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Chemistry HL
Chemistry HL
Sample Internal Assessment
Sample Internal Assessment

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Table of content
Rationale
Background Information
Hypothesis
Variables
Procedure
Conclusion
Evaluation
References

How does the mass of caffeine (in g) extracted from Tetley Black Tea leaves in its aqueous extract depends on the temperature of the water in which the tea leaves are brewed, determined using solvent extraction?

How does the mass of caffeine (in g) extracted from Tetley Black Tea leaves in its aqueous extract depends on the temperature of the water in which the tea leaves are brewed, determined using solvent extraction? Reading Time
10 mins Read
How does the mass of caffeine (in g) extracted from Tetley Black Tea leaves in its aqueous extract depends on the temperature of the water in which the tea leaves are brewed, determined using solvent extraction? Word Count
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Table of content

Rationale

Being an avid consumer of beverages like tea and coffee to stimulate my mind, caffeine is an important constituent of regular liquid diet. While finding another suitable ways to consume tea, I came across another staple dish of Japan known as ‘ochazuke’ which is actually a dish prepared by heating tea with boiled rice at very high temperature. I was intrigued to understand the health benefits of this particular dish and came to know that they boil tea at a very high temperature almost to dryness to get more caffeine out of it which gives a distinctive flavour and better nutritional values to the food. Thus, I had a question- Will we get more caffeine from tea if the temperature at which tea is extracted is increased? Food processing and preservation is an immensely important branch of chemistry. Extraction of naturally occurring pigments and other heterocyclic compounds like alkaloids, tarpenoids from vegetables or fruits or seeds of plants is a common practice. This process is mainly based on the principle of difference of solubility of organic compounds in different solvents and is also affected by multiple physicochemical conditions like temperature, polarity of solvent used, pH and many more. Researches have been done over time to elucidate the optimum conditions for brewing tea. This investigation is a part of that same exploration. My first objective behind this was to understand the chemical process which is involved in the extraction of caffeine from tea leaves; which I found to be simply a process of diffusion. Since middle school chemistry classes, I have studied that molecules diffuse at a faster rate at higher temperature. This investigation may allow me to illustrate or verify this understanding of mine. Moreover, specific characteristics of organic compounds and the variation they show in their properties and structure like aromatic compounds, heterocyclic compounds, alicyclic compounds were some of the terms I came across while exploring ideas to conduct this experiment and that fascinated me more towards this topic.

Background Information

Caffeine

Figure 1 - Structural Formula Of Caffeine (Omar)
Figure 1 - Structural Formula Of Caffeine (Omar)

Caffeine is a component of coffee, tea, chocolate, soft drinks, diet pills, and analgesic making it the most widely used psychoactive drug in the world which affects the same parts of the brain as cocaine(Daly et al.).

 

Caffeine belongs to the family of heterocyclic compounds, purines, with 1,3,7 - Trimethylpurine - 2,6 - dione as its IUPAC name, and C8H10N4O2 as its chemical formula(E.F). Caffeine is water-soluble like most purines; however, caffeine’s solubility varies with temperature. Caffeine can be classified as an alkaloid, and like most alkaloids, caffeine has powerful physiological effects on humans such as increased focus. It exists as a white crystalline solid at room temperature. It belongs to the class of ‘methylxanthine alkaloid’(PubChem)a class of the heterocyclic aromatic family of organic compound.

Chemical composition of tea leaves

Tea leaves are chemically composed of two main class of aromatic compounds other than carbohydrates-

  • Alkaloids
  • Flavonoids or tannins.

Caffeine belongs to the category of alkaloids. This compound is a covalent non-polar compound. It is insoluble in polar solvent like water and highly soluble in non-polar solvent like chloroform or dichloromethane. Chloroform is a better solvent as it does not react with the caffeine molecules and is easily volatile. Caffeine can also dissolve easily in ethanol.

