Activated carbon is among the most effective and widely used absorbent materials in numerous industries, including water treatment, air purification, pharmaceuticals, food processing, and chemicals. Thanks to its porous structure and large specific surface area, it possesses excellent absorption capacity for various substances. The quality and effectiveness of activated carbon are determined by several key criteria, including the methylene blue index .
The blue color is an important indicator of activated carbon’s ability to adsorb high-molecular-weight organic compounds (such as dyes, phenols, and some biological contaminants). This article examines in detail the definition of this indicator, measurement methods, its significance, applications, and its relationship to other properties of activated carbon.
What number is Blue?
The value highlighted in blue indicates the number of milligrams of methylene blue dye absorbed per gram of activated carbon. This indicator reflects the activated carbon’s ability to remove large organic compounds from the environment. The higher the blue value, the better the activated carbon can absorb substances into its micropores.
Why do we use methylene blue?
Due to its relatively high molecular weight and complex structure, methylene blue is ideally suited for the synthesis of large organic compounds such as tannins, proteins, dyes, and some biological contaminants. Therefore, the use of methylene blue in tests is an effective method for simulating the adsorption behavior of activated carbon towards these compounds.
Blue numerical units of measurement
Blue values are usually given in milligrams per gram:
mg MB/g AK
(Methylene blue absorbed per gram of activated carbon)
Some sources also give the blue values in milliequivalents, but the unit mg/g is more common and easier to understand.
Methods for measuring blue light intensity
Standard test procedures (based on ASTM or DIN standards):
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Prepare a methylene blue solution with the specified concentration (usually 0.1%).
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A specific amount of activated carbon ( e.g. 0.1 g) is added to a specific volume of the solution.
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Shake or stir the mixture for a specific period of time (e.g. 60 minutes) .
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Filter or centrifuge the solution.
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The absorption of the remaining solution was measured at a wavelength of 664 nm using an optical spectrometer.
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The amount of dye absorbed is calculated to determine the final blue value (unit: mg/g).
The test can also be performed using the calibration method or the graphical method (using a calibration curve).
The role of the blue number in identifying pore types in activated carbon
An important aspect of the “blue number” is that it indicates the adsorption capacity of intermediate pores. Intermediate pores are pores with diameters between 2 and 50 nanometers and are necessary for the adsorption of larger molecules.
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The iodine number reflects the adsorption capacity of micropores (diameter < 2 nanometers).
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The blue numbers indicate the adsorption capacity of medium-sized pores.
Therefore, carbon with a high blue dust value is better suited for the absorption of dyes , proteins, heavy organic compounds and biological pollutants.
Typical range of blue color values for different types of activated carbon.
| Types of activated carbon | Blue numbers (mg/g) | Frequently used |
|---|---|---|
| wooden base | 100 – 250 | Food and pharmaceutical industries |
| That depends on the type of coal. | 150 – 300 | Air and gas purification |
| coconut shell base | 70 – 150 | Water purification, removal of volatile organic compounds. |
| Absorption of specific synthetic dyes | 250 – 400 | Wastewater decolorization, textile dyes |
The higher the blue value, the better the carbon can remove complex compounds. In the printing and textile industries, carbon materials with blue values above 300 are in high demand.
Application of blue numbering in various industries
1. Treatment of colored wastewater
In industrial wastewater containing aniline, mineral, or organic dyes, the blue color plays a crucial role in selecting the appropriate type of carbon. Textile factories, dye works, printing plants , and food processing facilities pay particular attention to this property.
2. Pharmaceutical and food industries
The use of high-quality blue carbon can improve the clarity of the final product during the extraction purification process and remove unwanted dyes from beverages or liquids.
3. Predicting the age of carbon
The blue color value serves as an indicator of the effectiveness and shelf life of activated carbon. A decrease in the blue color after repeated use indicates that the pores of the material are saturated.
Comparison of the blue number with the iodine number
| Special feature | Blue number | Iodine number |
|---|---|---|
| Types of typical pores | Medium porosity (2-50 nanometers) | Micropores (<2 nanometers) |
| Types of absorbed compounds | Raw materials, dyes and proteins. | Volatile organic compounds |
| Units of measurement | mg/g | mg/g |
| Common uses | Wastewater treatment, bleaching | Water purification, air purification, gas absorption. |
Therefore, choosing the correct value between iodine and blue dye values is crucial, depending on the type of pollutant to be neutralized.
Rules regarding blue numbers
Important international standards for the identification and reporting of blue numbers include the following:
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ASTM D2330 : Test method for determining the dye absorption rate using activated carbon.
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DIN 53958 : Methylene blue absorption test
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AWWA Standard B600 : Requirements for activated carbon for water treatment
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ISO 9001 : Requirements for quality management systems in factories
How can blue numbering be used in purchasing and quality control processes?
When purchasing activated carbon, you should always request a certificate of analysis from the seller or manufacturer. The certificate should contain a blue serial number that corresponds to the intended use.
For example:
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The blue color plays no major role in the treatment of household water.
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When treating colored wastewater , the blue value should exceed 300 mg/g.
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In the field of pharmacy, it is necessary to maintain a balance between the value of iodine and the value of the blue color.
In conclusion
Methylene blue is an important indicator of the effectiveness of activated carbon in adsorbing complex macromolecular organic compounds. This value reflects the quality of the carbon’s medium-porous structure and is of great importance for applications such as decolorization, industrial wastewater treatment, food processing, and pharmaceuticals.
Selecting the right activated carbon without considering its “blue number” can reduce adsorption efficiency, increase costs, and lower the quality of the final product. Therefore, the “blue number,” along with other criteria such as iodine number, specific surface area, hardness, and ash content, is an important factor for quality control and improvement.