Filter paper. We all use it at one time or another. Whether we’re testing soil samples, removing dissolved gases from beer, or perhaps just soaking up that coffee spill.
But do you know why you’re using that grade of filter paper? Is it just because your protocol tells you to?
In this back-to-basics blog, we’ll review some of the most popular basic filter paper grades, and pick out a few key applications.
What is qualitative filter paper?
Qualitative filter papers are what most users would consider general purpose filter papers. There are a vast range of applications in and out of the laboratory. Composed of cellulose fiber, these filters are manufactured from high-quality cotton linters treated to maximize alpha cellulose.
Alpha cellulose is the most stable form of cellulose, with the highest degree of polymerization. Cellulose paper made from a high percentage of alpha cellulose, such as Whatman paper, is a sign of high quality and consistency.
Qualitative filter paper grades are based on various properties including particle retention, thickness, and weight. Unlike quantitative filter papers, of which ashless grades can be burned off for analytical applications, qualitative papers are more suited for general purpose use.
Different types of qualitative filter paper & their properties
The most common qualitative paper grades are 1-6, 595, 597, 598, and 602, which have applications across multiple industries. Below are summaries of the grade properties and common uses.
|Paper Grade||Available Sizes||Properties||Applications|
|Filter Paper Grade 1|| || ||Grade 1 is the most widely used filter paper in routine applications requiring medium retention |
and flow rate, and is most frequently used for clarifying liquids. Specific applications include:
|Filter Paper Grade 2|| || ||Grade 2 is slightly more retentive than Grade 1, with an equivalent increase in filtration time. Specific applications include: |
|Filter Paper Grade 3|| || ||Grade 3 is double the thickness of Grade 1, so it can hold more precipitate without clogging and has increased wet strength. It is designed for use in Büchner funnels, and the high absorbency makes Grade 3 well suited for sample transport after collection.|
|Filter Paper Grade 4|| || ||Grade 4 is a fast filter material that is well suited for retention of coarse particles and gelatinous precipitates, such as ferric hydroxide and aluminum hydroxide. Specific applications include: |
|Filter Paper Grade 5|| || ||Grade 5 provides the maximum degree of fine particle retention in the qualitative paper range, with an equivalent slow flow rate. Specific applications include: |
|Filter Paper Grade 6|| || ||Thick with a high loading capacity, Grade 6 provides similar particle retention to Grade 5 with twice the flow rate. It has high absorbency and increased wet strength, and is often specified for boiler water analysis. |
|Filter Paper Grade 591|| || ||A thick filter paper with very high loading capacity for fast filtration of medium to coarse precipitates. Offers high absorbency and increased wet strength. Also available prepleated as Grade 591 ½.|
|Filter Paper Grade 595|| || ||Popular thin filter papers, Grade 595 provides medium-fast flow rate and fine particle retention. It is used in a variety of routine analytical applications, including: |
|Filter Paper Grade 597|| || ||A medium-fast flow rate paper with medium-to-fine particle retention, Grade 597 is used in a variety of routine analytical applications, including: |
|Filter Paper Grade 597L|| || ||A qualitative filter paper with low fat content. Suitable for nitrate determination in foodstuffs to §35 LMBG* (* LMBG = German law for food and consumer products). |
|Filter Paper Grade 598|| || ||A medium-fast to fast flow rate paper with medium particle retention, Grade 598 has a high thickness, giving it a high loading capacity. |
|Filter Paper Grade 602h|| || ||Grade 602 h is a dense filter designed for collecting very fine particles. It is commonly used in the beverage industry for: |
1At 98% efficiency.
2Determined by ignition at 900°C.
Other guides and resources:
Guide to quantitative filter papers
Whatman Methods & Standards Search Tool
Laboratory Filtration Principles and Chemical Compatibility Chart (PDF)