Extraction of caffeine

The extraction of solid caffeine from tea leaves various stages:

  • Solid-liquid extraction - In this step, caffeine molecules are extracted from tea leaves in the aqueous layer. Soaking the tea leaves are soaked in hot water. In a more sophisticated analytical procedure, ‘Soxhlet extractor’ can also be used. This step is an example of a biological process of active transport mechanism where the caffeine molecules in the cells of tea leaves degrades down to release the phytochemicals inside it like tannins and flavonoids. These molecules then spread into the aqueous layer through the process of diffusion.
  • Acid-base liquid-liquid extraction: In this step, first anhydrous sodium carbonate is added to the aqueous extract and then an organic solvent like dichloromethane or chloroform is added to the same. Caffeine is transferred from the aqueous layer to the organic layer because of its high affinity towards the organic solvent. This step involves an acid base reaction between the base sodium carbonate and the acidic tannin molecules. It is termed as the liquid-liquid extraction as the caffeine gets transferred from one liquid layer (water) to another liquid layer (chloroform/dichloromethane). Chloroform and dichloromethane are both highly volatile and inert in nature; so they are the best choice as solvents for this step.
  • Liquid-Solid extraction: In this step, the organic layer is separated from the aqueous layer through a separating funnel. The solvent is the allowed to evaporate leaving behind the crude crystals of caffeine. This is then crystallised using ethanol as solvent.

Role of sodium carbonate

Tannins contains an aromatic ring with the phenolic OH group. Tannins react with sodium carbonate in the following way:

 

→     Ar - OH + Na2CO3              Ar - ONa + CO2 + H2O

 

This is an example of an acid base reaction which converts the acidic tannins into their water soluble phenolic salts. As the tannins becomes water soluble and the caffeine not, it becomes more likely for the caffeine to pass into the organic layer when a volatile organic solvent like dichloromethane or chloroform is added to the aqueous extract of caffeine. Thus in one way addition of caffeine separates the tannins from caffeine by converting the acidic tannin molecules into water soluble salts.

Literature review

In a research article titled as – ‘Effect of time and water temperature on caffeine extraction from coffee’ published in the journal of Pakistan Journal of Nutrition, the effect of brewing temperature on caffeine content of two coffee grains – ‘Coffee arabica’ and ‘Coffee robusta’ was studied. The range of temperature chosen was 60.00 °C to 120.00 °C. The caffeine content was found to have increased with the increase of temperature. The correlation of caffeine with temperature was expressed as a “linear equation y = 1.7647x - 1.5023 with a R2 value of 0.9963” (Nhan and Phu)

Hypothesis

This investigation is mainly focuses on the effect of temperature on the extent and rate at which caffeine is passing from the cells in the tea leaves to the aqueous medium in the solid-liquid extraction stage. It involves two process in sequence- degradation or decomposition of the cells in the tea leaves to release the phytochemicals it contains and diffusion of the caffeine thus released into the aqueous layer. Both of these process must occur at a faster rate as temperature increases. Thus, more caffeine must be extracted from the tea leaves if the temperature of extraction is increased. Thus, it is predicted that mass of caffeine extracted into the aqueous layer must increase as the temperature increases.

Variables

Independent variable

Brewing temperature in °C ; temperature to which the water was heated in which the tea leaves were soaked in.

 

For my experiment, I used Tetley Black Tea which has a caffeine content of 40 - 50 milligrams in a typical 8 - ounce cup(Price et al.). The standard value for caffeine content of an 8 - ounce cup is 47.4 milligrams (Cho).

 

Hence, Tetley Black Tea is a fair estimate for all Black Teas. Conventionally, Black Tea is consumed at a temperature of 99 °C(McNaughton et al.). However, this may not be the temperature at which caffeine content is maximised. At the same time, temperatures such as 200 °C may be impractical as they exceed the heat threshold of the human tongue. Hence, a temperature range of 60 °C to 120 °C had been chosen for this experiment. The temperature of water was varied using a Bunsen Burner. A beaker of distilled water was placed above the heater until the desired temperature was reached. The temperature was measured using a thermometer which was placed in the beaker of water.

