Effect of percentage concentration of NaCl on adsorption of the food color-Carmoisine Red Dye by dried bread crumps

Chemistry is the central science. The way the subject connects other branches of science and finds it’s applications in various aspects of life is really fascinating. Identifying a suitable topic and selecting an appropriate research question for Chemistry IA was a little challenging given the restrictions and challenges we had during this global pandemic. However, my inquisitiveness as a learner and reflective aptitude helped in me being successful to arrive at a suitable topic. The story begins from a recent article that I came across in my Facebook wall which shows that various bio-chemical waste like fruit peels, bio degradable domestic waste which when disposed in water bodies can acts as a purifier by adsorbing the heavy metal ions and other toxic organic molecules from it. Firstly, the term adsorption was new to me but what was enchanting was the fact that this is an easy and eco-friendly method of waste disposal and water purification. With my desire to have more in-depth knowledge about this, I researched on the topic of adsorption and also the various physical and chemical factors that may have an effect on the rate of adsorption. Almost all the articles or text books that I came across spoke about how the rate of adsorption depends on physical factors like temperature, pressure, surface area and many more. The inquirer inside me was bothered about the biological and chemical factors that may be relevant especially in connection to the example of use of biodegradable waste products in purifying water by adsorbing heavy metal ions and other toxic organic molecules. For example, all the sea and oceans are saline in nature and the level of salinity; concentration of NaCl would not be the same for all water bodies would not be the same. Would this difference in salinity level impact the efficiency of adsorption of heavy metals by biodegradable waste products in nay way? To answer this question, I decided to perform my investigation of Chemistry Internal Assessment based on this. Considering the restrictions of performing the experiment in home and the safety or ethical restrictions associated, I chose to study the adsorption of a dye which mimic a organic molecule at various salinity levels by using NaCl solutions of different concentrations using bread crumbs that represent the bio degradable waste product. Thus, I arrived at the research question narrated below:

How does the percentage extent of adsorption (mass of dye adsorbed by 100 g of the adsorbate) of the food color-Carmoisine Red Dye (Azorubine - C₂₀H₁₂N₂Na₂O₇S₂) by a definite mass of dried bread crumps in presence of aqueous NaCl depends on the molar concentration of NaCl solution used at constant temperature and surface area, determined using colorimetry?

Adsorption is a surface phenomenon where the molecules of a particular substance is more concentrated and sticks to the surface of another substance instead of entering the bulk of the matter. For example, if a plastic and a cotton is immersed in water, both of them adheres the water molecules but there is a difference. For the cotton substance, the water will enter the bulk of the matter and thus the entire cotton substance will absorb water. For the plastic substance the water molecule will stick to the surface of the object and not enter the bulk of it. This is an example of adsorption. Here, the water is the adsorbate and the plastic substance is the adsorbent. This investigation deals with the adsorption of the organic dye – Carmoisine Red Dye by bread crumbs. The organic dye solution is the adsorbate and the bread crumbs are the adsorbent. This process exists in a dynamic equilibrium as shown below:

Dye solution (adsorbate) + Bread crumbs (adsorbent ) ←---→ Dye-Bread crumbs

(adsorbate-adsorbent complex)

Adsorption are of two types:

Physisorption: Here there is only physical forces of attraction like Vander Wall forces between the molecules of adsorbate and adsorbent. This is a multilayer phenomenon and is reversible in nature.

Chemisorption: Here there are real chemical bonds between the molecules of adsorbate and adsorbent. This is a mono-layered phenomenon and is irreversible in nature.

The adsorption considered in this case is an example of chemisorption. Here, chemical bonds (covalent bonds) exists between the dye molecule and the C in the dried bread crumbs.

• Temperature: Adsorption is an exothermic process. Thus with the decrease of temperature, the equilibrium involved in adsorption shifts more towards the product and more molecules of adsorbate gets stick to the surface of the adsorbent. This in turn increases the extent as well as speed of adsorption.
• Pressure: Pressure plays a role only during the adsorption of gases on solid surfaces. Increase of pressure causes more gas molecules to be adsorbed on the surface of the solid used as adsorbent.
• Surface area: Higher the surface area of the adsorbent, more the molecules that can adhere to the surface and thus higher the extent of adsorption.

