29-01-2013, 10:44 AM
Quantitative Determination of the Acid Content of Fruit Juices
[attachment=48674]
Background
The sour taste of many fruit juices is due in large part to the presence of acids. Citric acid, C3H5O(COOH)3 is one of several acids present in these juices. Citric acid reacts with sodium hydroxide (NaOH), a base, as shown in Equation 1.
C3H5 O(COOH)3 + 3NaOH → C3H5O(COONa)3 + 3H2O (Eq. 1)
Sodium citrate
Equation 1 describes a neutralization reaction, in which an acid and a base react to form salt and water. Various acids are found in different amounts in different juices, but for the purpose of this experiment, we assume the acid is just citric acid.
We can determine the amount of acid in a given volume of fruit juice by titrating the juice with a standard NaOH solution. A standard solution is a solution of known concentration expressed usually in molarity.
The molarity (mol/ L, or M) of a solution is the number of moles of solute per liter of solution, as expressed by Equation 2.
molarity, M = ( number moles of solute) / volume of solution, L (Eq. 2)
Titration is the measurement of the volume of a standard solution re¬quired to completely react with a measured volume or mass of the substance being analyzed. We add the standard solution from a calibrated glass tube called a buret. Before beginning the titration, we add an indica¬tor to the titration mixture. An indicator is a substance that changes color at the point when the titration reac¬tion is complete. In this experiment, you will use phe¬nolphthalein as an indicator when you titrate fruit juice with NaOH solution. Phenolphthalein is a complex organic dye that is colorless in acidic solutions and pink in solutions that are slightly alkaline, or basic.
Assume that we want to determine the acidity C3H5O(COOH)3 content of an orange juice sample. We find that 39.62 mL of 0.106 M NaOH solution are re¬quired to titrate a 10.0-mL sample of orange juice. We determine the number of moles of NaOH required to neutralize the C3H5 O(COOH)3from the concentration and volume of NaOH solution used in the titration, and a rearrangement of Equation 2, shown as Equation 3. Note that in part of Equation 3 the NaOH volume is converted from milliliters to liters.
Procedure
Preparing the Fruit Juice for Titration
Note: The numbers appearing in parentheses indicate the specific lines on your Data Sheet on which the indicated data should be entered.
1. Obtain 50 mL of fruit juice from your laboratory in¬structor in a clean, dry 100-mL beaker. Record the type of fruit juice and the code identification of the juice sample on your Data Sheet (1, 2).
Note: If the juice has substantial amounts of pulp floating in it, filter the juice by pouring it from the beaker into another clean, dry 100-mL beaker through some glass wool placed in a conical fun¬nel. Otherwise, the pulp tends to obscure the ti¬tration end point.
2. Label two clean, 125-mL Erlenmeyer flasks “1” and “2.”
3. Measure 20.0 mL of your juice from the beaker into a 25-mL graduated cylinder. Transfer the juice into Erlenmeyer flask 1. Record on your Data Sheet the volume of juice transferred (3). Rinse the graduated cylinder twice, using 5 mL of distilled or deionized water each time. Transfer the rinses into Erlenmeyer flask 1.
4. Use the procedure in Step 3 to transfer a 20.0-mL juice sample to Erlenmeyer flask 2.
5. Add three drops of phenolphthalein indicator solu¬tion to the solution in each Erlenmeyer flask. Gently swirl each flask and its contents to thoroughly mix each solution.
[attachment=48674]
Background
The sour taste of many fruit juices is due in large part to the presence of acids. Citric acid, C3H5O(COOH)3 is one of several acids present in these juices. Citric acid reacts with sodium hydroxide (NaOH), a base, as shown in Equation 1.
C3H5 O(COOH)3 + 3NaOH → C3H5O(COONa)3 + 3H2O (Eq. 1)
Sodium citrate
Equation 1 describes a neutralization reaction, in which an acid and a base react to form salt and water. Various acids are found in different amounts in different juices, but for the purpose of this experiment, we assume the acid is just citric acid.
We can determine the amount of acid in a given volume of fruit juice by titrating the juice with a standard NaOH solution. A standard solution is a solution of known concentration expressed usually in molarity.
The molarity (mol/ L, or M) of a solution is the number of moles of solute per liter of solution, as expressed by Equation 2.
molarity, M = ( number moles of solute) / volume of solution, L (Eq. 2)
Titration is the measurement of the volume of a standard solution re¬quired to completely react with a measured volume or mass of the substance being analyzed. We add the standard solution from a calibrated glass tube called a buret. Before beginning the titration, we add an indica¬tor to the titration mixture. An indicator is a substance that changes color at the point when the titration reac¬tion is complete. In this experiment, you will use phe¬nolphthalein as an indicator when you titrate fruit juice with NaOH solution. Phenolphthalein is a complex organic dye that is colorless in acidic solutions and pink in solutions that are slightly alkaline, or basic.
Assume that we want to determine the acidity C3H5O(COOH)3 content of an orange juice sample. We find that 39.62 mL of 0.106 M NaOH solution are re¬quired to titrate a 10.0-mL sample of orange juice. We determine the number of moles of NaOH required to neutralize the C3H5 O(COOH)3from the concentration and volume of NaOH solution used in the titration, and a rearrangement of Equation 2, shown as Equation 3. Note that in part of Equation 3 the NaOH volume is converted from milliliters to liters.
Procedure
Preparing the Fruit Juice for Titration
Note: The numbers appearing in parentheses indicate the specific lines on your Data Sheet on which the indicated data should be entered.
1. Obtain 50 mL of fruit juice from your laboratory in¬structor in a clean, dry 100-mL beaker. Record the type of fruit juice and the code identification of the juice sample on your Data Sheet (1, 2).
Note: If the juice has substantial amounts of pulp floating in it, filter the juice by pouring it from the beaker into another clean, dry 100-mL beaker through some glass wool placed in a conical fun¬nel. Otherwise, the pulp tends to obscure the ti¬tration end point.
2. Label two clean, 125-mL Erlenmeyer flasks “1” and “2.”
3. Measure 20.0 mL of your juice from the beaker into a 25-mL graduated cylinder. Transfer the juice into Erlenmeyer flask 1. Record on your Data Sheet the volume of juice transferred (3). Rinse the graduated cylinder twice, using 5 mL of distilled or deionized water each time. Transfer the rinses into Erlenmeyer flask 1.
4. Use the procedure in Step 3 to transfer a 20.0-mL juice sample to Erlenmeyer flask 2.
5. Add three drops of phenolphthalein indicator solu¬tion to the solution in each Erlenmeyer flask. Gently swirl each flask and its contents to thoroughly mix each solution.