1.INTRODUCTION

Adsorption is the adhesion of atoms, ions, or molecules from a

gas, liquid, or dissolved solid to a surface. This process creates

a film of the adsorbate on the surface of the adsorbent. This

process differs from absorption, in which

a fluid (theabsorbate) permeates or is dissolved by a liquid or

solid (the absorbent). Note that adsorption is a surface-based

process while absorption involves the whole volume of the

material. The term sorption encompasses both processes,

whiledesorption is the reverse of adsorption. It is a surface

phenomenon.

Brunauer, Emmett and Teller's model of multilayer adsorption is a random distribution of molecules on the material surface

Similar to surface tension, adsorption is a consequence

of surface energy. In a bulk material, all the bonding

requirements of the constituent atoms of the material are filled

by other atoms in the material. However, atoms on the surface

of the adsorbent are not wholly surrounded by other adsorbent

atoms and therefore can attract adsorbates. The exact nature

of the bonding depends on the details of the species involved,

but the adsorption process is generally classified

[1]

as physisorption  or chemisorption . It may also occur due to

electrostatic attraction.

Adsorption is present in many natural physical, biological, and

chemical systems, and is widely used in industrial applications

such as activated charcoal, capturing and using waste heat to

provide cold water for air conditioning and other process

requirements (adsorption chillers), synthetic resins, increase

storage capacity of carbide-derived carbons for tunable

nanoporous carbon, and water purification. Adsorption, ion

exchange, and chromatography are sorption processes in which

certain adsorbates are selectively transferred from the fluid

phase to the surface of insoluble, rigid particles suspended in a

vessel or packed in a column. Lesser known, are the

pharmaceutical industry applications as a means to prolong

neurological exposure to specific drugs or parts thereof.

2.ISOTHERMS

Adsorption is usually described through isotherms, that is, the

amount of adsorbate on the adsorbent as a function of its

pressure (if gas) or concentration (if liquid) at constant

temperature. The quantity adsorbed is nearly always

normalized by the mass of the adsorbent to allow comparison

of different materials.

2.1 Freundlich

[2]

The first mathematical fit to an isotherm was published by

Freundlich and Küster (1894) and is a purely empirical formula

for gaseous adsorbates,

where   is the quantity adsorbed,   is the mass of the

adsorbent,   is the pressure of adsorbate and   and   are

empirical constants for each adsorbent-adsorbate pair at a

given temperature. The function has an asymptotic maximum

as pressure increases without bound. As the temperature

increases, the constants   and   change to reflect the empirical

observation that the quantity adsorbed rises more slowly and

higher pressures are required to saturate the surface.

2.2 Langmuir

In 1916, Irving Langmuir published a new model isotherm for

gases adsorbed to solids, which retained his name. It is a semi-

empirical isotherm derived from a proposed kinetic mechanism.

It is based on four assumptions:

1. The surface of the adsorbent is uniform, that is, all the

adsorption sites are equivalent.

2. Adsorbed molecules do not interact.

3. All adsorption occurs through the same mechanism.

4. At the maximum adsorption, only a monolayer is

formed: molecules of adsorbate do not deposit on

other, already adsorbed, molecules of adsorbate, only

on the free surface of the adsorbent.

[3]

These four assumptions are seldom all true: there are always

imperfections on the surface, adsorbed molecules are not

necessarily inert, and the mechanism is clearly not the same

for the very first molecules to adsorb to a surface as for the

last. The fourth condition is the most troublesome, as

frequently more molecules will adsorb to the monolayer; this

problem is addressed by the BET isotherm for relatively flat

(non-microporous) surfaces. The Langmuir isotherm is

nonetheless the first choice for most models of adsorption, and

has many applications in surface kinetics (usually

called Langmuir-Hinshelwood kinetics) and thermodynamics.

Langmuir suggested that adsorption takes place through this

mechanism:  , where A is a gas molecule and S is

an adsorption site. The direct and inverse rate constants are k

and k-1. If we define surface coverage,  , as the fraction of the

adsorption sites occupied, in the equilibrium we have:

or

where   is the partial pressure of the gas or the molar

concentration of the solution. For very low pressures   

and for high pressures 

 is difficult to measure experimentally; usually, the adsorbate

is a gas and the quantity adsorbed is given in moles, grams, or

[4]

gas volumes at standard temperature and pressure (STP) per

gram of adsorbent. If we call vmon the STP volume of adsorbate

required to form a monolayer on the adsorbent (per gram of

adsorbent),   and we obtain an expression for a straight

line:

Through its slope and y-intercept we can obtain vmon and K,

which are constants for each adsorbent/adsorbate pair at a

given temperature. vmon is related to the number of adsorption

sites through the ideal gas law. If we assume that the number

of sites is just the whole area of the solid divided into the cross

section of the adsorbate molecules, we can easily calculate the

surface area of the adsorbent. The surface area of an adsorbent

depends on its structure; the more pores it has, the greater the

area, which has a big influence on reactions on surfaces.

