The ACID/BASE Characteristics of Flower Pigments
Instructor: Oliver Seely, Jr., Professor of Chemistry
Revised September 22, 2014
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
For example: grape juice, brown tea,
some flower pigments.
These substances are called acid/base
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.
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.
|Name of flower||Starting color||Ending color||Number of drops to color change||pH of color change|
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
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
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:
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)
MISS ALL AMERICAN BEAUTY
If this experiment is to be done as a
demonstration only, here are the equipment and materials
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
instructor ought to have 100 mL of 1.0 M NaOH and 1.0 M HCl as stock solutions for refilling
the 4 ounce bottles if needed.
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.