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.