To determine the surface area of activated charcoal via adsorption of iodine from solution.
INTRODUCTION
The term of adsorption is used to specify that the adsorbate (gas or liquid) is bound onto the adsorbent (solid surface). This is different from absorption, in which the molecule goes into the interior of the solid. In the case of adsorption, the process is defined as physical adsorption if the adsorbate is held by weak, van der Waals type of forces on the surface. If bond formation between the adsorbate and the adsorbent is observed, the process is commonly known as chemical adsorption.
The amount adsorbed per gram of solid influenced by the specific area of the solid, the equilibrium solute concentration in the solution or pressure in the case of adsorption from the gas phase, the temperature, and the nature of the molecules involved. From measurements at constant temperature, one can obtain a plot of N, the number of moles adsorbed per gram of solid, versus c, the equilibrium solute concentration. This is called an adsorption isotherm.
In this experiment, we use activated charcoal sample as adsorbent and iodine as adsorbate. Adsorption of iodine will determine the surface area of activated charcoal. This method can be applied in determination of the surface area of powder drug in pharmaceutical industries. The surface area is very important in field of pharmacy where it is one of factor that affects the rate of dissolution and bioavailability of drugs that are absorbed in gastrointestinal tract.
MATERIAL AND APPARATUS
12 conical flask, 6 centrifuge tubes, measuring cylinder, analytical balance, Beckman J6M/E, retort stand and clamps, pasteur pipette, 0.05 M iodine solution, 0.1 M Potassium iodine, 1% w/v starch solution, 0.1 M sodium thiosulphate solution, distilled water and activated charcoal.
PROCEDURE
12 conical flasks labeled 1 to 12 was filled with 50 ml mixture of 0.05 M Iodine (solution A) and 0.1 M Potassium iodine ( solution B) by using measuring cylinder as stated in the table 1.
Set 1: Actual concentration of iodine in solution A (X)
For flasks 1-6, 1-2 drops of starch solution was added into the conical flask as an indicator. Next, the solution was then titrated with 0.1 M sodium thiosulfate solution until the colour of solution change from dark blue to colourless. The volume of sodium thiosulfate used was recorded.
Set 2: Concentration of iodine in solution A at equilibrium (C).
For flask 7-12, 0.1 g activated charcoal wad added into the conical flask and the flask was tightly cap and was swirl or shake every 10 minutes for 2 hours. Next, the solutions were transferred into the centrifuge tubes and labeled accordingly after 2 hours. The solutions were then centrifuge at 3000 rpm for 5 minutes and the resulting supernatants were transferred into new conical flask accordingly. Steps 1, 2, and 3 were repeated for flask 1-6 in Set 1.
RESULT
TITRATION EQUATION
I₂ + 2Na₂S₂O₃ = Na₂S₄O₆ + 2 NaI
Na₂S₂O₃= ½ I₂
1. Calculate N for iodine in each flask.
The value of N is calculated by using the formula
N = (X - C) x 50/1000 x 1/y
Where y = 0.1g
2. Plot amount of iodine adsorbed (N) versus balance concentration of solution (C) at equilibrium to obtain adsorption isotherm.
3. According to Langmuir theory, if there is no more than a monolayer of iodine adsorbed on the charcoal,
C/N = C/Nm + I/KNm
Where, C = concentration of solution at equilibrium
Nm = number of mole per gram charcoal required
K = constant to complete a monolayer
Plot C/N versus C, if Langmuir equation is followed, a straight line with slope of 1/Nm and intercept of 1/KNm is obtained. Obtain the value of Nm, and then calculate the number of iodine molecule adsorbed on the monomolecular layer. Assume that the area covered by one adsorbed molecule is 3.2x10-19 m2, Avogadro no.= 6.023x1023 molecule, calculate the surface area of charcoal in m2g-1
Value of 1/KNm (y-int.) = 0.60
Value of Nm :
1/Nm = (y2-y1)/(x2-x1)
=(4.1250-0.7500) / (0.0080-0.0040)
1/ Nm =843.75
Nm =0.00119 mol g-1
No. of molecules of charcoal = Nm x Avogadro no.
