Question

Week 2 Pre-Lab Organizational Questions and Activities 1. What is the approximate pH of white grape juice? 2. a. What sources did you use to find this information? b. How do you assess the reliability of the sources you consulted? 3. How will you measure the actual pH of white grape juice? 4. The indicator you will use (phenolphthalein) turns from colorless to pink at pH-8. It is very important that the color change is noticed immediately. How would the optical clarity of the grape juice affect your experiment and what can be done to enhance the visibility of the color change? 5. Given the variety of juice samples, devise a testable question that can be answered by your inquiry into acid content. You may want to think about the characteristics of the juice samples in order to do this (e.g. "light/diet," store brand vs. national brand, etc.) 6. Given your testable question, make a prediction about the outcome? (e.g. Juice A will be more or less because...) 7. Outline a preliminary plan to determine the concentration of acid of various samples of white grape juice to answer your testable question.

Answer 1

1. The approximate pH of white grape juice is 3.3 (2.9-3.9, in general).

2. a. I have used the ADA (American Dental Association) website.

b. The reliability of the sources I consulted may be assessed by comparing and contrasting the data. The selection of authorized sources like ADA is important.

3. A well calibrated pH meter can be used. The pH of each beverage may be measured in triplicate immediately after it was opened at a temperature of 25 C.

4. Since the end point is colorless to immediate pink, the optical clarity of the juice is significant. Presence of any color in the juice may adversely affect the assessment of the end point. The juice should be filtered before the experiment.

5. Phenolic content of grape skin varies with cultivar, soil composition, climate, geographic origin, and cultivation practices or exposure to diseases, viz., fungal infections. You may ask: Is Tropicana grape juice made from Niagara grapes?

6. Tropicana grape juice typically has a pH of ~ 3.3. Normally, white grapes have a low acidity level (pH ~ 3.3).

7. Total Acidity or TA measures all of the combined acids in the juice. This is more precise and accurate than pH, which is a measure of how acids balance out against buffering compounds, viz., potassium. Higher the TA, more is the acidity, whereas lower the pH, more is the acidity.