions of a wash solution are drawn through the funnel to wash the precipitate.
• This may be repeated several times to ensure that only pure precipitate remains.
• Allow air to be drawn through the filter paper for several minutes to dry the precipitate partially.
6. Lift a corner of the filter paper with a spatula or scoopula and then use the corner to pry up the rest of the filter paper. Carefully transfer the filter paper and precipitate to a previously weighed watch glass. Allow the precipitate to air dry, or if it has a moderate melting point (above 70^oC), put the watch glass and precipitate in an oven to dry.

Source: General Chemistry Lab, St. Mary's College of Maryland

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