1. Comparison of the Acidity of Fruit Juices
I. 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)
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2. Citric acid is capable of donating 3 protons when dissolved in water acid. Salt and water
are 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 of
hydrogen ion will be taken by the sodium ion (Na+) which was dissociated from sodium
hydroxide. For regulatory purposes and for their own manufacturing specifications, industries
must know the concentration of the citric acid in juices they produce. This is important since
fruit 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.
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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:
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4. III. Procedure
To determine the concentration of the citric acid in two brands of fruit juices, titration
was performed.
The NaOH solutions that will be used as the titrant were prepared. To set a 0.1 M NaOH
solution, weighted 0.40 g of NaOH pellet was placed on a pre-weighted 250-mL beaker. Ten
milliliters of distilled water was added and this was stirred until all solids were dissolved. This
was allowed to cool in tap water. This solution was quantitavely transferred to a 100-mL
volumetric 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 a
pure form because, it has a tendency to absorb water from air and its solution reacts with carbon
dioxideit is necessary to determine the accurate concentration of NaOH solution in the process of
standardization(Chang, 2010). Two Erlenmeyer flasks were filled with 0.25 g of potassium acid
phthalate and 50 mL water. Its components were dissolved and 2-3 drops of phenolphthalein was
added to it. These solutions were titrated with NaOH previously prepared.
Then, 5 mL of each fruit juice samples were transferred to their respective Erlenmeyer
flask using a pipette. Fifty milliliters of water and 2-3 drops of phenolphthalein were then added
to each flask. These solutions were titrated with the standardized NaOH solution. Each fruit juice
had been prepared an individual NaOH solution that was standardized to suffice the needed
amount 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 were
interpreted.
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5. IV. Data and Observation
Table 1.a. Preparation of NaOH Solution for Zest-O
Zest-O Initial After 2 minutes
Mass of Beaker, g 145.93
Mass of Beaker + NaOH, g 146.320 146.34
Mass of NaOH, g 0.390 0.410
Volume of distilled water, mL 100.0 100.0
Table 1.b. Preparation of NaOH Solution for Plus
Plus Initial After 2 minutes
Mass of Beaker, g 126.45
Mass of Beaker + NaOH, g 126.85 126.87
Mass of NaOH, g 0.400 0.420
Volume of distilled water, mL 100.0 100.0
Table 2.a. Standardization of the NaOH solution for Zest-O
Zest-O Trial 1 Trial 2
Mass of KHC8H404, g 0.250
Final buret reading, mL 35.80 24.00
Initial buret reading, mL 24.00 11.90
Volume of NaOH used, mL 11.80 12.10
Molarity of NaOH, n/L 0.104 0.101
Ave. molarity of NaOH, n/L 0.103
Table 2.a. Standardization of the NaOH solution for Plus
Plus Trial 1 Trial 2
Mass of KHC8H404, g 0.250
Final buret reading, mL 11.20 37.30
Initial buret reading, mL 0.00 26.60
Volume of NaOH used, mL 11.20 10.70
Molarity of NaOH, mL 0.109 0.114
Ave. molarity of NaOH, n/L 0.112
Table 3.a. Determination of the Acidity of Fruit Juice Sample 1 (Zest-O)
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6. Zest-O Trial 1 Trial 2
Volume of Fruit Juice, mL 5.00 5.00
Final buret reading, mL 4.10 6.20
Initial buret reading, mL 3.00 5.20
Volume of NaOH used, mL 1.10 1.00
Molarity of NaOH, n/L 0.103 0.103
Molarity of Citric acid, n/L 0.00755 0.00687
Average Molarity of Citric acid, n/L 0.00721
Table 3.b. Determination of the Acidity of Fruit Juice Sample 2 (Plus)
Plus Trial 1 Trial 2
Volume of Fruit Juice, mL 5.00 5.00
Final buret reading, mL 2.80 7.30
Initial buret reading, mL 1.20 6.20
Volume of NaOH used, mL 1.60 1.10
Molarity of NaOH, n/L 0.112 0.112
Molarity of Citric acid, n/L 0.0119 0.0158
Average Molarity of Citric acid, n/L 0.0138
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7. V. Discussion
To compare the acidity of two commercially available fruit juice drink, titration was
performed.
Two standardized NaOH solutions were simultaneously prepared to suffice the needed
amount of NaOH solution in determining the acidity of citric acid in each fruit juice. Using the
eq. 1 below and the data in tables 2.a and 2.b, the NaOH solutions were standardized. The
molarities of the citric acid present in each fruit juice sample were also computed with the data
and 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 fruit
juice 1 with NaOH solution. Five milliliters of fruit juice was neutralized by 1.10 mL of NaOH
solution in the first trial while 1.00 mL of NaOH solution in the second trial. The molarities of
the citric acid in first and second trials were computed and it have values of 0.00755 mol/L and
0.00687 mol/L, respectively. The average molarity of citric acid in fruit juice 1, Zest O is
0.00721mol/L.
Table 2.b shows that the standardized of NaOH solution used in the titration of fruit juice
2, Plus has the average molarity of 0.112 mol/L. Table 3.a shows the data in titration of fruit
juice 2 with standardized NaOH solution. In the first and second trial, 5.00mL of fruit juice was
neutralized by 1.60 mL and 1.10 mL of NaOH solution, respectively. The molarities of the citric
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8. acid in first and second trials were computed and it has values of 0.0119 mol/L and 0.0158
mol/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 the
measurements 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 be
determined correctly. Since citric acid was not he only acid present in the fruit juices, the other
acids will be reacting with the NaOH as well, so the results will not show the exact concentration
of citric acid.
VI. Conclusions and Recommendations
Titration of samples was performed to determine the concentration of citric acid in brands
of fruit juices.
Computing for the molarity of the concentration of the acid, fruit juice 1, Zest O and fruit
juice 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 all
the acids present in the fruit juice will be conducted to establish the actual acidity of each fruit
juice. Another, it was suggested that another flavor of fruit juice will be studied since both fruit
juices that were used were flavored apple. To be able to examine the citric acid content of other
fruit juices which are colored, it was propose that another indicator will be used.
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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:
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10. C. Determination of the Acidity of each Fruit Juices
Molarity of Citric Acid:
Trial 1:
Trial 2:
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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. 208
Chang, 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. 145
Torio, M. A. O. Laboratory Instruction Manual for CHEM 16.1 General Chemistry 1 Laboratory.
5th ed. College. University of the Philippines, Los Banos
Wikipedia. 2012. Citric Acid. <http://en.wikipedia.org/wiki/Citric_acid>. Accessed 07 October
2012.
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