EXPERIMENT 8b

[phcab.lab]

OBJECTIVES:

1. To relate the relative acidity of a solution with the numerical value of pH .

2. To be able to use indicators to determine the pH of solutions.

3. To be able to use a pH meter for measuring pH.

4. To learn the pH of some common substances.

DISCUSSION:

In this version of the experiment, we will use red cabbage (RC) extract as the indicator. It covers all of the pH range, ranging from red for very acid to yellow for very basic solutions. To save time, the Instructor will prepare the RC extract ahead of time. However, you should know how to do this if necessary.

To decide the pH of a sample, we can first use buffer solutions of known pH from 1 to 13 and add the RC to these. Each will be a slightly different color. Then we can use these colors to compare with the Samples to decide the pH of the Samples. We did this before and made a pH color chart for each pH. A pH of 1.0 is very acidic; 7.0 is neutral; 14.0 is very basic (or alkaline).

EXPERIMENTAL PROCEDURE:

Part A. Preparing the Red Cabbage extract.

1. Cut up red cabbage leaves. Place them in a large beaker and cover with distilled water. Boil until the liquid is strongly colored. This liquid is the "extract".

2. Filter the extract (through a glass wool plug or a coarse paper filter) into a flask. Keep the extract refrigerated until used.

Part B. Work alone or as directed by the Instructor.

1. To a clean test tube, add about 2 mL of the RC extract. To this add 2 mL (or one dropper-full) of vinegar. Record the color. Match this color with the color on the pH chart and record the pH of the vinegar. If the pH chart is not available, we will use a series of Buffer Solutions.

2. Repeat Step 1 for each sample available. Be sure that the test tubes you use are clean. They do not have to be dry. Record the color seen and the pH in each case.

Suggested: Drano, club soda, vinegar, Rolaids, Borax, baking soda, lemon juice.

QUESTIONS

1. What is the pH range of most cleaning products? What ion is probably responsible?

2. What will be the pH values of handsoaps and detergents for delicate fabrics?

3. Account for the difference in pH between 0.1 M HCl, 0.1 M HC₂H₃O₂ (vinegar) and 0.1 M H₃BO₃ (eye wash solution). All three have the same number of molecules in a given volume of solution.

4. List 3 foods or drinks that you would expect to be in the acid range and 3 in the neutral range. Do not list those already tested.

ACIDIC FOODS NEUTRAL FOODS

1. 1.

2. 2.

3. 3.

( There are very few that are basic !! )