Dependent variable

Mass of caffeine extracted in g

 

The caffeine content was the dependent variable of the experiment which varied with temperature of water and hence represented the link between temperature and the caffeine content of tea. The mass of re- crystallised caffeine obtained through solvent extraction was measured using a mass balance after the solvent extraction process.

Controlled variable

Variable Name
How it impacts?
How was it controlled?
Apparatus Used
Tea type and quantity
Different tea brands can have different caffeine contents which can influence the relationship between caffeine and temperature. More the mass of tea leaves taken more the mass of caffeine extracted.

Two teabags of Tetley Black Tea were used for all readings.
All the tea bags contained same mass of tea leaves.

NA
Brewing Time
The time taken for the dissolution of caffeine was kept constant as an increase in time would have resulted in an increase in the caffeine content of the tea.
The brewing time was fixed at 120.00 ± 0.01 seconds.
Digital stopwatch
Volume of dichloromethane used
An increase in the volume of organic solvent- dichloromethane used would have increased the caffeine which dissolves and hence the caffeine content. More the organic solvent more the caffeine molecules that disperses to it.

To ensure that 10.00 ± 0.05 cm3 of dichloromethane was added to aqueous extract of caffeine in all trails, a graduated pipette of 10 cm3 was taken.

Graduated pipette 10 cm3

Mass of Na2CO3 Added

A change in the mass of Na2CO3 would have resulted in a change in the tannin content of the solution which would have created discrepancies in the caffeine reading.

1.00 ± 0.01 g of Na2CO3 weighed using a digital mass balance was added to the aqueous extract.

Digital mass balance
Volume of Ethanol
A change in the volume of ethanol would have caused a change in the mass of caffeine which was recrystallised by influencing its solubility.

10.00 ± 0.05 cm3 of ethanol was added during the crystallisation process.

Graduated pipette
Volume of Water
Water is used as the solvent in the first stage; solid-liquid extraction. More the solvent more the caffeine that dissolves and more the mass of caffeine extracted.

100.0 ± 0.5 cm3 of distilled water was added using a graduated measuring cylinder.

Graduated measuring cylinder
Figure 2 - Table On Controlled Variable

Apparatus

Apparatus Name
Quantity
Capacity
Least Count
Uncertainty
Burette
1

50.00 cm3

0.10 cm3

±0.05cm3

Glass Beaker
2

250.00 cm3

-
-
Watch Glass
2
-
-
-
Temperature probe
1
---
0. 01°C
±0.01°C
Bunsen Burner
1
-
-
-
Stopwatch
1
-
-
-
Tongs
1
-
-
-
Mass balance
1
-
0.01g
±0.01g
Spatula
1
-
-
-
Separatory Funnel
1

50.00 cm3

-
-
Clamp Stand
1
-
-
-
Dropper
1

10.00 cm3

-
-
Graduated Cylinder
1

100.00 cm3

1cm3

±0.5cm3

Figure 3 - Table On Apparatus

Materials required

Material Name
Quantity
Source
Distilled Water
1.5 L
School Laboratory
Tea Bags
30.00 g
School Laboratory
Sodium Carbonate
20.00 g
School Laboratory
Dichloromethane

500 cm3

School Laboratory
Anhydrous Sodium Sulphate
50.00 g
School Laboratory
Pure Ethanol

500 cm3

School Laboratory
Ice
200.00 g
School Laboratory
Figure 4 - Table On Materials Required

