Determination of Phosphoric Acid Content in Cola Beverages

Nicole Pavlichich & Michele Awad

April 10/17/24, 2012

CHEM 1516L

Tuesday 11:00 am – 2:00 pm

ABSTRACT

In order to determine the phosphoric acid content in cola beverages, an acid base titration

was used. A standardized sodium hydroxide solution was titrated against a known volume of

decarbonated cola to determine the pKa, Ka, and molarity of the phosphoric acid content.

Measures were taken with a drop counter and pH probe for Pepsi, Dr. Pepper, and Coke. Four

trials were run for each solution in which the best three were recorded. It was found that Pepsi is

the most basic cola whereas Coke was found to be the most acidic. Furthermore, the

concentration of phosphoric acid was greatest in the Coke and smallest in the Pepsi.

INTRODUCTION

If one were to select a cola beverage from any given vending machine, few things are

certain. Whether the choice is Coke, Pepsi, or Dr Pepper, many ingredients are guaranteed to

overlap. The focus here is on the ingredient phosphoric acid. Phosphoric acid, H3PO4, has many

applications; two of which are adding a sour flavor to cola drinks and serving as a preservative.

It is composed of three hydroxide groups and an oxygen atom bonded to a phosphorus atom.

Though the concentration of phosphoric acid in any given soda is small, larger

concentrations are used in dentistry to clean plaque off of teeth.1 However, it is shown that

excessive consumption of any soda beverages erodes the enamel from the teeth from repetitive

exposure to acidity. The average American consumes approximately 50 gallons of soda in a year

which translates to over 500 cans total and almost 2 cans per day. 2 Though phosphoric acid has

many practical implications, it also has been shown to provide negative health effects.

Phosphorus aids in the absorption of calcium; however, too much phosphorus renders the

intestines incapable of absorbing calcium. This leads to calcium being broken down from the

1 Khosravi, K., E. Ataei, M. Mousavi, and N. Khodaeian. "Effect of Phosphoric Acid Etching of Enamel Margins on the Microleakage of a Simplified All-in-One and a Self-etch Adhesive System." Operative Dentistry 34.5 (2009): 531-36.2 Luntz, Frank I. What Americans Really Want . . . Really: The Truth about Our Hopes, Dreams, and Fears. New York: Hyperion, 2009.

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bones, causing a decrease in bone mineral density.3 In this magnitude, it is important to discover

the acidity of the cola in which Americans have become so accustomed to drinking through acid-

base titrations.

Acid-base titrations are a chemical technique used to determine the concentration of a

known volume of a solution by adding a known volume of a known concentration of a different

solution. If the concentration of the base is known, one can calculate the concentration of the

acid by determining the equivalence point. By monitoring the pH throughout the titration, the

resulting titration curve can be used to determine the equivalence point. At the equivalence

point, the number of moles of the acid is equal to the number of moles of the base. Also at the

midpoint, pH = pKa.

The experiment was set up as an acid-base titration using a known concentration of

sodium hydroxide. Data was collected via the use of a Microlab system connected to a pH probe

and a drop counter. Therefore, the purpose of this experiment was to determine the pH, Ka, and

pKa of Coke, Pepsi, and Dr Pepper by use of titration.

EXPERIMENTAL PROCEDURE

Before the titration could take place, preparatory experiments were conducted. The first

preparatory experiment was the dilution of sodium hydroxide. A large Florence flask was filled

with de-ionized water (300 mL) to which concentrated sodium hydroxide (8 mL, 6M) was added.

This solution was then standardized by obtaining two burets filled with the sodium hydroxide

solution (50 mL) and a hydrochloric acid solution (50 mL, 0.203M). Some HCl (15 mL) was 3 Rienecke, Kimberly. "The Effects of Phosphoric Acid in Soda on Your Body." LIVESTRONG.COM. The Lance Armstrong Foundation, 26 July 2011.

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drained into an Erlenmeyer flask and the initial and final readings were taken. Phenolphthalein

indicator (4 drops) was added to the flask. The sodium hydroxide was titrated into the acidic

solution until the solution was a light pink color, showing the equivalence point. This titration

was repeated for three trials in which the molarity (0.171M) was found, as shown in Table 1.

The secondary preparatory experiment was the calibration of a drop counter. The drop

counter was attached to a Microlab system. NaOH (~2.0 mL) was drained from the buret drop

wise through the counter. By comparing the volume of NaOH drained with the number of drops

counted, the volume (4.66 x 10-2 mL) of each drop was calculated as shown in Table 2.

