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What Is Filter Paper? Grades, Sizes & How to Use It
If you've ever separated sand from water in a school science lab, you've used filter paper. It's one of those items that looks almost too simple to matter, yet it shows up in nearly every practical class and quantitative lab procedure, from a Grade 8 CAPS experiment to a full water quality analysis. Not all filter paper is the same, though, and picking the wrong grade or size can leave you with a filtrate full of fine particles or a filtration that takes twenty minutes longer than it should.
This guide covers what filter paper actually is, how the different grades compare, how to choose the right size, and how to fold and use it properly for gravity and vacuum filtration.
What Is Filter Paper?
Filter paper is a sheet of porous material, almost always made from purified cellulose fibres, used to separate solids from liquids or gases. When a mixture is poured through it, the liquid (or filtrate) passes through the tiny gaps between the fibres while solid particles larger than those gaps get trapped on the surface or within the paper itself. This process is called filtration, and it's one of the oldest and most widely used separation techniques in both the lab and everyday life.
The size of the gaps between fibres, referred to as pore size or particle retention, is what determines how fine a filter paper is. A finer paper traps smaller particles but lets liquid through more slowly. A coarser paper filters faster but lets more fine material pass through with the filtrate. Choosing a grade is really about finding the right balance between retention and speed for the job at hand.
Qualitative vs Quantitative Filter Paper
Filter paper is generally split into two broad categories:
| Type | Ash Content | Typical Use |
|---|---|---|
| Qualitative | Higher (paper is not ashless) | Routine filtration, clarifying liquids, classroom and general lab work where the paper itself doesn't need to disappear during analysis |
| Quantitative | Very low, "ashless" | Gravimetric analysis, where the filter paper and any residue collected on it are later ignited and weighed, so the paper can't leave behind measurable ash |
Nearly everything covered in this guide, and the vast majority of what gets used in South African school and university labs, falls under qualitative filter paper. That's the type SmartLabs stocks across its Ahlstrom-Munksjö and student-grade ranges.
Choosing the Right Grade
Qualitative filter paper is sold in different grades, and each one is built for a slightly different job. SmartLabs stocks Ahlstrom-Munksjö grades, manufactured to be closely equivalent to the Whatman grading system used worldwide. Here's how they compare, based on the particle retention figures published by Cytiva, the current owner of the Whatman brand:
| Grade | Whatman Equivalent | Particle Retention | Flow Speed | Best For |
|---|---|---|---|---|
| Student Grade (613) | Basic teaching grade | Coarser, general purpose | Fast | Classroom demonstrations, basic separations like sand and water, everyday teaching use |
| 601 | Grade 1 | 11 µm | Medium | General purpose qualitative work, the standard "reach for this first" grade for routine filtration |
| 642 | Grade 2 | 8 µm | Medium, slightly slower than 601 | Finer precipitates where you need better retention and can allow for a slower filtration |
| 631 | Grade 4 | 20 to 25 µm | Fast | Coarse particles and gelatinous precipitates, such as ferric or aluminium hydroxide, where speed matters more than fine retention |
If you're not sure which grade to pick, 601 is the safest general-purpose choice for most qualitative work. Reach for 642 when you specifically need finer retention, and 631 when you're filtering something coarse or gelatinous and want it done quickly.
Choosing the Right Size
Filter paper circles are sold in a range of diameters, and matching the paper to your funnel matters more than most people expect. As a rule of thumb, the flat filter paper disc should be roughly the same diameter as the top of your funnel, or slightly smaller, so that once folded into a cone, the top edge of the paper sits just below the funnel's rim rather than overhanging it. Paper that's too small won't seal properly against the funnel wall and can let unfiltered liquid slip past the edges. Paper that's too large will fold over the rim and won't sit securely.
SmartLabs stocks common diameters from 90mm up to 150mm for standard funnels, plus smaller 35mm to 65mm circles for the Ahlstrom-Munksjö grades used with smaller glass and Büchner funnels.
How to Fold Filter Paper
There are a few folding methods, but the simple cone fold is what you'll use for almost every gravity filtration in a school or general lab setting:
- Fold the circular filter paper exactly in half.
- Fold it in half again, so you're left with a quarter-circle wedge.
- Open one side of the folded wedge into a cone shape, with three layers of paper on one side and one layer on the other.
- Place the opened cone into your funnel, and moisten it slightly with a few drops of the solvent you're using (usually water) so it seats against the glass and holds its shape.
For faster filtration, some labs use a fluted or pleated fold instead, which creates more surface area in contact with the funnel and speeds up the flow rate. It takes a bit more practice to get a neat result, so it's worth mastering the simple cone fold first.
