effect of ph on caffeine extraction

8/15/2019 Effect of pH on caffeine extraction

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The effect of pH on the mass of caffeine extracted from brewed coffee.

Submitted by: Karol Buda

Candidate Number: 000708-0007

Session: May 2015

Subject Area: Chemistry

Word Count: 3010


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Contents

Abstract................................................................................................................1

Introduction......................................................................................................2-7

Context........................................................................................................................................2

Background............................................................................................................................2-7

Significance.................................................................................................................................7

Method..............................................................................................................6-8

Materials................................................................................................................................6-7

Methodology..............................................................................................................................7

Dichloromethane Procedure.................................................................................................7-8

Dilution Procedure....................................................................................................................8

Data Presentation.............................................................................................8-9

Results.........................................................................................................................................8

Analysis.......................................................................................................................................9

Evaluation.......................................................................................................9-11

Assumptions.............................................................................................................................10

Limitations and Improvements.......................................................................................10-11

Conclusion.....................................................................................................11-12

Unresolved Questions.............................................................................................................11

Further Investigation..............................................................................................................12

Works Cited..................................................................................................13-15

Appendix A...................................................................................................16-18


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Karol Buda000708-0007

1

Abstract

The purpose of this experiment was to discover the effect of acid concentration (M)

(converted to pH) changes of the water used for brewing coffee on the mass of caffeine

(g±0.0001g) extracted from the coffee. The significance of this relationship is the potential to

utilize pH changes in order to increase the potency of a cup of coffee in terms of its

rejuvenating properties (as a result of increased caffeine content). In this experiment, the

coffee was brewed in different concentrations of hydrochloric acid (4M; 1M; 0.5M; 0.25M;

Pure water), filtered, and dissolved in the organic solvent: dichloromethane. The

dichloromethane solution was then boiled away to leave a residue of caffeine, along with a

few impurities. The resulting data reveals a correlation between a decrease in acid

concentration (M) and an increase in caffeine (g) extracted. This, however, implies that the

caffeine remaining in the coffee solution when brewed acidic solutions will actually be greater

in mass, due to the fact that the strong acid protonates the caffeine and causes it to dissolveand remain in the water, as opposed to migrating to the dichloromethane. Dichloromethane is

a non-polar solvent, and therefore less of the caffeine from the more acidic solution will be

dissolved. The significance of this experiment is however unclear, as the concentrations of

acid used were very high, and cannot be ingested by a coffee consumer. Therefore an

unanswered question is whether a few drops of lemon juice (containing citric acid) will

increase the mass of caffeine present in the coffee. The greatest limitation experienced in the

lab was the extraction of impurities along with the caffeine, and an improvement to the

procedure by using a supercritical CO2 apparatus, followed by infrared spectroscopy would

allow for a more precise means of measuring the caffeine in the coffee solution.

Word Count: 300


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2

Introduction

Context

When I woke up, on a regular school day, I went to the kitchen to make myself some coffee. Iwas feeling particularly tired and I told my mom that I may need to drink two cups of coffee.

She mentioned, however, that I can just brew my coffee using water mixed with some lemon

juice and the coffee is then stronger 1. This left me puzzled. I thought to myself that stronger

coffee would either imply a more bitter taste, or higher caffeine content. I assumed that it was

just the sour taste of the lemon juice that made the coffee taste more bitter and therefore

appear to be ‘stronger’, however I wondered if the latter could also be true. Then, I wondered

what about the lemon juice could possibly increase the coffee’s caffeine content. I realized

that lemons contain citric acid (C6H8O7)2, and this made me wonder: What effect does the pH

of the solution (pH) used for brewing coffee, have on the mass (g) of caffeine extracted? Is ittrue that the lower the pH of the liquid used for brewing the coffee, the higher the caffeine

content of the drink?

Background

The first evidence of coffee consumption dates back to the mid-15th century, in the

monasteries of Yemen. It is now a very popular drink all over the world, both for its taste as

well as its vitalizing abilities3. The ingredient in coffee that acts as a stimulant is called

caffeine (C8H10 N4O2), represented in Figure 1.14.

1"Kawa Z Cytryną, Energetyk Albo Lek Na Gardło - Jak Walczymy Ze Snem I Czym to Grozi." NaTemat.pl . Web. 3 Feb. 2015.