The adsorption extent is a quantitative tool to measure how much of the adsorbate has been adsorbed by the surface of the adsorbent. It is measured according to the equation stated below:

Percentage Adsorption extent = \(\frac{mass \ of\ dye\ adsorbed}{mass \ of\ adsorbent\ (bread\ crumb)\ used}\)×100

It may be defined as the mass of the adsorbate adhered to the surface of the 100 g of the adsorbent.

It is an azo dye formed by diazotisation of 4-aminonapthalene sulphonic acid and 4-hydroxy naphthalene sulphonic acid. The IUPAC name of the dye is Disodium 4-hydroxy-3-(4-sulfonato-1- naphthylazo) naphthalene-1-sulfonate. It is a dark red crystal at room temperature and soluble in water. It is used as a food additive as a colouring agent. It is also popular by the name of Azorubine where the prefix ‘Azo’ refers to the N = N present in the dye.

In a research based on the effect of the salt KCl on the ‘adsorption of organic molecules like phenol, toluene and benzene on activated C it was found that as the concentration of KCl increases, the adsorption coefficient of the activated charcoal was found to decrease.

This was mainly described by the electro neutralization of the surface charges on the surface of the activated charcoal that was used as a adsorbent in this case.

There is no correlation between the percentage adsorption extent of adsorption of Carmoisine Red Dye by bread crumbs and the molar concentration of NaCl solution in which the adsorption has occurred.

There is a negative correlation between the percentage adsorption extent of adsorption of Carmoisine Red Dye by bread crumbs and the molar concentration of NaCl solution in which the adsorption has occurred.

As the molar concentration of NaCl increases, the number of moles of NaCl in the solution increases which eventually occupies or binds to many molecules of the surface of the bread crumbs. This blocking of sites allows less space on the surface for the molecules of the Carmoisine Red dye to make a bond with the molecules on the surface of the bread crumbs. Thus, the mass of the dye adsorbed decreases and the percentage adsorption extent as well. The relationship between the variables as expected is illustrated in the schematic Figure-2.

Safety precautions:

• A safety masks was used so that any toxic vapors is not inhaled.
• A protective clothing was used so that any of the chemicals is not exposed to skin.
• A safety gloves was also used so that spillage of solutions does not harm the skin.
• Though performed at home, the investigation was performed at a suitable place where there are no disturbances and under the supervision of a responsible adult.

The methodology used does not result in the emission of any toxic gases or by-products that may harm the environment in any possible way.

All the unused chemicals were diluted and then disposed of. The waste chemicals were segregated into solid waste and liquid waste. The liquid waste chemicals were stored in a sealed plastic container, large amount of water was added to it to reduce the toxicity and then disposed into the sink. The solid waste products were taken to an open space and burnt.

• Take a 100 cc glass beaker.
• Place a watch glass on the digital mass balance and tare the reading of the balance to 0.00 ± 0.01 g.
• Transfer the food colour-Carmoisine Red Dye using a digital mass balance on a watch glass until the balance reads 0.50 ± 0.01 g (0.001 moles) using a spatula.
• Transfer the weighed solid from the watch glass to the beaker.
• Add 100 cc distilled water into the same beaker using a graduated measuring cylinder.
• Take a glass rod and stir the solution to obtain a clear red color solution.
• Transfer 1.00 ± 0.05 cm³ of the same solution from the beaker to a cuvette using a 1.00 cc glass pipette.
• Set up the wavelength of the colorimeter at 516 nm⁵
• Record the absorbance.
• 10. Repeat steps 6 - 8 for four more times.
• Repeat steps 1 - 9 with other values of mass of the dye – 1.00 ± 0.01g (0.002 moles), 1.50 ± 0.01g (0.003 moles), 2.00 ± 0.01g (0.004 moles), 2.50 ± 0.01g (0.005 moles), 3.00 ± 0.01g (0.006 moles), 3.50 ± 0.01g (0.007 moles), 4.00 ± 0.01g (0.008 moles), 4.50 ± 0.01g (0.009 moles) and 5.00 ± 0.01g (0.010 moles).

