Today we will study the scale which measures acidity of a solution. It is called the pH scale. Then we will determine the pH at which a flower pigment changes color.

Many common substances can be classified as acids or bases.

Examples. Acids: Vinegar, wine, grape juice

Bases: Shampoo, soap, drano, household ammonia, window cleaner.

Neutral: Water, blood, urine, saliva.

Some naturally colored substances change colors when the acidity (or basicity) of their environment changes.

For example: grape juice, brown tea, some flower pigments.

These substances are called acid/base indicators.

Acidity and basicity are determined on the pH scale, which varies from 1 to 14. pH 7 is neutral. Anything that has a pH less than 7 is said to be acidic. Solutions with a pH greater than 7 are said to be basic.

Before starting: PUT ON YOUR SAFETY GLASSES.

PROCEDURE

1. Put on your safety glasses.

2. Add 200 mL acidified alcohol to a 250 mL beaker.

3. Add pieces of two or three flowers to alcohol in beaker.

4. Set aside until you get to step 10 below.

5. Rinse out a 50 mL erlenmeyer flask with distilled water.

6. Add 20 mL 0.005 Molar phosphoric acid to flask.

7. Add 0.5 mL Universal Indicator to flask.

8. Half-fil 50 mL beaker with 0.1 Molar sodium hydroxide (NaOH).

9. Add dropwise sodium hydroxide to erlenmeyer flask solution.

Record number of drops at each color change.

10. Discard solution and rinse erlenmeyer flask with distilled water.

11. Add 20 mL of the flower extract prepared in step 2.

12. Add dropwise sodium hydroxide until the color changes.

Record the number of drops.

13. Determine the pH of the color change.

pH TABLE Flower Data

Drop # Color pH Name of Flower

______ Red 4.0 ___________________________

______ Orange-red 5.0

______ Orange 5.5 Number of drops required to

produce a color change:

______ Yellow-orange 6.0

__________________

______ Yellow 6.5

______ Yellow-green 7.0 What pH does that number

of drops represent?

______ Green 7.5

__________________

______ Dark green 8.0

______ Blue-green 8.5

______ Blue 9.0

______ Violet 9.5

______ Red-violet 10.0

Materials Required

Each student:

1 wash bottle with distilled water

1 50 mL erlenmeyer flask

1 polycarbonate eye dropper (pipette)

1 sheet white paper

1 50 mL beaker

1 pair safety glasses

Each row:

1 set of flowers for every five students

keep stems in water until used

1 250 mL beaker for every five students

1 heavy duty stirring rod for each beaker

Each session of 30 students

2000 mL 0.005 Molar H₃PO₄ in bottle set to dispense 20 mL for each student.

1 4-oz amber bottle of Universal Indicator for each row, with polycarbonate pipette attached at side of bottle.

2000 mL ethanol with 10 mL 1.0 Molar H₃PO₄ added and set to dispense 200 mL.

3000 mL 0.1 M NaOH

For instructor:

3 250 mL erlenmeyer flasks at instructors desk

3 sturdy stirring rods

1 4 oz bottle with eyedropper containing 1M NaOH

1 4 oz bottle with eyedropper containing 1M HCl

Bottle of grape juice

2 tea bags

FLOWERS THAT DON'T WORK:

YELLOW DAISIES

DAFFODILS

DANDILIONS

OTHER THINGS THAT DON'T WORK

RED HAWAIIAN PUNCH

FLOWERS GUARANTEED TO WORK:

CSUDH LILIES AT STAGE OF YELLOW OR RED

LIGHT BLUE DAISIES

HIBISCUS(yellow and red)

RED ROSES

PINK ROSES

JACQUERANDA BLOSSOMS

SWEET WILLIAMS

PURPLE MUMS

MISS ALL AMERICAN BEAUTY

EUROPIANA

If this experiment is to be done as a demonstration only, here are the equipment and materials needed:

1 wash bottle & distilled water

1 150 mL erlenmeyer flask

1 eye dropper

1 sheet white paper

1 50 mL graduate

1 10 mL burette

1 L .005 M H₃PO₄

1 4 oz bottle of universal indicator & dropper

1 L ethanol with 5 mL 1.0 M H₃PO₄ added

1 L 0.1 M NaOH

3 500 mL graduates

6 250 mL beakers

1 4 oz bottle with dropper containing 1.0 M NaOH

1 4 oz bottle with dropper containing 1.0 M HCl

For each hour demonstration the 1.0 M NaOH and 1.0 M HCl need a quantity of 100 mL of each as stock solutions for refilling.

The demonstrator uses 60 mL of the phosphoric acid and 60 mL of the alcohol solution of pigment instead of the 20 mL listed. The demonstrator uses the 1 M acid and base exclusively to show color transitions. Only if the group is small can he use the 50 mL burette to illustrate the slow transition of color and the pH at which it occurs.