.

2"Citric Acid." PubChem. Web. 3 Feb. 2015. .

3Weinberg, Bennett Alan, and Bonnie K. Bealer. The World of Caffeine: The Science and Culture of the World's Most Popular Drug . New

York: Routledge, 2001. 3. Print.

4"Caffeine." Wikimedia Commons. Web. 3 Feb. 2015. .


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3

When brewing coffee, the caffeine is extracted by dissolving it in the boiling water. The

caffeine can then be extracted from aqueous solutions using a water-immiscible 5 substance

called dichloromethane6. An immiscible solution means that the two liquids in it are not

soluble. The reason dichloromethane and water are immiscible solutions is due to their

polarities. Dichloromethane (CH2Cl2) is a polar molecule with a diagram showing its net

dipoles in Figure 1.2 below7.

5"Immiscible." Merriam-Webster . Merriam-Webster. Web. 3 Feb. 2015. .

6"Dichloromethane." Sigma Aldrich. Web. 3 Feb. 2015. .

7"Difference of Dipole Moments of Dichloromethane and Trichloromethane?" Chemistry Beta. Web. 3 Feb. 2015.

.

Figure 1.1

Figure 1.2

0.3 D

δ- δ+

Net Dipole: 1.54 D

1.56 D


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4

The electronegativity8 between carbon and chlorine atoms is greater than the electronegativity

difference between the carbon and hydrogen atoms. This causes a slight internal dipole within

the molecule, where the hydrogen bonded parts will be more positively charged due to the

larger tendency for electrons to reside closer to the chlorine atoms. In contrast, a water molecule can be seen below in Figure 1.3 below9.

Due to the water molecule’s stronger net dipole10, it is more polar than dichloromethane, and

therefore is less likely to form intermolecular bonds with dichloromethane in the presence of

more water molecules. Another reason for its immiscibility is caused by the preference of

hydrogen bond formation between the water molecules as opposed to dipole-dipole

interactions with the dichloromethane molecules. The caffeine dissolves in the

dichloromethane more effectively, due to caffeine’s polar nature which can be seen in Figure

1.4 below11.

8"Electronegativity and Molecular Dipoles." Yale.edu. Web. 3 Feb. 2015.

.

9 "Do Water Molecules Change When You Talk to Them?" Skeptics. Web. 3 Feb. 2015.

.

10"Polarity of Bonds and Molecules." D. W. Brooks. Web. 3 Feb. 2015.

.

11"Molecular Modelling Analysis of the Metabolism of Caffeine." Science Alert . Web. 3 Feb. 2015.

.

1.5 D 1.5 D

δ+

δ-

Net Dipole:

1.84 D

Figure 1.3


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Caffeine’s polarity allows it to be dissolved in both water and dichloromethane, however it is

the organic solvent that can dissolve this compound more effectively, due to the large non-

polar portion of the caffeine which can bond with the non-polar elements of the

dichloromethane. When dissolved in water, the caffeine mainly interacts by forming hydrogen

bonds with the oxygen atom of the water 1213.

pH stands for potential Hydrogen, and it measures acidity of an aqueous solution14. The

equation in Figure 1.4 can be used to effectively portray pH. The lower the pH, the more

acidic a solution, due to the increased concentration of hydrogen ions.

pH = -log[H+]

12 "What Are the Properties of the Caffeine Molecule?" MadSci Network . Web. 3 Feb. 2015. .

13"Hydrogen Bonding, Dipole-Dipole & Ion-Dipole Forces: Strong Intermolecular Forces." Education Portal . Web. 3 Feb. 2015.

.

14Helmenstine, Ph.D. "PH Definition." About.com. Web. 3 Feb. 2015. .

Figure 1.5

Figure 1.4

δ+ δ-


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Caffeine is weakly basic in solution. (pKa = 0.52±0.70)15. This is most likely due to a nitrogen

in the compound that can act as a proton acceptor. The possible mechanism for the reaction

can be seen below in Figure 1.6.