Procedure

  • A 250 cm3 glass beaker was taken and filled with 50 cm3 of drinking water using a graduated measuring cylinder.
  • The beaker was placed on the tripod stand with the Bunsen burner below it to heat the water.
  • A temperature probe was inserted into the water to check its temperature and a Vernier Logger pro was connected to the temperature probe.
  • The water was heated until the temperature reaches 60.00 ± 0.01°C.
  • Two tea bags were taken and dipped into the water and the stop-watch was started.
  • The tea bags were dipped until the stop-watch reads 120.00 ± 0.01 s.
  • The tea bags were then squeezed completely using a glass rod to drain out all water inside it.
  • The tea bags were taken out and discarded.
  • The tea extract was allowed to cool down. It was kept in an ice bath for this.
  • The temperature of the tea extract was measured using the temperature probe and the cooling process was continued until the temperature of the tea extract reaches to or below 20.00 ± 0.01°C.
  • 10.00 ± 0.05 cm3 of chloroform was added to the tea extract using a graduated pipette keeping the tea extract in the ice water bath.
  • The extract was then transferred into a separating funnel.
  • A 50.00 cm3 glass beaker was taken and its mass was recorded using a digital mass balance.
  • The organic layer was extracted out in a 50.00 cm3 glass beaker.
  • The beaker was covered with a filter paper. The filter paper was tied with a rubber band and small holes were pricked in the filter paper using a sharp pointed pencil.
  • The beaker was placed in a safe place and the solvent was allowed to evaporate.
  • In the following day, crude crystals of caffeine were observed in the beaker.
  • The crystals were dissolved in 10.00 ± 0.05 cm3 of ethanol added to the same beaker using a graduated pipette.
  • The beaker was tied with a filter paper and tied with a rubber band as done earlier. Small holes were pricked in the filter paper allowing the solvent(ethanol) to evaporate.
  • In the following day, pure shinning white crystals of caffeine was obtained in the beaker and the mass of the beaker was recorded again using a mass balance.
  • Steps 1 - 20 were re-iterated for four more times to collect data in five trial sets.
  • All of the above steps were repeated for other temperature values - 70.00 ± 0.01°C, 80.00 ± 0.01°C, 90.00 ± 0.01°C, 100.00 ± 0.01°C, 110.00 ± 0.01°C and 120.00 ± 0.01°C

Safety precautions

  • During the heating processes, gloves, lab coat, facemask and eye-goggles were worn to ensure that human skin does not come in contact with high temperatures and consequently burn.
  • The aforementioned pieces of equipment were also worn during the addition of chemicals such as 100% Ethanol, which is highly flammable; Sodium Carbonate, which is a strong skin irritant; Dichloromethane, which is highly toxic; and Anhydrous Sodium Sulphate, which can severely dehydrate your skin.
  • The facemask was particularly important during the addition of these substances, and during the transfer of caffeine into the vial, considering the fact that caffeine can even be lethal in a very small amount if consumed.
  • To further reduce the risk of burns, metallic tongs were used for the movement of apparatus to and away from the heat source.
  • Whilst any substance was heating, vision was entirely focussed on the heating process to minimise the chance of any mishaps at school.
  • Hands were cleaned with water and soap after every trial rather than sanitiser which is flammable.
  • The experiment was conducted in a fairly secluded space with minimal movement which improved the overall safety of it.

Ethical considerations

No living organisms were harmed during this experiment, and all chemicals were disposed of in a very careful manner.

Environmental concerns

No harm was done to the environment, and the waste chemicals were disposed carefully.

Qualitative data

Observation
Inferences
Bubbles were observed when sodium carbonate was added to the aqueous extract of caffeine.
Reaction of sodium carbonate with tannins in the aqueous tea extract released carbon dioxide.
White lumpy solid was left in the beaker as the chloroform evaporated.
Formation of crude caffeine.
White shinning crystals were observed after the ethanol evaporated.
Formation of crystals of caffeine.
Two distinct layers of liquid was observed when chloroform was added to the aqueous extract.
Chloroform and water are immiscible with each other.
Figure 5 - Table On Qualitative Data

Quantitative data

Figure 6 - Table On Mass Of Caffeine Obtained At 60.00 ± 0.01°C
Figure 6 - Table On Mass Of Caffeine Obtained At 60.00 ± 0.01°C

Sample calculation

Mass of caffeine crystals = (Mass of beaker + caffeine) – (Mass of beaker)

 

= (21.30 ± 0.01g) – (20.50 ± 0.01g) = 0.80 ± 0.02 g

 

Average mass of caffeine crystals = \(\frac{0.80+0.79+0.78+0.80+0.80}5{}\) = 0.79 g

 

