Effect of pH on adsorption of Carmoisine Red Dye by activated charcoal

How does the % adsorption extent of food color- ‘Carmoisine-14720’ by activated charcoal in acidic medium
depends on the pH (1.00 to 6.00) of the medium, determined using colorimetry?

Chemistry is a central science where observations lead to inquiries which are then answered using logical explanations and experimental evidences to create or produce new knowledge in the subject. The inception of this investigation follows the same pattern. It was a simple incident of cut on the cheeks I had while shaving using the new razor I bought. Soon, my father got me a solid white ice like thing and surprisingly after applying that on my skin, the bleeding stopped immediately. On asking my father he said that solid was a potash alum. I started my own research to understand what the substance was actually and how did it stop bleeding. I got to know that potash alum is a hydrated double salt of potassium sulphate and aluminum sulphate with 24 molecules water of crystallization. This salt can act as a coagulating agent and coagulates the platelets of blood through the preferential adsorption of the cations – K⁺ and Al³⁺ on the surface of the negatively charged colloidal particles of blood (Mohammed and Rashid). This is how, I was introduced to a new term- ‘adsorption’ which was not studied as a part of the DP Chemistry course. Soon on further research, I got to know about more real-life applications of adsorption like that in adsorption of drugs in target cells, use of activated charcoal for removal of toxic gases in gas chambers and so on (El maguana et al.). The factors that mainly impacts the kinetics and thermodynamics of adsorption are temperature, pressure, surface area and so on. While reading the concept of buffer solutions in Topic-8, an idea came to my mind that if controlling the pH of a medium is so important in colloids like shampoos, moisturizers that buffer solutions are used in them, does pH have a role to play in the process of adsorption? Thus, I thought of choosing a simple case of adsorption where the adsorption extent can be easily measured and vary the pH of the medium to see if pH has any effect on adsorption mechanism or not. The easiest case I could refer to use an organic dye as measuring the concentration of the dye is easy using a simple photo-colorimeter. The school laboratories were closed, the easiest dye that I could access was the food coloring stuff. And thus, I arrived at the research question stated above.

Adsorption is a surface phenomenon where the molecules of a matter deposits itself on the surface of another matter without entering into the bulk of it (Li et al.). The substance that sticks to the surface is an adsorbate and the surface on which it sticks is an adsorbent. For an adsorption of the Hydrogen gas by activated charcoal, the hydrogen gas is adsorbate and the activated charcoal is an adsorbent. This investigation deals with Carmoisine red dye as an adsorbate and activated charcoal as an adsorbent. Often the two terms – adsorption and absorption are confusing. A simple example to illustrate the difference is that water entering into a cotton pad when the cotton pad is added to a beaker of water is an example of absorption of water by cotton. A wooden block when added to the same beaker of water would not allow the water molecules to enter the bulk of the matter rather it will just stick to the surface of the wood and that is a case of adsorption. There are various real-life applications of adsorptions like – transfer of nutrients and glucose molecules from villi (projections of small intestine) to blood is an example of adsorption or preferably biosorption, the use of potash alum or porous structures like zeolites to purify water through removal of toxic organic impurities and so on (Končar-Djurdjević).

The adsorption of organic dyes on the porous surface of activated charcoal is an example of physisorption.

Here, the activated charcoal has been chosen as an adsorbent because (Ramirez et al.)

• It is highly porous in nature where it can accommodate more adsorbate molecules on the surface of the pores.
• It has a high ability to retain the molecules adsorbed.
• Adsorption on activated charcoal has high thermal resistance and is not usually affected by small changes in temperature.

It is an organic dye with the IUPAC name – “disodium;4 - hydroxy - 3 - [(4 - sulfonatonaphthalen -1 - yl)diazenyl]naphthalene - 1 - sulfonate”. It contains of a 4 - sulphonato napthol bonded with naphthalene sulphonic acid through an azo (- N = N -) linkage (Shahabadi et al.). It falls under the category of azo dye and is made by a coupling reaction where one sulphonato substituted alpha napthol ring couples with another 4 - nitro naphthalene sulphonic acid. It is used as a food colour under the name-Carmoisine –14720 (David and Moldovan). It is a red crystalline solid at room temperature and miscible with water.

 

This molecule can act as a chromophore due to the presence of extended conjugation of the two bicyclic naphthalene rings along with the azo system. This makes the molecule to absorb electromagnetic radiation in the UV region by reducing the gap between its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) . This molecule has been reported to show a peak of maximum absorbance at 516 nm within the visible region.

The ability of a chromophore to absorb an electromagnetic radiation is measured in terms of optical density or absorbance which is measured in absorbance units (AU). This data is collected by using a device known as photo-colorimeter where the electromagnetic radiation of a particular wavelength is passed across the sample contained in a cylindrical glass tube known as cuvette which is opaque on two sides and transparent on theother two. The absorbance of a sample is a logarithmic ratio of the light transmitted by the sample and the light incident on it. It is mathematically expressed as (Swinehart):

 

Absorbance (A) \(= log \frac{I}{I_0}; I =\) intensity of transmitted light, Io = intensity of incident light

According to Beer-Lambert law (Mayerhöfer et al.), this absorbance depends on two factors- concentration of the chromophore (c) in mol dm^-3 and path length of the sample (l) in dm.

 

A ∝ c × l

 

A = ∈× c × l

 

∈ = molar absorptivity constant in AU mol^-1dm²

 

This final form of the equation is known as Beer-Lambert law.

pH of the medium in the acidic region

 

The pH of the medium of adsorption will be varied from 1.00, 2.00, 3.00, 4.00, 5.00 and 6.00 in the acidic region. To do this, a stock solution of 11 mol dm^-3 concentrated HCl will be used to make 0.10 mol dm^-3 HCl solution and the other solutions will be made by serial dilution of that. A graduated pipette will be used to measure the volume of the HCl solution and 100 cm³ glass beaker will be used to make the solutions. The simple dilution formula,C₁V₂ = C₂V₂, where C₁ is the concentration of the stock solution and C₂ is the concentration of the solution to be made. V₁ is the volume of the stock solution used and V₂ is the total volume of the diluted solution.

% adsorption extent

 

It will be calculated using the formula:

 

\(\text{% adsorption extent = }\frac{mass \ of\ dye\ adsorbed\ in\ g}{mass\ of\ the\ adsorbent\ (activated \ charcoal)\ in\ g} = 100\)

 

A standard calibration curve will made by measuring the absorbance of some known solutions of the dye and thus a scatter plot will be made of the absorbance against concentration. This will give a linear trend line with an equation that can be used to calculate concentration of unknown solutions of the dye. A definite mass of the dye will be added to the HCl solution of desired pH followed by addition of activated charcoal of definite mass. After a specific time, the absorbance of the solution will be measured and the value obtained will be used to calculate the mass of the dye not adsorbed with the help of the equation obtained from the calibration curve. A digital photo-colorimeter will be used to record the absorbance of the sample.

Null hypotheses: There is no correlation between the % adsorption extent of adsorption of Carmoisine Red dye by activated charcoal and the pH at which it occurs.

 

Alternate hypotheses: There is a correlation between the % adsorption extent of adsorption of Carmoisine Red dye by activated charcoal and the pH at which it occurs.

• HCl is a corrosive liquid and may burn skin. Inhaling flames of concentrated HCl may cause breathing issues and nausea.
• Activated charcoal is a carcinogenic substance. If ingested, it may cause constipation and other digestive disorders.

• A laboratory coat, a safety gloves and a safety mask was always used.
• All solutions were prepared under careful guidance of an expert.
• No eatables were allowed in the work station.

To avoid the use of toxic substances and offer a minimum budget for the investigation, food coloring dye was used instead of any other carcinogenic inorganic dye.

All waste chemicals were diluted before they were disposed of into the waste bin.

• Take a 1.00 cm³ of graduated pipette and transfer 0.90 cm³ of 11.00 moldm^-3 (the concentrated HCl present in laboratory is usually of this strength) into a 100 cm³ glass beaker.
• Add distilled water till the mark of 100 cm³.
• Stir with a glass rod.

• Take a 10 cm³ graduated pipette and transfer 10.00 ± 0.05 cm³ of 0.10 moldm^-3 into a 100 cm³ glass beaker.
• Add distilled water till the mark of 100 cm³
• Stir with a glass rod.

• Take a 10 cm³ graduated pipette and transfer 10.00 ± 0.05 cm³ of 1.0 × 10^-2 moldm^-3 into a 100 cm³ glass beaker.
• Add distilled water till the mark of 100 cm³.
• Stir with a glass rod.

• Take a 10 cm³ graduated pipette and transfer 10.00 ± 0.05 cm³ of 1.0 × 10^-3 mol dm^-3 into a 100 cm³ glass beaker.
• Add distilled water till the mark of 100 cm³.
• Stir with a glass rod.

• Take a 10 cm³ graduated pipette and transfer 10.00 ± 0.05 cm³ of 1.0 × 10^-4 moldm^-3 into a 100 cm³ glass beaker.
• Add distilled water till the mark of 100 cm³.
• Stir with a glass rod.

• Take a 10 cm³ graduated pipette and transfer 10.00 ± 0.05 cm³ of 1.0 × 10^-6, moldm^-3 into a 100 cm³ glass beaker.
• Add distilled water till the mark of 100 cm³.
• Stir with a glass rod.

• Take a 100 cm³ glass beaker.
• Take a digital mass balance, plug it on and put an empty watch glass on it.
• Tare the reading of the balance to 0.00 ± 0.01 g.
• Use a spatula and transfer the solid ‘red dye’ until the balance reads 0.50 ± 0.01 g (0.001 moles)
• Transfer the weighed dye into the glass beaker.
• Add distilled water till the mark of 100 cm³ .
• Use a glass rod to stir the solution and dissolve the Red dye.
• Set up the colorimeter and plug it on.
• Set the wavelength at 516 nm (or the value closest to it).
• Take the cuvette and fill it with distilled water.
• Insert it into the colorimeter.
• Adjust the absorbance reading to 0.000 abs to calibrate the colorimeter.
• Take out the cuvette
• Throw the water and wipe off the cuvette using soft tissue.
• Transfer 1.00 cm³ of the solution using a 1.00 cm³ graduated pipette into the cuvette.
• Insert the cuvette into the colorimeter.
• Record the absorbance reading.
• Take out the cuvette, wash it with distilled water and wipe it off with soft tissue.
• Repeat Steps 15 to 18 for two more times.
• Repeat steps 1-19 using other values of mass – 1.00± 0.01 g (0.002 moles), 1.50 ± 0.01 g (0.003 moles), 2.00 ± 0.01 g (0.004 moles) and 2.51 ± 0.01 g (0.005 moles) in Step-4.

• Take a 100 cm³ glass beaker.
• Transfer 20.00 ± 0.05 cm³ of 0.01 moldm^-3 HCl using a graduated 20.00 cm³ pipette.
• Weigh 0.50 ± 0.01 g (0.001 moles) of the ‘Red dye’ in an empty watch glass using a spatula and a digital mass balance.
• Use a glass rod to stir the solution and dissolve the solid.
• Weigh 1.00 ± 0.01 g of activated charcoal using a watch glass and a spatula on a digital mass balance.
• Transfer the weighed solid into the beaker.
• Wait for the activated charcoal to settle down at the bottom.
• Start the stop-watch.
• Set up the colorimeter.
• Take a cuvette and fill it with 0.01 moldm^-3 HCl.
• Set the wavelength at 516 nm (or the value close to it).
• Adjust the wavelength to 0.000 abs.
• Take out the cuvette and throw the acid taken.
• Wash the cuvette with distilled water, wipe it with a soft tissue and dry it using a hair dryer.
• As soon as the stop watch reads 600.00 s, take a 1.00 cm³ graduated pipette and fill up the cuvette with 1.00 ± 0.05 cm³ of the solution in the beaker.
• Record the absorbance.
• Take out the cuvette, throw the liquid, wash with distilled water, wipe it with soft tissue and dry it with a hair dryer.
• Repeat steps 15-17 for four more times.
• Repeat all of the above steps using 1.0 × 10^-2 mol dm^-3, 1.0 × 10^-3 mol dm^-3, 1.0 × 10^-4 mol dm^-3, 1.0 × 10^-5 mol dm^-3 and 1.0 × 10^-6 mol dm^-3 of HCl.

The red color of the dye solution was found to fade out with time.

Mass of dye added = 0.50 ± 0.01 g

 

Volume of solution  \((V) = 100.00 \, \,cm^3 =\frac{100}{1000}\, \,dm^3 = 0.10 \, \,dm^3\)  (the absolute uncertainty in volume has been ignored as a glass beaker was used to prepare the solution)

 

\(Number \,of moles \,of \,dye = \frac{mass\ of\ dye\ taken}{molecular\ mass\ of\ taken}= \frac{0.50\ ±\ 0.01}{502.40}= 0.0009 ≅ 1.00 × 10-4 ± 0.01\)  

 

\(Molar \,concentration \,= \frac{number\ of\ mole}{volume}=\frac{1.00\ ×\ 10 ^{-4}\ ±\ 0.01}{0.10}​  = 1.00 × 10-3 ± 0.01 mol dm-3 ​\)  

 

\(Average \,absorbance\, = \frac{(0.212\ ±\ 0.001)\ + (0.217\ ±\ 0.001)\ +\ (0.215\ ±\ 0.001)}{3}= 0.215 ± 0.001 AU\) 

 

\(\frac{(Standard \,deviation (SD) = 0.212\ - \ 0.215)^2\ +\ (0.217\ -\ 0.215)^2\ +\ (0.215\ -\ 0.215)^2}{3}= 0.003\) 

The graph above is a calibration curve where the mean absorbance of the standard solutions of the dye at 516 nm (the wavelength at which the dye shows the maximum absorbance) has been plotted against the molar concentration of the dye taken. As indicated the mean absorbance increases with the increase in the molar concentration of the dye taken and this is in agreement with the Beer- Lambert law. A linear equation of trend line has been also displayed in the graph and the equation has been displayed in the graph. The equation is: y = 0.215x where y represents the mean value of absorbance in ± 0.001 abs and x represents the molar concentration of the dye in 10^-3 moldm^-3

 

\(y = 0.215 x ; x = \frac{y}{0.215}\)

 

 \(Molar \,concentration \,of \,the \,dye (10-3 mol dm-3) =\frac{mean \ absorbance\ in\ ±\ 0.001\ abs}{0.215}\)

Mean absorbance of the solution after adsorption = 0.915 ± 0.001 abs

 

Molar concentration of the solution after adsorption of the dye  \(=\frac{0.915}{0.215}= 4.25 × 10^{-3} \, \,mol \, \,dm^{-3}\) 

 

Number of moles of dye left in the solution after adsorption = comcentration × volume

 

\(= 4.25 × 10-3 ×\frac{20}{1000}= 85.11 × 10-6\) 

 

Mass of dye left in the solution after adsorption = moles × molar mass

 

= 85.11 × 10^-6, × 502.40 = 42759.26 × 10^-6, g = 0.0427 g ≅ 0.042 g

 

Mass of dye initially taken = 0.50 ± 0.01 g

 

Mass of dye adsorbed = 0.50 – 0.042 ≅ 0.46 g (answered up to two significant figures)

 

Mass of activated charcoal taken = 2.00 ± 0.01 g

 

 \(Adsorption \,extent \,percentage =\frac{mass\ of\ dye\ adsorbed}{mass\ of\ activated\ charcoal\ taken}× 100=  \frac{0.46}{2.00}× 100 = 22.98\)

For pH = 1.00,

 

Absolute error in mean absorbance (∆ abs) = ±0.001 abs

 

Absolute error in volume of the solution taken (∆V) = ±0.05cm³

 

Molar concentration of the dye after adsorption = 4.25 × 10^-3( ± 0.001 mol dm^-3

 

(As the molar concentration has been calculated using the value of mean absorbance and the equation from the standard calibration curve; the sources of error from the calibration curve have been ignored for convenience).

 

Absolute error in moles of the dye left after adsorption

 

\(= (\frac{absolute\ error\ in\ molar\ concentration}{molar\ concentration }\ +\ \frac{absolute\ error\ in\ volume}{volume})× moles of dye\) 

 

\(= (\frac{0.001}{4.25\ × 10^-3}\ +\ \frac{0.05}{20.00}) × 85.11 × 10-6 = ±2.03 × 10-5\)

 

Mass of dye left after adsorption = 0.042 ± 2.03 × 10^-5 (error in molar mass has been ignored)

 

Mass of dye adsorbed = (0.50 ± 0.01) – (0.042 ± 2.03 × 10^-5) = 0.46 ± (0.01+ 2.03 × 10^-5) = 0.46 ± 0.01 g g

 

Mass of activated charcoal used = 2.00 ± 0.01 g

 

Percentage error in % adsorption extent

 

\(= (\frac{absolute\ error\ in\ mass\ of\ dye\ adsorbed}{mass\ of\ dye\ adsorbed}\ +\ \frac{absolute\ error\ in\ mass\ of\ activated\ charcoal\ used}{mass\ of\ activated \ charcoal\ used})× 100\) 

 

 \(=(\frac{0.01}{0.46}\ +\ \frac{0.01}{2.00})× 100 = 2.67\) 

 

Above is a scatter graph that depicts the variation of percentage adsorption extent of the dye- Carmoisine Red by activated charcoal against pH in the acidic region (from pH=1.00 to pH=6.00). The graph clearly shows that as pH increases from 1.00 to 6.00, the adsorption extent increases from 22.98% to 23.97%. It indicates that as the pH increases, the medium becomes less acidic and thus lesser number of dye molecules are adsorbed on the surface of activated charcoal.

Above is a scatter graph that depicts the variation of percentage adsorption extent of the dye-Carmoisine Red by activated charcoal against pH in the acidic region (from pH = 1.00 to pH = 6.00). The graph clearly shows that as pH increases from 1.00 to 6.00, the adsorption extent increases from 22.98% to 23.97%. It indicates that as the pH increases, the medium becomes less acidic and thus lesser number of dye molecules are adsorbed on the surface of activated charcoal. The increase in the adsorption extent is not gradual in nature. As pH changes from 3.00 to 4.00, a jump in the value of adsorption extent from 23.26% to 23.66% has been observed. This shows that the effect of pH or acidity rather on the adsorption of the dye by activated charcoal is not uniform; a pH of 3.00 can be considered as a breakpoint as the data points below pH of 3.00 are gradual while that after pH of 4.00 are again gradual. However, this can also be an outcome of a systematic error of the investigation. The graph above also displays a linear trend line and the equation obeyed it. The equation is y = 0.2092 x + 22.727 where y is the % adsorption extent and x denote the pH of the medium. The gradient of the graph is 0.2092 which is a positive value and thus it confirms a positive correlation between the % adsorption extent and the pH of the medium. However, the magnitude of the gradient is small enough to claim that the effect of change in pH in the acidic region on the magnitude of % adsorption extent is not that significant. As the pH changes from 1.00 to 6.00, by (6.00 -1.00) = 5 units, the % adsorption extent increases by (23.97 – 22.98) ≅ 1.00 unit. This shows that a five-fold change in the pH in the acidic region alters the % adsorption extent by one- fold. This again confirms the claim that the effect of change in pH of the medium on the % adsorption extent of the dye by activated charcoal is not significant enough. The equation of linear trend line shows a y-intercept value of 22.727. This means that at x = 0.00, the value of y is 22.727. Theoretically it indicates that at a pH of 0.00, the % adsorption extent is 22.727 which can allow us to theoretically claim that the adsorption of the dye by activated charcoal can occur even in a non acidic medium where there are no H⁺ ions in the medium. Thus, qualitatively it can be claimed that the presence of H⁺ ions is not required for the adsorption of the dye by activated charcoal.

The above graph shows a correlation coefficient (R² ) of 0.9715 which means that there is 97.15% correlation between the % adsorption extent and the pH of the medium (from 1.00 to 6.00) for the adsorption of the dye - Carmoisine Red by activated charcoal. Thus, a strong correlation has been confirmed. Moreover, the value of the gradient (0.2092) being positive indicates that the correlation is positive in nature. Thus, the null hypotheses has been rejected and the alternate hypotheses has been accepted.

Activated charcoal is a porous adsorbent with residual charges on the surface. The adsorption of large molecule like – ‘Carmoisine Red Dye’ on activated charcoal is an example of physisorption where there are intermolecular interactions between the molecules on the surface of the activated charcoal and that on the dye molecule. Among many others like- H bonds, disulphide linkages, Vander Waal forces, electrostatic forces of attraction between opposite charges also plays a major role here. The Carmoisine Red dye has two sulphonate – (SO₃ ^- ) groups attached to the benzene ring. It usually exists as a disodium salts where the anionic O atom of the SO₃ ^- group makes ionic bond with Na⁺ ions. As soon as the dye is dissolved to make an aqueous solution, SO₃Na part dissociate and the Na⁺ ions are lost and the sulphonate groups exist as SO₃ ^- only. However, in presence of H⁺ ions in the medium, the SO₃ ^- group combines with the H⁺ ions and exist as SO₃H. Thus, the dye molecule loses the two unit of negative charge it had because of the two SO₃ ^- groups. Because of this loss of charge, the electrostatic interaction between the charged dye molecule and the surface of the activated charcoal with positively charged pores decreases. This decreases the number of moles of the dye adsorbed on the surface of the dye.