If more than one gas adsorbs on the surface, we define   as

the fraction of empty sites and we have:

Also, we can define   as the fraction of the sites occupied by

the j-th gas:

[5]

where i is each one of the gases that adsorb.

2.3 BET

Often molecules do form multilayers, that is, some are

adsorbed on already adsorbed molecules and the Langmuir

isotherm is not valid. In 1938 Stephen Brunauer, Paul Emmett,

and Edward Teller developed a model isotherm that takes that

possibility into account. Their theory is called BET theory, after

the initials in their last names. They modified Langmuir's

mechanism as follows:

A(g) + S ⇌ AS

A(g) + AS ⇌ A2S

A(g) + A2S ⇌ A3S and so on

Langmuir isotherm (red) and BET isotherm (green)

The derivation of the formula is more complicated than

Langmuir's (see links for complete derivation). We obtain:

[6]

x is the pressure divided by the vapor pressure for the

adsorbate at that temperature (usually denoted  ), v is the

STP volume of adsorbed adsorbate, vmon is the STP volume of

the amount of adsorbate required to form a monolayer and c is

the equilibrium constant K we used in Langmuir isotherm

multiplied by the vapor pressure of the adsorbate. The key

assumption used in deriving the BET equation that the

successive heats of adsorption for all layers except the first are

equal to the heat of condensation of the adsorbate.

The Langmuir isotherm is usually better for chemisorption and

the BET isotherm works better for physisorption for non-

microporous surfaces.

2.4 Kisliuk

Two adsorbate nitrogen molecules adsorbing onto a tungsten adsorbent from the precursor state around an

island of previously adsorbed adsobate (left) and via random adsorption (right

In other instances, molecular interactions between gas

molecules previously adsorbed on a solid surface form

significant interactions with gas molecules in the gaseous

phases. Hence, adsorption of gas molecules to the surface is

more likely to occur around gas molecules that are already

present on the solid surface, rendering the Langmuir adsorption

isotherm ineffective for the purposes of modelling. This effect

[7]

was studied in a system where nitrogen was the adsorbate and

tungsten was the adsorbent by Paul Kisliuk (b. 1922-d. 2008) in

1957. To compensate for the increased probability of

adsorption occurring around molecules present on the

substrate surface, Kisliuk developed the precursor state theory,

whereby molecules would enter a precursor state at the

interface between the solid adsorbent and adsorbate in the

gaseous phase. From here, adsorbate molecules would either

adsorb to the adsorbent or desorb into the gaseous phase. The

probability of adsorption occurring from the precursor state is

dependent on the adsorbate’s proximity to other adsorbate

molecules that have already been adsorbed. If the adsorbate

molecule in the precursor state is in close proximity to an

adsorbate molecule which has already formed on the surface, it

has a sticking probability reflected by the size of the SEconstant

and will either be adsorbed from the precursor state at a rate of

kEC or will desorb into the gaseous phase at a rate of kES. If an

adsorbate molecule enters the precursor state at a location

that is remote from any other previously adsorbed adsorbate

molecules, the sticking probability is reflected by the size of the

SD constant.

These factors were included as part of a single constant termed

a "sticking coefficient," kE, described below:

As SD is dictated by factors that are taken into account by the

Langmuir model, SD can be assumed to be the adsorption rate

[8]

constant. However, the rate constant for the Kisliuk model (R’)

is different to that of the Langmuir model, as R’ is used to

represent the impact of diffusion on monolayer formation and is

proportional to the square root of the system’s diffusion

coefficient. The Kisliuk adsorption isotherm is written as follows,

where Θ(t) is fractional coverage of the adsorbent with

adsorbate, and t is immersion time:

Solving for Θ(t) yields:

2.5 Henderson-Kisliuk

The Henderson-Kisliuk adsorption equation prediction of normalised impedance as a function of adsorption time, where

the first peak corresponds to the formation of an adsorbent surface that is saturated with MPA in its "lying down" structure.