Mass of NaCl to be used = moles × molar mass = concentration × volume × molar mass

= 0.20 × \(\frac{100}{1000}\) × 58.05 = 1.16 g

• Take a watch glass and place it on the top pan digital mass balance.
• Adjust the reading of the balance to 0.00 ± 0.01 g.
• Transfer solid NaCl from the reagent bottle to the watch glass using a spatula until the balance reads 1.16 ± 0.01 g.
• Take a 100 cm³ volumetric flask and place a funnel on it.
• Transfer the weighed solid through the funnel.
• To transfer the solid completely, wash the watch glass with distilled water (from a long neck distilled water bottle) and collect the washing in the volumetric flask. This will ensure that any weighed solid remaining on the wall of the funnel or on the watch glass have been transferred to the volumetric flask.
• Add distilled water till the mark of 100 cm³
• Close the lid of the flask and shake it gently to dissolve the solid completely and obtain a clear NaCl solution.

Repeat the same process to prepare 0.40 moldm^-3 , 0.60 moldm^-3 , 0.80 moldm^-3 and 1.00 moldm^-3 NaCl solutions. Use 2.32 ± 0.01 g, 3.48 ± 0.01 g, 4.64 ± 0.01 g and 5.80 ± 0.01 g of NaCl respectively.

• Take a 100 cm³ glass beaker.
• Transfer 100 cm³ of 0.20 moldm^-3 NaCl solution into the beaker using a graduated measuring cylinder.
• Use a digital mass balance, a watch glass and a spatula to weigh 2.00 ± 0.01 g of the dye and transfer it to the same beaker.
• Stir the solution with a glass rod to dissolve the dye completely.
• Weigh 5.00 ± 0.01 g of powdered bread crumps using a watch glass, a digital mass balance and a spatula.
• Transfer the weighed bread crumps into the same beaker slowly so that it can settle down at the bottom of the beaker.
• Start the stop-watch.
• As soon as the stop-watch reads 60.00 ± 0.01 mins, use a graduated pipette to transfer 1.00 cm³ of the supernatant liquid to a cuvette.
• The colorimeter must be previously calibrated using a NaCl solution at 516 nm.
• Insert the cuvette into the colorimeter and record the absorbance at 516 nm.
• Repeat steps 9 - 10 for 4 more times to collect data in five trials.
• Repeat steps 1 - 11 for other solutions of NaCl – 0.40 moldm^-3 , 0.60 moldm^-3 , 0.80 moldm^-3 and 1.00 moldm^-3 . To collect the data for control, follow the same procedure using distilled water.

• The dye solutions were clear and red in colour.
• The intensity of the colour of the dye solutions increased as the mass of dye added increased.
• After the addition of bread crumbs into the dye solutions, the red colour was found to fade slowly.

For Row-1:

Mass of dye taken = 0.50 ± 0.01 g

Molar mass of dye taken = 502.44

Volume of solution = 100 cc

Molar concentration of dye solution =\(\frac{moles \ of\ dye}{Volume \ of \ solution}\) = \(\frac{\frac{mass\ of\ dye}{molar\ mass\ of\ dye}}{\frac{Volume\ of\ solution\ in\ cc}{1000}}\) = \(\frac{\frac{0.50}{502.44}}{\frac{100}{1000}}\)

≅ 1.00 × 10^-2 mol dm^-3

Mean absorbance = \(\frac{0.064+0.061+0.062+0.060+0.064}{5}\) = 0.062 ± 0.001 abs

Standard deviation (SD) = \(\frac{(0.064-0.062)^2+(0.061-0.062)^2+(0.062-0.062)^2+(0.060-0.062)^2+(0.064-0.062)^2}{2}\) = 0.002

As per the trend line obtained in the graph, the absorbance (± 0.001 abs) and the molar concentration (10-2 mol dm^-3 ) are related according to the equation:

y = 0.0644 x

x = \(\frac{y}{0.0644}\)

Molar concentration (x) = \(\frac{absorbance\ (±0.0001\ abs)}{0.0644}\) × 10^-2 mol dm^-3 (equation - 1)

According to Beer-Lambert law, A = ∈× c × l , the absorbance and the concentration are directly related. Thus, a straight line passing through the origin of the form y = mx is expected when absorbance is plotted against molar concentration, where y is the absorbance, x is the molar concentration and slope (m) is the product of molar absorptivity constant (∈) and the path length (l). Thus, the graph obtained here is in agreement with the Beer-Lambert law.

