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EXPERIMENT 6: ADSORPTION

Natorilla, Haziel May C.Mendoza, Enrico

Chem 157.1 - DEMr. Leonardo Dante P. Yambot

Department of Physical Sciences and MathematicsCollege of Arts and Sciences

University of the Philippines Manila=====================================================================

Theoretical Framework

When molecules or particles interact with the surface, they bind through a process called adsorption. Adsorption is a surface phenomenon that occurs upon accumulation of gas or liquid solute on the surface of a solid, or sometimes, on a liquid called adsorbent. Adsorption from solution is usually monomolecular, i.e. adsorption ceases when the surface is completely covered with a layer one molecule thick.

Adsorption differs from absorption. In absorption, a substance diffuses into a solid or a liquid to form a solution, and the bind on the surface is usually weak and reversible. In adsorption, fluids that dissolve or that suspends the material of interest are bound. Coloured compounds and compounds containing taste or odour also tend to bind strongly, and compounds with chromogenic groups are strongly adsorbed on activated carbon.

Some common industrial adsorbents that present large surface areas per unit weight include activated carbon, silica gel, and alumina.

Some common sources of activated carbon that are produced by roasting organic material to decompose it into carbon granules include coconut shells, wood, and bones. Active carbon is specially made so as to achieve a very huge internal surface (between 500-1500 m2/g), and such huge surface makes active carbon ideal for adsorption.

The amount of adsorption varies with the concentration of the solution. Of course, a freshly prepared activated carbon has a clean surface. Now, if a surface is heavily contaminated by adsorbates, the activated carbon is less likely able to accommodate additional binding.What are the two types of adsorption? How is adsorption different from absorption?What are the significances of the constants k and n?

Two equations for this relationship for monomolecular adsorption:

Freundlich Equation

an adsorption isotherm, which is a curve relating the concentration of a solute on the surface of an adsorbent, to the concentration of the solute in the liquid with which it is in contact.

The Freundlich Adsorption Isotherm is mathematically expressed as:

x/m = Kp( 1/ n ), or x/m = Kc( 1/ n ), or ln (x/m) = (1/n) ln C + ln k

where x = mass of adsorbate, m = mass of adsorbent, p = Equilibrium pressure of adsorbate, and c = Equilibrium concentration of adsorbate in solution.

http://en.wikipedia.org/wiki/Concentration

http://en.wikipedia.org/wiki/Pressure

http://en.wikipedia.org/wiki/Equilibrium

http://en.wikipedia.org/wiki/Adsorbent

http://en.wikipedia.org/wiki/Adsorbate

http://en.wikipedia.org/wiki/Mass

http://en.wikipedia.org/wiki/Adsorbent

http://en.wikipedia.org/wiki/Solute


http://en.wikipedia.org/wiki/Isotherm

http://en.wikipedia.org/wiki/Adsorption

K and 1/n are constants for a given adsorbate and adsorbent at a particular temperature.

Langmuir Equation (or Langmuir isotherm or Langmuir adsorption 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. It is expressed as:

C/y = (1/ab) + (1/b)Cwhere y = (x/m), C=concentration, a= constant related to energy or net enthalpy of absorption, and b= amount adsorbed per gram adsorbent

*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 sitesWhat do the constants a and b mean? (besides that which is written in this report)

Assumptions of Langmuir equation The surface of the adsorbent is uniform, meaning all the adsorption sites are equal.

Adsorbed molecules do not interact.

All adsorption occurs through the same mechanism.

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.

Methodology

Objectives:1. Determine the saturation value for monomolecular coverage for the adsorption of acetic acid

by activated charcoal.2. Determine the values of the constants k and n in the Freundlich equation.3. Determine the values of the constants a and b in the Langmuir’s equation.

Procedure: Revise this into a simpler flowchart – too wordyPerform Serial Dilution: Glacial Acetic Acid (17M)

1.0 M 0.50M 0.25 M 0.125 M 0.0625 M

Run 1: Two 250-ml E. flask

50 ml of 1.0 M sol’n + 1.0 g activated charcoalSHAKE mixture; Equilibriate (1hr) Agitate during equilibrium process

E1-a: Original Solution (no activated C) perform while waiting for E1-b to equilibriate

Pipet 5.0 ml to E. flaskTITRATE w/ 0.1 M NaOH (phenolphthalein end point)

E1-b: Solution with activated charcoal perform after equilibriating for 1 hour

Filter off activated CPipet 5.0 ml to E. flaskTITRATE w/ 0.1 M NaOH (phenolphthalein end pt)




Pipet 5.0 ml to E. flask; TITRATE w/


Filter off activated C


http://en.wikipedia.org/wiki/Gas_pressure


0.1 M NaOH (phenolphthalein end pt) Pipet 5.0 ml to E. flaskTITRATE w/ 0.1 M NaOH (phenolphthalein end pt)




Pipet 10.0 ml to E. flask; TITRATE w/ 0.1 M NaOH (phenolphthalein end pt)















Calculate equilibrium concentration (C2) and specific adsorption (y)

Plot y vs C1, ln y vs C1, and (C1/y) vs C1Also plot y vs C2, ln y vs C2, and C2/y vs C2 ; check with other books because I think C stands for final or

equilibrium concentration

Results and DiscussionsCompare and contrast the graphs made vs C1 and those made using C2 instead of C1.Present the data of other groups and perform statistical analyses between data.Reminder, groups 7-8 and 4-6 did not use the shaker; only groups 1-3 used the shaker during the experiment; compare their results and see if there is a correlation – how much error does the fact that the shaker was not used creates?

