How does the pseudo first order rate constant of adsorption (measured in min-1) of the dye –Allura Red (Carmoisine-a food color) from a solution of it in aqueous NaCl by activated charcoal (as adsorbent) depends on the molar concentration of the NaCl used, determined using colorimetry?
Inquiry leads to production of knowledge and the process gains optimum output when that is connected to the real-life observation of a knower – This concept that I studied in TOK class has always evoked me to ask questions about any fact and observations that I come across and use my best scientific judgements to justify them. Chemistry was not an exception either. Though the global pandemic has brought an end to the aspirations of life in mankind but it has healed the nature as well. I realized this while coming across the fact that the turbidity of River Yamuna in the city of Delhi, India has decreased during the times of pandemic which made the issue of accumulation of domestic waste polluting the water bodies more prominent. One of the most effective way to clean these biological waste or domestic waste from water bodies is adsorption. In within the water bodies. Off-late recent research by an environmentalist in Japan led to the introduction of a material prepared from the waste non-biodegradable plastics which acted as an adsorbent of the toxic and carcinogenic organic products involved. However, the same method was not found to be effective enough when used for Arabian Sea in the Mumbai belt of India. Investigations and analysis showed that there were several factors related to the differences in composition of river water and sea water which can be accounted for this. This brought me to the question, by any chance is salinity the factor that makes the adsorption process as a cleansing method less effective for adsorption. Does the presence of a foreign ionic impurity like NaCl will have an effect on the adsorption kinetics of a dye? The chemical waste products present in sea water are mostly heterocyclic organic compounds or azo compounds. To mimic that in a laboratory set up, the food color-Allura Red dye, an azo dye was chosen. Activated charcoal is one of the most widely used adsorbent due to its highly porous surface. Thus, I finally arrived at the research question stated above.
Allura Red dye is a food color popularly known as Carmoisine Red (E 122). It has the molecular formula – C18H14N2Na2O8S2. It is a synthetic azo dye where the term azo indicates the presence of the N = N. It is made by the coupling reaction of 6 - hydroxy naphthalene - 2 - sulphonic acid and 4 - Amino - 5 -methoxy - 2 - methylbenzene sulfonic acid. At room temperature, it appears as a red amorphous powder with a melting point of 300.000C. This dye is used as a food colour in various soft drinks, chocolates and even preparations of processed meat products. As indicated from the molecular structure given below, the dye consists of two anionic sulphonic acid groups – SO3 - which makes an ion pair with Na+ ions and thus the dye exists as a disodium salt. Despite having three hydrophobic phenyl rings, the dye is highly soluble in water due to the presence of a phenolic OH group and the O- atoms of the SO3- groups which can make intermolecular H bond with the water molecules.
Adsorption is a surface phenomenon where the molecules of a substance (adsorbate) adhere to the surface of another substance (adsorbent). In this investigation, the food coloring dye- Allura Red Dye (ARD) is the adsorbate and activated charcoal is the adsorbent. Activated charcoal acts as a potential adsorbent because it is highly porous in nature. Adsorption finds applications in various fields like delivery of drugs, chelation therapy where removal of heavy metals by complex ligands, blood coagulation. There are two types of adsorption – physisorption and chemisorption9. In physisorption, the adsorbate molecules are bonded with the adsorbent molecules via physical forces of attractions like – Van derWaal forces of attraction, dipole-dipole forces, Hydrogen bonds and so on. In chemisorption, the adsorbate molecules are bonded to the surface of the adsorbent via covalent bonds. Physisorption is a monolayered phenomenon while chemisorption is a multilayered phenomenon. Adsorption of Allura Red Dye (ARD) by activated charcoal is an example of chemisorption.
ARD (adsorbate) + AC (adsorbent) ---------→ ARD - AC (adsorbed complex)
ARD = Allura Red Dye
AC = Activated charcoal
The rate of this adsorption reaction is found to be of overall second order. The orders with respect to ARD and AC are 1.
rate = k [ARD][AC]
If the concentration of AC is taken in huge excess of ARD, the reaction can be considered to be of pseudo first order with respect to ARD.
if [A] ≫ [ARD]
[AC] ≅ constant
rate = k' [ARD] where k'' = pseudo first order rate constant = k × [AC]
\(rate=k'[AC]\frac{-d[ARD]}{dt}=k'[ARD]\)
\(\frac{-d[ARD]}{[ARD]}k'dt\)
\(\displaystyle\int\frac{-d[ARD]}{[ARD]}\displaystyle\int k'dt\)
- ln [ARD] = k't + c.................equation (1)
At time (t) = 0 [ARD] = initial concentration of ARD = [ARD]0
- ln [ARD]0 = c
- ln [ARD] = k't - ln [ARD]0
- ln [ARD]0 - ln [ARD]0 = k't
\(\text{in } \frac{[ARD]_0}{[ARD]}=k't.................equation (1)\)
According to Beer-Lambert law
Absorbance (A) = ∈ × c × l
If the same sample is taken, the molar absorptivity constant ∈ will remain constant and the same colorimeter is used, the path-length (l) will also remain same.
Therefore, Absorbance (A) ∝ molar concentration (c)
Thus, equation-2 can be re-written as:
\(ln\frac{A_0}{A}k't\)
Ao = initial absorbance of ARD
A = absorbance of ARD at time t
Thus, if ln \(\frac{A_0}{A}\) against time(t), the gradient of the curve obtained is the pseudo first order rate constant.
In a research paper titled – “Effectiveness of Alkali-Acid Treatment in Enhancement the Adsorption Capacity for Rice Straw: The Removal of Methylene Blue Dye” by Nady Fathy it was reported that the removal of the dye methylene blue by rice straw used as adsorbent was found to decrease from 90 % to 82% as the molar concentration of NaCl increases from 0.05 moldm-3 to 0.20 moldm-3. This shows that the increase in the concentration of NaCl has caused the adsorption of methylene blue by rice straw to occur at a slower rate.
Type | Variable | How is it measured or varied? | Apparatus used |
---|---|---|---|
Independent | The molar concentration of the aqueous solution of NaCl | Aqueous solutions of NaCl of molar concentrations – 0.10 moldm-3, 0.20 mol dm-3, 0.30 moldm-3, 0.40 moldm-3 and 0.50 moldm-3 were used. These solutions were prepared by adding requisite mass of NaCl within 100 cm3 of distilled water. The variables has been chosen in this range as the average salinity of ocean is reported to be 0.30 moldm-3 in terms of NaCl. | Digital mass balance Graduated measuring cylinder |
Dependent | Pseudo first order rate constant | The absorbance of the aqueous solution of ARD in NaCl will be measured against time. A scatter plot of ln \(\frac{A_0}{A}\) against time will be plotted and the pseudo first order rate constant will be calculated from the gradient of the graph. To optimize the values of absorbance recorded, the wavelength of the colorimeter will be fixed at the 504 nm which is the wavelength at which Allura Red dye shows maximum absorbance. | Colorimeter |
Variable | Why is it controlled? | How is it controlled? | Apparatus used |
---|---|---|---|
Mass of ARD added | ARD is the azo dye which is acting as the adsorbate used. More the mass of adsorbate used, more the molecules of adsorbate adhering to the surface of the adsorbent. | 4.96 ± 0.01 g (0.01 moles) of ARD was used in all trials. | Digital mass balance |
Mass of activated charcoal used | Activated charcoal (AC) is the adsorbent added. More the mass of adsorbent used, more the number of surface sites available for the ARD to bind with the adsorbent. Thus, faster the rate of adsorption. | 1.20 ± 0.01 g (0.10 moles) of activated charcoal was added in all trials. | Digital mass balance |
Temperature | Adsorption of ARD on AC is an exothermic process. Higher the temperature, the equilibrium of adsorption shifts towards the reactant and thus lower the rate of adsorption. However, from a kinetics aspect, more the temperature, more the fraction of reactant molecules with energy greater than activation energy and thus faster the rate of the reaction. | All trials were conducted at room temperature. | None |
Surface area | Larger the surface area, more the sites available for the ARD to bind on the surface of AC. Thus, faster the rate. | A 100 cm3 glass beaker was used in all trials. | 100 cm3 glass beaker |
Time of adsorption | Longer the time, the adsorbate and adsorbent are in contact with each other more the number of dye molecules adsorbed. | In all trials, the adsorption was carried out for 25 minutes. | Stop - watch |
Variable | Why is it controlled? | How is it controlled? | Apparatus used |
---|---|---|---|
Apparatus | 1 | 1.00 cm3 | ±0.50 cm3 |
Graduated measuring cylinder | 1 | 0.10 cm3 | ± 0.05 cm3 |
Graduated pipette | 1 | 0.10 g | ± 0.10 g |
Digital mass balance | 1 | 0.001 abs | ± 0.001 abs |
Digital colorimeter | 1 | 0.01 s | ± 0.01 s |
Digital stop-watch | 1 | --- | --- |
Glass rod | 1 | --- | --- |
Watch glass | 1 | --- | --- |
Spatula | 1 | --- | --- |
Soft tissues | 1 | --- | --- |
Cuvette | 1 | --- | --- |
100 cm3 glass beaker | 1 | --- | --- |
Concerns | Precautions |
---|---|
Activated charcoal if exposed to skin may cause irritations and redness. | A laboratory coat was worn at all times. Safety masks, safety gloves were used. All solutions were prepared carefully under the guidance of an adult. Any eatables were not allowed in the laboratory. |
The experiment was executed using the least possible amount of chemicals.
