Titration of Fruit Juices

Titration of Fruit Juices 18 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). OH OH HOOC CH2 C CH2 COOH COOH Citric acid HOOC CH CH OH COOH HOOC CH2 CH COOH OH Tartaric acid Malic acid Figure 1. Organic Acids in Fruits 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. OH OH – HOOC CH2 O2 C CH2 C CH2 CO2 – + 3H2 O C CH2 COOH + 3NaOH Equation 1 COOH Citric acid CO2 – Citrate ion 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. Laboratory Experiments for General, Organic and Biological Chemistry 1 Titration of Fruit Juices continued Materials Fruit juices, 50 mL each (orange, white grape, pineapple, and grapefruit, etc.) Buret, 50-mL Buret clamp Phenolphthalein indicator solution, 0.5 or 1%, 2 mL Erlenmeyer flask, 125-mL Sodium hydroxide (standard) solution, NaOH, 0.100 M, 100 mL Graduated cylinder, 25- or 50-mL Water, distilled or deionized Ring stand Beaker, 50-mL, or small cup Test tube, large Beakers, 100- or 150-mL, 2 Wash bottle Beral-type pipet, graduated White paper for background Safety Precautions  ilute sodium hydroxide solution is irritating to skin and eyes. Notify the instructor and clean up all spills D immediately. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemicalresistant 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. 23 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. 23.45 mL (not 24.55) 24 Figure 2. 2 Laboratory Experiments for General, Organic and Biological Chemistry Titration of Fruit Juices continued 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. Laboratory Experiments for General, Organic and Biological Chemistry 3 Pre-Laboratory Assignment 18 Name:_____________________________________________   Section:__________   Date:___________ Titration of Fruit Juices 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, 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. a. Calculate the number of moles of sodium hydroxide required to reach the endpoint. b. 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. c. M  ultiply 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. d. T  he 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? 4 Laboratory Experiments for General, Organic and Biological Chemistry Laboratory Report 18 Name:_____________________________________________   Section:__________   Date:___________ Titration of Fruit Juices Juice 1: ____________________ Juice 2: ____________________ Rough Titration Data (Step 4) Estimated Volume of NaOH Needed to Titrate 20.0 mL of Juice Precise Molarity of NaOH Standard Solution Trial 1 Trial 2 Trial 1 Trial 2 Initial Buret Reading Final Buret Reading Volume of NaOH Added at Endpoint* *Per 20.0 mL of juice. 1. Determine the volume of sodium hydroxide added at the endpoint for each trial and enter the results in the data table. 2. Calculate the number of moles of sodium hydroxide required to reach the endpoint for each trial. 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. Laboratory Experiments for General, Organic and Biological Chemistry 5 Titration of Fruit Juices continued 4. Calculate the mass in grams of citric acid in 20.0 mL of juice for each trial. 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? 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 Pre-Lab Question 1. 6 Laboratory Experiments for General, Organic and Biological Chemistry