The curve then tends to an impedance value that is representative of an adsorbent saturated with "standing up"

structure

This adsorption isotherm was developed for use with the new

field of Self Assembling Monolayer (SAM) adsorption. SAM

[9]

molecules adsorb to the surface of an adsorbent until the

surface becomes saturated with the SAM molecules'

hydrocarbon chains lying flat against the adsorbate. This is

termed "lying down" structure (1st structure). Further

adsorption then occurs, causing the hydrocarbon chains to be

displaced by thiol groups present on the newly adsorbed SAM

molecules. When this adsorption step takes place, electrostatic

forces between the newly adsorbed SAM molecules and the

ones previously adsorbed, causes a new structure to form,

where all of the SAM molecules are occupying a "standing up"

orientation (2nd structure). As further adsorption takes place,

the entire adsorbent becomes saturated with SAM in a standing

up orientation, and no further adsorption takes place.

The SAM adsorbate is usually present in a liquid phase and the

adsorbent is normally a solid. Hence, intermolecular

interactions are significant and the Kisliuk adsorption isotherm

applies. The sequential evolution of "lying down" and "standing

up" mercaptopropionic acid (MPA) SAM structures on a gold

adsorbent, from a liquid MPA-ethanol adsorbate phase, was

studied by Andrew P. Henderson (b. 1982) et al. in 2009.

Henderson et al. used electrochemical impedance spectroscopy

to quantify adsorption and witnessed that the 1st structure had

different impedance properties to the 2nd structure and that

both structures evolved sequentially. This allowed four rules to

be expressed:

[10]

That the amount of adsorbate on the adsorbent surface

was equal to the sum of the adsorbate occupying 1st

structure and 2nd structure.

The rate of 1st structure formation is dependent on the

availability of potential adsorption sites and

intermolecular interactions.

The amount of 1st structure is depleted as 2nd

structure is formed.

The rate of second structure formation is dictated by

the amount of adsorbate occupying 1st structure and

intermolecular interactions at immersion time, t.

From these statements, Henderson et al. used separate terms

to describe rate of fractional adsorption for 1st structure [Θ1(t)]

and 2nd structure [Θ2(t)] as a function of immersion time (t).

Both of these terms were dictated by the Kisliuk adsorption

isotherm, where variables with a subscript of 1 relate to 1st

structure formation and a subscript of 2 relates to 2nd structure

formation.

These terms were combined in the Henderson adsorption

isotherm, which determines the total normalised impedance

detection signal strength caused by the adsorbate monolayer

(z(t)) as a function of Θ1(t), Θ2(t), φ1 and φ2. Values of φ are

weighting constants, which are normalized signal values that

would result from an adsorbent covered entirely with either 1st

structure or 2nd structure. This isotherm equation is shown

below:

[11]

Although the Henderson-Kisliuk adsorption isotherm was

originally applied to SAM adsorption, Henderson et al.

hypothesised that this adsorption isotherm is potentially

applicable to many other cases of adsorption and that Θ1(t) and

Θ2(t) can be calculated using other adsorption isotherms, in

place of the Kisliuk model (such as the Langmuir adsorption

isotherm equation).

2.6 Adsorption enthalpy

Adsorption constants are equilibrium constants, therefore they

obey van 't Hoff's equation:

As can be seen in the formula, the variation of K must be

isosteric, that is, at constant coverage. If we start from the BET

isotherm and assume that the entropy change is the same for

liquefaction and adsorption we obtain  ,

that is to say, adsorption is more exothermic than liquefaction.

3.ADSORBENTS

3.1 Characteristics and general requirements

Activated carbon is used as an adsorbent

[12]

Adsorbents are used usually in the form of spherical pellets,

rods, moldings, or monoliths with hydrodynamic diameters

between 0.5 and 10 mm. They must have high abrasion

resistance, high thermal stability and small pore diameters,

which results in higher exposed surface area and hence high

surface capacity for adsorption. The adsorbents must also have

a distinct pore structure which enables fast transport of the

gaseous vapors.

Most industrial adsorbents fall into one of three classes:

Oxygen-containing compounds – Are typically

hydrophilic and polar, including materials such as silica

gel and zeolites.

Carbon-based compounds – Are typically hydrophobic

and non-polar, including materials such as activated

carbon and graphite.

