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.

Whatman Filter Paper: 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 Whatman 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
  • Circles: 10 mm to 150 cm (available prepleated)
  • Sheets: 26 x 31 mm to 600 x 600 mm
  • Particle retention1: 11µm
  • Nominal thickness: 180 µm
  • Nominal basis weight: 87 g/m2
  • Nominal ash content2: 0.06%
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:
  • Qualitative analytical separation for lead sulfate, calcium oxalate, and calcium carbonate
  • Soil analysis and seed testing
  • Separating foodstuffs from associated liquid or extracting liquid
  • Collecting atmospheric dust for photometric measurement
  • Gas detection through the addition of a chromogenic reagent
Filter Paper Grade 2
  • Circles: 42.5 mm to 500 mm (available prepleated)
  • Sheets: 460 x 570 mm to 580 x 680 mm
  • Particle retention1: 8µm
  • Nominal thickness: 190 µm
  • Nominal basis weight: 97 g/m2
  • Nominal ash content2: 0.06%
Grade 2 is slightly more retentive than Grade 1, with an equivalent increase in filtration time. Specific applications include:
  • Plant growth trials, where the extra absorbency is utilized to hold soil nutrient
  • Contaminant monitoring in air and soil
Filter Paper Grade 3
  • Circles: 23 mm to 320 mm (available prepleated)
  • Sheets: 26 x 31 mm to 600 x 600 mm
  • Particle retention1: 6µm
  • Nominal thickness: 390 µm
  • Nominal basis weight: 185 g/m2
  • Nominal ash content2: 0.06%
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
  • Circles: 27 mm to 400 mm (available prepleated)
  • Sheets: 38 x 114 mm to 580 x 580 mm
  • Particle retention1: 25µm
  • Nominal thickness: 210 µm
  • Nominal basis weight: 92 g/m2
  • Nominal ash content2: 0.06%
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:
  • Air pollution monitoring where the collection of fine particles is not critical
  • Routine clean-up of biological fluids or organic extracts
Filter Paper Grade 5
  • Circles: 25 mm to 320 mm (available prepleated)
  • Particle retention1: 2.5µm
  • Nominal thickness: 200 µm
  • Nominal basis weight: 100 g/m2
  • Nominal ash content2: 0.06%
Grade 5 provides the maximum degree of fine particle retention in the qualitative paper range, with an equivalent slow flow rate. Specific applications include:
  • Retaining fine precipitates in chemical analysis
  • Clarifying cloudy suspensions in soil and water analysis
Filter Paper Grade 6
  • Circles: 42.5 mm to 240 mm (available prepleated)
  • Particle retention1: 3µm
  • Nominal thickness: 180 µm
  • Nominal basis weight: 100 g/m2
  • Nominal ash content2: 0.06%
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.
  • Plant growth trials, where the extra absorbency is utilized to hold soil nutrient
  • Contaminant monitoring in air and soil
Filter Paper Grade 591
  • Sheet: 580 x 580mm
  • Particle retention1: 12µm
  • Typical thickness: 180 µm
  • Basis weight: 161 g/m2
  • Nominal ash content2: 0.06%
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
  • Circles: 110 mm to 150 mm (available prepleated)
  • Sheet: 580 x 580mm
  • Particle retention1: 7µm
  • Typical thickness: 180 µm
  • Basis weight: 68 g/m2
  • Alpha cellulose: 98% minimum
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:
  • Particle separation from food extracts
  • Filtration of solids from digested environmental samples for iCP/AAs analysis
Filter Paper Grade 597
  • Circles: 12.7 mm to 320 mm (available prepleated)
  • Particle retention1: 4-7µm
  • Typical thickness: 180 µm
  • Basis weight: 85 g/m2
  • Alpha cellulose: 98% minimum
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
  • Circles: 40mm x 100mm
  • Particle retention1: 7µm
  • Typical thickness: 180 µm
  • Basis weight: 82 g/m2
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).
  • Particle separation from food extracts
  • Filtration of solids from digested environmental samples for iCP/AAs analysis
Filter Paper Grade 598
  • Circles: 90 mm (available prepleated)
  • Sheet: 580 x 580mm
  • Particle retention1: 8-10µm
  • Typical thickness: 320 µm
  • Basis weight: 140 g/m2
A medium-fast to fast flow rate paper with medium particle retention, Grade 598 has a high thickness, giving it a high loading capacity.
  • Particle separation from food extracts
  • Filtration of solids from digested environmental samples for iCP/AAs analysis
Filter Paper Grade 602h
  • Circles: 125 mm to 240 mm (available prepleated)
  • Particle retention1: <2µm
  • Typical thickness: 160 µm
  • Basis weight: 84 g/m2
Grade 602 h is a dense filter designed for collecting very fine particles. It is commonly used in the beverage industry for:
  • Residual sugar determination
  • Acidic spectra
  • Refractometric analysis
  • HPLC

1At 98% efficiency.
2Determined by ignition at 900°C.

Hopefully, this summary of properties and applications of these popular qualitative filter papers clarifies why you might be using one or more of these paper grades in your work. For more information on the wider range of filter paper grades, download our Filter Paper and Membranes guide.

How to Choose the Best Whatman Filter Paper for your Application

There are many grades of paper, each with its own unique properties and applications.

If you’re unsure whether you’re using the right paper for your application, utilize our Filter Paper Properties Chart or check our Whatman filter selector to find your filter fast.