The third preparatory experiment was the calibration of a pH probe. To do so, the probe

was connected to the Microlab system. The probe was rinsed with de-ionized water, dried, and

submerged into a buffer solution (pH 4.0). The probe was rinsed, dried again, and then

submerged into a second buffer solution (pH 7.0).

The final preparatory experiment was the decarboxylation of the soda sample to remove

the carbonic acid from the sample so that only phosphoric acid was present. This was done by

boiling each solution until there were no more bubbles remaining, and in essence, the soda was

“flat”.

The setup consisted of an upright burette containing the standardized NaOH solution

(0.171M) above a beaker containing the solution. Between the two were a drop counter and a

pH probe which were both connected to a Microlab system for analysis. The first solution,

Pepsi, was titrated against the standardized NaOH solution (0.171M) by adding the base drop

wise into the Pepsi (10.00 mL). This was done until the reaction passed the equivalence point.

This was conducted for a total of four trials in which the best three were recorded. The data of

this is shown in Table 3. The second solution, Coke, followed the same experimental procedure.

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However, the Coke was too acidic to begin. Therefore, deionized water (49.00 mL) was added

to the dilute the Coke (1.00 mL). The data of this is shown in Table 4. The Dr. Pepper solution

followed the same procedure as the Pepsi solution except for the amount of the cola used (20.00

mL). The data of this trial is shown in Table 5.

RESULTS

Table 1: Standardization of NaOH Solution

Trial 1 Trial 2 Trial 3Initial Volume of HCl, mL 49.80 34.70 19.70Final Volume of HCl, mL 34.70 19.70 4.70Volume of HCl used, mL 15.10 15.00 15.00Initial Volume of NaOH, mL 47.30 48.70 30.90Final Volume of NaOH, mL 29.40 30.90 13.05Volume of NaOH used, mL 17.90 17.80 17.85Molarity of NaOH 0.171M 0.171M 0.171MAverage Molarity of NaOH 0.171M

Table 2: Calibration of Drop Counter

Trial 1 Trial 2 Trial 3Final Volume of NaOH Titrant, mL 37.20 35.20 33.20Initial Volume of NaOH Titrant, mL 39.20 37.20 35.20Volume of NaOH added, mL 2.00 2.00 2.00Number of Drops Added 43 45 41Volume per Drop, mL 4.65 x 10-2 4.44 x 10-2 4.88 x 10-2

Average Volume per Drop, mL 4.66 x 10-2

Table 3: Titration of Pepsi

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Trial 1 Trial 2 Trial 3 Trial 4Volume of Cola, mL 10.00 10.00 10.00 10.00Drops NaOH at EP 22.00 13.00 18.00 13.00Volume NaOH added at EP, mL 1.03 0.606 0.839 0.606pH at EP 10.28 8.35 7.82 8.46Drops NaOH at MP 12.00 4.00 5.00 4.00Volume NaOH added at MP, mL 0.559 0.186 0.233 0.186pH at MP 8.53 4.45 2.02 4.41pKa Value 8.53 4.45 2.02 4.41[H+] at MP, moles/L 2.95 x 10-9 3.55 x 10-5 9.55 x 10-3 3.89 x 10-9

Ka Value 2.95 x 10-9 3.55 x 10-5 9.55 x 10-3 3.89 x 10-9

Table 4: Titration of Coke

Trial 1 Trial 2 Trial 3 Trial 4Volume of Cola, mL 1.00 1.00 1.00 1.00Drops NaOH at EP 34.00 9.00 28.00 160.00Volume NaOH added at EP, mL 1.584 0.419 1.303 7.456pH at EP 5.24 5.54 5.09 5.87Drops NaOH at MP 15.00 3.00 8.00 60.00Volume NaOH added at MP, mL 0.699 0.140 0.373 2.796pH at MP 0.500 1.53 2.69 3.11pKa Value 0.500 1.53 2.69 3.11[H+] at MP, moles/L 0.316 2.95 x 10-2 2.04 x 10-3 7.76 x 10-4

Ka Value 0.316 2.95 x 10-2 2.04 x 10-3 7.76 x 10-4

Table 5: Titration of Dr. Pepper

Trial 1 Trial 2 Trial 3 Trial 4Volume of Cola, mL 20.00 20.00 20.00 20.00Drops NaOH at EP 46 50 30 31Volume NaOH added at EP, mL 2.144 2.33 1.398 1.445pH at EP 6.00 6.15 6.20 6.85Drops NaOH at MP 4 6 4 4Volume NaOH added at MP, mL 0.186 0.280 0.186 0.186pH at MP 2.09 2.05 2.27 2.60pKa Value 2.09 2.05 2.27 2.60[H+] at MP, moles/L 8.13 x 10-3 8.91 x 10-3 5.37 x 10-3 2.51 x 10-3