Standard deviation (SD) = \(\frac{(0.80-0.79)^2+(0.79-0.79)^2+(0.78-0.79)^2+(0.80-0.79)^2+(0.80-0.79)^2}{5}\) = 0.01

Figure 7 - Table On Mass Of Caffeine Obtained At 70.00 ± 0.01°C
Figure 7 - Table On Mass Of Caffeine Obtained At 70.00 ± 0.01°C
Figure 8 - Table On Mass Of Caffeine Obtained At 80.00 ± 0.01°C
Figure 8 - Table On Mass Of Caffeine Obtained At 80.00 ± 0.01°C
Figure 9 - Table On Mass Of Caffeine Obtained At 90.00 ± 0.01°C
Figure 9 - Table On Mass Of Caffeine Obtained At 90.00 ± 0.01°C
Figure 10 - Table On Mass Of Caffeine Obtained At 100.00 ± 0.01°C
Figure 10 - Table On Mass Of Caffeine Obtained At 100.00 ± 0.01°C
Figure 11 - Table On Mass Of Caffeine Obtained At 110.00 ± 0.01°C
Figure 11 - Table On Mass Of Caffeine Obtained At 110.00 ± 0.01°C
Figure 12 - Table On Mass Of Caffeine Obtained At 120.00 ± 0.01°C
Figure 12 - Table On Mass Of Caffeine Obtained At 120.00 ± 0.01°C

Data processing

Tempertaure ±0.010 °C
Average mass of caffeine obtained ± 0.02 g
Fractional error ±
Percentage error ±
60.00
0.79
0.03
2.53
70.00
1.43
0.01
1.40
80.00
1.66
0.01
1.20
90.00
2.19
0.01
0.91
100.00
2.64
0.01
0.76
110.00
3.06
0.03
2.53
120.00
3.47
0.01
1.40
Figure 13 - Table On Average Mass Of Caffeine Crystals Against Temperature

Impact of uncertainty

Absolute error = ± 0.02

 

Average mass of caffeine crystals obtained = 0.79 ± 0.02 g

 

Fractional error = \(\frac{0.02}{0.79}\) = 0.0253 = 0.03 (rounded off to two decimal places)

 

Percentage error = 0.0253 × 100 = 2.53

Figure 14 - Mass Of Caffeine Obtained Against Temperature
Figure 14 - Mass Of Caffeine Obtained Against Temperature

Features of the graph

The average mass of caffeine increases from 0.79 ± 0.01 g to 3.47 ± 0.01 g as the temperature increases from 60.00 ± 0.01°C to 120.00 ± 0.01°C. This indicates that as the temperature of the water in which the tea leaves are soaked in the mass of caffeine extracted increases.

 

The data points follow a linear trend line which is y = 0.0439x – 1.77. This brings us to the claim that the increase of mass of caffeine extracted from tea leaves with temperature is linear in nature. Almost all the data points are equally spaced which again confirms that the increase is mostly gradual in nature.

Inferring the equation of trend line

At y = 0

 

0.0439x − 1.77 = 0

 

x\(\frac{1.77}{0.0439}\) = 40.31

 

It means that if the trend line is extrapolated it would intersect the x axis at 40.31. This indicates that at a temperature of 40.31 ± 0.01°C, the average mass of caffeine extracted will be zero. Thus, it can be said that if the temperature of the water is brought down to or below 40.31 ± 0.010 °C, caffeine will not be extracted from the tea leaves to water. This means that the minimum temperature required for the extraction is 40.31 ± 0.01°C

Scientific justification of the trend

  • The trend obtained can be explained from two different perspectives- thermodynamic perspective (to what extent) and kinetic (at what rate). It is clear from the trend that with the rise of temperature both the amount of caffeine extracted from tea leaves extracted and the rate at which they are extracted increases. The fact that rate at which extraction is carried out increases with temperature can be supported by the arguments that as temperature increases, rate at which caffeine molecules spread from the tea leaves into the aqueous layer increases and thus more caffeine molecule enters the aqueous layer within the same time frame (which was 2 minutes; the brewing time).
  • As temperature rises, the diffusion of caffeine molecules from tea leaves into the aqueous layer increases bringing more molecules into the aqueous layer. This is because of the reason that temperature rises the kinetic energy of the particles and causes them to move faster.
  • Mass of caffeine extracted is also related to the solubility of caffeine in the aqueous layer which again increases with the increase of temperature and thus allows more caffeine molecules to be in the aqueous layer. This happens because solubility of molecules increases with the increase in the temperature.

