Comparison of the acidity of fruit juices

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Comparison of the acidity of fruit juices

  1. 1. Comparison of the Acidity of Fruit JuicesI. Introduction A. Principle The sour taste of many fruit juices is due to the presence of acids. Citric acid is one of several acids present in these juices. Citric acid, a naturally occurring acid, exists in greatest trace amounts in a variety of fruits and vegetables, most notably citrus fruits as implied in the name. Citric acid contains three carboxylic acid functional groups and has a molecular formula of C3H5O(COOH)3 (Citric acid, 2012). Titration, the common procedure to assess the concentration of acid or base, was used in the determination of the concentration of the citric acid in fruit juices. In this process, a solution of accurately known concentration, standard solution (NaOH solution in this experiment), is added gradually to another solution of unknown concentration (the fruit juices in this experiment), until the chemical reaction between the two solutions is complete as shown by the indicator. In this experiment, phenolphthalein was used as an indicator. Phenolphthalein is a complex organic dye that varies its appearance in depending on acidity: colorless in acidic solutions while pinkish in basic solutions. Given the volumes of the standard and unknown solutions used in the titration as well as the concentration of the standard solution, the concentration of the unknown solution can be calculated (Chang, 2010). In this experiment, it was considered that all the acid in the fruit juice samples is in the form of citric acid even it was known that ascorbic, malic and folic acids are also present. Below is the equation in the determination of the concentration of citric acid using the standardized NaOH solution. C3H5 O(COOH)3(aq) + 3NaOH(aq) → C3H5O(COONa)3(aq) + 3H2O(l) 1
  2. 2. Citric acid is capable of donating 3 protons when dissolved in water acid. Salt and waterare formed by the neutralization of the base added to it. Water is formed when hydrogen ion(H+)from the acetic acid will react with one hydroxide ion (OH-) from the NaOH. The place ofhydrogen ion will be taken by the sodium ion (Na+) which was dissociated from sodiumhydroxide. For regulatory purposes and for their own manufacturing specifications, industriesmust know the concentration of the citric acid in juices they produce. This is important sincefruit juices is a common item in everyday living. The experiment was conducted in October 5, 2012 at the Institute of Chemistry, UPLB,College, Laguna. B. Objectives: In this experiment, the concentration of the citric acid in the two brands of fruit juices.The specific objectives of the study were the following: 1. To determine the acidity of each brand of fruit juice using the standardized NaOH solution; and 2. To determine which of the two fruit juices contains a higher acidity. 2
  3. 3. II. Materials A. Reagents: 10 mL of each commercial fruit drink 1 g Potassium acid phthalate 1 mL Phenolphthalein 2 g NaOH pellets 1 L distilled Water B. Apparatus: two 250 mL beaker two wash bottles 1 100 mL graduated cylinder one10 mLpipet four 250 mL Erlenmeyer flasks one stirring rod top loading balance two sets of Iron stand and buret holder two 50 mL buret two100 mL volumetric flask C. Other materials: white background 3
  4. 4. III. Procedure To determine the concentration of the citric acid in two brands of fruit juices, titrationwas performed. The NaOH solutions that will be used as the titrant were prepared. To set a 0.1 M NaOHsolution, weighted 0.40 g of NaOH pellet was placed on a pre-weighted 250-mL beaker. Tenmilliliters of distilled water was added and this was stirred until all solids were dissolved. Thiswas allowed to cool in tap water. This solution was quantitavely transferred to a 100-mLvolumetric flask. Water was added to it up to the 100-mL mark. Next step is standardization. Since it is difficult to obtain solid sodium hydroxide in apure form because, it has a tendency to absorb water from air and its solution reacts with carbondioxideit is necessary to determine the accurate concentration of NaOH solution in the process ofstandardization(Chang, 2010). Two Erlenmeyer flasks were filled with 0.25 g of potassium acidphthalate and 50 mL water. Its components were dissolved and 2-3 drops of phenolphthalein wasadded to it. These solutions were titrated with NaOH previously prepared. Then, 5 mL of each fruit juice samples were transferred to their respective Erlenmeyerflask using a pipette. Fifty milliliters of water and 2-3 drops of phenolphthalein were then addedto each flask. These solutions were titrated with the standardized NaOH solution. Each fruit juicehad been prepared an individual NaOH solution that was standardized to suffice the neededamount of solution in determining the acidity of citric acid in each fruit juice. All data were recorded in tables, necessary calculations were done and results wereinterpreted. 4
  5. 5. IV. Data and ObservationTable 1.a. Preparation of NaOH Solution for Zest-OZest-O Initial After 2 minutesMass of Beaker, g 145.93Mass of Beaker + NaOH, g 146.320 146.34Mass of NaOH, g 0.390 0.410Volume of distilled water, mL 100.0 100.0Table 1.b. Preparation of NaOH Solution for PlusPlus Initial After 2 minutesMass of Beaker, g 126.45Mass of Beaker + NaOH, g 126.85 126.87Mass of NaOH, g 0.400 0.420Volume of distilled water, mL 100.0 100.0Table 2.a. Standardization of the NaOH solution for Zest-OZest-O Trial 1 Trial 2Mass of KHC8H404, g 0.250Final buret reading, mL 35.80 24.00Initial buret reading, mL 24.00 11.90Volume of NaOH used, mL 11.80 12.10Molarity of NaOH, n/L 0.104 0.101Ave. molarity of NaOH, n/L 0.103Table 2.a. Standardization of the NaOH solution for PlusPlus Trial 1 Trial 2Mass of KHC8H404, g 0.250Final buret reading, mL 11.20 37.30Initial buret reading, mL 0.00 26.60Volume of NaOH used, mL 11.20 10.70Molarity of NaOH, mL 0.109 0.114Ave. molarity of NaOH, n/L 0.112Table 3.a. Determination of the Acidity of Fruit Juice Sample 1 (Zest-O) 5