H+ (aq) + C8H10 N4O2 (s) → [C8H11 N4O2]+ (aq)

As seen above, caffeine can be protonated16, or react with an acid to produce its conjugate

acid. The resulting conjugate acid is a polyatomic ion. Ionic compounds are more readily

dissolved in polar solvents, such as water, due to their electrostatic attraction to either the

positive or negative poles of the solvent molecule. In the case of caffeine’s conjugate acid, it

is the negative end of the water molecule. Therefore if the protonation of the caffeine is

successful, then more of the caffeine should dissolve in the water when brewing the coffee.

In this experiment, the pH will be changed by using different concentrations of hydrochloric

acid (HCl). Increasing the HCl concentration will increase the potential hydrogen ion

concentration in the coffee solution, which should protonate more caffeine and cause it to

more readily dissolve in the water. However, ionized caffeine will not dissolve as well in the

dichloromethane. This is because the protonation of caffeine will allow for a strong

interaction between the positively charged nitrogen atom in the caffeine with the relatively

15"Caffeine." ChemSpider . Web. 3 Feb. 2015. .

16"Illustrated Glossary of Organic Chemistry - Protonate." Chem UCLA. Web. 3 Feb. 2015.

.

H+

H

+

Figure 1.6


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7

negative oxygen atom in the water molecule. Therefore, a lower mass of caffeine extracted in

the experiment implies a lower amount of caffeine dissolved in the dichloromethane, and the

assumption is made that the majority of the caffeine became protonated and stayed dissolved

in the water instead of migrating to the dichloromethane.

Significance

The significance of this question leads to either proving or disproving the effect of lemon

juice on the caffeine content of a beverage. Although HCl is corrosive and cannot be ingested,

it used to illustrate the relationship between pH and caffeine extraction, and therefore this

experiment cannot fully determine the effect of lemon juice on the caffeine content of a cup of

coffee. However, if the pH does correlate with caffeine extraction, then it is assumed that

lemon juice will also affect the caffeine extraction due to its acidic pH. This could prove to be

very efficient when attempting to brew coffees with a higher caffeine content. A significant

difference in the mass of caffeine extracted with different pH levels due to increases in acid

concentrations could potentially serve as useful information for consumers of coffee. An

example would be addition of lemon juice or other acidic products to the water used for

brewing coffee, to ensure the creation of a more caffeinated beverage.

Method

Materials

Coffee powder, weighing boat, analytical digital balance (±0.0001 g), thermometer (±0.1ºC),

10ml (±0.05 ml) and 50ml (±0.5 ml) graduated cylinder, 6M HCl stock solution, Distilled

water, dichloromethane (CH2Cl2), 250 ml Erlenmeyer flask, ring stand, filter paper, funnel,

ring stand, graduated 10 ml pipette (±0.05 ml), hot plate, fume hood, 25 ml Beakers, 150 ml

Beaker, 1L volumetric flask, gloves, safety goggles

Methodology

In this lab, dilutions will be necessary to create the optimal molarities of acid needed. To do

this the following equation must be used:

ncv

Figure 2.1 contains 3 variables, c representing concentration (in this case of H+ ions in mol d

m-3), v is volume in L, and n is moles. All dilutions will be performed with a 6.0M HCl stock

solution and distilled water. The assumptions made in this experiment are that the coffee in

each bag has the same amount of caffeine. Also, for the purpose of this experiment it is

assumed that only caffeine will dissolve in the dichloromethane, and therefore the impurities

will be neglected and assumed to be part of the caffeine powder. The independent variable in

this experiment will be the concentration (M) of HCl acid used in each trial, and the

Figure 2.1


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dependent variable will be the mass of caffeine extracted using the dichloromethane

procedure. The controlled variables include: the mass (20.00g±0.01g) of coffee used per trial,

to ensure equivalent amounts of caffeine available for extraction during the brewing process,

the volume (20.0ml±0.1ml) of dichloromethane used to control the amount of caffeine thatwill dissolve into the organic solvent, and the temperature (101ºC±1ºC) and time

(300sec±10sec) of the boiling were controlled to maintain the same conditions for the

extraction of coffee into the solution for each trial.

Dichloromethane Procedure

This procedure is a modified version of an online caffeine extraction procedure17 without the

use of anhydrous sodium sulfate or a distillation apparatus.

