Lab Activity No. 6

pH OF FAMILIAR PRODUCTS

There are many naturally occurring substances that change color in the presence

of an acid or a base. For example, litmus, a blue coloring matter that is extracted from

lichens, turns red in the presence of an acid and is restored to the blue color by alkali or

base. Phenolphthalen, another indicator, is commonly used in acid-base titrations. This

indicator is colorless in acidic solution but turns pink in basic solution.

The indicator you will use in this experiment is derived from red cabbage

(Brassica oleracea). The cabbage contains a purple dye, which is a mixture of different

anthocyanins. These molecules change color in the presence of acid or base.

Anthocyanins are benzopyrilium salts present in many plants. These are the

compounds responsible for many of the foliage colors, the red color of strawberries,

poppies and red cabbage and the blue color of blueberries. Like most indicators the

anthocyanins are weak acids or weak bases. The molecules exhibit one color when a

hydrogen ion is attached to the molecule and a different color when the hydrogen ion is

removed. Anthocyanins in red cabbage change color from red in acidic solution to purple

and green in slightly basic solution and then to yellow in strongly basic solution.

The acidity of a substance is determined by its ability to produce hydronium H3O+

ions in solution. Basicity on the other hand is determined by the ability of a substance to

increase the number of hydroxide OH- ions in solution.

The acidity or basicity of a solution is often expressed in terms of its hydronium

ion concentration [H3O+].

[H3O+] > [OH

-] for acidic solutions

[OH-] > [H3O

+] for basic solutions

[H3O+] = [OH

-] for neutral solutions

However, since the value of [H3O+] can be very high or very low, it is often more

convenient to take the logarithms of the concentration. This is called the pH of the

solution. The pH is defined as – log [H3O+]. A solution containing [H3O

+] = 1.0 x 10

-5,

for example, would have a pH of 5.0. At room temperature solutions with pH less than 7

are acidic, those with pH greater than 7 are basic and those with pH equal to 7 are neutral.

OBJECTIVE: To determine the pH of common household products and classify each

based on its acidic or basic properties using the pH scale.

MATERIALS: stirring rod, spot plates or watch glass, 50 mL beakers, small test tubes,

distilled water, buffer solutions, at least ten different household products (preferably

liquid), test tubes, droppers, beakers, stirrers, osterizer, strainer, buffer solutions, test

reagents (0.5 M sodium chloride, 0.5 M sodium hydroxide, 0.5 M acetic acid, 0.5 M

hydrochloric acid, 0.5 M potassium nitrate, 0.5 M ammonia, 0.5 M phosphoric acid)

Wear protective glasses and an apron at all times. Avoid skin contact with solids and

solutions. Read the labels on all products, record any warnings and take appropriate

precautions for handling. Dispose of all solutions in the containers provided by your

teacher. Wash your hands before leaving the laboratory.

PROCEDURE:

1. ALREADY PREPARED. Extract the anthocyanin dye from red cabbage. This

can be done by pulverizing the cabbage with distilled water in a food blender or

by boiling the cabbage, cut into small pieces, in a 50:50 mixture of ethanol and

water. The longer the pulverization or the boiling, the better the extraction will

be. After straining the mixture, the anthocyanin solution will be ready for use.

2. Some buffer solutions, each with a different pH, will be provided. To 2 mL of

each solution add one full dropper of the anthocyanin indicator. Once the

reference set is complete record each color as descriptively as possible in Table 1.

Use colors or terms that you think most accurately describe each solution.

3. Place 2 mL of each of the following solutions in a test tube and add one full

dropper of the red cabbage indicator to each. Note the colors and record your

results in Table 2.

distilled water 0.5 M hydrochloric acid

0.5 M sodium chloride 0.5 M potassium nitrate

0.5 M sodium hydroxide 0.5 M ammonia

0.5 M acetic acid 0.5 M phosphoric acid

4. Compare the colors of the test reagents against your reference solutions in Table 1

and estimate the pH of each solution. Make a rational classification of each of the

solutions as strongly acidic, weakly acidic, neutral, weakly basic or strongly

basic.

5. Use the household reagents you have brought for testing. Test each product with

the anthocyanin indicator (2 mL of test solution + 1 full dropper of indicator).

Describe the color, and estimate the pH of the solution (Table 3).

Group No. _____ Date: _______

Names: _____________________________

_____________________________

_____________________________

pH of FAMILIAR PRODUCTS

Table 1. Reference anthocyanin colors vs pH

pH Color pH Color

Table 2. pH of Test Reagents

Reagent Indicator Color Estimated

pH

Acid-base

classification

Table 3. pH of some household chemicals

Household Product (kind/brand) Indicator

color pH Classification

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

QUESTIONS:

1. Choose one of the reagents in Table 2 that causes a color change in the indicator.

Is there any way to change the color of the indicator back to the original? Which

other reagent in Table 2 would you choose to do this? What happens in the

solution when you do this?

2. What conclusion(s) can you draw from your results on the acid-base nature of

some household chemicals?

3. Are the pH values of the products tested consistent? Do they make sense? Why or

why not?

4. Predict the mouth - pH range most likely to promote tooth decay. Provide an

answer obtained through library research.

5. Look up the pH value or range found on human skin. Also try finding the

meaning of "pH balanced". (A skin-care book may be helpful.) Were any products

"pH balanced?" Which one(s)? Cite any reference(s) used.

6. State which, if any, pH values surprised you. Why were you surprised?