Chemistry 101. Chemistry Experiments for the Home Acidity Determination Using Indicators

Transcription

1 Chemistry 101 Chemistry Experiments for the Home Acidity Determination Using Indicators I. Objective: To determine the acidity of a variety of common substances by the use of indicators. To prepare your own indicators from naturally occurring substances. II. Discussion: Acid-base indicators are generally large organic molecules that react with acids and bases. When they react, the structure of the molecules change and so does the color. Many colored solutions found around the home can be used as acid-base indicators: fruit and vegetable juices, food colorings, inks, and even tea. Some indicators such as litmus and phenolphthalein change color near the neutral point as a solution changes from acidic to basic or vice versa. Others are one color in strongly acidic solution and a different color in weakly acidic and basic solutions. Still others pass through a variety of color changes as the ph changes. Natural indicators have been used for hundreds of years, long before chemists made synthetic acid-base indicators. Robert Boyle described indicators extracted from roses and other plant materials in his book; "The Experimental History of Colors" published in Boyle listed among the properties of acids, their ability to turn plant juices red. Red cabbage contains two principal types of plant dyes, anthocyanin and flavonol. The anthocyanin pigments are red in strongly acidic solution, blue in neutral and weakly basic solutions and colorless in strongly basic solutions. Weakly acidic solutions contain some of the red form and some of the blue form, and thus appear purple. Flavonol pigments are colorless in acidic and neutral solutions, and yellow in basic solutions. Weakly basic solutions thus contain both blue (anthocyanin) and yellow (flavonol) dyes and appear to be green. The color chart for red cabbage indicator is:

2 Red cabbage is not the only natural indicator that goes through a variety of color changes. Cherry juice, for example, may be red (ph = 2.5), orange (ph = 4.5), brown (ph = 7), or green (ph = 10). Litmus is a pigment obtained from lichens (a combination of algae and a fungus). It is red (pink) under acidic conditions and blue under basic conditions. Soaking paper in a litmus solution makes litmus paper. Turmeric is a spice, which is made from ground turmeric tuber, and can be found alone or as an ingredient in curry powder. Turmeric is yellow in acid solutions and pink in basic solutions. Phenolphthalein is a chemical used by chemists as an acid-base indicator, and sold by pharmaceutical companies as a laxative. It is colorless in acid solutions and pink in basic solutions. Goldenrod paper contains a dye that is blue in concentrated solutions of strong acids, gold in other acidic solutions, and red in basic solutions. In this experiment you will test a variety of household substances with some of the above indicators to determine acidities and basicities. III. Materials: Red and blue litmus paper, turmeric, 5 Ex-Lax tablets, rubbing alcohol (isopropyl alcohol), toothpicks, red cabbage, plastic cups, white Styrofoam egg carton with the top removed, vinegar, baking soda solution (1 teaspoon of baking soda in a cup of distilled water), colorless soda (e.g., 7-Up), Milk of Magnesia solution (1 antacid tablet in a cup of distilled water), lemon juice, aspirin (3 tablets in a cup of distilled water), tap water, distilled water, salt solution (1 teaspoon in a cup of distilled water), detergent solution (1 teaspoon liquid detergent in a cup of distilled water), household ammonia water (make sure you wear eye protection when working with ammonia water; also work in a well ventilated area), plastic stirrers, plastic bag, straws. IV. Procedure: A. Red and Blue Litmus: Obtain pieces of red and blue litmus paper from the chemistry lab or from your instructor. Before you test the solutions listed in Table 1, try to predict if they will be acidic, basic, or neutral. Test the solutions with red and blue litmus paper. To test a solution with red litmus paper, transfer a drop of the solution onto the paper with the stirrer. Record the color that is produced in the Table. Wash off the stirrer with a little distilled water before you proceed to the next solution. A piece of litmus paper can be used to test two solutions. One test can be done on each end of the paper. Repeat the tests using blue litmus in the same fashion.