Obtain the coffee powder, begin massing 3 sets of 20.00g±0.01g then place them in their

respective 150ml beaker. Fill the beakers with 100ml of distilled water using the 50mlgraduated cylinder (100ml±1ml). Connect and turn on the hot plate(s), making sure that the

setting on the hot plates are the same (in this experiment the setting ‘4’ was used). Place the

beaker(s) with the coffee solution onto the hot plate(s) and begin to stir them. As the solution

in the beaker begins to boil, start the stopwatch (±0.1sec) and wait for 300 seconds±10

seconds. Use a thermometer to ensure that the solutions are boiling at the same temperature

(101ºC±1ºC). Using a side arm flask, tubing, a Buchner funnel and filter paper, assemble a

vacuum filtration system. Pour the contents of the beaker into the Buchner funnel and begin

the vacuum filtration. After all the liquid has been filtered, transfer the solution into a

separatory funnel (100ml). CAUTION. Dichloromethane is a volatile carcinogenic substanceand should be handled with care. Add 20ml of dichloromethane, using the 10 ml±0.1ml

graduated cylinder, into the separatory funnel (20.0ml±0.1ml). Place a stopper on the top

opening of the separatory funnel and tilt it back and forth, mixing the solution, however not to

violently to prevent an emulsion from forming. Allow the solution to then sit until it forms

two distinct layers, the clear dichloromethane solution on the bottom and the coffee solution

on the top. Mass a 25ml beaker. Decant the bottom solution using the separatory funnel, into

the massed beaker. Inside of the fume hood, boil away all the dichloromethane on a hot plate

(at setting ‘3’), and once the solution has boiled away, allow the beaker to sit in the fume

hood for 3 minutes. Allowing it sit will ensure that any remaining solution will boil away, and

will prevent toxic fumes from escaping. Mass the beaker on the analytical digital balance

(±0.0001 g) to determine the mass of the solid caffeine. 3 trials should be performed for each

treatment, and the remaining 4 treatments are 4M, 1M, 0.5M and 0.25M HCl acid instead of

distilled water. CAUTION. Handle acids while wearing gloves and safety goggles as they are

17"Extraction of Caffeine." Indiana State University. Indiana State University. Web. 3 Feb. 2015.

.


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corrosive. The molarities were achieved through dilutions of a 6M stock solution, for which

the procedure can be seen below.

Dilution procedure

The 6M stock solution is to be diluted in order to produce enough 4M, 1M, 0.5M, and 0.25M

HCl acid for 3 trials. The dilution should be performed using an appropriate volumetric flask

(500ml flasks were used, as only 300ml of acid was needed per treatment) and a graduated

cylinder (50.0ml±0.5ml) to measure out and pour the acid. CAUTION. It is important to pour

a volume of distilled water equal to that of the acid, before adding the acid. This ensures that

the strongly exothermic neutralization of the acid does not result in a splash back.

Calculations for dilutions are present in Appendix A.

Data Presentation

Results

4M (~pH -0.6) 1M (pH 0) 0.5M (0.3 pH) 0.25M (0.6 pH) Distilled Water

(pH 7)

Mass of

caffeine

(Average)

0.0148g 0.0476g 0.1145g 0.1367g 0.1187g

Uncertainty 0.0071g 0.0175g 0.0382g 0.0844g 0.0226g


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Raw data table along with uncertainties, calculations and qualitative remarks present in

Appendix A.

Analysis

The data shows that an increase in acid concentration seems to decrease the amount of

caffeine extracted. The data follows a consistent negative trend, except for the distilled water

treatment that resulted in a lower caffeine mass than the 0.25M treatment. The uncertainty of

the treatments also decreased as acid concentration increased, with the exception of distilled

water, whose uncertainty is lower than the 0.25M and 0.5M treatments.

Evaluation

In the study, the expected results were that an increase in concentration of acid would increase

the mass of caffeine extracted. This is because, through protonation of the caffeine, it would

be more soluble in water and therefore more could be extracted through the experiment.

However, the trend seen on the graph seems to indicate the contrary, the mass of caffeine

extracted seemed to increase at lower acid concentrations (higher pH). This is most likely

because the solvent used for the caffeine extraction was dichloromethane, an organic solvent,

less polar than water. At high acid concentrations (low pH) the caffeine became protonated in

the solution. It readily dissolved in the water due to its increased polarity in the ionic state.

