EXPERIMENT 8a

[pH.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:

Every aqueous (water) solution is acidic, basic (alkaline) or neutral. These adjectives are not very useful, since we often need to know HOW acid a solution is. For example, lemon juice, boric acid and hydrochloric acid are all acidic, but they are not equally acidic. Boric acid can be used as an eye wash, since it is a very mild acid. Lemon juice would irritate the eye, but it is not strong enough to hurt us if we swallow it. To help us judge how acidic or basic a solution is, scientists have invented the pH scale, which ranges from 0 to 14.

On the pH scale, 0 = very strong acid

5 = weak acid

7 = neutral (pure water)

9 = weak base

14 = very strong base

You may have used litmus paper to test for acids. Litmus is an "indicator" that changes color with acids and bases. It is red in acids and blue in bases. It is useful, but it really is not a very good indicator. Many natural substances act as indicators, too. For example, tea and red cabbage leaves will change color in acids and bases.

The indicators we will use (and their abbreviations) are:

thymol blue (TB) methyl orange (MO)

methyl red (MR) bromthymol blue (BTB)

phenolphthalein (PP) thymolphthalein (TP)

tropeolin O (TO)

Each of these indicators has a specific pH range over which it changes color. Before and after this range, the colors remain constant. In Part A, you will work on one or two indicators. One or two drops of your assigned indicator will be added to each of the "buffer" solutions (these are solutions of known pH). You will record the color in each case and put this on the blackboard on the master chart. You will be able to see what the RANGE of each indicator is.

In Part B, each group of students will find the pH of an unknown solution as a demonstration in front of the class. The instructor will also demonstrate the use of a pH meter at this time.

EXPERIMENTAL PROCEDURE and LAB REPORT: Name ___________________ Sec ___

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

1) Clean 13 test tubes. Add one buffer solution to each test tube. Use about 1-2 mL. Label these pH 1 to pH 13 .

2) Find your assigned indicator. Add 1-2 drops of this indicator to each test tube. List the colors of the indicator at each pH.

3) Record these colors on the blackboard. Use the following abbreviations: red= R, orange= O, blue= B, yellow= Y, green = G and colorless= C.

4) Copy the other students' work for the other indicators.

Part B. 1-6: To be done in groups; 7: To be done alone.

1. Select ANY indicator and add it to 1-2 mL of the solution to be tested. From the color, you can decide that the pH is greater than or less than some pH value.

2. Choose another indicator and determine the possible pH.

3. Repeat until you are sure of the pH. You should NOT have to use more than 4 indicators.

Suggested: Drano, 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/drinks that you would expect to be in the acid range and 3 in the neutral range. Do not list those tested.

ACIDIC FOODS NEUTRAL FOODS

1. 1.

2. 2.

3. 3.

( There are very few that are basic!! )