For 0.00 mol dm^-3 of NaCl (distilled water),

Average absorbance of the solution after 1 hour of adsorption of the red dye by dried bread crumbs

= 0.195 ± 0.001 abs

Using equation-1,

Molar concentration (x) = \(\frac{absorbance\ (±0.001\ abs)}{0.0644}\) × 10^-2 mol dm^-3

molar concentraton = \(\frac{0.195}{0.0644}\) = 3.02 × 10^-2 mol dm^-3

Moles of dye un-adsorbed

= molar concentration of the dye after 1 hour of adsorption × Volume

= 3.02 × 10^-2 × \(\frac{100}{1000}\) = 3.02 × 10^-3

Mass of dye un-adsorbed = moles of dye un - adsorbed × molar mass = 3.02 × 10^-3 × 502.44 = 1.52 g

Mass of dye adsorbed

= initial mass of dye taken - mass of dye unadsorbed = 2.00 - 1.52 = 0.48 g

Percentage adsorption extent = \(\frac{mass \ of \ dye\ adsorbed}{mass \ of \ dried\ bread\ crumbs\ taken}\) × 100 = \(\frac{0.48}{5.00}\) = 9.57

As the molar concentration is calculated from the value of absorbance, Absolute error in molar concentration (∆c) = absolute error in absorbance (∆A) = ±0.001 As the number of moles of dye un-adsorbed is calculated using the molar concentration of the dye solution after it was adsorbed for 1 hour and the total volume of the solution used, Fractional error in moles of dye un-adsorbed = fractional error of molar concentration of the dye after adsorption for 1 hour + fractional error in total volume of the solution

=\(\frac{±\ 0.001}{3.02\ ×\ 10^-2}\) + \(\frac{±\ 0.50}{100}\) ±0.04

As the mass of dye un-adsorbed was calculated from the moles of dye un-adsorbed, the fractional error in mass of dye un-adsorbed and that in the moles of dye un-adsorbed.

Fractional error in mass of dye un-adsorbed = ± 0.004

Absolute error in mass of dye un-adsorbed

fractional error of dye un - adsorbed × mass of dye unadsorbed = ± 0.004 × 1.52 = ± 0.006

Mass of dye adsorbed = Initial mass of dye added – Mass of dye un-adsorbed

= 2.00 − 1.52 ± (0.01 + 0.006) = 0.48 ± 0.02 g

Fractional error in adsorption extent

= \(\frac{absolute \ error\ in\ mass\ of\ dye\ adsorbed}{mass\ of\ dye\ adsorbed}\) + \(\frac{absolute\ error \ in\ mass\ of\ breadcrumbs\ added}{mass\ of\ breadcrumbs\ added}\) = \(\frac{±0.02}{0.48}\) + \(\frac{±0.01}{5.00}\) 

Percentage error in adsorption extent = \(\bigg(\frac{±0.02}{0.48}+\frac{±0.01}{5.00}\bigg)\) × 100 = ± 4.37

As indicated in Figure - 11, the values of percentage adsorption extent is decreasing from 9.57 to 4.74 as the molar concentration of NaCl increases from 0.00 moldm^-3 to 1.00 moldm^-3. The fact that, there is a sharp fall in the value of percentage adsorption extent from 9.57 to 6.92 as we shift from control (0.00 moldm^-3 , distilled water) to 0.02 moldm^-3 NaCl solution allows us to claim that the presence of NaCl decreases the adsorption of the dye from the solution by the adsorbent bread crumbs. The shape of the curve obtained clearly affirms that the decrease in percentage adsorption extent is not regular or linear. At the concentration of 0.60 moldm^{- 3} of NaCl solution, the concentration of NaCl becomes a limiting factor and has no major impact on the percentage adsorption extent as the curve gets almost parallel to the x axes. The values for 0.60 moldm^-3 NaCl to 1.00 moldm^-3 NaCl is almost along a straight line showing that after the threshold value of 0.60 moldm^-3 NaCl, the decrease in percentage adsorption is independent of the molar concentration of NaCl and reaches a constant value. Mathematically, it will be an extremely high concentration of NaCl solution when the adsorption of the dye by the bread crumbs will absolutely cease down.

