The Colors of Cabbage

Alexandria Chisum

Chemistry 130

Lab Report

12/13/16

Title:

The Colors of Cabbage

Introduction:

        In most cases, we cannot determine the pH, or hydrogen ion concentration of a solution. Instead we use another solution called an indicator. Indicators are solutions that change color at a specific pH. You have already used the indicator phenolphthalein; this indicator turns bright pink in basic solutions. A universal indicator has many color changes at different pH values. The purple extract from red cabbage leaves provides a rainbow of different colors at different colors at different pH values. Universal indicator is the solution used to make pH paper, this solution also has a variety of color changes at different pH values.

        Buffers are special solution that contain a mixture of a weak acid and its conjugate base. Buffers act to resist changes in pH when acid or base are added to the solution. The weak acid ions react with any added base to form additional conjugate base ions (remember acid + base = water + salt, the conjugate base ion is the salt.) The conjugate base ions react with any added acid to form additional weak acid molecules. However, if too much acid is added then all the conjugate base ions will be used up. Once that happens, any additional acid will lower the pH quickly. The same happens if too much base is added, the weak acid molecules are used up and the pH rises.

        In this lab exercise you will extract the red cabbage to create your own universal indicator and use it to test a variety of household substances. Then you will test the effectiveness of a simple buffer system.

Materials:

Procedure:[pic 1]

1.  pH and concentration:

1. The pH of a substance lets us know if it is an acid or a base. The pH is based on the concentration of hydrogen ions. The more hydrogen ions in a solution, the more acidic the solution is.

1. Preparation of Red Cabbage Indicator:

1. Take 2 cabbage leaves and cut them into small pieces.
2. Place the pieces into a mortar and pestle and cover with 10mL of isopropyl alcohol.
3. Grind the leaves until the solution becomes a dark purple.
4. Add an additional 10mL of isopropyl alcohol to the mortar and pestle.
5. Carefully pour only the purple alcohol solution into 50mL beaker, discard the cabbage leaves in the trash.

1. pH of Household Items

1. Take 10 small test tubes and arrange them in a single row in a test tube rack.
2. Using disposable dropper add 10 drops of the red cabbage indicator to each test tube.
3.  Using a clean disposable dropper add 10 drops of one household substances into the first test tube.
4. Compare the color of the solution to the colors on the “Cabbage Juice pH Chart”, determine the pH of the solution. Record the name, color, and pH of the solution in the Part B data table.
5. Repeats steps 3 and 4 for each of the household substances. Use clean disposable dropper and a separate test tube for each household substance.

1. Buffers

1. Rinse the test tubes used in Part B with distilled water. Remove as much water from inside the test tubes as possible, they do not need to be completely dry.
2. Take 8 small test tubes and arrange them in two rows of four in attest tube rack.
3. Place ~3mL of distilled water into the first test tubes in each row, ~3mL of pH 4 buffer into the second test tubes, ~3mL of 7 pH buffer into the third test tubes, and ~3mL of pH 10 buffer into the fourth test tubes. You may wish to label the test tubes with a marking pen or grease pencil.
4. Add 5 drops of red cabbage indicator to each test tube. Record the color of each solution in part D data table
5. To the first row of test tubes, add 0.1M NaOH one drop at a time until a stable color change is observed, color should remain the same for 30 seconds.
6. To the second-row test tubes, add 0.1M HCI one drop at a time until a stable color change is observed, color should remain the same for 30 seconds. Stir the test tube after each drop is added, stirring can be done by gently shaking the test tube side to side or by using a clean disposable dropper to bubble air through the solution.
7. Record the number of drops needed and the new color of the solution in the Part C data table.

Observations and results:

1.  pH and Concentration:[pic 2]