  6. 6. Zest-O Trial 1 Trial 2Volume of Fruit Juice, mL 5.00 5.00Final buret reading, mL 4.10 6.20Initial buret reading, mL 3.00 5.20Volume of NaOH used, mL 1.10 1.00Molarity of NaOH, n/L 0.103 0.103Molarity of Citric acid, n/L 0.00755 0.00687Average Molarity of Citric acid, n/L 0.00721Table 3.b. Determination of the Acidity of Fruit Juice Sample 2 (Plus)Plus Trial 1 Trial 2Volume of Fruit Juice, mL 5.00 5.00Final buret reading, mL 2.80 7.30Initial buret reading, mL 1.20 6.20Volume of NaOH used, mL 1.60 1.10Molarity of NaOH, n/L 0.112 0.112Molarity of Citric acid, n/L 0.0119 0.0158Average Molarity of Citric acid, n/L 0.0138 6
  7. 7. V. Discussion To compare the acidity of two commercially available fruit juice drink, titration wasperformed. Two standardized NaOH solutions were simultaneously prepared to suffice the neededamount of NaOH solution in determining the acidity of citric acid in each fruit juice. Using theeq. 1 below and the data in tables 2.a and 2.b, the NaOH solutions were standardized. Themolarities of the citric acid present in each fruit juice sample were also computed with the dataand the equation (eq. 2) its reaction. HC8H4O4(aq) + NaOH(aq) → KNaC8H4O4(aq) + H2O(l) (eq. 1) C3H5 O(COOH)3(aq) + 3NaOH(aq) → C3H5O(COONa)3(aq) + 3H2O(l) (eq. 2) As seen in table 2.a, the standardized NaOH solution used in the titration of fruit juice1,Zest O has the average molarity of 0.103 mol/L. Table 3.a shows the data in titration of fruitjuice 1 with NaOH solution. Five milliliters of fruit juice was neutralized by 1.10 mL of NaOHsolution in the first trial while 1.00 mL of NaOH solution in the second trial. The molarities ofthe citric acid in first and second trials were computed and it have values of 0.00755 mol/L and0.00687 mol/L, respectively. The average molarity of citric acid in fruit juice 1, Zest O is0.00721mol/L. Table 2.b shows that the standardized of NaOH solution used in the titration of fruit juice2, Plus has the average molarity of 0.112 mol/L. Table 3.a shows the data in titration of fruitjuice 2 with standardized NaOH solution. In the first and second trial, 5.00mL of fruit juice wasneutralized by 1.60 mL and 1.10 mL of NaOH solution, respectively. The molarities of the citric 7
  8. 8. acid in first and second trials were computed and it has values of 0.0119 mol/L and 0.0158mol/L, respectively. The average molarity of citric acid in fruit juice Plus is 0.0138mol/L. The errors committed in the experiment are attributed in many factors. One is on themeasurements of the reagents and other materials. Another is in the standardization of the NaOH.If those solutions were not standardized properly, the concentration of the citric acid will not bedetermined correctly. Since citric acid was not he only acid present in the fruit juices, the otheracids will be reacting with the NaOH as well, so the results will not show the exact concentrationof citric acid.VI. Conclusions and Recommendations Titration of samples was performed to determine the concentration of citric acid in brandsof fruit juices. Computing for the molarity of the concentration of the acid, fruit juice 1, Zest O and fruitjuice 2, Plus, have molarity of 0.0119 mol/L and 0.0158 mol/L, respectively. Given these values,it was inferred that fruit juice 2, Plus has higher concentration of citric acid than fruit juice 1,Zest O so it has higher acidity. It was further recommended that a study regarding the percentage of the citric acid in allthe acids present in the fruit juice will be conducted to establish the actual acidity of each fruitjuice. Another, it was suggested that another flavor of fruit juice will be studied since both fruitjuices that were used were flavored apple. To be able to examine the citric acid content of otherfruit juices which are colored, it was propose that another indicator will be used. 8
  9. 9. VII. Sample Calculations A. Preparation of NaOH solution Mass of NaOH pellets to be dissolved: B. Standardization of the NaOH solution Volume of NaOH used: Molarity of NaOH solution: Trial 1: Trial 2: Average Molarity of NaOH Solution: 9
  10. 10. C. Determination of the Acidity of each Fruit Juices Molarity of Citric Acid: Trial 1:Trial 2: 10
  11. 11. VIII. References<http://chemlab.truman.edu/CHEM100Labs/THE%20DETERMINATION%20OF%20C ITRIC%20ACID.pdf>. Accessed 02 October 2012.Brown, T.L., H.E. Lemay, et al. 2012. CHEMISTRY: The Central Science. 12th ed. USA: Pearson Prentice Hall.Chang, R. 2010. Chemistry. 10th ed. New York: The McGraw-Hill Companies, Inc. p. 208Chang, R.; J. O. 2011. General Chemistry: The Essential Concepts. 6th ed. New York: The McGraw-Hill Companies, Inc.Masterton, W. L., Hurley, C. N., Neth, E. J. 2009. Chemistry: Principles and Reactions. 7th ed. California: Brooks/Cole, Cengage Learning. P. 145Torio, M. A. O. Laboratory Instruction Manual for CHEM 16.1 General Chemistry 1 Laboratory. 5th ed. College. University of the Philippines, Los BanosWikipedia. 2012. Citric Acid. <http://en.wikipedia.org/wiki/Citric_acid>. Accessed 07 October 2012. 11

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