All waste materials were disposed safely.
Any toxic gases were not evolved.
The red color of the solution started to fade out with the progress of time. As the concentration of NaCl has increased, the disappearance of color with time was slower.
Molar concentration of NaCl (aq) in moldm-3 | Equation of trend line | Pseudo first order rate constant (k’) in min-1 |
---|---|---|
0.10 | y = 0.0269x | 26.90 |
0.20 | y = 0.0204x | 20.40 |
0.30 | y = 0.0202x | 20.20 |
0.40 | y = 0.0093x | 09.30 |
0.50 | y = 0.0044x | 04.40 |
Formula used: Pseudo first order rate constant (k’) in min-1 = Gradient = Co-efficient of x in the equation of trend line
As the molar concentration of NaCl increases from 0.10 moldm-3 to 0.50 moldm-3, the pseudo first order rate constant decreases from, the pseudo first order rate constant decreases from 26.90 × 10-3 min-1 to 4.40 × 10-3 min-1. This shows that as the NaCl solution used as medium is more concentrated, the rate at which the dye molecules are adsorbed by the surface of the activated charcoal decreases.
The decrease in the pseudo first order rate constant is not gradual in nature as the distance between the consecutive points are not the same. The major difference is found in between the values of pseudo first order rate constant at 0.30 moldm-3 and 0.40 moldm-3. Moreover, the trend line plotted using MS-Excel clearly indicates that the point at 0.30 mol dm-3 is the most deviated point and this indicates that there is a systematic error in this investigation.
The equation of trend line has been obtained using MS-Excel and it indicates that the pseudo first order rate constant and the molar concentration of NaCl is related according to the equation: y = -56.10 x + 33.07 where y represents the pseudo first order rate constant and x indicates the molar concentration of NaCl.
The gradient as indicated in the equation of trend line is -56.10 and the negative value confirms that there is a negative correlation between the pseudo first order rate constant and the molar concentration of NaCl.
The value of regression coefficient has also been obtained in MS-Excel and the value is 0.9395 which confirms that there is a 93.95% correlation between the pseudo first order rate constant and the molar concentration.
As the molar concentration of NaCl increases, there are more number of Na+ and Cl- ions in the medium. Thus, most of the surface sites are occupied by the ions and there are none available for the dye molecules to bind with. As a result, less number of dye molecules are adsorbed on the surface of the activated charcoal, the decrease of absorbance slows down. This is why the decrease of absorbance within the same time frame is less. For example, at 0.10 moldm-3 NaCl, the absorbance decreases from 0.964 ± 0.001 abs to 0.565 ± 0.001 abs while at 0.50 moldm-3 NaCl, the decrease is from 0.964 ± 0.001 abs to 0.867 ± 0.001 abs. (Refer to Table - 1). This proves the claim that as the molar concentration of NaCl increases, more area in the surface are blocked by the foreign ions and thus there are less free surface sites available to make chemical bonds with the dyes which eventually decreases the rate of adsorption.
How does the pseudo first order rate constant of adsorption (measured in min-1) of the dye –Allura Red (Carmoisine-a food color) from a solution of it in aqueous NaCl by activated charcoal (as adsorbent) depends on the molar concentration of the NaCl used, determined using colorimetry?
Type | Source | Effect | Improvements |
---|---|---|---|
Random | Digital mass balance has been used to weigh the dye as well as NaCl. It has uncertainty. The stop-watch has been used which also has an uncertainty. | Data collected is not precise enough. | Collect data in repeated trials and use average values. |
Systematic error | The absorbance has been measured using the colorimeter. The colorimeter has an instrumental uncertainty associated with it. | Data collected for absorbance is not accurate. | The colorimeter must be calibrated before use. To do this, the absorbance at the wavelength at which the absorbance readings has been taken was set at 0.000 ± 0.001 abs. |
Methodological error | The stop-watch was used to record the time intervals. There must be a gap between the time stop-watch reads 5.00 minutes and the absorbance was recorded. | Data collected is inaccurate. | The solution can be kept inside the colorimeter and a Vernier Logger Pro can be used to record the change of absorbance against time. |
Apart from presence of foreign impurities, there are other factors like pH which also has an impact on the rate of adsorption. I would like to perform an investigation to study the effects of pH on rate of adsorption. To do this, I would like to perform the adsorption of a dye in presence of solutions with different buffer tablets. The absorbance of the solution can be recorded as a function of time using a colorimeter. The scatter plot of absorbance versus time can give us the rate of adsorption from the value of gradient. Thus, the rate of adsorption at different pH values can be measured. This has a real life application as bio-sorbents used to remove toxic metals from wastewater depends on the pH at which it is carried out.
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