Polymer-based compounds - Are polar or non-polar

functional groups in a porous polymer matrix.

3.2 Silica gel

Silica gel is a chemically inert, nontoxic, polar and

dimensionally stable (< 400 °C or 750 °F) amorphous form of

SiO2. It is prepared by the reaction between sodium silicate and

acetic acid, which is followed by a series of after-treatment

processes such as aging, pickling, etc. These after treatment

methods results in various pore size distributions.

Silica is used for drying of process air (e.g. oxygen, natural gas)

and adsorption of heavy (polar) hydrocarbons from natural gas.

[13]

3.3 Zeolites

Zeolites are natural or synthetic crystalline aluminosilicates

which have a repeating pore network and release water at high

temperature. Zeolites are polar in nature.

They are manufactured by hydrothermal synthesis of sodium

aluminosilicate or another silica source in an autoclave followed

by ion exchange with certain cations (Na+, Li+, Ca2+, K+, NH4+).

The channel diameter of zeolite cages usually ranges from 2 to

9 Å (200 to 900 pm). The ion exchange process is followed by

drying of the crystals, which can be pelletized with a binder to

form macroporous pellets.

Zeolites are applied in drying of process air, CO2 removal from

natural gas, CO removal from reforming gas, air separation,

catalytic cracking, and catalytic synthesis and reforming.

Non-polar (siliceous) zeolites are synthesized from aluminum-

free silica sources or by dealumination of aluminum-containing

zeolites. The dealumination process is done by treating the

zeolite with steam at elevated temperatures, typically greater

than500 °C (930 °F). This high temperature heat treatment

breaks the aluminum-oxygen bonds and the aluminum atom is

expelled from the zeolite framework.

3.4 Activated carbon

Activated carbon is a highly porous, amorphous solid consisting

of microcrystallites with a graphite lattice, usually prepared in

[14]

small pellets or a powder. It is non-polar and cheap. One of its

main drawbacks is that it is reacts with oxygen at moderate

temperatures (over 300 °C).

Activated carbon nitrogen isotherm showing a marked microporous type I behavior

Activated carbon can be manufactured from carbonaceous

material, including coal (bituminous, subbituminous, and

lignite), peat, wood, or nutshells (e.g., coconut). The

manufacturing process consists of two phases, carbonization

and activation. The carbonization process includes drying and

then heating to separate by-products, including tars and other

hydrocarbons from the raw material, as well as to drive off any

gases generated. The process is completed by heating the

material over 400 °C (750 °F) in an oxygen-free atmosphere

that cannot support combustion. The carbonized particles are

then "activated" by exposing them to an oxidizing agent,

usually steam or carbon dioxide at high temperature. This

agent burns off the pore blocking structures created during the

carbonization phase and so, they develop a porous, three-

dimensional graphite lattice structure. The size of the pores

developed during activation is a function of the time that they

[15]

spend in this stage. Longer exposure times result in larger pore

sizes. The most popular aqueous phase carbons are bituminous

based because of their hardness, abrasion resistance, pore size

distribution, and low cost, but their effectiveness needs to be

tested in each application to determine the optimal product.

Activated carbon is used for adsorption of organic substances

and non-polar adsorbates and it is also usually used for waste

gas (and waste water) treatment. It is the most widely used

adsorbent since most of its chemical (e.g. surface groups) and

physical properties (e.g. pore size distribution and surface area)

can be tuned according to what is needed. Its usefulness also

derives from its large micropore (and sometimes mesopore)

volume and the resulting high surface area.

4.HOW ADSORPTION OCCURS?

The process of adsorption arises due to presence of unbalanced

or residual forces at the surface of liquid or solid phase. These

unbalanced residual forces have tendency to attract and retain

the molecular species with which it comes in contact with the

surface. Adsorption is essentially a surface phenomenon.

Adsorption is a term which is completely different from

Absorption .While absorption means uniform distribution of the

substance throughout the bulk, adsorption essentially happens

at the surface of the substance. When both Adsorption and

Absorption processes take place simultaneously, the process is

called sorption.

[16]

Adsorption process involves two components Adsorbent and

Adsorbate. Adsorbent is the substance on the surface of which

adsorption takes place.Adsorbate is the substance which is

being adsorbed on the surface of adsorbent. Adsorbate gets

adsorbed.

