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?

Rationale

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.

Background information

Figure 1 - Structural Formula Of Allura Red Dye

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

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.

Adsorption of allura red dye (ARD) by activated charcoal (AC)

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

\(\text{in } \frac{[ARD]_0}{[ARD]}=k't.................equation (1)\)

Beer-lambert law

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:

\(ln\frac{A_0}{A}k't\)

A_o = initial absorbance of ARD

A = absorbance of ARD at time t

Figure 2 - Determination Of Pseudo First Order Rate Constant Using Graphical Representation

Thus, if ln \(\frac{A_0}{A}\) against time(t), the gradient of the curve obtained is the pseudo first order rate constant.

Literature review

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.

Variables

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

Figure 3 - Table On Variables

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

Figure 4 - Table On List Of Controlled Variables

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	---	---

Figure 5 - Table On List Of Apparatus Required

Figure 6 - Table On List Of Materials Used

Considerations

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.

Figure 7 - Table On Safety Considerations

Ethical considerations

The experiment was executed using the least possible amount of chemicals.

Environmental considerations

All waste materials were disposed safely.

Any toxic gases were not evolved.

Experimental procedure

A 100 cm³ glass beaker was taken.
A watch glass was placed on a top pan digital mass balance and the reading was tared to 0.00 ± 0.01 g.
Solid NaCl was transferred to the watch glass from the reagent bottle to a watch glass using a spatula until the digital mass balance reads 0.58 ± 0.01 g (0.01 moles).
The weighed mass of NaCl was exactly transferred to the glass beaker.
4.96 ± 0.01 g (0.01 moles) of Allura Red Dye (ARD) was weighed using the digital mass balance and added to the same beaker.
Distilled water was added to the same beaker using a graduated measuring cylinder till the mark of 100 cm³.
1.20 ± 0.01 g of activated charcoal was weighed using the digital mass balance and added to the same beaker.
The weighed mass of activated charcoal was transferred to the beaker slowly along a glass rod so that it settles down at the bottom.
As soon as the activated charcoal settles down at the bottom, the stop-watch was started.
A graduated pipette was used to extract 1.00 ± 0.05 cm³ of the supernatant liquid and transferred to a cuvette. The absorbance of the solution was recorded using a digital colorimeter setting the wavelength at 504 nm. The colorimeter was calibrated using distilled water at the same wavelength and the reading was adjusted to 0.000 ± 0.001 abs.
As soon as the stop-watch reads 5.00 ± 0.01 mins, step-10 was repeated.
Step 11 was repeated at 10.00 ± 0.01 mins, 15.00 ± 0.01 mins, 20.00 ± 0.01 mins and 25.00 ± 0.01 mins.
Steps 10-12 were repeated for three more times.
Steps 1-13 were repeated using 1.16 ± 0.01 g, 1.74 ± 0.01 g, 2.32 ± 0.01 g and 2.90 ± 0.01 g of NaCl.

Qualitative data

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.

<p>Figure 8 - Table On Data Of Absorbance For 0.10 Moldm<sup>-3</sup> NaCl</p> — Figure 8 - Table On Data Of Absorbance For 0.10 Moldm^-3 NaCl

<p>Figure 9 - Table On Data Of Absorbance For 0.20 Moldm<sup>-3</sup> NaCl</p> — Figure 9 - Table On Data Of Absorbance For 0.20 Moldm^-3 NaCl

<p>Figure 10 - Table On Data Of Absorbance For 0.30 Moldm<sup>-3</sup> NaCl</p> — Figure 10 - Table On Data Of Absorbance For 0.30 Moldm^-3 NaCl

<p>Figure 11 - Table On Data Of Absorbance For 0.40 Moldm<sup>-3</sup> NaCl</p> — Figure 11 - Table On Data Of Absorbance For 0.40 Moldm^-3 NaCl

<p>Figure 12 - Table On Data Of Absorbance For 0.50 Moldm<sup>-3 </sup>NaCl</p> — Figure 12 - Table On Data Of Absorbance For 0.50 Moldm^-3NaCl

<p>Figure 13 - ln <span class="math-tex">\(\frac{A_0}{A}\)</span> versus time (t) for 0.10 moldm<sup>-3</sup>, 0.20 moldm<sup>-3</sup>, 0.30 moldm<sup>-3</sup>, 0.40  moldm<sup>-3</sup> and 0.50 moldm<sup>-3</sup> of NaCl</p> — Figure 13 - ln \(\frac{A_0}{A}\) versus time (t) for 0.10 moldm^-3, 0.20 moldm^-3, 0.30 moldm^-3, 0.40 moldm^-3 and 0.50 moldm^-3 of NaCl

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

Figure 14 - Table On Determination Of Pseudo First Order Rate Constant Against Various Molar Concentration Of NaCl

Formula used: Pseudo first order rate constant (k’) in min^-1 = Gradient = Co-efficient of x in the equation of trend line

Figure 15 - Variation Of Pseudo First Order Rate Constant Of Adsorption Against Molar Concentration Of NaCl

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.

Evaluation of hypotheses

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.

Scientific justification

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.

Conclusion

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.
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 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.
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.
The qualitative observation that the color of the solution fades with time supports the conclusion.

Evaluation

Strengths

The independent variable chosen has a wide range and is continuous in nature. The difference between the consecutive values are same. This allows a fair and accurate analysis.
The data processing is coherent. For example, Figure - 8 to 12, the raw data table indicates that as the molar concentration increases, the decrease of absorbance within the specific time interval is lesser. Figure - 15 indicates that as molar concentration increases, the pseudo first order rate constant decreases which means that the number of dye molecules adsorbed within 25 minutes gets lesser and thus the fading of color is slow.
The factors-temperature, pH which may affect the reliability of the data has been controlled.

Sources of error(s) and improvements

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.

Figure 16 - Table On Sources Of Error(s) And Improvements

Further scope of analysis

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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