- Acid-base indicators are used in titrations to let you know when the solution has been neutralized, and has a pH of about 7. This indicates the end point of the titration, and we can then calculate the concentration of our unknown, from the concentration and volume of the known. We have many acid-base indicators available to us today, but the first indicators were made from natural compounds.
- The indicator that you will be making comes from purple cabbage. And is simply called cabbage extract indicator.
Part 1: Making the indicator
MATERIALS:
Purple cabbage
Distilled water
250-mL beaker
Ring stand
Large ring
wire gauze
BurnerPROCEDURE:
1. Take a few leaves of purple cabbage and tear them into small pieces.
2. Fill a 250-mL beaker about three-fourths full of the cabbage leaf pieces, and add distilled water to the 200mL mark.
3. Place the beaker on the wire gauze of a ring stand and heat with a burner to a slow boil for about 20 minutes. (You can use a hot plate in place of the burner and ring stand.)
4. The water will turn from a clear color to a purple color. When the water is very purple, remove the beaker from the heat. The purple juice which you have made is cabbage extract indicator, and can be used for acid-base titrations.
5. Pour the cabbage extract indicator solution into a labelled stock bottle. If you are going to keep the solution for a while, put it in the refrigerator.
Part 2: Testing the indicator
- You now have an indicator which can tell you of you have a solution that is an acid or a base, and can help you determine the end point of a titration. You need to test some acid and base solutions so you will know what color it turns in each.
MATERIALS:
Cabbage extract indicator
Transfer pipette (eye dropper) for indicator
7 test tubes 13 X 100mm
Test tube rack
Acid solutions:
- Vinegar
- Hydrochloric acid solution
- Sulfuric acid solution
- Lemon juice
Basic solutions:
- Baking soda solution
- Ammonia
- Sodium hydroxide solution
- Potassium hydroxide solution
PROCEDURE:
1. Put a small amount of the test solutions in test tubes that are clearly marked, and place them in the test tube rack.
2. Using the dropper, add 3 drops of indicator to each test solution. Observe the color change and compare the test solutions to the indicator alone. Record the color changes in the DATA TABLE.
DATA TABLE: TEST SOLUTION: ACID or BASE: COLOR CHANGE: 
QUESTIONS:
1. What color does the indicator turn in acidic solutions?
2. What color does the indicator turn in basic solutions?
Part 3: Using the indicator to titrate a base with an acid.
You will now determine the concentration of a base that has an unknown concentration with an acid that has a known concentration.
MATERIALS:
Transfer pipette (eye dropper)
Watch glass or very small test tube 13 X 100mm
Cabbage indicator
Base with unknown concentration*
0.1 M hydrochloric acid*** Dissolve 1.0 gram of Sodium hydroxide, NaOH, in enough distilled water to make a total volume of 500 mL.
** Add 8.6 mL of concentrated HCl in about 250 mL of distilled water, then add distilled water to a total volume of 1 L.
PROCEDURE:
1. Place 10 drops of the unknown base solution into a watch glass with a dropper.
2. Add two drops of the cabbage indicator to the watch glass.
3. Clean the dropper you used for the unknown with distilled water, then rinse it several times with the acid solution you are going to use. You need to use the same dropper so the drops will be the same volume.
4. Using the prepared dropper, slowly add 0.1 M hydrochloric acid drop-by-drop until the color of the indicator changes. Swirl the watch glass gently between drops. When it changes, you have reached the equivalence point and you are finished with the titration.
CALCULATIONS:
5. Calculate the molarity of the base using the following formula:
With M for molarity, and V for volume in drops; with 1 the acid and 2 the base:- M1V1 = M2V2 rearrange to: M2 = M1V1 / V2
- This simple formula only works if the equivalence of the acid and base are the same. If not, you must use normality in place of molarity.
Part 4: Using the indicator to titrate an acid with a base.
You will now determine the concentration of an acid that has an unknown concentration with a base that has a known concentration.
MATERIALS:
Transfer pipette (eye dropper)
Watch glass or very small test tube 13 X 100mm
Cabbage indicator
Acid with unknown concentration*
0.1 M sodium hydroxide*** Add 4.3 mL of concentrated HCl in about 250 mL of distilled water, then add distilled water to a total volume of 1 L.
** Dissolve 4.0 grams of NaOH in about 250 mL of distilled water, then add distilled water to a total volume of 1 L.
PROCEDURE:
1. Place 10 drops of the unknown acid solution into a watch glass with a dropper.
2. Add two drops of the cabbage indicator to the watch glass.
3. Clean the dropper you used for the unknown with distilled water, then rinse it several times with the base solution you are going to use. You need to use the same dropper so the drops will be the same volume.
4. Using the prepared dropper, slowly add 0.1 M sodium hydroxide drop-by-drop until the color of the indicator changes. Swirl the watch glass gently between drops. When it changes, you have reached the equivalence point and you are finished with the titration.
CALCULATIONS:
5. Calculate the molarity of the acid using the following formula:
With M for molarity, and V for volume in drops; with 1 the base and 2 the acid:- M1V1 = M2V2 rearrange to: M2 = M1V1 / V2
- Again, this simple formula only works if the equivalence of the acid and base are the same. If not, you must use normality in place of molarity.
Back to Chemistry Table of Contents
Back to Lab Dad's Laboratory
- The indicator that you will be making comes from purple cabbage. And is simply called cabbage extract indicator.
MAKING AN ACID-BASE
INDICATOR
From Purple Cabbage
INTRODUCTION: