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).
Physisorption | Chemisorption |
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The molecules of adsorbate and adsorbent have weak physical forces of attraction between them which may include but not limited to Vander Waal forces of attraction, electrostatic forces of attraction and so on . | The molecules of adsorbate and adsorbent makes covalent bonds between them. |
It is a multi-layered phenomenon. There can be multiple layers of adsorbate molecules on the surface of the adsorbent. This layering keeps extending as long as the residual physical forces on the surface of the adsorbent. are able to reach the molecules of adsorbate. | It is a monolayered phenomenon. There is only one layer of the adsorbate on the surface of the adsorbent. |
This process is reversible in nature. Thermal or mechanical excitation of the surface of the adsorbent may cause the adsorbate molecules to leave the surface of the adsorbent and that process is known as desorption. | This is an irreversible process. |
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 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.
Variable | Why is it controlled? | How is it controlled? | Apparatus used |
---|---|---|---|
Contact time between dye and activated charcoal | Longer the adsorbent and adsorbate are in contact with each other, more the molecules of adsorbate that gets to the surface of the adsorbent. | In all cases, the solution of the dye with activated charcoal was kept undisturbed for 10 minutes. | Digital stop- watch. |
Mass of activated charcoal added | More the mass of activated charcoal added longer the surface area available, higher the value of % adsorption extent. | For all trials, 2.00 ± 0.01 g of activated charcoal was added. | Digital mass balance. |
Mass of dye used | More the mass of dye added, more the molecules of adsorbate available and greater the adsorption extent. | For all trials, 0.50 ± 0.01 g of the dye was added. | Digital mass balance. |
Temperature | Adsorption is an exothermic process. As temperature increases, the equilibrium between the adsorbate, adsorbent and the adsorbed complex shifts more towards the product. Thus, % adsorption extent increases. | All trials were conducted at room temperature. | None. |
Surface area of activated charcoal | Larger the surface area of the adsorbent, more the adsorbate molecules adsorbed and greater the % adsorption extent. | In all cases, a 100 cm3 glass beaker was used for adsorption. | 100 cm3 glass beaker. |
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.
Materials | Quantity | Source |
---|---|---|
Distilled water | 1000 cm3 | School laboratory |
Carmoisine Red Dye | 10.00 g | Bought from local superstore |
Concentrated HCl (analytical grade) | 10.00 cm3 | School laboratory |
Apparatus | Quantity | Least count | Uncertainty |
---|---|---|---|
Digital mass balance | 1 | 0.01 g | ± 0.01 g |
10.00 cm3 graduated pipette | 1 | 0.10 cm3 | ± 0.10 cm3 |
1.00 cm3 graduated pipette | 1 | 0.10 cm3 | ± 0.10 cm3 |
20.00 cm3 graduated pipette | 1 | 0.10 cm3 | ± 0.10 cm3 |
Colorimeter | 1 | 0.001 abs | ± 0.001 abs |
Glass rod | 1 | --- | --- |
100 cm3 glass beaker | 5 | --- | --- |
Watch glass | 1 | --- | --- |
Spatula | 1 | --- | --- |
Soft tissue | 1 | --- | --- |
Stop-watch | 1 | 0.01 s | ± 0.01 s |
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.
Preparation of 0.10 moldm-3 dilute HCl
Preparation of 1.0 × 10-2mol dm-3 of dilute HCl
Preparation of 1.0 × 1.0-3 mol dm-3 dilute HCl
Preparation of 1.0 × 10-4 mol dm-3 dilute HCl
Preparation of 1.0 × 10-5 mol dm-3 of dilute HCl
Preparation of 1.0 × 10-6 mol dm-3 dilute 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
\(y = 0.215 x ; x = \frac{y}{0.215}\)
\(Molar \,concentration \,of \,the \,dye (10-3 mol dm-3) =\frac{mean \ absorbance\ in\ ±\ 0.001\ abs}{0.215}\)
pH | Mean absorbance of the solution (±0.001 abs) | Adsorption extent % |
---|---|---|
1.00 | 0.915 | 22.98 |
2.00 | 0.860 | 23.11 |
3.00 | 0.796 | 23.26 |
4.00 | 0.623 | 23.66 |
5.00 | 0.578 | 23.77 |
6.00 | 0.492 | 23.97 |
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?
Source of error | How does it affect? | How can it be improved? |
---|---|---|
Uncertainty in the volume of the dye solution added, mass of the dye initially taken and the mass of activated charcoal added | All these values – volume of dye solution, mass of dye and mass of activated charcoal are all involved in the process of calculating the % adsorption extent and thus uncertainties in them will reduce the precision of the result. | solution, mass of dye and mass of activated charcoal are all involved in the process of calculating the % adsorption extent and thus uncertainties in them will reduce the precision of the result. Use a graduated pipette instead of a graduated measuring cylinder to record the volume. Calibrate the mass balance using a standard mass before use. Take repeated trials of same value and use average values. |
The entire investigation is based on readings of absorbance taken from the digital photo- colorimeter. This device itself has a standard instrumental error. | Reduces the accuracy of the absorbance readings and the values collected are either more than actual or less than it. | The colorimter must be calibrated using the solvent (water in this case) at the wavelength of the measurement by adjusting the absorbance at that wavelength (516 nm) as 0.000 ± 0.001 AU |
Source of error | How does it affect? | How can it be improved? |
---|---|---|
The entire investigation is based on readings of absorbance taken from the digital photo- colorimeter. This device itself has a standard instrumental error. | Reduces the accuracy of the absorbance readings and the values collected are either more than actual or less than it. | The colorimter must be calibrated using the solvent (water in this case) at the wavelength of the measurement by adjusting the absorbance at that wavelength (516 nm) as 0.000 ± 0.001 AU |
Activated charcoal is a porous adsorbent. It is essential to have an uniform surface of it for the most effective adsorption. | Any uneven distribution of the powdered solid reduces the extent of adsorption. | The activated charcoal must be added to the beaker slowly and allowed to settle down at the bottom to make the surface as much uniform as possible. |
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.
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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