Surface phenomena ADSORPTION OF ACETAMINOPHEN ON ACTIVATED CHARCOAL AIM The aim of the experiment is to analyze the adsorption process of acetaminophen on activated charcoal. Spectrophotometry is used to determine the concentration of paracetamol in solution before and after adsorption process on activated charcoal. The obtained results will serve to verify if the Freundlich isotherm can be used for quantitative description of the adsorption of acetaminophen on activated charcoal. REQUIRED KNOWLEDGE Adsorption ad solid interfaces, physical and chemical adsorption, adsorption isotherms (Langmuir, Freundlich and BET), hysteresis phenomena, adsorbents. INTRODUCTION The attachment of particles to a surface is called adsorption. The substance that adsorb is called the adsorbate. The adsorbate attaches to the surface of the solid substance that is called adsorbent. The reverse of adsorption is desorption. Adsorption of material at solid surfaces can take place from either adjacent liquid or gas phase. Molecules and atoms can attach to surfaces in two ways. Considering the nature of those interactions, the adsorption process is classified as physical adsorption (physisorption) and chemical adsorption (chemisorption). It is important to note that there are several differences between physical and chemical adsorption: The physisorption is associated with van der Waals interactions or hydrogen bonds that are weak but have a long range. In chemisorption, the adsorbate is sticked to the adsorbent by forming a chemical bond (usually a covalent one). Therefore, the chemical adsorption is considered to be the specific process. The energy released as the chemisorption occurs is much greater when compared to physical adsorption. The physical adsorption is an exothermic process. Therefore, when the temperature increases the physisorption decreases (the desorption takes place). In opposite, increase in temperature results in increasing chemisorption. The physical adsorption is reversible (desorption can take place). In the case of chemical adsorption, it is assumed that it is irreversible (it is very difficult to break the chemical bonds). In chemical adsorption, due to specific chemical bonds, only monolayer coverage of the adsorbent is possible. Monolayer formation is rare in physisorption where formation of several layers of adsorbate molecules on adsorbent surface is more common. Adsorption isotherms are the equations that describe mathematically the equilibrium state between an adsorbate and a solid surface (adsorbent) at constant temperature. They represent the amount of the 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
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adsorbent to allow comparison of different materials. Those equations can be derived theoretically (assuming the specific model of adsorption process) or empirically (considering experimental data). The Langmuir isotherm Irving Langmuir presented the simplest adsorption isotherm. It is a semi-empirical isotherm derived from a proposed kinetic mechanism. It is based on following assumptions: Adsorption cannot proceed beyond monolayer coverage. The surface of the adsorbent is uniform, that is, all the adsorption sites are equivalent. There are no interactions between adsorbed molecules. The Langmuir isotherm describes the variation of fractional coverage ϴ with pressure at constant temperature (derivation of Langmuir isotherm is presented in Appendix): (1) where: ϴ K p
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fractional coverage (the ratio of a number of adsorption sites occupied and a number of adsorption sites available on the surface) constant value; the ratio of adsorption rate constant and desorption rate constant gas pressure
Replacing ϴ by a/am where a is the mass of gas adsorbed per gram of adsorbent at pressure p and at constant temperature and am is the mass of gas that 1 gram of adsorbent can adsorb when monolayer is complete gives the formula: (2) The plot reflecting Langmuir isotherm:
Figure 1. The Langmuir isotherm.
It is characteristic that in the initial part of the graph (at low pressure) the directly proportional increase in adsorption with increasing pressure takes place. Then, with the increasing pressure of adsorbate (if gas), all the sites available for adsorption at the surface are occupied. Thus, despite the
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further increase in pressure, the adsorption does not change because the entire surface of the adsorbent is occupied (and the Langmuir model assumed only the monolayer coverage).
By inverting the equation (2) becomes: (3)
A plot 1/a against 1/p yields a straight line and the am and K values can be calculated from the slope and Y-intercept:
Figure 2. Linear form of the Langmuir isotherm.
The BET isotherm Often molecules do form multilayers, that is, some are adsorbed on already adsorbed molecules and then the Langmuir isotherm is not valid. Stephen Brunauer, Paul Emmett, and Edward Teller developed a model isotherm that takes that possibility into account. They considered that the initial adsorbed layer can act as a substrate for further adsorption. The BET isotherm serves as the basis for an important analysis technique for the measurement of the specific surface area of a material. (4) where: ϴ C
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Z
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fractional coverage (the ratio of a number of adsorption sites occupied and a number of adsorption sites available on the surface) a constant connected with the enthalpies of desorption from monolayer and vaporization of the liquid adsorbate the ratio of the gas pressure (p) and the saturated vapour pressure (p*)
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Figure 3. The BET isotherm.