 

Thus, as pH decreases from 6.00 to 1.00, medium becomes more acidic, there are a greater number of H⁺ ions in the medium, a greater number of SO₃ ^- groups of the dye are converted into SO₃H group, charge on the dye decreases, electrostatic interaction between the dye and surface of the charcoal decrease, less number of moles of the dye are adsorbed by the same mass of activated charcoal, mass of dye adsorbed decreases and thus eventually the % adsorption of the dye decreases.

How does the % adsorption extent of food color- ‘Carmoisine-14720’ by activated charcoal in acidic medium depends on the pH (1.00 to 6.00) of the medium, determined using colorimetry?

• As pH increases from 1.00 to 6.00, the adsorption extent increases from 22.98% to 23.97%. It indicates that as the pH increases, the medium becomes less acidic and thus lesser number of dye molecules are adsorbed on the surface of activated charcoal.
• A five-fold change in the pH in the acidic region alters the % adsorption extent by one- fold. This confirms the claim that the effect of change in pH of the medium on the % adsorption extent of the dye by activated charcoal is not significant enough.
• The effect of pH or acidity rather on the adsorption of the dye by activated charcoal is not uniform; a pH of 3.00 can be considered as a breakpoint as the data points below pH of 3.00 are gradual while that after pH of 4.00 are again gradual.
• A strong correlation has been confirmed. Moreover, the value of the gradient (0.2092) being positive indicates that the correlation is positive in nature. Thus, the null hypotheses have been rejected and the alternate hypotheses has been accepted.
• As pH decreases from 6.00 to 1.00, medium becomes more acidic, there are a greater number of H⁺ ions in the medium, a greater number of SO₃ ^- groups of the dye are converted into SO₃H group, charge on the dye decreases, electrostatic interaction between the dye and surface of the charcoal decrease, less number of moles of the dye are adsorbed by the same mass of activated charcoal, mass of dye adsorbed decreases and thus eventually the % adsorption of the dye decreases.

Carmoisine Red dye reports a peak at 516 nm while recording its absorbance in the UV-Visible region. However, in the colorimeter used in this investigation, this particular wavelength was not available. Thus, a wavelength of 520 nm was used. This slight change in the wavelength may cause to compromise recording the exact absorbance of the dye solution after adsorption which is the main basis of finding % adsorption extent.

• As shown in the raw data table,Figure - 6 and Figure - 8, the values of standard deviation is not significantly high. This shows that the raw data values for absorbance measured by using the colorimeter both in case of calibration curve and determining the adsorption coefficient are precise enough.
• The error analysis reports a sample percentage error of 2.67% for pH = 1.00. Thus, it can be assumed that the percentage error for other pH values were also in the same range. This indicates that there was no major systematic error in the investigation. Probably, using a calibration curve and calibrating the colorimeter to set the instrumental error of the device at 0.000 has been the key factor behind this.
• Instead of using a calibration curve from a secondary source, a calibration curve has been made in the investigation. This makes the methodology more reliable and the data more accurate. Because, solvent plays a major role in deciding the absorbance of a dye or any other chromophore. The same dye may have different values of absorbance when measured in water or an organic solvent like Dichloromethane (DCM). Thus, to obtain a fair result, it is essential that the calibration curve is made using the solvent which will be used during the investigation.

Adsorption is a surface phenomenon and apart from pH of a medium, it can also depend on other factors like presence of ionic impurities within the medium, organic molecules and so on. Removal of toxic organic substance by bio adsorbents like fruit peel is an important research topic in environmental chemistry now. It has also been reported that the process is not equally effective in all kinds of water bodies and vary largely with the change in salinity level of the water or the number of dissolved ions in them. I would like to measure the adsorption extent of an organic dye like malachite green by activated charcoal in different molar concentration of NaCl solution. To do this, aqueous solution of NaCl can be made of various strengths and malachite green and activated charcoal can be added to it. The absorbance of the solution after a definite time can be measured using a digital photo-colorimeter. Using that value and a calibration curve the adsorption extent at various values of molar concentration of NaCl can be determined. This would allow us to investigate, how does salinity (expressed as molar concentration of aqueous solution of NaCl) has an effect on the adsorption extent of malachite green by activated charcoal. To make it more realistic, any bio-adsorbent like fruit peels, vegetable skins can be used instead of activated charcoal. Malachite green has been chosen as a representative of the toxic organic matter in water bodies that are removed by adsorption. However, other such heterocyclic dyes like – bromocresol green, crystal violet can also be used.

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