pH Indicator Lab

Introduction:

Acid-base indicators are substances that change color as a function of pH, usually over a range of 1 or 2 pH units. These indicators are themselves weak acids or weak bases. If we symbolize a weak acid indicator as HIn, then its conjugate base would be In-. The equilibrium between the two forms is shown by

HIn(aq) <--> H+(aq) + In-(aq).

Indicators work because HIn and In- are different colors. For example, the HIn form of litmus is red, and its conjugate base, In-, is blue. When litmus is put in acid solution, LeChatelier's Principle tells us that the above equilibrium shifts toward the left. This will produce more of the red form of litmus. A basic solution will remove H+ ions causing a shift to the right. Then litmus will be predominantly in its blue form.

For litmus, the shift in color occurs between pH 5 and pH 7. Litmus is often used to determine if a solution is acidic or basic. This type of classifying is why the phrase "Litmus Test" came about. Other indicators change color over different pH ranges; some are in the middle of the pH range, while others are at the ends. A large selection of indicators is available, and there are indicators which change color for nearly every value on the pH scale. These different indicators are useful for finding the pH of many different solutions. The earliest indicators to be used were extracted from plants. Many of the compounds that lend flowers and leaves their color are weak acids or bases, and thus can be used as indicators.

Finally, you will use the indicator information in order to determine the pH of different solutions.

Purpose:

What are some common acid-base indicators, over what pH range does each indicator change color, and what are the colors?

Procedure. Part 1.

1. Determine the color of each indicator (1 drop) with each pH solution (2 drops).

Make a data table for the information:

There are 5 indicators: methyl red, bromthymol blue, phenolphthalein, orange IV, and alizarin yellow R. There are 7 pH solutions: 2, 4, 6, 7, 8, 10, 12.

2. Draw 5 horizontal lines in your lab notebook that are 15 squares long. Label the start of the line with 0 and the appropriate place for 14. Label each line with a different name of indicator. In the correct number placement, fill out a color line for each indicator. Figure out what pH the transition color is for each indicator.

3. Decide which indicator best determines whether the tested substance is an acid or a base.

4. Clean the well plates.

Part 2.

5. Chose four unknowns. Put ten drops of each unknown in three separate wells.

6. Using the indicator you chose in step 3, determine whether the substance was an acid or a base.

7. If the unknown was an acid, chose two more indicators that would best help you determine what the pH range of the substance would be, and use them in the other two wells.

8. If the unknown is a base, chose two other indicators that would best help you determine what the pH range of the substance would be, an use them in the remaining wells.

9. After cleaning all of the supplies, put the materials in the proper places.

Picture 1: pH Buffer solutions with 5 different indicators

Picture 2: Unknowns with selected indicators.

Note: All of the unknowns were mixed with bromthymol blue in the first well. The next well had 2 of the remaining 4 indicators. The indicators were chosen based on the results from the reaction with the bromthymol blue.