The Freundlich isotherm Freundlich gave an empirical expression representing the variation of adsorption of a quantity of substance adsorbed by unit mass of solid adsorbent with pressure (if adsorbate is a gas) or concentration (if the adsorbate is liquid) at constant temperature. In Freundlich model there is no assumption on monolayer coverage only, so it can describe multilayer adsorption process. The Freundlich isotherm equation is: (5) where: a k
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n p
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the mass of a substance adsorbed per gram of adsorbent the constant connected to adsorption energy; it is the adsorption value when the adsorbate concentration equals 1 mol/dm3 the constant that reflects the interactions between adsorbate molecules gas pressure
The Freundlich adsorption isotherm is a curve that reflects adsorption increase with increasing pressure:
Figure 4. The Freundlich isotherm.
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Constants k and n can be evaluated from linear form of Freundlich equation. Therefore, the decimal logarithms should be taken from equation (5). That gives: (6) The plot of loga against logp results a straight line where 1/n is a slope and logk is an Y-intercept.
Figure 5. The linear form of the Freundlich isotherm .
In the case of the adsorption from the solutions, the Freundlich isotherm becomes (c is a concentration of adsorbate in the solution): (7)
The adsorption phenomenon is widely used in laboratory practice and in the industry. It is the basis of processes of purification, separation, analysis and isolation of compounds present in mixtures. Moreover, it plays a major role in adsorption chromatography. Several compunds used in treatment because of their strong abilities to adsorption. Medical charcoal (activated charcoal) is used as an “universal antidote” in reducing the effects of poisoning by the oral route. Activated charcoal and diosmectite are widely used in the treatment of diarrhea (they adsorb water, bacterial toxins and patogens such as bacteria or rotaviruses). From the other hand, adsorption may result in drug interactions when the molecules of one drug (eg. antihypertensive) are adsorbed on the surface of co-administered drug (eg. medical charcoal, antacid inorganic salts). It may cause the reduced effectiveness of pharmacotherapy.
REFERENCES 1. 2. 3. 4.
Atkins P., de Paula J.: Physical pharmacy. Oxford University Press, Oxford 2006. Hermann T.W.: Farmacja fizyczna. Wydawnictwo Lekarskie PZWL, Warszawa 1999. Hermann T.W.: Chemia fizyczna. Wydawnictwo Lekarskie PZWL, Warszawa 2007. Atwood D., Florence A.T.: Physical pharmacy.Pharmaceutical Press, London 2008. 5
EQUIPMENT AND MATERIALS Spectrophotometer UV-Vis Thermo-shaker Pipettes 0.1 M HCl Acetaminophen solution in 0.1 M HCl of concentration 12.5 mg/mL Acetaminophen solution in 0.1 M HCl of concentration 12.5 µg/mL Activated charcoal EXPERIMENTAL The task is to determine the acetaminophen concentration in solution (applying spectrophotometry) before and after adsorption (after 45-minute incubation of the drug with activated charcoal). 1. Prepare a series of 6 samples (in plastic tubes with the cap) that contain: Sample 1 2 3 4 5 blank
Mass of activated charcoal [mg] 100 100 100 100 100 100
Volume [mL] of acetaminophen solution 12.5 mg/mL 8.0 6.0 4.0 2.7 0.8 -------
Volume of 0.1 M HCl [mL] -----2.0 4.0 5.3 7.2 8.0
2. Tightly close the tubes, vortex thoroughly and place in thermoshaker. Let the samples incubate for 45 minutes at the temperature 37˚C. The speed of shaking should be stated at 700 cycles per minute. 3. During the incubation of samples with activated charcoal, prepare a calibration curve of absorbance against the concentration of acetaminophen in solution: a. Prepare a series of 6 samples that contain following acetaminophen concentrations: 12.5 µg/mL; 10.0 µg/mL; 7.5 µg/mL; 5.0 µg/mL; 2.5 µg/mL; 1.25 µg/mL. Use the stock solution of acetaminophen of concentration 12.5 µg/mL. Prepare the samples in 25 mL volumetric flasks by diluting the stock solution of paracetamol (12.5 µg/mL) with 0.1 M HCl solution. Consult the results obtained in calculations of volume of acetaminophen stock solution with the teacher. b. Measure the absorbance of the obtained solutions at λmax = 243 nm. Calibrate the spectrophotometer using 0.1 M HCl solution as a blank sample. c. Plot the calibration curve A = f (c) and determine the equation of a straight line (consult with the teacher the significance of the Y-intercept). 4. When the incubation is finished, remove the samples from the thermoshaker and centrifuge the samples for 10 minutes at 3500 rpm. 6