= 0.00119 mol g-1 x (6.023x1023 molecules per mole)
= 7.167x1020 molecules g-1
Surface area of charcoal = (3.2 x 10-19 m2 molecules-1) x (7.167x1020 molecules g-1)
= 2293.44 m2 g-1
4. How do you determine experimentally that equilibrium has been reached after shaking for two hours?
After been shaken for two hours, the solutions been titrated by using starch solution as indicator. By referred to the flask labelled 1 to 6, compared the volume of Sodium thiosulphate, Na2S2O3 used with the flask labelled 7 to 12. The solution will change colour from dark blue to colourless.
DISCUSSION
Adsorption is the sticking of molecules from the gas or liquid phase onto the surface of a solid and it is different from absorption which is the filling of pores in a solid. A molecule that undergoes adsorption is referred to as the adsorbate, and the solid is the adsorbent. In the case of adsorption, the process is defined as physisorption if the adsorbate is held by weak, van der Waals type of forces on the surface. If bond formation between the adsorbate and the adsorbent is involved chemical bond, the process is commonly known as chemisorption. Physical adsorption involves low heats of adsorption (less than 40 kJ/mol) and is enhanced by lowering the temperature while chemical adsorption involves higher heats of adsorption (80 to 400 kJ/mol of molecules) and is enhanced by raising the temperature. Similar to surface tension, adsorption is a consequence of surface energy. In a bulk material, all the bonding requirements (be they ionic, covalent, or metallic) 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.
Adsorbents are used usually in the form of spherical pellets, rods, moldings, or monoliths. 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 that enables fast transport of the gaseous vapors. The most common industrial adsorbents are activated carbon, silica gel, and alumina, because they present enormous surface areas per unit weight.
The amount of a substance that can be adsorbed onto activated charcoal depends on nature of adsorbate and adsorbent, the surface area of adsorbent, activation of adsorbenta and experimental conditions such as temperature. Adsorption process is usually studied through graphs known as adsorption isotherm. That is the amount of adsorbate on the adsorbent as a function if its pressure or concentration at constant temperature. Example of isotherm is Langmuir equation and it is most common isotherm equation to use due to its simplicity and its ability to fit a variety of adsorption data.
In this experiment, Langmuir equation is used to estimate the surface area of activated charcoal sample. Langmuir derived an equation which explained the relationship between the number of active sites of the surface undergoing adsorption and pressure. This equation is called Langmuir isotherm equation.
Cf/N = (1/Nm) Cf + (1/kNm)
Where (1/Nm) is the slope, and (1/kNm) is the intercept, when Cf/N is plotted versus the concentration Cf. The inverse of the slope is Nm,and this represents the moles adsorbed at monolayer coverage. Nm can be used to determine the specific surface area of a solid. The basic limitation of Langmuir adsorption equation is that it is valid at low pressure only.
The actual concentration in solution A (X) and concentration of iodine in solution A at equilibrium was recorded and amount of iodine adsorb (N) was calculated from the data that was collected. Graph amount of iodine adsorb (N) versus balance concentration of solution (C) at equilibrium is not linear graph and shows that the number of iodine adsorb gradually increase in the solution. This is because the greater the solubility, the stronger are the solute-solvent bonds and hence the smaller the extent of the adsorption of iodine onto the activated charcoal. The linear plot graph with slope of 1/Nm and intercept of 1/KNm was obtained from graph C/N versus C. This shows that the Langmuir equation is followed.
The possible errors that occur while conducting this experiment is the amount of sodium thiosulphate titrated recorded are inaccurate because the position of eye s not perpendicular to the solution so it will overall changes result of the experiment. Other than that, that amount of charcoal added is more than needed and not distributed equally over the solution. This cause the solution not achieved equilibrium. Next, the charcoal maybe included in supernatant that will affect the amount of sodium thiosulphate needed to change colour from dark blue to colourless.
Conclusion
From this experiment, the adsorption of iodine solution in charcoal follows the Langmuir theory of adsorption isotherm. The result shows that the adsorption decrease as the concentration of the iodine solution decrease. From the experimental result, the surface area of charcoal is 2293.44 m2 g-1.
References
1. Patrick J. Sinko, Lippincott Williams and Wilkins. Martin’s Physical Pharmacy and Pharmaceutical Sciences (page 39,40), 5th Edition.
2. Alexander T Florence and David Attwood. (2006). Physiocochemical Principles of Pharmacy (page 194-197). 4th Ed. Palgrave. USA. 3. http://www.slideshare.net/ionizer_88/adsorption-from-solutions-acetic-acid-on-charcoal-14522884
No comments:
Post a Comment