Materials Included In Kit

Additional Materials Required

Safety Precautions

Dilute sodium hydroxide solution is irritating to skin and eyes. Keep citric acid on hand to clean up any spills. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a lab coat or chemical-resistant apron. All food-grade items that have been brought into the lab are considered laboratory chemicals and are for lab use only. Do not taste or ingest any materials in the chemistry laboratory. Do not remove any remaining food items from the lab after they have been used in the lab. Please consult current Safety Data Sheets for additional safety, handling and disposal information. Remind students to wash their hands thoroughly with soap and water before leaving the lab.

Disposal

Consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Excess sodium hydroxide solution may be neutralized accordng to Flinn Suggested Disposal Method #10. The titrated solutions are neutral and may be rinsed down the drain with water according to Flinn Suggested Disposal Method #26b.

Lab Hints

• The laboratory work for this experiment can reasonably be completed within a typical 2-hour lab period. For best results, review the Prelab Assignment with students. These questions introduce the calculations that will be needed to complete the lab report.
• The phenolphthalein endpoint color will be different for every juice, depending on the initial color of the juice and any natural indicators that may be present in the juice itself. This is frustrating for students, who are unsure what color they should be titrating to. Instructors may find it helps the lab run more smoothly if they prepare a set of before-and-after color controls in test tubes for comparison. The endpoint colors will generally fade within 10–20 minutes—“top off” the endpoint controls before each lab period.
• “No-pulp” juices are recommended for this activity. Juices with a lot of pulp should be filtered through cheesecloth or qualitative filter paper before use. Orange, white grape, grapefruit and pineapple juices gave the most consistent results in this experiment. Apple juice does not give a stable endpoint color with phenolphthalein and is not recommended. Also, since the principal acid in apple juice is malic acid, it would be misleading to translate the total acidity in terms of the concentration of citric acid.
• The “titratable acidity” of fruit juices is measured to define the sugar/acid balance in different juices and to provide a quality control specification. It is hard to find accurate numerical data, however, for the actual citric acid content of different juices. According to patent data for “low-acid” orange juice, the average citric acid concentration in regular orange juice is 0.62–0.82 g per 100 mL. FDA guidelines stipulate a citric acid concentration of < 1.35 g per 100 mL for pineapple juice.
• Collect class data to allow students to compare the acidity in different types of fruit juices. Enlist your students’ help in this regard—encourage students to bring in their favorite type of juice for analysis. Most students really appreciate the opportunity to have some input into what they are testing in the lab. You may find there are interesting variations in acid content for frozen, bottled, canned or boxed juices.
• The sugar content in different juices can be analyzed by thin layer chromatography using an ethyl acetate–isopropyl alcohol–water solvent system. Use glucose, fructose, sucrose and maltose as reference standards.

Further Extensions

Supplementary Information: Standardization of Sodium Hydroxide Solution

1. Obtain a sample of potassium hydrogen phthalate (KHP) that has been previously dried in an oven at 110 °C for two hours and stored in a desiccator.
2. On an analytical balance, accurately weigh 0.4 to 0.6 g of KHP.
3. Transfer the KHP into an Erlenmeyer flask or beaker.
4. Add about 40 mL of distilled water to the flask and swirl until all the KHP is dissolved.
5. Obtain 0.10 M sodium hydroxide, NaOH, solution. This is the nominal concentration.
6. Rinse and fill a buret with the NaOH solution.
7. Add three drops of phenolphthalein solution to the KHP solution in the flask.
8. Titrate the KHP solution using the nominal 0.1 M NaOH solution. At least three trials should be conducted and the data or results averaged.
9. Calculate the concentration of NaOH. See the following sample calculation: Assume 0.500 g of KHP was used, and the average volume of NaOH titrant required was 25.50 mL. KHP, KHC₈H₄O₄, molar mass = 204.23 g/mole. The mole ratio for the neutralization reaction is one mole of KHP per mole NaOH.
   {14041_Extensions_Equation_2}

Sample Titration Curve
“Everything you ever wanted to know about an acid or base can be found in its titration curve.” This statement can be proven by extending the titration procedures to measure pH as a function of the amount of NaOH added. The resulting plot is called a titration curve and can be used to answer the following questions:

• What is the initial pH of the juice and how does this relate to the [H₃O⁺] concentration?
• What is the pH value at the equivalence point? Compare this value to the indicator transition pH for phenolphthalein.
• What is the pH value at the “half-equivalence point” in the titration? How is this value related to the identity of the acid?

The titration curve for analysis of 20.0 mL of pineapple juice using 0.0971 M sodium hydroxide as titrant is shown.

