Lab: pH and Indicators

PROBLEM: Observe and record the colors of acid-base indicators at various pH values. Measure and record

the pH values at which indicators change colors. Identify the pH of a couple unknown solutions.

INTRODUCTION: An acid-base indicator is usually a weak acid with a characteristic vivid color. Upon the

addition of base, this acid is converted into its conjugate base, which is a different color. When the weak acid

and its conjugate base exist in solution in approximately equal concentrations, an intermediate color is evident.

For example bromthymol blue is a weak acid that is yellow; its conjugate base is blue. When the weak acid and

its conjugate base are in equal concentration the resulting color is green. A mixture of yellow and blue. This is

illustrated by the following chemical equation:

HBTB + H2O <-------> H3O+ + BTB

-

yellow blue

Using Le Chatelier’s think how this reaction would respond with excess acid or excess base.

PROCEDURE:

1. Clean and dry with a paper towel two 24 well plates.

2. Orientate the well plate so that you have 4 wells across and 6 wells down, side by side.

3. You have 6 solutions (pH 1,3,5,7,10,12) and 6 indicators ( Orange IV, Methyl Orange, Methyl Red,

Bromthymol blue, Thymol blue, Phenolphthalein). You will be using 3-5 drops of the solutions and 1-2 drop of

indicator. Set them up as illustrated below.

pH 1 pH 3 pH 5 pH 7 pH 10 pH 12 unk 1 unk 2

Orange IV

Methyl

Orange

Methyl Red

Bromthymol

Blue

Thymol blue

Phenolphthalein

4. Note the pH at which each indicator changes color (significant change).

5. Now test your unknown solution(s). Estimate the pH of the unknown by comparing with the known

reactions.

CLEAN UP: Carefully clean the wells by absorbing the contents onto a paper towel. Rinse the wells and dry

them with a paper towel. Dispose of the paper towels in the waste bin. WASH YOUR HANDS

THOROUGHLY WITH SOAP AND WATER.

QUESTIONS AND CALCULATIONS:

1. At what pH range does each indicator change? Make a list of the indicators and the pH range at which they

change.

2. Write a balanced equation for each indicator using the equation on the front as a guide. Determine the Acid,

Base, Conjugate Acid, and Conjugate Base for each equation. Finally include the color of the acid and it’s

conjugate base.

3. Identify the pH of each unknown. Explain (using specific examples) how you arrived at you conclusion.