Acid content of juices

ACID CONTENT OF JUICES INTRODUCTION Sour fruit juices are common sources of weak acids. The fact that fruit juices are acidic can be verified with a pH meter. The pH meter can then be used to monitor the neutralization of the acidic fruit juice with an NaOH solution. OBJECTIVES 1. To use the pH meter to determine the acidity of various fruit juices. 2. To observe the change in pH as NaOH solution is added to the fruit juice. 3. To compare the pH of the various juices and the amount of NaOH solution required to neutralize them. SAFETY 1. Wear your goggles and apron at all times during this experiment. MATERIALS 25.0 mL of fruit juice NaOH solution (approximately 0.10M) 250 mL beaker 50 mL buret ring stand buret clamp magnetic stirring motor and stirring bar graduated cylinder PROCEDURE 1. Obtain about 100 mL of NaOH solution in a clean, dry beaker. This beaker should be labeled. Never pour any solution back into this beaker. Once the solution has been poured into the buret, it should be discarded into the sink or waste container. 2. Rinse your buret with distilled water. Then use a small amount of the NaOH solution to rinse the buret. (Pour about 10 ml of the HCl solution into the buret. Let some of it flow through the tip. Pour the rest of the NaOH solution out the top of the buret, rotating the buret as you pour.) The rinsing solution should be discarded into the sink. Repeat this rinsing procedure twice more. Fill the buret to some point higher than the markings with the NaOH solution and then carefully let the NaOH solution out into a waste container until the bottom of the meniscus is on the 0.00 line. 3. Standardize the pH meter (Refer to the specific instructions for the pH meter that you will be using): 4. Using a graduated cylinder, measure out 25.0 mL of the fruit juice of your choice. (Your teacher will tell you the options.) Pour the fruit juice into a 250 mL beaker. 5. Carefully drop a magnetic stirring bar into the beaker. Set the beaker on the magnetic stirring motor and position the buret containing the NaOH solution and the pH electrode as shown in the diagram. Carefully turn on the stirring motor and make sure that the stirring bar does not hit the electrode. Adjust the stirring speed as directed by your teacher. 6. Measure and record the pH of the fruit juice. 7. Add 1 mL of the NaOH solution and record the pH. Continue to add the NaOH in 1 mL increments until the equivalence point has been passed. This will be noted by a more rapid rise in the pH and then a slowing down. Estimate the middle of the more rapid rise and record this as the mL of NaOH required to neutralize the 25 mL of fruit juice. 2 NOTE: Because the juices are weak acids, the salts formed at the equivalence point are weak bases. Therefore the pH at the equivalence point will be greater than 7. 8. The solutions may be poured down the drain. Remove the pH electrode from the solution and rinse it with distilled water. If time permits, fill up the buret with NaOH solution so that the meniscus is a 0.00, and repeat the titration procedure (steps 6 and 7) with another fruit juice. 9. When all titrations have been completed, remove the pH electrode from the solution, rinse it with distilled water, and store it as directed by your teacher. 10. All solutions may be discarded down the sink. Rinse the buret with distilled water and place it upside down in the buret holder to drain. DATA ANALYSIS 1. Rank the juices in order of increasing acidity. 2. Is there a relationship between the amount of NaOH added to reach the equivalence point and the pH of the fruit juice? What is it? 3. Name several other common beverages that would be acidic. 3