07 Oct

Filtration

Whatman PaperOne of the most common items in the lab is the ever-present cellulose filter paper. Filter papers are depth filters – during filtration they retain particles within the filter matrix not just on the surface like membrane filters. Not all filter paper is the same. There are great differences depending on the application. What is important to you, the filtrate or the residue? If the fluid coming through the filter paper is more important a qualitative filter paper would be sufficient. If the sample retained by the filter paper must be quantified, a quantitative filter paper must be used. The paper may be dried and burned in a crucible as for heavy metals testing. In this case a low ash or ashless filter would be needed.  Some filtration requires retaining particles larger than say 2um and allowing the rest to go with the filtrate. Filter paper grade is determined by the size of particle retention. Filter paper is also graded by the speed at which an aqueous solution will pass through. The smaller the particle retention the slower the speed.

The support for filter paper can be a cone shaped funnel; in this case the paper would be fluted. With a a fluted filter and cone funnel gravity is allowed to carry the sample through the filter. Another type of funnel, Buchner is used with the filter flat over a perforated support. In this case a vacuum flask with a side arm is used to contain the filtrate. The vacuum pulls the sample through the filter.

Whatman filter papers are designed for many uses from soil analysis to malt and beer filters. Whatman’s website is very helpful in choosing the right filter paper for your application. Ahlstrom paper is also available in many grades.

Robin Prymula