Determination of phosphoric acid content in pepsi light soft drink PDF

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POTENTIOMETRIC DETERMINATION OF PHOSPHORIC ACID CONTENT IN PEPSI LIGHT® SOFT DRINK Karen Xiomara Galeano Contreras 1, Valentina Sayari Sierra Navarrete 1 1

Departamento de Ingeniería Química y Ambiental. Facultad de Ingeniería. Universidad Nacional de Colombia, Bogotá, D.C., Colombia

ABSTRACT This paper presents the quantitative analysis of phosphoric acid content in Pepsi light sugar-free® soft drink through potentiometric methods, being sodium hydroxide the acid-neutralizing substance. As the added strong base reacted with the acid, the pH values in the solution increased gradually, becoming volume a function of the pH value. That relationship was plotted showing up the concerning points from the practice. In that way, the titration curve revealed two points of equivalence at 5.10 mL and 12.50 mL of added titrant, with pH values within the acidic range at 4.10 and the alkaline range at 8.30, respectively. The plot also illustrated two pKa values which were further from the expected, being pKa 1 = 2.40 and pKa2 = 6.30. The resulting relative errors for these values were 14.3% for the first one and 12.6% for the second pKa value. Considering the first equivalence point by First Derivative Criteria, it was found that there were 460 mg of phosphoric acid in one liter of the solution, which is equivalent to 184 mg in the 400 mL net content reported on the label. With a relative error of 2.56%, the results demonstrated great accuracy. Keywords: potentiometric, phosphoric acid, titration, pH value, concentration. 1. INTRODUCTION Cola soft drinks generally have a very small content of phosphoric acid to give them their tartness [1]. Phosphoric acid is a triprotic acid, which means it gives up three H+ ions in aqueous solution. Potentiometric titration is the most accurate analytical technique to find its concentration by adding a known volume of an acid-neutralizing substance to the decarbonized cola beverage (carbonic acid must be removed so that phosphoric acid can be measured). In this case, sodium hydroxide will act as the standard solution and will react with the moles of acid in the soft drink as it is shown below: − − ⇌ 𝐻2 𝑃𝑂4(𝑎𝑞) + 𝐻2 𝑂(𝑙) (1) 𝐻3 𝑃𝑂4(𝑎𝑞) + 𝑂𝐻(𝑎𝑞) − − 2− 𝐻2 𝑃𝑂4(𝑎𝑞) + 𝑂𝐻(𝑎𝑞) ⇌ 𝐻𝑃𝑂4(𝑎𝑞) + 𝐻2 𝑂(𝑙) (2) − 3− ⇌ 𝑃𝑂4(𝑎𝑞) + 𝐻2 𝑂(𝑙) (3) 𝐻𝑃𝑂42− + 𝑂𝐻(𝑎𝑞) All three species of the acid are weak acids (represented with the double arrows), and each neutralization reaction has its corresponding equivalence point [2], which is the volume of sodium hydroxide added to completely neutralize the cola soft drink. However, the point of concern will specifically be the first equivalence point since it provides the H3PO4 concentration (equation 1). In a potentiometric titration, the titrant volume acts as a function of the pH, which is given by the pH meter [3]. This relationship can be plotted (titration curve) and this is how the equivalence points of the neutralizing reaction can be estimated.

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On the other hand, the relative acidity of an acid or a base is usually expressed in terms of pKa, where Ka is the dissociation constant for the reaction [4]. The lower the pKa value, the stronger the acid. This laboratory practice is, in general terms, quite usual. However, it was found an article from Benue State University and University of Agriculture, Makurdi, Nigeria, which provides useful background information on the topic [5]. In that way, the present study is aimed to determine the acidity of Pepsi light® soft drink by considering the first equivalence point to find its phosphoric acid concentration. 2. EXPERIMENTAL PROCEDURE 2.1. Degassing and pre-treatment It was taken a trade sample identified as Pepsi Light sugar-free® soft drink 400 mL with expiration date on March 23, 2019, lot number 2L0002951182 and Health Registration Number RSA-0050982017, made by Postobón S.A. A week prior to the practice, the cola drink was left opened and occasionally shaken in order to remove the remaining carbonic acid, as it is shown below: 𝐻2 𝐶𝑂3(𝑎𝑞) ⇌ 𝐶𝑂2(𝑔) + 𝐻2 𝑂(𝑙) (4) 2.2. Potentiometric titration The potentiometric titration apparatus was constructed and the pH meter was plugged into the port of LabQuest 2 calculator. The pH sensor was previously calibrated with two buffer solutions (pH values of 4 and 7, respectively), and the titrant was formerly standardized. At the first step, the 250 mL beaker was filled with 100.0 mL of the decarbonized soft drink by using the graduated cylinder. Next, 25.00 mL of Sodium hydroxide 0.092 M we re added to the burette. The membrane from the Vernier pH sensor must be completely submerged in the solution but not touching the rotating stirring magnet. Once pH initial value became constant, 0.02 mL of NaOH were added from the burette to the beaker, and each volume was registered with its corresponding stabilized pH value. The neutralizing reaction of the process corresponds to the equation 1, 2 y 3. It was later plotted in a curve of titration, showing up the equivalence points and the pKa values for phosphoric acid. Then, phosphoric acid concentration could be determined by stoichiometric calculations. 3. RESULTS AND DISCUSSION Initially, it was registered a low pH value for the cola drink and it gradually increased as Sodium hydroxide was added to the solution. This relationship is herewith presented in Figure 1. The initial pH value was at 2.21, whereas the standard phosphoric acid titration curve [4] has initial pH values between 1.48 – 1.55. As can be seen, the equivalence points were estimated through First Derivative Criteria. The third equivalence point could not be seen due to the low concentration of phosphoric acid in cola drinks [6].