Adsorbate + Adsorbent gives rise to Adsorption

Oxygen molecules (red) adsorb on a bimetallic surface of

platinum (purple) and cobalt (green). From P. B. Balbuena, D.

Altomare, L. Agapito, and J. M. Seminario, “Theoretical analysis

of oxygen adsorption on Pt-based clusters alloyed with Co, Ni,

or Cr embedded in a Pt matrix,” J. Phys. Chem. B (in press).

Some modern techniques have been used to study surface.

1. Low energy electron diffraction (LEED).

2. Photo electron spectroscopy (PES).

3. Scanning Tunneling microscopy (STM).

[17]

Adsorption in liquids

Adsorption can be understood by considering a simple

example. In case of liquid state, water molecule present on the

surface is attracted inwards by the molecules of water present

in the bulk. This gives rise to surface tension. While the

molecule of water present within the bulk is equally attracted

from all the sides and the net force experienced by the water

molecule in bulk is zero. This clearly shows that particles at

surface and particles at the bulk are in different environment.

Water molecule on surface experiencing unbalanced forces as compared to molecule inside which experiences forces from all

direction.

Adsorption in solids

In case of solid state these residual forces arises because a of

unbalanced valence forces of atoms at the surface. The

generation of these forces on solid surface can be explained

diagrammatically as follows:

[18]

Cleavage of a big crystal into smaller unit

Due to cleavage of a big crystal into smaller unit, residual

forces or vacancies gets generated on the surface of the solid.

Occupancy of these vacancies by some other molecular species

results into Adsorption.

4. FACTS ABOUT ADSORPTION PROCESS

Adsorption is a spontaneous process

For reaction or process to be spontaneous, there must be

decreases in free energy of the system i.e. ΔG of the system

must have negative value.

Also we know, ΔG = ΔH – TΔS

And during this process of adsorption, randomness of the

molecule decreases which ΔS is negative. We can rewrite above

equation as

Therefore for a reaction to be spontaneous ΔH has to be

negative and

[19]

Adsorption is an exothermic process

Adsorption process takes place by adsorbate getting adsorbed

on adsorbent .Forces of attraction exist between adsorbate and

adsorbent and due to these forces of attraction, heat energy is

released. So adsorption is an exothermic process.

5.TYPES OF ADSORPTION

Adsorption can be classified into two categories as described

below,

(1) Depending upon the concentration : In adsorption the

concentration of one substance is different at the surface of the

other substance as compared to adjoining bulk or interior

phase.

(i) Positive adsorption : If the concentration of adsorbate is

more on the surface as compared to its concentration in the

bulk phase then it is called positive adsorption.

Example :When a concentrated solution of KCl is shaken with

blood charcoal, it shows positive adsorption.

(ii) Negative adsorption : If the concentration of the adsorbate

is less than its concentration in the bulk then it is called

negative adsorption.

Example :When a dilute solution of KCl is shaken with blood

charcoal, it shows negative adsorption.

(2) Depending upon the nature of force existing between

adsorbate molecule and adsorbent

(i) Physical adsorption : If the forces of attraction existing

between adsorbate and adsorbent are Vander Waal’s forces,

[20]

the adsorption is called physical adsorption. This type of

adsorption is also known as physisorption or Vander Waal’s

adsorption. It can be easily reversed by heating or decreasing

the pressure.

(ii) Chemical adsorption :If the forces of attraction existing

between adsorbate particles and adsorbent are almost of the

same strength as chemical bonds, the adsorption is called

chemical adsorption. This type of adsorption is also called as

chemisorption or Langmuir adsorption. This type of adsorption

cannot be easily reversed.

Comparison between physisorption and chemisorption

Physisorption(Vander Waal's

adsorption)

Chemisorption(Langmuir adsorption)

Low heat of adsorption usually in range of 20-40kJ/mol  

High heat of adsorption in the range of 50-400 kJ/mol

Force of attraction areVander Waal's forces.

Forces of attraction arechemical bond forces.

It is reversible It is irreversibleIt is usually takes place atlow temperature and decreases with increasing temperature.

It takes place at high temperature. 

It is related to the case of liquefication of the gas.

It is not related.

It forms multimolecular layers. 

It forms monomolecular layers.

It does not require any activation energy.

It requires high activation energy.

High pressure is favourable. Decrease of pressure causesdesorption

High pressure is favourable. Decrease of pressure does not cause desorption.

[21]

It is not very specific. It is highly specific.

