Filter Paper Materials

Filter paper is manufactured from cotton fibers. Its surface features countless tiny pores that allow liquid particles to pass through, while larger solid particles are unable to traverse them. This property enables the separation of mixed liquid and solid substances.

Filter Paper Specifications

For the technical specifications prescribed for qualitative filter paper products, please refer to the National Standard (GB 1515). The approximate pore sizes for these papers are 80–120 micrometers, 30–50 micrometers, and 1–3 micrometers, respectively.

The Function of Filter Paper

Because the cotton fibers within filter paper exhibit a strong affinity for water, the paper can absorb approximately 22% of its weight in water; of this amount, 6% to 7% consists of water molecules bound to the cellulose via hydrogen bonds. Under normal conditions, this bound water is extremely difficult to remove. In contrast to its affinity for water, the cotton fibers in filter paper have a weaker affinity for organic solvents. Consequently, in most applications of paper chromatography, the bound water within the filter paper fibers serves as the stationary phase, while the mobile phase consists primarily of organic solvents. As the organic solvent (the mobile phase) flows through the sample, a continuous partitioning process occurs between the water and the organic phase. As the mobile phase continues to advance, the various chemical components undergo repeated redistribution, thereby facilitating the separation and purification of the substances. This specific application of filter paper is commonly encountered in chemical laboratories.

Classification of Filter Paper

Filter paper is generally classified into two categories: qualitative and quantitative. In analytical chemistry applications, when inorganic compounds are filtered to separate precipitates, the residue collected on the filter paper can be used to calculate the loss rate incurred during the experimental process. Qualitative filter paper tends to shed a relatively higher amount of cotton fibers after filtration; therefore, it is suitable only for qualitative analysis. Quantitative filter paper—particularly the “ashless” grade—undergoes special treatment processes that render it more resistant to chemical reactions; as a result, it generates fewer impurities and is suitable for use in quantitative analysis.

What is the Difference Between Qualitative and Quantitative?

The concepts of “quantitative filter paper” and “qualitative filter paper” apply exclusively to cellulose-based filter papers; they do not apply to other types of filter media, such as glass microfiber filter paper.

  1. Quantitative Analysis Filter Paper

During the manufacturing process of quantitative analysis filter paper, the pulp is treated with hydrochloric acid and hydrofluoric acid, and subsequently washed with distilled water to remove the majority of impurities from the paper fibers. Consequently, it leaves behind very little residual ash upon ignition, exerting a negligible influence on analytical results; thus, it is suitable for use in precise quantitative analysis.

Currently, quantitative analysis filter paper produced domestically is classified into three categories: fast, medium, and slow. On the filter paper packaging boxes, these categories are distinguished by specific color-coded bands: white (fast), blue (medium), and red (slow). The filter paper is available in two shapes: circular and square. Circular quantitative filter paper comes in various specifications based on diameter, including 9 cm, 11 cm, 12.5 cm, 15 cm, and 18 cm. Square quantitative filter paper is available in sizes of 60 cm × 60 cm and 30 cm × 30 cm.

  1. Qualitative Analysis Filter Paper

Qualitative analysis filter paper generally contains a relatively higher amount of residual ash. It is intended solely for general qualitative analysis and for filtering precipitates or suspended solids from solutions; it is not suitable for use in quantitative analysis.

The types and specifications of qualitative analysis filter paper are fundamentally identical to those of quantitative analysis filter paper; however, instead of using color-coded bands to indicate fast, medium, or slow filtration speeds, these categories are designated by printing the specific text labels—”Fast,” “Medium,” or “Slow”—directly onto the packaging. However, the packaging boxes for filter paper often lack specific instructions regarding the precautions to be observed when using quantitative and qualitative filter papers to separate precipitates:

① Generally, gravity filtration is employed, utilizing the physical structure of the filter paper and its capacity to retain solid particles to achieve the separation of liquid and solid phases;

② Since filter paper possesses relatively low mechanical strength and toughness, vacuum filtration (suction filtration) methods should be avoided whenever possible. If it becomes necessary to accelerate the filtration rate—and to prevent filter failure caused by “breakthrough” (solids passing through the filter)—specific measures should be taken: when using an air pump for suction, 2 to 3 layers of filter paper may be stacked in the funnel, depending on the strength of the suction force; when using a vacuum pump, a layer of dense filter cloth should first be placed in the funnel to serve as a backing, with the filter paper then placed on top of it for filtration;

③ Filter paper should not be used to filter hot, concentrated sulfuric acid or nitric acid solutions;

Distinctions in Filter Paper Flow Rates

Slow Speed: Pore size of 1–3 micrometers. Spots do not diffuse easily. Suitable for samples with low RF values ​​and mobile phases (eluents) with low viscosity; however, the development time is relatively long.

Medium Speed: Pore size of 30–50 micrometers. Represents an intermediate range between the other two categories.

Fast Speed: Pore size of 80–120 micrometers. Due to the loose texture of the paper, spots tend to diffuse easily. Suitable for samples with high RF values ​​and mobile phases with high viscosity.

Basic Properties of Filter Paper

When selecting appropriate filter paper, the decision must be based on the following four factors: First is hardness; since filter paper becomes wet during the filtration process, a relatively tough and durable paper should be selected for applications involving prolonged filtration. Second is filtration efficiency; this is primarily determined by the density of the pores within the filter paper and the actual size of those pores. Next is capacity; the density of the pores largely determines the capacity (or dust-holding capacity)—the denser the pores, the higher the capacity. Finally, there is suitability (or applicability); some types of filter paper undergo highly specialized manufacturing processes, so particular attention must be paid to their specific intended applications.

Selection Criteria for Filter Paper

Several key points should be primarily considered when selecting filter paper:

  1.  Effective Area: A larger effective area—meaning a larger usable surface area of ​​the filter paper—results in a higher dust-holding capacity, lower resistance to flow, and a longer service life; naturally, however, the cost will increase correspondingly.
  2.  Fiber Diameter: The finer the diameter of the fibers comprising the filter paper, the more effective the interception of particles, and consequently, the higher the filtration efficiency.
  3. Binder Content in Filter Media: A higher binder content in the filter material results in greater paper tensile strength, higher filtration efficiency, reduced fiber shedding, lower inherent dust accumulation, and superior durability; however, airflow resistance increases correspondingly.

Relevant Data for Filter Paper:

  1. Classification: Fast, Medium, Slow
  2. Filtration Speed: ≤ 35 seconds, ≤ 70 seconds, ≤ 140 seconds
  3. Models: Qualitative Filter Paper (BIO-1, 2,4 5,6, 91,113); Quantitative Filter Paper (BIO-40,41,42,43,44)
  4. Separation Performance: Fe(OH)₃, ZnCO₃, BaSO₄ (Hot)
  5. Wet Burst Strength (mmH₂O): ≥ 130, ≥ 150, ≥ 200
  6. Ash Content: Qualitative Filter Paper ≤ 0.13%; Quantitative Filter Paper ≤ 0.0009%
  7. Iron Content: Qualitative Filter Paper ≤ 0.003%; Basis Weight (g/m²): 80 ± 4
  8. Diameter of Circular Sheets (cm): 5.5, 7, 9, 11, 12.5, 15, 18, 23, 27
  9. Dimensions of Square Sheets (cm × cm): 60 × 60, 30 × 30