Our Data - Groups 4-6Initial Conc (C1)

Final Conc. (C2)

wacetic acid

(g)mcharcoal

(g) y ln y ln C1 C1/y0.94 0.754 0.558 1 0.558 -0.583396317 -0.0618754 1.6845878140.578 0.44 0.414 1 0.414 -0.881889305 -0.54818141 1.3961352660.28 0.235 0.135 1 0.135 -2.002480501 -1.27296568 2.0740740740.113 0.092 0.063 1 0.063 -2.764620553 -2.18036746 1.7936507940.064 0.05 0.042 1 0.042 -3.170085661 -2.7488722 1.523809524

VNaOH VNaOH

(ml) (ml)47 37.728.9 2228 23.511.3 9.26.4 5

Plot of y versus C1

y = 0.1948Ln(x) + 0.5079

R2 = 0.8831

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.2 0.4 0.6 0.8 1

C1

y

y

Log. (y)

Remove the trendline. That equation is wrong. And then, to estimate the saturation of y for monomolecular coverage, plot and extrapolate. See where it will plateau or level off manually.Saturation value of y for monomolecular coverage: mg/g

Plot of ln y vs ln C1

y = 1.0094x - 0.5052

R2 = 0.9821-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

-3 -2 -1 0

ln C1

ln y ln C1

Linear (ln C1)

As is;

Freundlich’s constants: n = 0.991 k = 0.603372356r (Pearson correlation coefficient) = 0.991006264

Plot of C1/y vs C1

y = -0.0134Ln(x) + 1.6762

R2 = 0.0033

0

0.5

1

1.5

2

2.5

0 0.2 0.4 0.6 0.8 1

C1

(C1/

y) C1/y

Log. (C1/y)

Remove the trendline. That equation is wrong.Also, reformat the scale from 1 to 2.5 for the y axis so as to emphasize the data

Langmuir’s constants: a = 0.123156869 k = 0.5.417663961r (Pearson correlation coefficient) = 0.200328364

Sample Calculations:CNaOHVNaOH = Cacetic acidVacetic acid

Initial concentration: Final concentration:C1 = (0.1 moles/L x 47 ml)/ 5 ml = 0.9400 M C2 = (0.1 moles/L x 37.70 ml)/ 5 ml = 0.7540 MC1 = (0.1 moles/L x 28.9 ml)/ 5 ml = 0.5780 M C2 = (0.1 moles/L x 22 ml)/ 5 ml = 0.4400 MC1 = (0.1 moles/L x 28 ml)/ 10 ml = 0.2800 M C2 = (0.1 moles/L x 23.5 ml)/ 10 ml = 0.2350 MC1 = (0.1 moles/L x 11.3 ml)/ 10 ml = 0.1130 M C2 = (0.1 moles/L x 9.2 ml)/ 10 ml = 0.0920 MC1 = (0.1 moles/L x 6.4 ml)/ 10 ml = 0.0640 M C2 = (0.1 moles/L x 5 ml)/ 10 ml = 0.0500 M

y = (C1-C2) x MWacetic acid x Vsolution

mcharcoal

y = (0.9400 - 0.7540) moles/L x 60 g/mol x (50/1000) L = 0.5581.0000 g

Data From Other GroupsGroups 1-3Initial Conc (C1)

Final Conc. (C2)

wacetic acid

(g)mcharcoal

(g) y ln y ln C1 C1/y1.032 0.97 0.204 1.0056 0.202864 -1.595219667 0.031498667 5.0871529620.484 0.448 0.042 1.0005 0.107946 -2.226123927 -0.72567037 4.4837222220.2475 0.22 0.0825 1.0005 0.082488 -2.495106088 -1.3963447 3.0004473390.122 0.106 0.048 1.0001 0.047995 -3.036654273 -2.10373423 2.5419208590.059 0.048 0.033 1.0005 0.032984 -3.4117476 -2.83021784 1.788772741

Groups 7-8Initial Conc (C1)

Final Conc. (C2)

wacetic acid

(g)mcharcoal

(g) y ln y ln C1 C1/y1.042 1.002 0.12 1 0.12 -2.120263536 0.041141943 8.6833333330.526 0.51 0.048 1 0.048 -3.036554268 -0.64245407 10.958333330.265 0.255 0.03 1 0.03 -3.506557897 -1.32802545 8.8333333330.127 0.115 0.036 1 0.036 -3.324236341 -2.06356819 3.5277777780.0642 0.0545 0.0291 1 0.0291 -3.537017105 -2.74575207 2.206185567