The initial pH was 3.65, and the volume of NaOH required to reach the phenolphthalein endpoint was 20.60 mL. The gently rising “buffer region” in the pH range of 3.5–6.5 is characteristic of the titration of citric acid, which does NOT show separate or distinct inflection points for the first two ionizable hydrogens in its tripotic acid structure. Citric acid has the formula H₃C₆H₅O₇ (abreviated H₃A for convenience).
pK_a1 (H₃A/H₂A^–) = 3.1
pK_a2 (H₂A/HA^2–) = 4.8
pK_a3 (HA^2–/A^3–) = 6.4

Answers to Prelab Questions

1. Based on your previous observations of their taste, rank the following juices with respect to their acid content: orange, white grape, pineapple and grapefruit.

   Taste perceptions will vary. Note: Ask students to rely on their memory or their likes and dislikes of certain juices. Do not allow students to taste juices in the lab.

2. Using the structural formula of citric acid shown in Figure 1 in the Background, determine the molecular formula of citric acid and calculate its molar mass (g/mole).

   The molecular formula of citric acid is C₆H₈O₇. Molar mass = 192.0 g/mole.

3. A 10.0-mL sample of pineapple juice was titrated with 0.100 M sodium hydroxide solution. The average volume of NaOH required to reach the endpoint was 12.8 mL.
   1. Calculate the number of moles of sodium hydroxide required to reach the endpoint.

      Moles of NaOH = Molarity (moles/L) x Volume (L)

      {14041_PreLabAnswers_Equation_3}
   2. Using the mole ratio (number of moles of citric acid divided by the number of moles of sodium hydroxide) for the neutralization reaction shown in Equation 1, determine the number of moles of citric acid in 10.0 mL of pineapple juice.
      {14041_PreLabAnswers_Equation_4}
   3. Multiply the number of moles of citric acid by its molar mass to calculate the mass of citric acid in 10.0 mL of the juice.

      (4.27 x 10^–4 moles citric acid)(192.0 g/mole) = 0.0819 g

   4. The concentration of acid in juices is usually expressed in grams of acid per 100 mL of juice. What is the concentration of citric acid in pineapple juice?
      {14041_PreLabAnswers_Equation_5}

Sample Data

Laboratory Report

*Per 20.0 ml of juice.

*Per 20.0 ml of juice.

Results Table

*Per 20.0 ml of juice.

Answers to Questions

1. Determine the volume of sodium hydroxide added at the endpoint for each trial and enter the results in the data table.

   See sample titration data tables.

2. Calculate the number of moles of sodium hydroxide required to reach the endpoint for each trial.

   Sample calculation for orange juice, trial 1 (see the Sample Data results table for more results):

   {14041_Answers_Equation_6}
3. Based on the mole ratio for the neutralization reaction of citric acid with sodium hydroxide, determine the number of moles of citric acid present in 20.0 mL of juice.

   Sample calculation for orange juice, trial 1 (see the Sample Data results table for more results):

   {14041_Answers_Equation_7}
4. Calculate the mass in grams of citric acid in 20.0 mL of juice for each trial.

   Sample calculation for orange juice, trial 1 (see the Sample Data results table for more results):

   {14041_Answers_Equation_8}
5. What is the average concentration of citric acid in the fruit juice in units of grams of citric acid per 100 mL of juice?

   Sample calculation for orange juice, trial 1 (see the Sample Data results table for more results):

   {14041_Answers_Equation_9}
6. Compare the average citric acid concentration in different juices: Based on class data, rank the juices from most acidic to least acidic. Does this ranking agree with the predictions made in Prelab Question 1.

   From most acidic to least acidic: Grapefruit juice > Pineapple juice > Orange juice > White grape juice

Introduction

The refreshing taste of fresh fruit juices is due to a complex blend of flavors and fragrances. Fruit juices get their sweet taste from sugars, especially fructose and glucose, and their sour or tart taste from acids, such as citric acid and tartaric acid. The balance of sugar-to-acid content is one of the main factors responsible for the appealing taste of fruit juices—too much sugar, and the juice will taste bland, but too much acid, and the juice will taste sour. The “total acidity” of fruit juices is determined by titration with sodium hydroxide.

Concepts

• Acid–base neutralization
• Stoichiometry
• Titration
• Concentration and molarity

Background

The main acids present in fruits and fruit juices are citric acid (in citrus fruits), tartaric acid (in grapes) and malic acid (in apples).

The amount of citric acid in citrus fruit juices can be determined by titration with a standard solution of sodium hydroxide. (A standard solution is one whose concentration is accurately known). Citric acid is a tricarboxylic acid—it has three ionizable or “active” hydrogen atoms in its structure. One mole of citric acid therefore reacts with three moles of sodium hydroxide via the acid–base neutralization reaction shown in Equation 1. Acid–base titrations are a useful technique for determining the amount or concentration of an acid or base in a sample. In the titration of a citrus fruit juice with sodium hydroxide, a sodium hydroxide solution of known molarity is carefully added to a measured volume of fruit juice containing phenolphthalein as the indicator. The exact volume of sodium hydroxide that must be added to reach the phenolphthalein endpoint (pH > 7) is measured and then used to calculate the concentration of citric acid in the juice.

Experiment Overview

The purpose of this experiment is to compare the citric acid content in a variety of fruit juices. The concentration of citric acid in each juice will be determined by titration with sodium hydroxide solution. Phenolphthalein will be added as an indicator to detect the equivalence point or endpoint in the titration. This is the point at which all of the citric acid has been neutralized by reaction with sodium hydroxide.

Materials

Prelab Questions

1. Based on your previous observations of their taste, rank the following juices with respect to their acid content: orange, white grape, pineapple and grapefruit.
2. Using the structural formula of citric acid shown in Figure 1 in the Background, determine the molecular formula of citric acid and calculate its molar mass (g/mole).
3. A 10.0-mL sample of pineapple juice was titrated with 0.100 M sodium hydroxide solution. The average volume of NaOH required to reach the endpoint was 12.8 mL.
   1. Calculate the number of moles of sodium hydroxide required to reach the endpoint.
   2. Using the mole ratio (number of moles of citric acid divided by the number of moles of sodium hydroxide) for the neutralization reaction shown in Equation 1, determine the number of moles of citric acid in 10.0 mL of pineapple juice.
   3. Multiply the number of moles of citric acid by its molar mass to calculate the mass of citric acid in 10.0 mL of the juice.
   4. The concentration of acid in juices is usually expressed in grams of acid per 100 mL of juice. What is the concentration of citric acid in pineapple juice?

Safety Precautions

Dilute sodium hydroxide solution is irritating to skin and eyes. Notify the instructor and clean up all spills immediately. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a lab coat or chemical-resistant apron. All food-grade items that have been brought into the lab are considered laboratory chemicals and are for lab use only. Do not taste or ingest any materials in the chemistry laboratory. Do not remove any remaining food items from the lab after they have been used in the lab. Wash hands throroughly with soap and water before leaving the lab.

Procedure

1. Obtain about 50 mL of fruit juice in a 50-mL beaker or small cup and record the identity of the juice in the data table.
2. Obtain about 75 mL of standard sodium hydroxide solution in a 150-mL beaker and record the precise molarity of the solution in the data table.
3. Pour about 5 mL of fruit juice into a large test tube and add 1 drop of phenolphthalein.
4. Using a graduated, Beral-type pipet, add 0.1 M sodium hydroxide in 1-mL portions until the juice sample turns pink or red. Gently swirl the test tube while adding the sodium hydroxide solution. Record the volume of sodium hydroxide in the data table.
5. Based on this “rough titration” of 5 mL of juice, estimate and record the volume of sodium hydroxide that will be needed to titrate 20.0 mL of juice. Use this estimate as a guide to determine when to “slow down” in the titration (see step 13). Hint: If 3 mL of sodium hydroxide is needed to reach the “rough” endpoint, begin adding sodium hydroxide dropwise (step 13) after about 10 mL of NaOH have been added.
6. Rinse a clean 50-mL buret with two 5-mL portions of the sodium hydroxide solution.
7. Clamp the buret to a ring stand and place a “waste” beaker under the buret. Fill the buret to above the zero mark with the sodium hydroxide solution. Open the stopcock to allow any air bubbles to escape from the tip. Close the stopcock when the liquid level in the buret is between the 0- and 10-mL marks.
8. Record the precise level (initial volume) of the solution in the buret. Note: Volumes are read from the top down in a buret. Always read from the bottom of the meniscus and remember to include the appropriate number of significant figures (see Figure 2).
   {14041_Procedure_Figure_2}
9. Measure 20.0 mL of fruit juice in a graduated cylinder and transfer the juice into a 125-mL Erlenmeyer flask. Rinse the graduated cylinder with three 10-mL portions of distilled or deionized water and add the rinse solutions to the Erlenmeyer flask.
10. Add 2–3 drops of phenolphthalein indicator to the Erlenmeyer flask.
11. Position the flask and the buret so that the tip of the buret is inside the mouth of the flask. Place a piece of white paper under the flask to make it easier to detect the color change at the endpoint.
12. Open the stopcock to add the estimated amount of sodium hydroxide (see step 5) to the juice sample. Gently swirl the flask to mix the contents.
13. Continue to add sodium hydroxide slowly, drop by drop, while swirling the flask. Use a wash bottle to rinse the sides of the flask with distilled water during the titration.
14. When a faint pink color appears and persists for 15–20 seconds while swirling the flask, the endpoint has been reached. Close the stopcock and record the final buret reading (final volume) for trial 1 in the data table.
15. Pour the contents of the flask into the sink and rinse the flask with distilled water.
16. Perform a second trial by repeating steps 8–15 with a new 20.0-mL sample of the same fruit juice. Refill the buret if needed.
17. (Optional) If time permits, complete a set of titrations with a different fruit juice.

Student Worksheet PDF

14041_Student1.pdf