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1,50

11,00

Máx1. (5.10 mL, 1.40)

10,00

Máx2. (12.50 mL, 1.15)

1,20

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V2 = 12.50 mL, pH2= 8.30

7,00

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8,00 pKa2= 6.30

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V1 = 5.10 mL, pH1= 4.10

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pKa1= 2.40

2,00

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-0,30 Volumen de NaOH (mL)

Volumen de NaOH (mL)

Figure 1. Potentiometric titration curve of Pepsi Light for phosphoric acid and its equivalence points according to the First Derivative Criteria. In that sense, the volume which the H3PO4 was completely neutralized with, corresponds to 5.10 mL of NaOH. However in the buffer region, exactly halfway to the equivalence point, the concentration of acid and base is exactly the same. In other words, the pH values at these points are also the pKa values for phosphoric acid [3]. From the practice, the pKa values obtained were pKa1 = 2.40 and pKa2 = 6.30. This latter is approximately the average between the first and the second equivalence point, which corresponds to 12.50 mL of titrant. At this point the solution was in the alkaline range at 8.30. The pKa1 value was quite close to the expected one, which is 2.12. It differs three tenths from the real value, resulting in a relative error of 14.2%. Regarding to the pKa2, there was a higher absolute error with a difference of nine tenths from the expected value, which is 7.21. However, the relative error here was 12.6%, a little bit lower. The pH value of the first equivalence point was within the acidic range at 4.20. The equimolar conversion from NaOH moles to the H3PO4 moles in the first protolysis step (equation 1), made it possible to find the concentration of phosphoric acid considering the first NaOH volume. Thus, it was found that there is 460 mg of phosphoric acid in a litre of Pepsi light® soft drink. It corresponds to 0.0460%. In that way, for the trade sample used in the practice, there was 184 mg of phosphoric acid in the 400 mL reported volume of the bottle. PepsiCo, the international producer company of Pepsi drink, reported 53 mg of Phosphorus in 12 oz. of Pepsi light sugar free® [1]. That is 472 mg of phosphoric acid per liter. The results from the practice were greatly accurate considering a relative error of 2.56%. The recommended dietary allowance for adults is 700 mg of Phosphorus per day [7]. In that way, consuming one litre of Pepsi light per day would contribute approximately 150 mg of phosphorus from the phosphoric acid in the beverage, which represents 21.4% of what is allowed. That might be a high percent considering that phosphorus is also present in meats, milk, some fruits and vegetables [7].

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4. CONCLUSIONS The phosphoric acid content in the Pepsi Light soft drink® trade sample was 184 mg in the 400 mL reported on the label. The results demonstrated great accuracy since its relative error was 2.56%. There were found two equivalence points at 5.10 mL and 12.50 mL of the titrant, with pH values of 4.10 and 8.30, respectively. The points of inflection from the plot elucidated two pKa values for phosphoric acid, which were 2.40 and 6.30. The relative errors for the acid dissociation constants revealed a closer proximity for the second pKa value in contrast to the first one (12.6% to 14.3%, respectively). 5. BIBLIOGRAPHY [1] Official Site for PepsiCo Beverages; (2019). The facts about your favorite beverages. https://www.pepsicobeveragefacts.com/mobile/phosphorus [2] Chang, R. (2010). Chemistry 10th edition. Mc Graw Hill. New York, p. 153 - 154. [3] Moore J., Stanitski C., Jurs P. (2011). Chemistry the Molecular Science, Fourth edition. In: Chapter 16, Acids and Bases. USA: Brooks/Cole Cengaje learning. p. 770 - 772. [4] Massachusetts Institute of Technology; 5.310 Laboratory Chemistry: The potentiometric titration of an acid mixture. [5] Utange P., Wuana R., Akpoghol T. (2015). Potentiometric and spectrophotometric determination of phosphoric acid in some beverages. Pelagia Research Library. Department of Chemistry, Benue State University, University of Agriculture. Makurdi, Nigeria. [6] LD didactic. Chemistry Leaflets C3.6.4.2. In: Analytical chemistry, food analytics. Determination of the content of acid phosphoric in a cola drink. [7] Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D, and Fluoride. National Academies Press. Washington, DC, 1997. PMID: 23115811.

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