3 In the Table, record whether the solutions you tested are acidic, basic, or neutral. Remember that a neutral solution will not change the color of either red or blue litmus. Table 1 Solution Prediction: Acidic, Basic, or Neutral Color of red litmus paper Color of blue litmus paper Result: Acidic, Basic, or Neutral Distilled water Vinegar Ammonia water Baking soda Colorless soda Milk of Magnesia Lemon juice Tap water Detergent Aspirin solution Salt water

4 Turmeric Indicator: Prepare the turmeric indicator solution as follows. Add 1 teaspoon of turmeric to ¼ cup of rubbing alcohol. Stir and allow to stand for several minutes before using. Place a straw in a solution to be tested. Put you finger over the top of the straw. Remove the straw from the solution, and by slightly raising your finger, transfer 1 drop of the solution to one of the wells in the egg carton. Transfer 1 drop of the turmeric indicator solution to this well and mix with a toothpick. Observe the color, and record your observations in Table 2. Is the solution acidic or basic? In the same fashion, test the rest of the solutions, and record your observations in the Table. Table 2 Solution Color of indicator Acidic or Basic Distilled water Vinegar Ammonia water Baking soda Colorless soda Milk of Magnesia Lemon juice Tap water Detergent Aspirin solution Salt water

5 Phenolphthalein Indicator: Prepare the phenolphthalein indicator as follows. Crush 5 Ex-Lax tablets as fine as possible. If they are individually wrapped, you can crush them with a hammer before removing the wrappers. Mix the crushed tablets with 1 cup of rubbing alcohol. Place a straw in a solution to be tested. Put you finger over the top of the straw. Remove the straw from the solution, and by slightly raising your finger, transfer 1 drop of the solution to one of the wells in the egg carton. Transfer 1 drop of the phenolphthalein indicator solution to this well and mix with a toothpick. Observe the color, and record your observations in Table 3. Is the solution acidic or basic? In the same fashion, test the rest of the solutions, and record your observations in the Table. Table 3 Solution Color of indicator Acidic or Basic Distilled water Vinegar Ammonia water Baking soda Colorless soda Milk of Magnesia Lemon juice Tap water Detergent Aspirin solution Salt water

6 D. Red Cabbage Indicator: Prepare the red cabbage indicator as follows. Chop up 1 cup of red cabbage. Add it to a cup of water and heat the mixture on the stove or in a microwave until the water is a deep purple color. Pour the liquid away from the solid (a process called decanting) and allow it to cool. Transfer 2 teaspoonfuls of the solution to be tested to one of the wells in the egg carton. Transfer 1 teaspoonful of the red cabbage indicator solution to this well and mix with a plastic stirrer or straw. Observe the color, and record your observations in Table 4. Try to tell if the solution is strongly or weakly acidic or basic, or if it is neutral. Repeat the test with the other solutions and record your observations in the Table. Table 4 Solution Color of indicator Acid or Base Strength Distilled water Vinegar Ammonia water Baking soda Colorless soda Milk of Magnesia Lemon juice Tap water Detergent Aspirin solution Salt water

7 E. Other Natural Indicators: Decide which indicators to use from the list below (or others you may wish to try). Squash or rip up berries, petals, etc. and place them in a plastic bag. Add enough hot water to thoroughly soak the plant materials. Seal the bag and knead it, rubbing the contents together. When the plant dye has colored the water, open a corner of the bag and pour the liquid out into a container. Keep the solids in the bag. Possible indicator sources are: roses, violets, primroses, hydrangeas, cherries, beets, blueberries, turnip skins, plum skins, radish skins, rhubarb skins, red grapes, red wine, grape juice concentrate. Transfer 1 teaspoonful of the solution to be tested to one of the wells in the egg carton. Transfer 1 teaspoonful of the natural indicator solution to this well and mix with a plastic stirrer or straw. Observe the color of the resulting solution, and record your observations in Table 5. Table 5 Color of the Solution Plant indicator Test solution Strongly acidic (lemon juice) Weakly acidic (soda) Neutral (distilled water) Weakly basic (baking soda) Strongly acidic (ammonia water)

8 V. Questions: What color is litmus under acidic conditions? What color is litmus under basic conditions? Identify each of the follow ing solutions as being acidic, basic neutral: Both red and blue litmus paper end up blue. Neither color litmus paper changes color. Both red and blue litmus paper, end up red. Is red cabbage indicator more useful than an indicator such as litmus for determining the acidity or basicity of a solution? Is a solution which turns red cabbage indicator green, acidic or is it basic? Is a solution which turns red cabbage indicator violet, acidic or is it basic? If a solution turns red cabbage indicator bright yellow, is it safe to touch or taste? What color is turmeric under acidic conditions? What color is turmeric under basic conditions? What color is phenolphthalein under acidic conditions? What color is phenolphthalein under basic conditions? Which of the plant extracts that you chose for Part E could be used as acid-base indicators?