Experiment 6. Adsorption

Renon, Mike Angel T.

Tan, Elaine D.

The Phenomenon Called Adsorption The attachment of a substance to the surface

of a solid or a liquid.

Has two components: Adsorbent: the solid or liquid Adsorbate: the substance attached to the surface

Four Factors Affect Adsorption(Gas Adsorbed in a Solid) natures of the adsorbent and the gas being

adsorbed area of the adsorbent temperature of the gas pressure of the gas

Adsorption Has Two Types

Physisorption Chemical Adsorption

Adsorption Isotherms

describe the relation between the amount of substance adsorbed by an adsorbent and the equilibrium pressure or concentration at constant temperature

The 2 Common Adsorption Isotherms Freundlich Isotherm (1909)

empirical Langmuir Isotherm (1916)

theoretical

Objectives(Acetic Acid Adsorbed in Charcoal) To determine the saturation value for

monomolecular coverage for the adsorption of acetic acid by activated charcoal

To determine the values of the constants k and n in the Freundlich equation

To determine the values of constants a and b in the Langmuir’s equation

Methodology

Prepare 100 mL solutions of the ff. concentrations from 17 M acetic acid

using serial dilution: 1.0 M, 0.50 M, 0.25 M, 0.125 M, 0.0625 M

Measure 50 mL 1.0 M solution into 125-mL Erlenmeyer flask (A)

Add 1.0 g accurately weighed activated charcoal, shake the mixture and allow to equilibrate for one hour (B)

Agitate the mixture from time to time during the equilibration process (C)

While equilibration is taking place, titrate 5.0 mL original solution with 0.1 M NaOH to the phenolphthalein end point (D)

Calculate the original concentration of the solution from the titration data (E)

After equilibration, filter off the activated carbon

titrate 5.0 mL of the filtrate with 0.1 M NaOH to the phenolphthalein end point

Repeat A-E

For the titration of 0.25 M-0.0625 M analyte, use 10 mL

Calculate equilibrium concentration (C2) and the specific adsorption (y) for each concentration

Plot y versus C2 and determine the saturation value for monomolecular coverage

Plot ln y versus ln C2 and determine the values of K and n for Freundlich’s equation

Plot 1/y versus 1/C2 and determine the values of a and b for Langmuir’s equation

Results

Does the adsorption of acetic acid on activated charcoal obey the Freundlich equation? Support your answer. Yes it somehow obeys the Freundlich equation since the r value is close to 1.

Does the adsorption of acetic acid on activated charcoal obey the Langmuir equation? Support your answer. Yes it obeys the Langmuir equation since the r value is close to 1.

Which equation better describes the adsorption of acetic acid on charcoal? Explain your answer. The adsorption of acetic acid on activated charcoal is best explained by the Langmuir equation since the r value for Langmuir is closer to 1 than that of the r value from the Freundlich equation.

What gives rise to adsorption? Solids:

The intermolecular forces between the repeating units that make up the lattice are saturated

At the surface, these intermolecular forces are unsaturated

Consequently, any available molecule or ion is adsorbed to the surface.

The Two Components of Adsorption Adsorbent: solid phase Adsorbate:

system of the molecules that are adsorbed on the adsorbate

can be either a gas molecule or a solute molecule or ion

Adsorption is Different from Absorption Adsorption: surface accumulation Absorption: diffusion into the liquid or solid

The term “sorption” encompasses both processes.

The Case of Bonding Requirements solid :

bulk material bonding requirements of the atoms of the materials are

filled form of ionic, covalent or metallic bonds

Surface: bond deficiency is experienced

Thus, energetically favourable bonding with whichever available species is made possible.

Bonding Requirements and The Two Types of Adsorption Physisorption Chemisorption

Physisorption

Van der Waals adsorption adsorbate adheres to the

surface only through weak intermolecular interactions

Examples: Acetic Acid Adsorbed on

Charcoal The ability of geckos

(Uroplatus fibriatus) to climb walls

Chemisorption

molecule adheres to a surface and consequently forms a chemical bond

high activation energy monolayer form Examples:

Corrosion Metallic Oxidation

Activated Charcoal and Its Role on Adsorption Large surface area High surface area to mass ratio Highly porous

The Freundlich Isotherm

-Empirical basis

Where:

Y: specific adsorption (mg/g)

c: concentration (mol/L)

k and n: experimentally determined constants

In the experiment, we are to find k and n.

We need Y. Y (mg of adsorbate/ g of adsorbate)

We determine, from titration, the amount of acetic acid not adsorbed by charcoal.

The Freundlich Isotherm: A Derivation

At low pressure,

At high pressure,

Note that:

x: mass of adsorbate (mg)

m: mass of adsorbent (g)

x/m = Y = specific adsorption (mg/g)

At an intermediate value of pressure,

By using a constant of proportionality, k,

Note that n > 1.

By taking the logarithm of both sides, we obtain:

Upon plotting, we get:

The Limitations of The Freundlich Equation Fails at high pressures.

Current studies show that the Freundlich Isotherm can be derived from Kinetics

Langmuir Equation Relates the coverage or adsorption of molecules on a

solid surface to gas pressure or concentration of a medium above the solid surface at a fixed temperature.

b signifies the adsorption capacity/amount adsorbed per gram adsorbent while a is the Langmuir constant related to the adsorption energy/net enthalpy of adsorption.

The greater the a, the greater the affinity of the adsorbent with the adsorbate

The greater the b, the greater the number of available binding sites there are.

Assumptions of the Langmuir equationa) that the surface of the adsorbent is uniform

b) that the adsorbed molecules do not interact

c) that all adsorption occurs through the same mechanism

d) that at the maximum adsorption, only a monolayer is formed

Derivation of the Langmuir Isotherm: From Thermodynamics

S: the vacant surface sitesA: the gas phase moleculesSA: being the occupied surface sites, assuming there are a fixed number of surface sites present on the surface.

0 < θ < 1.

[SA] is proportional to the surface coverage of adsorbed molecules, or

proportional to θ

[S] is proportional to the number of vacant sites, or 1- θ

[A] is proportional to the pressure of gas, P.

Derivation of the Langmuir Isotherm: From Kinetics

The rate of adsorption will be proportional to the pressure of the gas and thenumber of vacant sites for adsorption. If the total number of sites on the surface isN,

The rate of change of the coverage due to the adsorbate leaving the surface(desorption) is proportional to the number of adsorbed species:

Ka and Kd are the rate constants for adsorption and desorption respectively, and P

is the pressure of the adsorbate gas. At equilibrium, the coverage is independent oftime and thus the adsorption and desorption rates are equal.

Conclusion

The saturation value for the monomolecular coverage for the adsorption of acetic acid on activated charcoal is 584 mg/g.

The Langmuir isotherm better describes the adsorption process because the Pearson coefficient is closer to 1.

Thank you!