Group 1-3: Graphs

Plot of y vs C1

y = 0.0565Ln(x) + 0.1742R2 = 0.893

0

0.05

0.1

0.15

0.2

0.25

0 0.5 1 1.5

C1

y


y = 0.6265x - 1.6729R2 = 0.9904

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

-3 -2 -1 0 1

ln C1

ln y

Plot of C1/y vs C1

y = 1.1998Ln(x) + 5.0659R2 = 0.9667

0

1

2

3

4

5

6

0 0.5 1 1.5

C1

C1/

y

Remove the trendline. That equation is wrong. And then, to estimate the saturation of y for monomolecular coverage, plot and extrapolate. See where it will plateau or level off manually.Only the plot for ln y vs ln c1 and ln y vs ln c2 has a linear trendline.

Group 7-8: Graphs

Plot of y vs C1

y = 0.0275Ln(x) + 0.0897R2 = 0.6278

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 0.5 1 1.5

C1

y


y = 0.4432x - 2.5076R2 = 0.7015

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

-3 -2 -1 0 1

ln C1

ln y

Plot of C1/y vs C1

y = 2.933Ln(x) + 10.795R2 = 0.7415

0

2

4

6

8

10

12

0 0.5 1 1.5

C1

C1/

y

In this experiment, it can be observed that the plot for specific adsorption (y) versus the initial concentration (c1) is nonlinear. It has a plateau or levelling effect. This is due to the limited amount of activated charcoal. At the point where it levels off, the activated carbon is already saturated, making it unable to adsorb on its surface. The saturation value of y for monomolecular coverage is ______ .

In the second plot, it can be observed that the plot for the natural logarithm of the specific adsorption (ln y) versus the natural logarithm of the initial concentration (ln c1) is nearly linear. This suggests that the system follows the Freundlich’s adsorption isotherm. On the other hand, it can be observed in the third plot that the plot for the initial concentration divided by the specific adsorption (C1/y) versus the initial concentration (c1) is nonlinear. This suggests that the system does not follow the Langmuir’s adsorption isotherm. This implies that the assumptions on Langmuir’s equation may have not been obeyed. Possibly, the surface of the adsorbent is not uniform, meaning the adsorption sites are not equal. Adsorbed molecules may have also interacted, and all adsorption occurs through the different mechanisms. Or, at the maximum adsorption, a monolayer was not formed.

Which equation best fits the data? What is the significance of this?How is Freundlich’s equation different from Langmuir equation?

Theoretical data for the specific adsorption of acetic acid with respect to concentration:C y ( mg/g)1 88.480.5 191.760.25 91.700.125 42.620.0625 28.24

Hindi sure ung theoretical data; please check

Possible Sources of Error: Not using the shaker during the experiment The solution may have not yet reached equilibrium in one hour

Possible Ways to Minimize Error: Use the shaker during the experiment Perform a pre-analysis of the system to ensure that the equilibrium is reached within the said

time. Perform the pre-analysis starting from 30 minutes, with a 30-minute interval, until optimum time for reaching equilibrium is determined.

Why did you use charcoal? What property of charcoal makes it a good adsorbent?What does it mean by the word “activated” ?

Answers to Questions

1. Does the adsorption of acetic acid on activated charcoal obey the Freundlich equation? Support your answer. Yes. The r value is near 1, meaning, it is nearly linear and it obeys the Freundlich equation.This answer is subject to examination of graphs again

2. Does the adsorption of acetic acid on charcoal obey Langmuir equation? Support your answer. No. The r value is far from 1, meaning, it is nonlinear and it does not obey the Langmuir’s equation.This answer is subject to examination of graphs again

3. Which equation better describes the adsorption of acetic acid on activated charcoal? Explain your answer. The Freundlich equation describes the adsorption of acetic acid on activated charcoal better. The high correlation coefficient suggests that the Freundlich adsorption isotherm is a better representation than the Freundlich isotherm for the adsorption of acetic acid on activated charcoal.

This answer is subject to examination of graphs again

References:

Atkins, Peter and de Paula, Julio. Atkins Physical Chemistry, 7th edition.Laidler. Meisler. Physical Chemistry, 3rd editionMaron, Samuel H. and Lando, Jerome B. Fundamentals of Physical Chemistry.http://www.lenntech.com/adsorption.htmhttp://www.rpi.edu/dept/chem-eng/Biotech-Environ/Adsorb/adsorb.htmhttp://en.wikipedia.org/wiki/Adsorptionhttp://en.wikipedia.org/wiki/Freundlich_equationhttp://en.wikipedia.org/wiki/Langmuir_equation

http://en.wikipedia.org/wiki/Langmuir_equation

http://en.wikipedia.org/wiki/Freundlich_equation


http://www.rpi.edu/dept/chem-eng/Biotech-Environ/Adsorb/adsorb.htm

http://www.lenntech.com/adsorption.htm

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