5. Prepare the appropriate dilutions of samples 1 – 4 (in volumetric flasks of 50 mL). For this purpose, measure out (using the pipette) the following volumes of the supernatant: Sample 1 (diluted 1000x) – 0.05 mL (50 µL) Sample 2 (diluted 1000x) – 0.05 mL (50 µL) Sample 3 (diluted 1000x) – 0.05 mL (50 µL) Sample 4 (diluted 100x) – 0.50 mL (500 µL) and fill up to 50 mL with 0.1 M HCl solution. 6. Measure the absorbance of the obtained solutions (after diluting) at λmax = 243 nm. The absorbance of the sample 5 shoud be measured directly (without diluting) by transferring to a cuvette about 3 mL of the supernatant. The spectrophotometer should be calibrated using the supernatant from the blank sample (without acetaminophen) transferred directly to the cuvette. Draw attention to not shake the sample and check if the obtained solution is clear (without particles of activated carbon) when pipetting the supernatant. If you notice any pollution in the supernatant, the sample should be centrifuged again and the clear supernatant should be collected. 7. Calculate the concentration of acetaminophen in samples after incubation with carbon using the equation of the calibration curve (the dilutions should be taken into account):
where: c – acetaminophen concentration at the equilibrium state (after incubation) [mg/mL] A – absorbance b – the Y-intercept of the calibration curve a – the slope of the calibration curve R – the sample dilution 8. Calculate the amount of acetaminophen (in mg): a. That was in the sample before incubation with activated charcoal b. That remained in the solution after incubation 9. Calculate the amount of adsorption a (how many mg of acetaminophen was adsorbed per 1 g of activated charcoal). 10. Plot the amount of adsorption a against acetaminophen concentration c (take into account the acetaminophen concentrations at the equilibrium state). Discuss the graph with the teacher. 11. Evaluate the linear Freundlich isotherm equation, calculate the values of k and n. Discuss with the teacher if Freundlich model is suitable for describing the process of acetaminophen adsorption on activated charcoal. 12. Analyze the amount of acetaminophen adsorbed on activated charcoal in the environment of the stomach (0.1 mol / l HCl) and at the temperature of human body. What clinical implications may 7
be related to co-administered acetaminophen and medical charcoal? Can the phenomenon of acetaminophen adsorption on activated charcoal be used in therapy? 13. Analyze the adsorption process in different temperature than 37˚C (considering the results obtained from the teacher). Which type of adsorption (physical or chemical) is the binding of acetaminophen onto the activated charcoal?
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LAB REPORT Acetaminophen adsorption on activated charcoal Purpose:……………………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………………………………… 1. Calibration curve of absorbance against acetaminophen concentration in the solution. Sample
Acetaminophen concentration [µg/mL]
1
12.5
2
10.0
3
7.5
4
5.0
5
2.5
6
1.25
Absorbance (λmax = 243 nm)
Calibration curve A = f(c) slope
a
Y-intercept
b
Y-intercept deviation
Sb
Correlation coefficient
r
The final calibration curve equation (considering the Y-intercept significance): …………………………………………………………………..
2. The calculation of initial amount of acetaminophen in the samples that were further incubated with activated charcoal. Sample
Acetaminophen concentration [mg/mL]
Acetaminophen amount [mg]
1 2 3 4 5 9
Incubation time: …………………………………………………….. Temperature of incubation: ……………………………………………. Shaking speed: ………………………………………………
3. The calculation of acetaminophen concentration in the samples at the equilibrium state (after incubation). Sample
Absorbance (λmax=243 nm)
dilution
Acetaminophen concentration at the equilibrium state
1
1000
2
1000
3
1000
4
100
5
---
- c [mg/mL]
4. The calculation of the mass of acetaminophen adsorbed on activated charcoal. Sample
Mass of activated charcoal
Initial amount of acetaminophen in the sample
amount of acetaminophen in the sample at the equilibrium state
amount of acetaminophen adsorbed on 100 mg of activated charcoal
a
[mg]
[mg]
[mg]
[mg]
[mg/1g of charcoal]
1
100
2
100
3
100
4
100
5
100
5. The Freundlich isotherm equation (at the temperature 37˚C). Sample 1
c [mg/mL]
logc
a [mg/1g of charcoal]
loga
2 3 4 5
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Linear equation of Freundlich isotherm: …………………………………………………………………… Correlation coefficient r: …………………………………………………………….. Constant k: …………………………………………………………. Constant n: ………………………………………………………….. 6. The Freundlich isotherm equation (at the temperature ……..˚C). Sample 1
c [mg/mL]
logc
a [mg/1g of charcoal]
loga
2 3 4 5
Linear equation of Freundlich isotherm: …………………………………………………………………… Correlation coefficient r: …………………………………………………………….. Constant k: …………………………………………………………. Constant n: ………………………………………………………….. 7. Enclosures: Graph of the calibration curve A = f(c) Graph of the Freundlich isotherm a = f(c) Graph of the linear form of the Freundlich isotherm loga = f(logc)
8. Summary and conclusions:
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