Evaluation of hypothesis

The correlation value is also mentioned in the graph. The value of R2 is 0.9946. As the value is positive, it supports the fact that mass of caffeine extracted increases as temperature increases. The value of R2 is high; 0.99 which shows that the correlation between the variables is strong. Moreover, the gradient of the linear trend line is 0.0439 which again supports that the correlation is positive in nature; mass of caffeine extracted increases as temperature increases. Thus, the hypothesis predicted stands valid.

Conclusion

How does the mass of caffeine (in g) extracted from Tetley Black Tea leaves in its aqueous extract depends on the temperature of the water in which the tea leaves are brewed, determined using solvent extraction?

  • The average mass of caffeine increases from 0.79 ± 0.01 g to 3.47 ± 0.01 g as the temperature increases from 60.00± 0.01°C to 120.00 ± 0.01°C. This indicates that as the temperature of the water in which the tea leaves are soaked in the mass of caffeine extracted increases.
  • The increase of mass of caffeine extracted from tea leaves with temperature is linear and gradual in nature.
  • A linear trend line following the equation; y = 0.0439x – 1.77 shows the correlation between the mass of caffeine extracted (y) and temperature (x).
  • The trend is scientifically justified based on the fact that caffeine molecules diffuse faster in the aqueous layer and its solubility in water also increases as the temperature increases.
  • The value of R2 (0.99) and the positive value of the gradient confirms that the hypothesis predicted is valid.
  • The qualitative observations like appearance of crystals also supports the conclusion made.

Evaluation

Strengths

  • The investigation has been designed in a simple and easy procedure which do not demands the use of any chemical or apparatus which is forbidden or difficult to procure.
  • The independent variable chosen has a real life significance and scientific justification. This has been done to collect the data in a range of temperature which is actually the brewing temperature that we come across in our daily life.
  • Any external factors that can question or alter or influence the reliability of the data like brewing time, volume of solvent, mass of sodium carbonate and so on have been controlled in a justified scientific procedure.
  • The magnitude of percentage error indicates that the data collected claims reliable accuracy and significant preciseness as well.

Limitations

Source
How does it impact?
How can it be minimized?
Uncertainty in apparatus
Reduces the accuracy and reliability of the mass of caffeine measured.
A burette and pipette were used instead of measuring cylinder. Temperature probe was used instead of a thermometer.
Delay in operating the stop-watch
The tea leaves gets brewed for longer leading to extraction of more caffeine than actual.
Data was collected for five times and average was calculated.
Figure 15 - Table On Random Error
Source of error
How does it impact?
How can it be minimized?
The tea bags may contain water within it.
Some of the caffeine may be lost with the water that is retained in the tea bag. This will gives us a lower value of the caffeine than actually obtained.
The tea bags were squeezed using a glass rod to drain out the water completely out of it.
Although the extraction is based on the fact that caffeine is more soluble in the chloroform than water yet it cannot be ignored that caffeine is soluble in water too.
It may happen that some of the caffeine molecules may still remain in the aqueous layer even after chloroform is added to it. This will produce a lower value for the mass of caffeine extracted than the actual.
Sodium carbonate was added to make the medium basic which makes caffeine more attracted towards the organic solvent over water.
Chloroform is used as a solvent here. It is extremely volatile and can evaporate easily. In case, it is mixed with hot water, it may evaporate.
This would make less amount of caffeine available for the caffeine to dissolve than the actually added volume.
The aqueous extract of caffeine was placed in an ice bath to cool it down before adding chloroform to it.
There is a chance of loss of mass of caffeine crystals while transferring it from one glass apparatus to another using a spatula.
This would reduce the mass of caffeine crystals obtained.
Same beaker was used to obtain the crystals of caffeine from evaporation of chloroform and to crystallize it using ethanol. Thus it was not required to transfer the caffeine crystals from one apparatus to another.
Figure 16 - Table On Systematic Error