However, when the coffee solution with the dissolved protonated caffeine was mixed with

dichloromethane, only small amounts of the ionic caffeine dissolved in the organic solvent

because ionic compounds are more easily dissolved by strongly polar compounds like water.This would indicate that the lower the amount of caffeine extracted from the dichloromethane,

means a high concentration of ionized caffeine dissolved in the coffee solution, due to a

decrease in migration of caffeine from the water to the organic solvent. This means that the

data supports the expected results, assuming that the protonation theory is in fact correct.

Assumptions

One of the largest assumptions that was necessary to undertake was the effect of pH on the

extraction of impurities from caffeine. As an organic solvent, dichloromethane will dissolve

other compounds that were dissolved in the coffee solution aside from caffeine. This explainsthe dark brown/orange color of the powder after the extraction. It is very possible that low pH

decreased the solubility of these compounds in water, and therefore the caffeine powder

contained more impurities as pH increased, and a higher mass of solid was achieved. Another

assumption made is that a significant portion of the caffeine will be protonated. The pKa

value for caffeine is (pKa = 0.52±0.70), meaning that the Kb (base dissociation constant)

ranges between 6.61x10-15 and 1.66x10-13. These values are very small, and therefore very

little of the caffeine will be protonated. Another assumption is the uncertainty associated with

the 6M stock solution of HCl acid. Due to being prepared by a technician, it was assumed that

the value is exact. Finally, a previously stated assumption is that bag of coffee powder used


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contained an equal amount of caffeine per gram of coffee. This is an assumption that cannot

be fully evaluated and controlled.

Limitations and Improvements

The large uncertainty values in the averages most likely occurred due to many factors in the

experiment. One of them was the formation of air bubbles in the separatory funnel, making it

difficult to effectively decant the two solutions. The amount of air bubbles formed by the

solution seemed to increase as the concentration of the acid decreased, explaining the reason

for an increase in uncertainty values across the table, however the air bubbles did not seem to

appear in the distilled water trial. The next difficulty arose due to the oxidation and

combustion of the solid caffeine while boiling the dichloromethane. Often the leftover

caffeine residue would appear black, as it was most likely combusted due to being heated for

too long. This caused the mass of the caffeine to be lower than the remainder of the trials. The

last problem was caused by the spill of coffee solution from the separatory funnel due to

adding the dichloromethane too rapidly. This occurred in one trial and was avoided from then

on out. Improvements to the procedure would include a lower hot plate setting used when

boiling the dichloromethane to decrease the likely hood of combustion or oxidation of the

caffeine. During multiple trials, the solid caffeine was combusted because the

dichloromethane had boiled away and the caffeine was subjected to heat and oxygen. Trials

with a black powder residue showed increased mass, and gave an inaccurate comparison to

the other trials. Using a more effective means of extracting the caffeine with less impurities

and testing the substance to ensure it is caffeine would be another improvement. The use of

dichloromethane as an organic solvent to extract the caffeine was not an efficient method of obtaining the caffeine, due to the impurities that were also dissolved in the solution and

remained in the beaker after boiling the dichloromethane away. A more effective means of

extracting the caffeine would have been the use of the Supercritical Fluid Extraction18, a

frequently used method of extracting organic compounds that is also used industrially for

decaffeination. Finally, to ensure that the remaining solid powder after boiling the organic

solvent was in fact caffeine, the use of infrared spectroscopy would allow testing for the

functional groups present in caffeine. Infrared Spectroscopy19 is a means of identifying the

functional groups in an organic compound by analyzing the wave length frequencies being

absorbed by it. Different functional groups have certain literature values of wavelengthfrequencies that they will absorb, and utilizing this information and comparing it to the

wavelengths absorbed by the functional groups in the caffeine would allow identification of

the extracted powder is in fact caffeine. The limitation to this improvement however is the

18"Supercritical CO2 Extraction System." Apeks Supercritical . Web. 3 Feb. 2015. .

19"Infrared Spectroscopy." Michigan State University. Michigan State University. Web. 3 Feb. 2015.

.


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fact that the extracted powder is brown/orange and not white, therefore it is visually certain

that the extract contains impurities.

