Cellulose Filter Paper

One of the most common items in the lab is the ever-present cellulose filter paper. Filter papers are depth filters - 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, 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.