Absorption (chemistry)

Not to be confused with Adsorption.

Other senses: for the amalgamation of gold into mercury, see absorption of gold into mercury. For the absorption of light, see Absorption (electromagnetic radiation).

Laboratory absorber. 1a): CO₂ inlet; 1b): H₂O inlet; 2): outlet; 3): absorption column; 4): packing.

In chemistry, absorption is a physical or chemical phenomenon or a process in which atoms, molecules, or ions enter some bulk phase – gas, liquid, or solid material. This is a different process from adsorption, since molecules undergoing absorption are taken up by the volume, not by the surface (as in the case for adsorption). A more general term is sorption, which covers absorption, adsorption, and ion exchange. Absorption is a condition in which something takes in another substance.

If absorption is a physical process not accompanied by any other physical or chemical process, it usually follows the Nernst partition law:

"the ratio of concentrations of some solute species in two bulk phases in contact is constant for a given solute and bulk phases"^{[citation needed]}:

$\frac{[x]_{1}}{[x]_{2}} = \text{constant} = K_{N(x,12)}$

The value of constant K_N depends on temperature and is called partition coefficient. This equation is valid if concentrations are not too large and if the species "x" does not change its form in any of the two phases "1" or "2". If such molecule undergoes association or dissociation then this equation still describes the equilibrium between "x" in both phases, but only for the same form – concentrations of all remaining forms must be calculated by taking into account all the other equilibria.

In the case of gas absorption, one may calculate its concentration by using, e.g., the Ideal gas law, c = p/RT. In alternative fashion, one may use partial pressures instead of concentrations.

In many processes important in technology, the chemical absorption is used in place of the physical process, e.g., absorption of carbon dioxide by sodium hydroxide – such acid-base processes do not follow the Nernst partition law.

For some examples of this effect, see liquid-liquid extraction. It is possible to extract from one liquid phase to another a solute without a chemical reaction. Examples of such solutes are noble gases and osmium tetroxide.

The process of absorption means that a substance captures and transforms energy. The absorbent distributes the material it captures throughout while and adsorbent only distributes it through the surface. The reddish color of copper is an example of this process because it is caused due to its absorption of blue light.

Types of absorption

Absorption is a process that may be chemical or physical.

Physical absorption

Physical absorption is made between a gas mixture or part of it and a liquid solvent. It involves the transfer of mass that takes place at the interface between the liquid and the gas and the rate at which the gas diffuses into a liquid. This type of absorption depends on the solubility of gases, the pressure and the temperature.

Chemical absorption

Chemical absorption or reactive absorption is a chemical reaction between the absorbed and the absorbing substances. Sometimes it combines with physical absorption. This type of absorption depends upon the stoichiometry of the reaction and the concentration of its reactants.