Conclusion

Unresolved Questions

The application of the data is highly theoretical, and not very applicable to everyday life. This

is because the HCl acid is a strong acid, meaning that it dissociates fully in water and it exists

as ions in solution. However the acid in lemon juice (citric acid) is a weak acid. It does not

dissociate fully (pKa=3.08)20 and therefore cannot protonate the caffeine as effectively. Using

the pKa value, and the estimated concentration of citric acid in lemon juice (0.3 mol dm-3)21 it

is estimated that the pH at STP of citric acid is roughly 1.80 (Calculations provided in

Appendix A). Without protonation, the caffeine remains unchanged in its net-charge and does

not dissolve more effectively in the water when brewed. Also, the volume of citric acid used

in brewing coffee (3 drops, equivalent to roughly 2 ml) will not be as acidic as 100 ml of

strong acid. Therefore, the unresolved question is whether such as small quantity of weak acid

will in fact have an effect on the difference in caffeine extraction.

Further Investigation

Aside from exploring the effect of much smaller volumes of a weaker acid, other factors

could be tested to determine the change in caffeine extraction. These factors include

temperature, time of brewing, increased collision frequency by changing surface area of the

coffee powder or stirring speed. In combination with pH, this could allow for brewing coffee

with a much higher caffeine content, and could be useful if the consumer desired a more

stimulating beverage.

20"Table of Acids with Ka and PKa Values." UC Santa Barbara. UC Santa Barbara. Web. 3 Feb. 2015.

.

21"Citric Acid." Wikipedia. Wikimedia Foundation. Web. 3 Feb. 2015. .


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Works Cited

"Caffeine." Wikimedia Commons. Web. 3 Feb. 2015. .

"Caffeine." ChemSpider . Web. 3 Feb. 2015. .

"Citric Acid." PubChem. Web. 3 Feb. 2015.

.

"Citric Acid." Wikipedia. Wikimedia Foundation. Web. 3 Feb. 2015. .

"Dichloromethane." Sigma Aldrich. Web. 3 Feb. 2015.

.

"Difference of Dipole Moments of Dichloromethane and Trichloromethane?" Chemistry Beta. Web. 3 Feb. 2015.

.

"Dipole Moments." UCDavis Chemwiki. Web. 3 Feb. 2015.

.

"Do Water Molecules Change When You Talk to Them?" Skeptics. Web. 3 Feb. 2015.

.

"Electronegativity and Molecular Dipoles." Yale.edu. Web. 3 Feb. 2015.

.

"Extraction of Caffeine." Indiana State University. Indiana State University. Web. 3 Feb. 2015.

.


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14

Helmenstine, Ph.D. "PH Definition." About.com. Web. 3 Feb. 2015.

.

"Hydrogen Bonding, Dipole-Dipole & Ion-Dipole Forces: Strong Intermolecular Forces." Education Portal .

Web. 3 Feb. 2015. .

"Illustrated Glossary of Organic Chemistry - Protonate." Chem UCLA. Web. 3 Feb. 2015.

.

"Immiscible." Merriam-Webster . Merriam-Webster. Web. 3 Feb. 2015. .

"Infrared Spectroscopy." Michigan State University. Michigan State University. Web. 3 Feb. 2015.

.

"Kawa Z Cytryną, Energetyk Albo Lek Na Gardło - Jak Walczymy Ze Snem I Czym to Grozi." NaTemat.pl .

Web. 3 Feb. 2015. .

"Molecular Modelling Analysis of the Metabolism of Caffeine." Science Alert . Web. 3 Feb. 2015.

.

"Polarity of Bonds and Molecules." D. W. Brooks. Web. 3 Feb. 2015.

.

"Supercritical CO2 Extraction System." Apeks Supercritical . Web. 3 Feb. 2015.

.


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15

"Table of Acids with Ka and PKa Values." UC Santa Barbara. UC Santa Barbara. Web. 3 Feb. 2015.

.

Weinberg, Bennett Alan, and Bonnie K. Bealer. The World of Caffeine: The Science and Culture of the World's

Most Popular Drug . New York: Routledge, 2001. 3. Print.

"What Are the Properties of the Caffeine Molecule?" MadSci Network . Web. 3 Feb. 2015.

.


