Question

33. What is sorption and describe three types of isotherm

Answer 1

1. Sorption is a physical and chemical process by which one substance becomes attached to another. Specific cases of sorption are :-

• Absorption –the incorporation of a substance in one state into another of a different state. (e.g., liquids being absorbed by a solid or gases being absorbed by a liquid);
• Adsorption – the physical adherence or bonding of ions and molecules onto the surface of another phase (e.g., reagents adsorbed to a solid catalyst surface);
• Ion exchange – an exchange of ions between two electrolytes or between an electrolyte solution and a complex.
• Sorption is the common term used for both absorption and adsorption. These terms are often confused. Absorption is the incorporation of a substance in one state into another of a different state (e.g., liquids being absorbed by a solid or gases being absorbed by water). Adsorption is the physical adherence or bonding of ions and molecules onto the surface of another molecule. It is the most common form of sorption used in cleanup.
• Different types of adsorption isothermal :-

• Type I Adsorption Isotherm:

• The above graph depicts Monolayer adsorption.
• This graph can be easily explained using Langmuir Adsorption Isotherm.
• If BET equation, when P/P₀<<1 and c>>1, then it leads to monolayer formation and Type I Adsorption Isotherm is obtained.
• Examples of Type-I adsorption are Adsorption of Nitrogen (N₂) or Hydrogen (H) on charcoal at temperature near to -1800°C.
• ​​​​​​Type II Adsorption Isotherm:-

  

• Type II Adsorption Isotherm shows large deviation from Langmuir model of adsorption.
• The intermediate flat region in the isotherm corresponds to monolayer formation.
• In BET equation, value of C has to be very large in comparison to 1.
• Examples of Type-II adsorption are Nitrogen (N₂ (g)) adsorbed at -1950°C on Iron (Fe) catalyst and Nitrogen (N₂ (g)) adsorbed at -1950°C on silica gel.

• Type III Adsorption Isotherm:

  

• Type III Adsorption Isotherm also shows large deviation from Langmuir model.
• In BET equation value if C <<< 1 Type III Adsorption Isotherm obtained.
• This isotherm explains the formation of multilayer.
• There is no flattish portion in the curve which indicates that monolayer formation is missing.
• Examples of Type III Adsorption Isotherm are Bromine (Br₂) at 790°C on silica gel or Iodine (I₂) at 790°C on silica gel.

• Type IV Adsorption Isotherm:

  

• At lower pressure region of graph is quite similar to Type II. This explains formation of monolayer followed by multilayer.
• The intermediate flat region in the isotherm corresponds to monolayer formation.
• The saturation level reaches at a pressure below the saturation vapor pressure. This can be explained on the basis of a possibility of gases getting condensed in the tiny capillary pores of adsorbent at pressure below the saturation pressure (P_S) of the gas.
• Examples of Type IV Adsorption Isotherm are of adsorption of Benzene on Iron Oxide (Fe₂O₃) at 500°C and adsorption of Benzene on silica gel at 500°C.