Ka Value 8.13 x 10-3 8.91 x 10-3 5.37 x 10-3 2.51 x 10-3

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Table 6: Average Values

Pepsi Coke Dr. PepperVolume of Cola, mL 10.00 1.00 20.00Drops NaOH at EP 14.67 23.67 36.67Volume NaOH added at EP, mL 0.684 1.102 1.662pH at EP 8.21 5.29 6.35Drops NaOH at MP 4.33 8.66 4.00Volume NaOH added at MP, mL 0.202 0.404 0.186pH at MP 3.63 1.573 2.32pKa Value 3.63 1.573 2.32[H+] at MP, moles/L 2.34 x 10-4 0.116 4.79 x 10-3

Ka Value 2.34 x 10-4 0.116 4.79 x 10-3

Molarity of H3PO4 1.17 x 10-2 0.188 1.42 x 10-2

Figure 1: Titration Curve of Pepsi: The equivalence point is found midway through the rise which was found to be on average at a pH of 8.21 and 0.684 mL of NaOH.

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Figure 2: Titration Curve of Coke: The equivalence point is found midway through the rise which was found to be on average at a pH of 5.29 and 1.102 mL of NaOH.

Figure 3: Titration Curve of Dr. Pepper: The equivalence point is found midway through the rise which was found to be on average at a pH of 6.35 and 1.662 mL of NaOH.

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CALCULATIONS

Calibration

M NaOH=( M HCl ) ×(mL HCl)

mL NaOH=

(0.203 M ) ×(15.10 mL)17.90 mL

=0.171 M

Volume per drop= mL NaOHnumber of drops

= 2.00 mL43 drops

=4.65 x10−2 mL /drop

Pepsi

Drops NaOH=4.66 x10−2mLdrop

x 13 drops=0.606 mL

p Ka=pH (midpoint )=4.45¿

Ka=antilog (−pK a )=antilog (−4.45 )=2.95 x10−2

Coke

Drops NaOH=4.66 x10−2mLdrop

x 9 drops=0.419 mL

p Ka=pH (midpoint )=1.53¿

Ka=antilog (−pK a )=antilog (−1.53 )=2.95 x 10−2

Dr. Pepper

Drops NaOH=4.66 x10−2mLdrop

x 46 drops=2.144 mL

p Ka=pH (midpoint )=2.09¿

Ka=antilo g (−pKa )=antilog (−2.09 )=8.13 x10−3

Molaritymol NaOH=( M NaOH ) ( L NaOH )=(0.171 M ) (0.000684 L )=1.17 x 10−4 mol NaOH

mol H 3 PO4 (at EP)=mol NaOH=1.17 x 10−4 mol H3 PO4

M H 3 PO4=mol H3 PO4

L H 3 PO4

=1.17 x 10−4 mol H 3 PO4

1.0 x10−2 L H 3 PO4

=1.17 x 10−2 M H 3 PO4

DISCUSSION

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The purpose of the experiment was to determine the pH, pKa, and Ka of Pepsi, Dr.

Pepper, and Coke through use of acid-base titration. Overall, the experiment was generally

successful, though there were a few sources of error which will be discussed later. Table 6

portrays the average values for each of the solutions. The true pKa value of phosphoric acid is

2.15 whereas the true Ka value is 7.1 x 10-3 4.

Of the three solutions, the Pepsi had the least number of complications. The first trial

was disregarded due to abnormal data; however, the second, third, and fourth trials had similar

data which yielded good results. The exclusion is highlighted in yellow. These results, as shown

in Table 3, show that Pepsi is the most basic of the three colas (pH 3.63). By finding the number

of moles of sodium hydroxide used which equals the number of moles of phosphoric acid at the

equivalence point, the molarity of the phosphoric acid was found (1.17 x 10-2 M). This shows

that the concentration of the acid in the cola is very weak which is important because too

concentrated of an acid would lead to bodily harm if ingested. Since the solutions were not pure

acids, the titration curves are not as distinct. Figure 1 demonstrates the titration curve for Pepsi.

Since the sodium hydroxide was significantly more basic than the Pepsi was acidic, the

equivalence point was in a basic range (pH 8.21). The equivalence point is different from the

neutrality point in which the pH would be 7.00. Since the pH is the same as the pKa at the

midpoint, the pKa was found (3.63) to be higher than the true pKa for pure phosphoric acid

(2.15). Since stronger acids have higher pKa values, this shows that pure phosphoric acid is a

stronger acid than the phosphoric acid in Pepsi which is what was to be expected.

The second solution, Coke, had to be diluted in order for the titration to be successful.

