APPLICATIONS OF ADSORPTION
Adsorption finds numerous applications both in the laboratory and industry. Some of these are
listed below.
(1) Production of high vacua
If a partially evacuated vessel is connected to a container of activated charcoal cooled with
Liquid air, the charcoal adsorbs all the gas molecules in the vessel. This results in a very high
vacuum. This process is used in high vacuum equipments as Dewar flask for storage of liquid
air or liquid hydrogen. Silica gel is also useful as an adsorbent in production of high vacua.
(2) Gas mask
All gas masks are devices containing an adsorbent (activated charcoal) or a series of adsorbents.
These adsorbents remove poisonous gases by adsorption and thus purify the air for breathing.
(3) Heterogeneous catalysis
In heterogeneous catalysis, the molecules of reactants are adsorbed at the catalyst surface where
they form an ‘adsorption complex’. This decomposes to form the product molecules which
then take off from the surface.
(4) Removal of colouring matter from solutions
Animal charcoal removes colours of solutions by adsorbing coloured impurities. Thus in the
manufacture of cane-sugar, the coloured solution is clarified by treating with animal charcoal
or activated charcoal.
(5) Froth Flotation process
The low grade sulphide ores (PbS, ZnS, Cu2S) are freed from silica and other earthy matter by
Froth Flotation Process. The finely divided ore is mixed with oil (pine oil) and agitated with
water containing a detergent (foaming agent). When air is bubbled into this mixture, the air
bubbles are stabilized by the detergent. These adsorb mineral particles wetted with oil and rise
to the surface.
Fig 1: The oil-wetted mineral particles are adsorbed by stabilized air-bubbles which rise to
the surface while gangue particles wetted by water settle down.
(6) Chromatographic analysis
Mixtures of small quantities of organic substances can be separated with the help of
Chromatography which involves the principles of selective adsorption. The mixture is
dissolved in a suitable solvent (hexane) and poured through a tube containing the adsorbent
(alumina). The component most readily adsorbed is removed in the upper part of the tube. The
next most readily adsorbed component is removed next, and so on. Thus the material is
separated into ‘bands’ in different parts of the tube. Now pure solvent is poured through the
tube. Each component dissolved in the solvent comes down by turn and is collected in a
separate receiver. Mixtures of gases can be separated by selective adsorption of gases by liquids
(Gas chromatography).
ION–EXCHANGE ADSORPTION
In recent years, many synthetic resins have been made which function as ion-exchangers. In
effect, the resin has one ion adsorbed on it. The resin releases this ion and adsorb another like
ion. The process is called ion-exchange adsorption. When cations are exchanged, the resin is
known as cation exchanger. When anions are exchanged, it is referred to as anion exchanger
Fig 2: Macromolecules of ion-exchange resins.
Cationic exchange
The cationic exchangers are high polymers containing acidic groups such as sulphonic acid
group, –SO3, H. The resulting macro-anion has adsorbed H+ ions. When solution of another
cation (Na+) is allowed to flow over it, H+ ions are exchanged for Na+ ions. This process in
fact, consist of desorption of H+ ions and adsorption of Na+ ions by the resin.
Since the above cationic exchange is reversible, the sodium ‘salt’ upon treatment with an acid
regenerates the original resin.
Anionic exchange
A resin containing a basic group such as quaternary ammonium hydroxide, –N+ R3 O– H, will
act as anion exchanger. It will, for example, exchange OH– ion for Cl–..
The original anion exchanger resin can be regenerated by treatment of the resin ‘chloride’ with
a base (OH– ions).
APPLICATIONS OF ION–EXCHANGE ADSORPTION
Ion-exchange adsorption has many useful applications in industry and medicine.
(1) Water softening
Hard water contains Ca2+ ions and Mg2+ ions. These form insoluble compound with soap and
the latter does not function as detergent. Hard water is softened by passing through a column
packed with sodium cation-exchanger resin, R–Na+. The Ca2+ and Mg2+ ions in hard water are
replaced by Na+ ions.
(2) Deionization of water
Water of very high purity can be obtained by removing all dissolved salts. This is accomplished
by using both a cation and anion exchanger resin. The water freed from all ions (cations and
anions) is referred to as Deionized or Demineralized water.
The water is first passed through a column containing a cation-exchanger resin, R–H+. Here
any cations in water (say Na+) are removed by exchange for H+. The water is then passed
through a second column packed with an anion-exchanger, R+ OH–. Any anions (Cl–) are
removed by exchange of OH– for Cl–.
The H+ and OH– ions thus produced react to form water.
Thus the water coming out of the second column is entirely free from ions, whether cations or
anions. The water is purer than distilled water and is called Conductivity water.
In another process, which is more common way, the tap water is passed into a column
containing both types of resin (cation and anion exchanger). Here cations and anions are
removed simultaneously.
Electrical demineralization of water
Ion-exchange resins supported on paper or fibre can be used as membranes through which only
cations or anions will pass. Such membranes are used in electrical demineralization of water
and they act as ionic sieves (Fig. 3). Upon application of the electric current, cations move
through the cation-exchanger membrane to the negative electrode. The anions move in the
opposite direction through the anion exchanger membrane. Thus the water in the middle
compartment is demineralized.
Fig 3 Electrical demineralization of saline water.
(3) Medical uses
Excess sodium salts can be removed from the body fluids by giving the patient a suitable ion-
exchanger to eat. Weakly basic anion-exchangers are used to remove excess acid or ‘acidity’
in the stomach.