6.FACTORS AFFECT THE ADSORPTION

Factors which affect the extent of adsorption : The following are

the factors which affect the adsorption,

(1) Nature of the adsorbate (gas) and adsorbent (solid)

(i) In general, easily liquefiable gases e.g., CO2, NH3, Cl2 and

SO2 etc. are adsorbed to a greater extent than the elemental

gases e.g. H2, O2, N2, He etc. (while chemisorption is specific in

nature.)

(ii) Porous and finely powdered solid e.g. charcoal, fullers earth,

adsorb more as compared to the hard non-porous materials.

Due to this property powdered charcoal is used in gas masks.

(2) Surface area of the solid adsorbent

(i) The extent of adsorption depends directly upon the surface

area of the adsorbent, i.e. larger the surface area of the

adsorbent, greater is the extent of adsorption.

(ii) Surface area of a powdered solid adsorbent depends upon

its particle size. Smaller the particle size, greater is its surface

area.

(3) Effect of pressure on the adsorbate gas

[22]

(i) An increase in the pressure of the adsorbate gas increases

the extent of adsorption.

(ii) At low temperature, the extent of adsorption increases

rapidly with pressure.

(iii) Small range of pressure, the extent of adsorption is found

to be directly proportional to the pressure.

(iv) At high pressure (closer to the saturation vapour pressure

of the gas), the adsorption tends to achieve a limiting value.

(4) Effect of temperature

(i) As adsorption is accompanied by evolution of heat, so

according to the Le-Chatelier’s principle, the magnitude of

adsorption should decrease with rise in temperature.

(ii)The relationship between the extent of adsorption and

temperature at any constant pressure is called adsorption

isobar.

[23]

(iii) A physical adsorption isobar shows a decrease in x/m

(where ‘m’ is the mass of the adsorbent and ‘x’ that of

adsorbate) as the temperature rises.

(iv) The isobar of chemisorption show an increase in the

beginning and then decrease as the temperature rises

7.APPLICATIONS OF ADSORPTION

Adsorption finds extensive applications both in research

laboratory and in industry. A few applications are discussed

below:

In preserving vacuum:

In Dewar flasks activated charcoal is placed between the walls

of the flask so that any gas which enters into the annular space

either due to glass imperfection or diffusion though glass is

adsorbed.

In glass masks:

All gas masks are devices containing suitable adsorbent so that

the poisonous gases present in the atmosphere are

preferentially adsorbed and the air for breathing is purified.

In clarification of sugar:

Sugar is decolorized by treating sugar solution with charcoal

powder. The latter adsorbs the undesirable colors present.

In paint industry:

The paint should not contain dissolved gases as otherwise the

paint does not adhere well to the surface to be painted and

thus will have a poor covering power. The dissolved gases are

therefore, removed by suitable adsorbents during manufacture.

[24]

Further, all surfaces are covered with layers of gaseous, liquid

or solid films. These have to be removed before the paint is

applied. This is done by suitable liquids which adsorbs these

films. Such liquids are called wetting agents. The use of spirit

as wetting agent in furniture painting is well known.

In chromatographic analysis:

The selective adsorbent of certain substances from a solution

by a particular solid adsorbent has helped to develop technique

for the separation of the components of the mixture. This

technique is called chromatographic analysis. For example: in

column chromatography a long and wide vertical tube is filled

with a suitable adsorbent and the solution of the mixture

poured from the top and then collected one by one from the

bottom.

In catalysis:

The action of certain solids as catalysts is best explained in

terms of adsorption. The theory is called adsorption theory.

According to this theory, the gaseous reactants are adsorbed

on the surface of the solid catalyst. As a result, the

concentration of the reactants increases on the surface and

hence the rate of reaction increases. The theory is also able to

explain the greater efficiency of the catalyst in the finely

divided state, the action of catalyst promoters and poisons.

In adsorption indicators:

Various dyes which owe their use to adsorption have been

introduced as indicators particularly in precipitation titrations.

[25]

For example: KBr is easily titrated with AgNO3 using eosin as

an indicator.

In softening of hard water:

The use of ion exchangers for softening of hard water is based

upon the principle of competing adsorption just as in

chromatography.

In removing moisture from air in the storage of delicate

instruments:

Such instruments which may be harmed by contact with the

moist air are kept out of contact with moisture using silica gel.

8.BIBLIOGRAPHY

[26]