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EXPERIMENT 6: Acids, Bases, and pH Objective The student will
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EXPERIMENT 6:

Acids, Bases, and pH

Objective

The student will learn how to determine the pH of a solution.

Introduction

An acid is a substance that dissolves in water to produce a hydronium ion, H3O+. In the laboratory we use strong acids such as hydrochloric acid (HCl), nitric acid (HNO3) and sulfuric acid (H2SO4).

HCl + H2O ® H3O+ + Cl?

You use acids and bases every day. There are acids in oranges, lemons, vinegar, and bleach. In this experiment we will use acetic acid (CH3CO2H). Acetic acid is the acid in vinegar and gives it a sour taste.

A base is a substance that accepts a proton, H +. Some household bases include ammonia, detergents, and oven-cleaning products. Some typical bases used in the laboratory are sodium hydroxide (NaOH) and potassium hydroxide (KOH). Most of the common bases dissolve in water and produce hydroxide ions, OH?.

NaOH ® Na+ + OH?

An important weak base found in the laboratory and in some household cleaners is ammonia. In water, it reacts to form ammonium and hydroxide ions:

NH3 + H2O ® NH4+ + OH?

The pH of a solution is a measure of the hydrogen ion (or hydronium ion) concentration in the solution and tells us whether a solution is acidic, basic, or neutral. Typically, the pH scale has values between 0 and 14. On this scale, pH values below 7 are acidic, a value equal to 7 is neutral, and a value above 7 is basic. The closer the pH is to 0, the more acidic it is, and the closer to 14 it is, the more basic it is.

Some natural substances contain pigments that change color when the pH changes. One example of such a substance is a pigment found in red cabbage leaves. The colored pigment in red cabbage is one of a number of naturally occurring acid-base indicator dyes (another is litmus, which is extracted from lichen). The red color of cabbage comes from a molecule called an anthocyanin. Many other foods contain anthocyanin, including cranberry juice, black currants, and strawberries. Like other indicator dyes, including phenolphthalein, the structure of the anthocyanin molecule changes between its acid or base forms.

In this experiment, we will extract (remove) this pigment from the leaves of red cabbage and use this natural pH indicator to examine pH values of various solutions. When the colored extract of red cabbage is obtained by boiling, it appears purple. However in acid it appears red, in sodium hydroxide, blue and in calcium hydroxide, green i.e. the purple extract of red cabbage changes to different colors in different pH values. These colors and approximate pH values are:

approximate pH:

2

4

6

8

10

12

color of extract:

red

red-violet

purple

blue

blue-green

green


Materials Needed

Blender (optional) Shampoo

24 well test plate Hair conditioner

Clear drinking glasses (same size) Detergent

Saucepan (with handle) Red Cabbage

pHydrion test paper Distilled water Bleach Bottled water

Mouthwash (clear, not colored) Soda (sprite or 7Up) Antacid Milk

Apple juice Aspirin

Vinegar Lemon juice

Baking soda Salt

Ammonia

A. Preparation of Red Cabbage Extract.

Procedure

1. Take a few leaves of red cabbage and tear or cut them into small pieces. Fill a saucepan about three-fourths full of the cabbage leaf pieces, and add distilled water to cover the cabbage pieces. Heat the mixture until the water is steaming. Continue heating just below boiling for about 20 minutes or until you have a dark purple extract. Stir the mixture occasionally. Be careful not to boil over the cabbage, or to let so much water evaporate that it burns. Allow the mixture to cool and then pour the contents through a strainer or filter paper and collect in a beaker or Erlenmeyer flask. The red cabbage extract appears dark purple, the color of anthocyanin at pH 7. This will be used in the next portion of the experiment. If you are going to keep the solution for a while, put it in the refrigerator.

Alternative Procedure. Cut up a quarter head of red cabbage and put it into a blender

or food processor. Just cover with water and blend the cabbage into slurry (1-3

minutes). Pour the slurry through a strainer or coffee filter collecting the juice in a

beaker. This method does not produce as strong an odor.

Making anthocyanin pH paper (optional).You may have noticed that the pH of a solution is often tested with litmus paper, pHydrion paper, or other similar pH sensitive paper. To prepare your own anthocyanin pH test paper, soak filter paper in freshly prepared dark purple red-cabbage extract. Remove the paper and hang on a clothesline or string to dry. Once dry, cut the paper into strips and test the pH of the solutions listed.

B. Preparation of Solutions.

Procedure

2. To test the pH of liquids (bottled water, soda, apple juice, milk, vinegar, mouthwash), place 2 ml of each liquid into a well in the 24 well plate. For concentrated liquids (shampoo, hair conditioner, detergent, bleach, ammonia, lemon juice) use 1- 4 teaspoon and dilute to 125 milliliters in a colorless drinking glass before placing 2ml of each sample into a well in the 24 well plate. To test the pH of solids (baking soda, salt, antacid, aspirin), crush the solid to a powder if necessary. Place 5 cubic centimeters (1 teaspoon) of the powder in a glass and add 125 milliliters or (½ cup) of water. Stir the mixture until the solid has dissolved. Then place 2ml of each sample into a well in the 24 well plate.

(Each sample of solution to be tested will be in its own well)

On each solution, perform the pHydrion test, first, followed by the Red Cabbage Indicator test.

C. Determination of pH Values

Procedure

3. Obtain several strips of pHydrion test paper, one for each of the solutions prepared above.

4. Using a clean plastic pipette for each solution, transfer one drop of the solution to a strip of the pH test paper.

5. Determine the pH of the solution by matching the color of the paper on which the solution was placed against the color chart on the pH test paper dispenser.

6. Test each solution prepared above and save for the next step.

7. After completing the pHydrion test, add 3 drops of red cabbage extract to each of the wells, stir, and note the color.

8. Record the color of each mixture and determine the pH from the Table above.

The colors will be easy to see if the 24 well plate is placed on a white sheet of paper.

EXPERIMENT 6: Acids, Bases, and pH

REPORT FORM: Name ________________________

Date _________________________

Determine the pH of as many of the following substances that are available with both the pHydrion test paper and the Red cabbage extract and identify if the .

Substance

pH with pHydrion test paper

Color with Red Cabbage Extract

pH. Acid, Basic, or Neutral?

Bottled water

Vinegar

Soda (sprite or 7Up)

Milk

Apple juice

Lemon juice

Mouthwash

Shampoo

Substance

pH with pHydrion test paper

Color with Red Cabbage Extract

pH. Acid, Basic, or Neutral?

Hair conditioner

Detergent

Ammonia

Bleach

Baking powder (NaHCO3) dissolved in distilled water

Sodium Chloride (NaCl) dissolved in distilled water

Aspirin crushed and dissolved in distilled water

Antacid


Substance pH value Colour Acid base or neutral Bottled water

 

Vinegar

 

Soda(sprite)

 

Milk

 

Apple juice

 

Lemon juice

 

Mouth wash

 

Shampoo 6

 

2

 

2

 

10

 

4

 

2

 

4

 

6 Purple

 

Red

 

Red

 

Blue-green

 

Red-violet

 

Red...

 







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