As the molar concentration of NaCl increases, there are more and more Na⁺ and Cl^- ions in the medium and these ions occupies the surface sites of the bread crumbs. Thus, less surface spaces are available for the dye molecules to form a chemical bond with the surface molecules of the bread crumbs and that eventually reduces the percentage of adsorption extent. Adsorption of dye molecules in a solid adsorbent surface of the bread crumbs in a liquid medium is an example of chemisorption. Thus, lesser the availability of the surface sites, less the molecules adsorbed and lower the percentage of adsorption extent.

How does the percentage extent of adsorption (mass of dye adsorbed by 100 g of the adsorbate) of the food
color-Carmoisine Red Dye (Azorubine- C₂₀H₁₂N₂Na₂O₇S₂) by a definite mass of dried bread crumps in
presence of aqueous NaCl depends on the molar concentration of NaCl solution used at constant temperature and surface area, determined using colorimetry?

• The values of percentage adsorption extent decreases from 9.57 to 4.74 as the molar concentration of NaCl increases from 0.00 moldm^-3 to 1.00 moldm^-3 . After the threshold value of 0.60 moldm^-3 NaCl, the decrease in percentage adsorption is independent of the molar concentration of NaCl and reaches a constant value. This is because, with the increase in molar concentration of NaCl, more of the surface sites were blocked by the Na⁺ and Cl^- ions and thus the surface of activated charcoal available to adsorb the dye molecules will be less.
• The qualitative observations are also in support of this. It was observed that the red color of the solution faded with time after the addition of bread crumbs.
• As shown in Figure - 11, there is a correlation between the percentage of adsorption extent and the molar concentration of the NaCl solution used. This allows us to reject the null hypotheses and accept the alternate hypotheses.
• The investigation involves the use of a standard calibration curve obtained by using dye solutions of known strength. The curve thus obtained was used to generate an equation that relates the absorbance of the solution with the molar concentration of it. This equation was used to determine the molar concentration of the unknown dye solutions (dye solutions containing bread crumbs) and thus calculate the percentage of extent of adsorption.

• The independent variable chosen covers a wide range and thus offers more scope of analysis leading us towards a more general and acceptable conclusion.
• The procedure adopted is simple and can be executed simply at home using simple apparatus and chemicals.
• The value of percentage error calculated is quite low and that indicates a high level of accuracy and precision in the investigation.
• The raw data tables display a low value of standard deviation claiming a huge precision in the raw data collected.

The process of adsorption and its thermodynamics depends on various factors like pH, temperature, pressure and many more. I would like to the study of adsorption of the dye molecules by activated charcoal at various temperatures to study the effect of temperature on the rate of adsorption of dye molecules by activated charcoal. I would use a solution of dye molecules by activated charcoal in a solution of dye at various temperature using a water bath. The absorbance of the dye molecule can be measured at various time intervals and a scatter plot of absorbance of the dye against time can be used to calculate the rate of the adsorption using the gradient of the graph obtained. Thus, this investigation can address the research question – how does the temperature affect the rate of adsorption of dye molecules by activated charcoal.

Arafat, Hassan A., et al. “Effect of Salt on the Mechanism of Adsorption of Aromatics on Activated Carbon.” Langmuir, vol. 15, no. 18, Aug. 1999, pp. 5997–6003. ACS Publications, doi:10.1021/la9813331.

EUR-Lex - 32012R0231 - EN - EUR-Lex. https://eur-lex.europa.eu/legal- content/EN/ALL/?uri=CELEX%3A32012R0231. Accessed 20 May 2021.

Gupta, Vinod K., et al. “Adsorption of Carmoisine A from Wastewater Using Waste Materials—Bottom Ash and Deoiled Soya.” Journal of Colloid and Interface Science, vol. 335, no. 1, July 2009, pp. 24– 33. DOI.org (Crossref), doi:10.1016/j.jcis.2009.03.056.

PubChem. Carmoisine. https://pubchem.ncbi.nlm.nih.gov/compound/19118. Accessed 20 May 2021.