What do the iodine number and CTC value tell us about the quality of activated carbon – and what do they not tell us?

Almost every technical data sheet for activated carbon lists an iodine number, often together with a CTC value. What do these numbers tell us about the quality and adsorption capacity of activated carbon?

The iodine number is one of the oldest and most widely applied quality indicators for activated carbon, especially for applications in liquids. This is because it is a relatively simple and inexpensive method that is widely used.

The iodine number indicates how much iodine (I₂) a particular type of activated carbon can adsorb from a solution. The iodine number is determined using the standardised ASTM D4607 method. As iodine molecules are very small, with a kinetic diameter of approximately 0.55 nm or 0.00000055 mm, the test correlates well with the available surface area in micropores, especially for micropores around 1 nm or smaller.

Based on the iodine number, activated carbon can be roughly classified into different qualities:

• 300–600 mg/g: low quality
• 600–900 mg/g: universal, average quality
• 900–1200 mg/g: high quality

Due to the relative simplicity of determining the iodine number, this parameter is often used as an approximation of the BET value, i.e. an indication of the internal surface area of the activated carbon. It can be stated that:

• an iodine number of 600 mg/g corresponds to ± 600–700 m²/g.
• an iodine number of 900 mg/g corresponds to ± 900–1000 m²/g.
• an iodine number of 1100 mg/g corresponds to ± 1100–1200 m²/g.

However, this correlation is only indicative and loses much of its value with respect to activated carbon types in which mesopores are dominant. The reason is that iodine will not penetrate these larger pores.

The CTC value is often used as a quality indicator for activated carbon in air and gas purification applications. ‘CTC’ stands for carbon tetrachloride (or CCl₄). The CTC value of an activated carbon type is determined using the standardised ASTM D3467 method. Here, the activated carbon is exposed to a gas flow charged with CCl₄ until equilibrium is reached (no further uptake of CTC by the activated carbon). The absorbed weight percentage of CCl₄ is expressed relative to the dry mass of activated carbon.

Based on the CTC value, activated carbon can be roughly classified into different qualities:

• 20–40%: low quality
• 40–70%: universal, average quality
• 70–120%: very high quality

Values above 100% are indeed possible because activated carbon can adsorb more than its own weight in CCl₄.

As with the iodine number, there is a rough correlation between the CTC value and the BET value (i.e. an indication of the internal surface area of the activated carbon), so the CTC value also gives an indication of the total adsorption capacity of activated carbon. It can be stated that:

• a CTC value of 50% corresponds to ± 800–900 m²/g.
• a CTC value of 60% corresponds to ± 1000 m²/g.
• a CTC value of 70% corresponds to ± 1100–1200 m²/g.

Nevertheless, the pore size distribution of the activated carbon reduces the correlation between the two. This is because the CTC value, due to the dimensions of CCl₄, mainly focuses on pores within the range of 1–5 nm and is therefore primarily sensitive to micropores and the smallest mesopores.

Both the iodine number and the CTC value are well-known quality indicators for activated carbon; the iodine number primarily for water applications, CTC rather for gas applications. However, because both methods use a specific molecule with fixed dimensions to determine the parameter, these indicators mainly tell us something about this range of pores (in which the test molecule fits). Extrapolation to the specific surface area and therefore to the adsorption capacity of the carbon type must therefore be done with caution. To obtain a complete and correct picture of the quality and applicability of a particular type of activated carbon, additional quality parameters such as pore distribution and surface chemistry can be determined.