Though the total volume of the solution was 50mL, there was only 1 mL of Coke used for each

trial. Even with this dilution, Coke was found to be the most acidic at the midpoint (pH 1.57)

4 Ripin, D. H., and D. A. Evans. "pKa's of Inorganic and Oxo-Acids." Harvard, 4 Nov. 2005.

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and was still acidic at the equivalence point (pH 5.29). As shown in Table 4, the fourth trial was

discarded due to outlying values, as highlighted in yellow. The equivalence point, as shown in

Figure 2, was not as clearly defined as that of the Pepsi solutions. This could be because of the

dilution. Using the equivalence point, the molarity of the phosphoric acid in Coke was found

(0.188M). This shows that Coke has the strongest phosphoric acid content of the three colas.

The midpoint was more difficult to determine by looking at the graph as shown in Figure 4.

However, the midpoint was still found to be very acidic. This is evident in the rumors that Coke

can take rust off of pennies and cars. The pKa value (1.57) of the Coke is less than that of pure

phosphoric acid (2.15); however, this could be due to human error.

The third solution tested was Dr. Pepper. To get more accurate readings by having the

pH probe more submerged, we increased the amount of cola used (20.00 mL); however, it did

not need to be diluted like the Coke. As shown in Table 5, the second trial was omitted from

average values, as highlighted in yellow. Dr. Pepper values were discovered to be in between

those of Coke and Pepsi in all aspects. The pH of the Dr. Pepper at the equivalence point (pH

6.35), though still acidic, was much closer to the neutrality point than the other solutions.

Furthermore, the pKa value of Dr. Pepper (2.32) was the closest to that of pure phosphoric acid

(2.15). The molarity (0.0142M), calculated from the equivalence point shown in Figure 3, was

in between the molarity of Coke and Pepsi.

In comparison of the three colas, Coke is the most acidic whereas Pepsi is the least acidic.

Practical applications and knowledge of this are important for individuals with oral or peptic

ulcers. Gastroesophageal reflux disease (heart burn) is caused by stomach acid going upwards

into the esophagus. A treatment for this is to decrease acid intake which in turn decreases the

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acidity of the stomach contents5. Therefore, if someone with GERD really wanted to drink a

cola, it is better for them to drink Pepsi instead of Coke or Dr. Pepper.

Sources of error were present throughout the experiment. To make the results more

accurate, each solution should have been tested in the same quantity. Therefore, each solution

should have been diluted like the Coke solution for best comparison. Furthermore, the drop

counter malfunctioned at times and there is a good possibility that it did not count every drop

titrated into the solution which would affect the volume of sodium hydroxide used. Moreover,

the pH probes used initially recorded a negative pH of the diluted Coke solution which may have

contributed to error. It is also possible the buffer solutions used to calibrate the pH probe were

contaminated. In many of the titration curves, the midpoint and equivalence point were not

clearly defined; therefore, it is possible that the true points were slightly different. Since dark

colas contain not only phosphoric acid but carbonic acid as well, the colas had to be boiled to

remove the carbonic acid. However, if some carbonic acid remained in the cola solution, it could

vary the pH.

ACKNOWLEDGEMENTS

I would like to thank my lab partner, Michele Awad, for helping me so much in

chemistry lab. She kept me from throwing equipment out the window on the sixth floor when it

would not function properly (which happened more often than not). I would also like to thank

Cory Muraco, my teaching assistant, for not throwing large quantities of any dangerous

5 Dean, Brian. "Ulcer and Heartburn: The Foods to Avoid." LIVESTRONG.COM. The Lance Armstrong Foundation, 26 Oct. 2010.

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chemicals on me due to the large volume of questions I ask him in any given lab. This, again,

was a frequent occurrence.

REFERENCES

Khosravi, K., E. Ataei, M. Mousavi, and N. Khodaeian. "Effect of Phosphoric Acid Etching of

Enamel Margins on the Microleakage of a Simplified All-in-One and a Self-etch

Adhesive System." Operative Dentistry 34.5 (2009): 531-36.

Luntz, Frank I. What Americans Really Want . . . Really: The Truth about Our Hopes, Dreams,

and Fears. New York: Hyperion, 2009.

Rienecke, Kimberly. "The Effects of Phosphoric Acid in Soda on Your Body."

LIVESTRONG.COM. The Lance Armstrong Foundation, 26 July 2011.

Ripin, D. H., and D. A. Evans. "pKa's of Inorganic and Oxo-Acids." Harvard, 4 Nov. 2005.

Dean, Brian. "Ulcer and Heartburn: The Foods to Avoid." LIVESTRONG.COM. The Lance

Armstrong Foundation, 26 Oct. 2010.

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