Calibration of a Color pH Indicator Spectrum

Intro: In this lab we will use pH probes and red cabbage juice to determine the pH of the red cabbage juice when other bases and acids are added to it.

Materials:

red cabbage juice

acid solution

base solution

2-250ml beakers

7-clear test tubes

test tube rack

2-1ml plastic pipettes (droppers)

1-pH probe (and associated hardware/software)

Steps:

PART 1:

1. add 100 ml of cabbage juice to a 250 ml beaker. Note the color.

2. insert the Pasco pH probe into the beaker and after a minute, note the pH

of this solution (note: the cabbage juice was prepared using distilled water).

3. pipette 5 ml of this solution into a clean test tube and record the color and pH of this solution

PART 2:

1. With the pH probe remaining in the beaker, add the acid solution, drop by drop (counting the drops), until a pH change of one whole value occurs.

2. Pipette 5 ml of this solution into a new clean test tube, note the color along with the pH and place it alongside the first test tube. you should now have two test tubes in your rack.

3. Again, with the pH probe remaining in the same beaker, continue adding drops of acid to the solution until a second change of one whole value occurs (and again, record the number of drops added).

4. Once again, pipette 5 ml of this solution into a new clean test tube and note the color along with the pH. you should now have three test tubes alongside one another containing different colored solutions, representing 3 specific pH values.

5. Repeat this procedure one more time for a fourth pH value.

Results:

The results we yielded from these experiments were quite interesting. Here they are for each test tube:

Test Tube 1 (Just the Cabbage Juice)

pH of Cabbage Juice: 7.45

Color of juice is purple

Test Tube 2

1 drop more of HCl for -1 pH change to 6.6

Lighter Purple

Test Tube 3:

5 drops more of HCl for -2 pH change to 5.5

Darker Pink

Test Tube 4

11 drops more of HCl for -3 pH change to 4.5

Light Pink

Part C:

7.5 initial pH (Just Cabbage Juice)

Color Purplish Blue

Test Tube 5:

2 drop more of NaOH to raise to 8.6 pH

Teal

Test Tube 6:

7 drops more of NaOH to raise to 9.4

Greenish Teal

Test Tube 7:

7 drops more of NaOH to raise to 10.42 pH

Dark Green

As you can see, adding an acid makes the cabbage juice more acetic and adding a base makes it more basic. The amount of change in ph is also proportional to the amount of drops you add as well as the compound you add. In addition, I would say the acids changed the pH more drastically than the bases since cabbage juice is already slightly basic on its own.