Gravity Filtration vs Vacuum Filtration
There are two common ways to filter using filter paper, and the right one depends on how much liquid you're dealing with and how quickly you need it done.
| Gravity Filtration | Vacuum (Büchner) Filtration | |
|---|---|---|
| Equipment | Glass or plastic funnel, filter paper cone, support stand, collection beaker or flask | Büchner funnel, filter flask (side-arm/Kitasato flask), vacuum source, rubber tubing |
| Speed | Slower, relies on gravity alone | Much faster, vacuum pulls liquid through |
| Typical Use | Routine, small-volume filtration; most classroom work | Larger volumes, or when you need to collect and dry a solid quickly |
For most school-level practicals, gravity filtration with a simple cone-folded filter paper is all you need. Vacuum filtration through a Büchner funnel becomes worthwhile once you're regularly filtering larger volumes or need to isolate a solid precipitate as quickly as possible, since the paper sits flat against a perforated plate rather than being folded into a cone.
Step-by-Step: How to Filter Using Filter Paper
- Fold your filter paper into a cone and fit it into a clean funnel.
- Moisten the paper slightly so it seats against the funnel.
- Place the funnel over a clean beaker or flask, using a support stand and clamp to hold it steady.
- Pour the mixture into the funnel slowly, aiming for the centre of the paper cone and keeping the liquid level below the top edge of the paper.
- Allow the liquid to drain through fully before adding more, rather than overfilling the cone.
- Once filtration is complete, the filtrate collects in the beaker below, while any solid residue remains on the filter paper.
Real-Life Uses of Filter Paper
Filter paper isn't just a school lab staple. It's used in:
| Field | Application |
|---|---|
| Education | CAPS-aligned practicals separating mixtures, precipitate reactions, and basic chemistry demonstrations |
| Water testing | Removing suspended solids before water samples are analysed for clarity or chemical content |
| Food and beverage | Clarifying liquids such as wine, oil, or syrup during production and quality testing |
| Analytical chemistry | Gravimetric analysis, where quantitative ashless paper is used to isolate and weigh a precipitate |
| Pharmaceutical and industrial labs | Routine filtration of solutions before further testing or processing |
Shop the Filter Paper Range
SmartLabs stocks the full range covered in this guide, ready to order:
- Filter Paper, Student Grade, for classroom and general teaching use
- Filter Paper, general purpose qualitative grade
- Filter Paper, Ahlstrom-Munksjö Grade 601, the Whatman Grade 1 equivalent for routine qualitative work
- Filter Paper, Ahlstrom-Munksjö Grade 642, for finer precipitates
- Filter Paper, Ahlstrom-Munksjö Grade 631, for coarse and gelatinous precipitates
Frequently Asked Questions
What is the difference between filter paper grades?
Grades differ mainly in particle retention and flow speed. A lower-numbered or finer grade, like 642 (Whatman Grade 2 equivalent), retains smaller particles but filters more slowly. A coarser grade, like 631 (Whatman Grade 4 equivalent), filters faster but lets more fine material through.
Can I reuse filter paper?
No. Filter paper is designed for single use. Once it's wet and has trapped particles, it loses its structural integrity and filtering efficiency, and reusing it risks contaminating your next sample with residue from the last one.
Why is my filtration taking so long?
This usually comes down to grade, fold, or funnel fit. A finer grade will naturally filter more slowly, especially if the liquid contains very fine particles that start to clog the paper. Check that your funnel and paper size match, that the paper isn't folded too tightly, and consider a coarser grade or vacuum filtration if speed matters for your application.
What's the difference between filter paper and chromatography paper?
They're both made from cellulose, but they're built for different jobs. Filter paper is designed to physically trap solid particles while liquid passes through. Chromatography paper is designed to let liquid travel through it via capillary action so that dissolved substances separate based on how they move with the solvent. You can read more in our chromatography paper guide.
Do I need special filter paper for a Büchner funnel?
You need filter paper cut to fit flat against the perforated plate inside the Büchner funnel, rather than folded into a cone. The paper should just cover the holes in the plate without curling up the sides.
What grade of filter paper should schools use?
For general CAPS practical work, Student Grade filter paper is more than adequate and is the most cost-effective option for classroom use. Save the Ahlstrom-Munksjö 601, 642, and 631 grades for more precise practical or research-level work.
The Final Word
Filter paper is simple in concept but worth getting right in practice. Match your grade to the job, size it correctly for your funnel, fold it properly, and pick gravity or vacuum filtration based on how much liquid you're working with. Get those four things right and there's very little that can go wrong.