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Appendix A

Trial

(g±0.0001g) 4.0M±0.1M 1M±?M 0.5M±?M 0.25M±?M Distilled Water

Initial

Mass

Final

Mass

Initial

Mass

Final

Mass

Initial

Mass

Final

Mass

Initial

Mass

Final

Mass

Initial

Mass

Final

Mass

149.7102 49.7199 32.1375 32.1680 28.3487 28.4587 50.5278 50.7489 51.3422 51.4443

0.0097 0.0305 0.1100 0.2211 0.1021

2 50.5093 50.5312 30.0080 30.0731 30.0045 30.0852 51.2699 51.3620 50.5087 50.6500

0.0219 0.0651 0.0807 0.0921 0.1413

351.2697 51.2825 28.2695 28.3166 29.5300 29.6827 49.7076 49.8045 28.2690 28.3816

0.0128 0.0471 0.1527 0.0969 0.1126

Average 0.0148g±0.0071g 0.0476g±0.0175g 0.1145g±0.0382g 0.1367g±0.0844g 0.1187g±0.0226g

Qualitative Remarks

Trial 4M 1 0.5M 0.25M Distilled Water

1

Rapid bubble

formation in

dichloromethane

after addition of

coffee. Air

bubbles make it

difficult to decant.

Minor

dichloromethane

solution

explosion under

the fume hood

- Dark brown

powder

No air bubble

formation in the

separatory

funnel

2

Black, ashy

residue instead of

orange like

powder.

Spill of

dichloromethane

solution when

pouring in coffee,

through a rapid

formation of

emulsion.

- - -

3 - Very dark brown

residue

Dark brown

residue - -


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Sample massing calculations

4M:

Trial 1:

Beaker’s initial mass = 49.7102±0.0001 g, Beaker’s final mass = 49.7199±0.0001 g

49.7199±0.0001 g - 49.7102±0.0001 g = 0.0097±0.0002 g (mass of caffeine)

Trial 2:



Trial 3:



Average for Trials 1-3 in 4M treatment: [(0.0097±0.0002 g) + (0.0219±0.0002 g) +

(0.0128±0.0002 g)] / 3 = 0.0148±0.0071 g (Uncertainty as a result of greatest difference

between average from the range of data points)

0.0219 - 0.0148 = 0.0071 (greatest uncertainty used)

0.0097 - 0.0148 = -0.0051

0.0128 - 0.0148 = -0.0020

Sample HCl dilution

c1v1 = c2v2

(6 mol dm-3)v1 = (4 mol dm-3)(0.5000L±0.0002L)

v1 = 0.3334L±0.0002L

Therefore 334 ml of acid must be diluted with 500ml-334ml (166ml) of distilled water to

receive 0.5000L of 4M HCl acid.

3 x 100.0ml±0.5ml graduated cylinder used to transfer the acid therefore uncertainty is

300.0ml±1.5ml

34ml added using a 10.00ml±0.05ml graduated cylinder therefore 34.0ml±0.2ml

300.0ml±1.5ml + 34.0ml±0.2ml = 334.0ml±1.7ml


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The uncertainty of the volumetric flask is negligible, therefore the volumetric flask contains

500ml±1.7ml and since 6M is certain, the uncertainty of the prepared solution is...

(1.7ml/500ml)*100 = 0.34%

4M±0.34% = 4.000M±0.136M and with sig figs 4.0M±0.1M

Sample concentration to pH conversion

A strong acid will fully dissociate in solution, meaning its concentration is equal to the [H+]

therefore -log[HCl] = pH

Sample: -log(4.0 mol dm-3) ≈ -0.60

Citric Acid pH

pKa = 3.08

[H3C6H5O7] (citric acid) = 0.3 mol dm-3

pKa = -log(Ka)

10-3.08 ≈ 8.32 x 10-4

According to weak acid equilibrium dissociation:

pKa = [H+][H2C6H5O7-]/[H3C6H5O7]

8.32 x 10-4 = x2 / (0.3 - x) However x on the bottom is negligible

sqrt[(8.32 x 10-4)(0.3)] = x

x ≈ 1.57 x 10-2

pH = -log(x) where x is [H+]

-log(1.57 x 10-2) ≈ 1.80

effect of ph on caffeine extraction

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