Materials Included In Kit

Additional Materials Required

Safety Precautions

Sodium hydroxide solution is irritating to skin and eyes. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles and chemical-resistant gloves and 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 review 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 laboratory.

Disposal

Please 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 with acid and disposed of according to Flinn Suggested Disposal Method #10. The waste solutions may be flushed down the drain with plenty of excess water according to Flinn Suggested Disposal Method #26b.

Lab Hints

• The laboratory work for this experiment can reasonably be completed within a typical 50-minute lab period. The Prelab Questions introduce the calculations that will be needed to complete the lab write-up. They may be assigned as homework in preparation for lab or used as part of a cooperative class discussion before lab.
• 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. Teachers may find it helpful to make 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 control 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. Grapefruit juice is not available in canned form and is thus not included in the kit. Grapefruit juice may be purchased in the refrigerated section of the grocery store. Apple juice does not give a stable endpoint color with phenolphthalein and is not recommended. (The principal acid in apple juice is malic acid, not 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.

Teacher Tips

• Collect class data allowing students to compare the acidity in different types of fruit juices. As an optional extension of this experiment, 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

Standardization of Sodium Hydroxide Solution
The sodium hydroxide solution supplied in this kit is 0.1 M. If desired, this solution may be standardized by titration with reagent-grade potassium hydrogen phthalate (KHC₈H₄O₄), a primary standard. (A primary standard is a compound that is available in pure form, is soluble in water, and is not hygroscopic.)

Potassium hydrogen phthalate reacts with sodium hydroxide in a 1:1 mole ratio, according to the following balanced equation.

KHC₈H₄O₄(aq) + NaOH(aq) → NaKC₈H₄O₄(aq) + H₂O(l)

Procedure

1. Measure and record the mass of a weighing paper or weighing dish.
2. Add about 0.5 g of potassium hydrogen phthalate (KHP) to the weighing paper. Measure and record the exact mass of the paper and KHP used.
3. Using a powder funnel, carefully transfer the KHP solid into a clean and dry 125-Erlenmeyer flask. Rinse the paper and funnel with distilled or deionized water from a wash bottle.
4. Add distilled or deionized water to the flask to obtain a final volume of about 50 mL. Swirl to dissolve the solid.
5. Add 2–3 drops of phenolphthalein indicator solution to the flask.
6. Fill a 50-mL buret with the sodium hydroxide solution and record the initial volume.
7. Slowly add sodium hydroxide solution to the KHP solution in the flask until a faint pink color persists in the solution.
8. Record the final volume.

Calculations 

• Divide the mass of KHP used by its molecular weight (204. 2 g/mole) to determine the moles of KHP used (moles of KHP = moles of NaOH).

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. For extra-credit homework, students may work in groups to conduct blind taste tests, if desired.

2. Using the structural formula of citric acid shown in Figure 1, 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)

      {12581_Answers_Equation_2}
   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.
      {12581_Answers_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?
      {12581_Answers_Equation_6}

Sample Data

Titration of Fruit Juices

*Post-Lab Calculation 1.

Results Table

*Per 20.0 mL of juice.

Answers to Questions

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

   See the Sample 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:

   {12581_Answer_Equation_7}

   See the Results Table for all results.

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:

   {12581_Answer_Equation_8}

   See the Results Table for all results.

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:

   {12581_Answer_Equation_9}

   See the Results Table for all results.

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:

   {12581_Answer_Equation_10}

   See the Results Table for all results.

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 based on taste made in PreLab Question 1.

   From most acidic to least acidic:

   Grapefruit juice > Orange juice > Pineapple juice > White grape juice

   Student perceptions of taste will vary.

References

This activity was adapted from Flinn ChemTopic™ Labs, Volume 23, The Chemistry of Food; Cesa, I., Editor; Flinn Scientific: Batavia, IL (2003).

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, primarily 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. In this activity, the “total acidity” of fruit juices will be 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) (see Figure 1).

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. 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 endpoint (the point at which all of the citric acid has been neutralized).

Materials

Prelab Questions

1. Based on your previous observations of their taste, rank the following juices with respect to their perceived acid content: orange, white grape, pineapple and grapefruit.
2. Using the structural formula of citric acid shown in Figure 1, 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

Sodium hydroxide solution is irritating to skin and eyes. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles and chemical-resistant gloves and 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 thoroughly with soap and water before leaving the lab. Please review current Safety Data Sheets for additional safety, handling and disposal information.

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, using a Beral-type pipet, 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 for 5 mL of juice, begin adding sodium hydroxide dropwise (step 13) to the 20 mL of juice after about 8 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 mark.
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).
   {12581_Procedure_Figure_2_How to read a buret volume}
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 water 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. Repeat the titration (steps 8–15) with a second 20.0-mL sample of the same fruit juice.
17. (Optional) If time permits, complete a second set of titrations with a different fruit juice.

Student Worksheet PDF

12581_Student1.pdf