Methodological limitation

The methodology of extraction relies on the fact that caffeine is more soluble in organic solvent like chloroform than polar solvent like water. Thus it cannot be ensured that all caffeine molecules were transferred from the aqueous layer to the organic layer. To avert this limitation, the methodology needs to be altered. The quantity of caffeine obtained can be monitored using a UV-Visible spectrophotometer. Caffeine shows ‘strong absorbance at 205 nm and 273 nm’(CSUN).Thus, if the absorbance of the solution is measured at any of these two wavelengths and compared in reference to a calibration curve, the amount of caffeine can be calculated.

Further scope

This experiment can be conducted for different types of beverages such as different types of tea, soft drinks, chocolates, and other caffeine containing items. Other factors which may influence caffeine content can also be assessed such as the pH of a solvent. Caffeine can be extracted in acidic medium instead of using drinking water. Dilute HCl solutions of various concentrations can be used for this. As the concentration of HCl changes, the pH of the medium would also change. Thus, the effect of pH on the mass of caffeine extracted can be investigated.

References

References has been done in MLA 8 format. As per MLA 8 guidelines the citations are listed below in alphabetical order and the in-text citations are inserted at appropriate places. Access date and time are not mentioned for research articles as per latest MLA 8 guidelines.

 

Cho, Hae-Wol. “How Much Caffeine Is Too Much for Young Adolescents?” Osong Public Health and Research Perspectives, vol. 9, no. 6, Dec. 2018, pp. 287–88. DOI.org (Crossref), doi:10.24171/j.phrp.2018.9.6.01.

 

CSUN. High Performance Liquid Chromatography. 10 June 2020.

 

Daly, J. W., et al. “[Is caffeine addictive? The most widely used psychoactive substance in the world affects same parts of the brain as cocaine].” Lakartidningen, vol. 95, no. 51–52, Dec. 1998, pp. 5878–83.

 

E.F, Olasehinde. “Corrosion Inhibition Behaviour for Mild Steel by Extracts of Musa Sapientum Peels in HCl Solution: Kinetics and Thermodynamics Study.” IOSR Journal of Applied Chemistry, vol. 2, no. 6, 2012, pp. 15–23. DOI.org (Crossref), doi:10.9790/5736-0261523.

 

McNaughton, David, et al. “‘Augmentative and Alternative Communication (AAC) Will Give You a Voice’: Key Practices in AAC Assessment and Intervention as Described by Persons with Amyotrophic Lateral Sclerosis.” Seminars in Speech and Language, vol. 39, no. 05, Nov. 2018, pp. 399–415. DOI.org (Crossref), doi:10.1055/s-0038-1669992.

 

Nhan, Pham Phuoc, and Nguyen Tran Phu. “Effect of Time and Water Temperature on Caffeine Extraction from Coffee.” Pakistan Journal of Nutrition, vol. 11, no. 2, Feb. 2012, pp. 100–03. DOI.org (Crossref), doi:10.3923/pjn.2012.100.103.

 

Omar, Waid. “Experimental Investigations of Adipic Acid Agglomeration Behavior under Different Operating Conditions Using Image Analysis Technique QICPIC Software.” Particulate Science and Technology, vol. 38, no. 6, Aug. 2020, pp. 740–46. DOI.org (Crossref), doi:10.1080/02726351.2019.1620386.

 

Price, Karen Overstreet, et al. “How Much Caffeine Is Too Much in Athletes?” American Journal of Health- System Pharmacy, vol. 47, no. 2, Feb. 1990, pp. 303–303. DOI.org (Crossref), doi:10.1093/ajhp/47.2.303a.

 

PubChem. Caffeine. https://pubchem.ncbi.nlm.nih.gov/compound/2519. Accessed 31 July 2020.