Filter Paper Aperture and Size
Filter paper is a kind of paper with good filtration performance. The paper is loose and has strong absorption properties for liquids. The filter paper commonly used in the analytical laboratory is used as a filter medium to separate the solution from the solid. Filter paper mainly includes quantitative analysis filter paper, qualitative analysis filter paper, and chromatographic qualitative analysis filter paper.
Quantitative analysis of filter paper, divided into fast, medium, and slow speed categories, on the filter paper box (fast: Quantitative Ashless Filter Paper Grade: BIO-41), blue belt (medium speed: Quantitative Medium Flow Filter Paper Grade: BIO-40), red belt (slow speed: Quantitative Laboratory Filter Paper Grade: BIO-42 ) for the classification. The shape of the filter paper is round and square. The specifications of the circular paper are 5.5cm, 7cm, 9cm, 11cm, 12.5cm, 15cm and 18cm, 18.5cm, 24cm, and 27cm. The square quantitative filter paper is 60 cm x 60 cm and 30 cm x 30 cm.
The type and specification of the qualitative analysis filter paper are the same as the quantitative analysis filter paper, indicating fast, medium speed, and slow speed, but are printed with fast, medium speed, and slow speed. The chromatographic qualitative analysis filter paper has a filtration speed of 35 seconds or less, and less than or equal to 70 seconds, and less than or equal to 140 seconds.
Filter paper is categorized by its pore size or aperture, which refers to the size of the openings in the paper that allow liquids or particles to pass through while retaining larger particles. The aperture size is usually expressed in micrometers (µm) or millimeters (mm) and determines the size of particles the paper can effectively filter.
1. Aperture (Pore Size)
There are several categories of filter paper based on aperture size:
| Product Name | Speed | Max Aperture（um） | Filter Speed（s） | Ash % | Quantitative（g/m2） |
|---|---|---|---|---|---|
| Qualitative filter paper | Fast | 20~25 | <35 | 0.15 | 80±4 |
| Medium | 15~20 | 35~70 | 0.15 | 80±4 | |
| Slow | 10~15 | 70~140 | 0.15 | 80±4 | |
| Quantitative filter paper | Fast | 20~25 | <35 | ≤0.01 | 80±4 |
| Medium | 15~20 | 35~70 | ≤0.01 | 80±4 | |
| Slow | 10~15 | 70~140 | ≤0.01 | 80±4 |
Coarse Filtration vs. Fine
- Coarse filtration uses filter papers with larger pore sizes (ranging from 25 µm to 80 µm or more) for separating larger particles, such as sand or debris. Coarse filters work faster but do not capture smaller particles effectively.
- Medium filtering filter paper typically has a 25-10 µm pore size range. It is commonly used for general filtration, separating crystalline substances, and collecting fine precipitates.
- Fine filtration requires filter papers with smaller pore sizes, generally between 10-2.5 µm, to capture fine particles, such as bacteria or small particulate matter. These are typically used in applications where high purity is needed, like microbiological analysis or fine chemical synthesis.
- Very fine filtering filter paper has microscopic pores (below 2.5 µm) and filters very fine particles, colloids, and microbial cultures.
Grade of Filter Paper
The grade of filter paper corresponds to its aperture size. For example, qualitative filter papers (used in general filtration) come in different grades with varying pore sizes, while quantitative filter papers (which are used for more accurate analysis) are made with very fine pores to minimize any particle passage.
Types of Apertures
Smooth vs. Rough: Filter papers can have smooth or rough surfaces, affecting the filtration process. A smoother surface usually results in better filtration of fine particles because there is less surface area for particles to adhere to. A rougher surface, on the other hand, may speed up filtration by offering more area for the passage of liquids.
Custom Apertures: Some specialized filter papers, like those used in industrial or specific laboratory settings, can be custom-made with apertures tailored for very precise needs (such as micro-filtration or ultra-filtration).
The choice of aperture size depends on the nature of the particles you want to separate, the clarity of the filtrate required, and the speed of filtration.
2. Size (Dimensions)
Filter paper comes in various sizes, usually denoted by its diameter. Common sizes include 70 mm, 90 mm, and 125 mm. The choice of size depends on the filtration apparatus you’re using and the volume of liquid you’re filtering. Larger sizes are suitable for filtering larger volumes of liquid, while smaller sizes are used for smaller quantities.
- Standard Sizes: Filter paper comes in various standard sizes, which can range from 50 mm to 500 mm in diameter, depending on the application. For example:
- Small sizes (50–150 mm) are often used for laboratory-scale filtration, such as in qualitative or semi-quantitative analysis.
- Larger sizes (300–500 mm) are used for larger-volume filtration, such as in industrial applications or larger-scale research work.
- Surface Area and Filtration Efficiency: The larger the filter paper, the more surface area is available for the liquid to pass through, which can increase filtration efficiency. Larger filter papers allow faster filtration when there is a high volume of liquid to be processed. Conversely, smaller filter papers are ideal for low-volume filtration and more focused applications.
- Folding: Some filter papers are pre-folded, which increases their surface area without increasing their physical size. This is especially useful for applications where high filtration speed is needed or when the paper is used in specific filtration setups, like in Buchner funnels.
When selecting the appropriate size of filter paper, consider the diameter of the filtration apparatus (funnel or holder) you’re using. The filter paper should fit snugly without folding or creasing.
Keep in mind that the aperture size and diameter of filter paper are related but distinct characteristics. You’ll need to choose both the right aperture size to suit your filtration needs and the appropriate size to fit your equipment.
Remember to always check the manufacturer’s specifications and guidelines for the filter paper you’re using, as well as any specific filtration requirements for your experiment or application.
