Name: _____________________________ Assignment #: ________ 06-07
Date: ________________ Period: ______ Lab Partner: _______________
Lab: Creating a pH Scale Using Natural Indicators
Introduction:
Roses are red, violets are blue – or are they? Red roses, as well as many other flowers,
fruits and vegetables, contain natural indicators that are sensitive to acids and bases. The
color of natural acid-base indicator depends on pH. One of the most well known effects
of natural indicators in plants occurs in the hydrangea or snowball plant. Hydrangea
flowers are blue when grown in acidic soils, pink or red in basic soils. How do the colors
of natural indicators vary with pH? We will investigate this question using purple
cabbage as our indicator.
Background:
Indicators are dyes or pigments that can be isolated from a variety of sources, including
plants, fungi, and algae. Almost any flower, for example, that is red, blue, or purple in
color contains a class of organic pigments called anthocyanins that change color with pH.
The use of a natural dyes as acid-base indicators was first reported in 1664 by Sir Robert
Boyle in his collection of essays Experimental History of Colours. Indeed, Boyle made
an important contribution to the early theory of acids and bases by using indicators for
the experimental classification of these substances. The idea, however, may actually
have originated much earlier-medieval painter used natural dyes treated with vinegar and
limewater to make different color watercolor paints.
Acid-base indicators are large organic molecules that behave as weak acids-they can
donate hydrogen ions to water molecules to form conjugate bases. The distinguishing
characteristic of indicator is that the acid and conjugate base forms are different colors.
Natural indicator solutions are obtained by treating flowers and fruits with a solvent to
remove (dissolve) the soluble components. This process, called extraction, is similar to
the procedure used to make a cup of tea using a tea bag. The solid is crushed or ground
and extracted with an appropriate solvent, such as boiling water.
The color of an acid-base indicator depends on the concentration of H3O+ ions, which is
most conveniently expressed using the pH scale. The mathematical relationship between
pH and [H3O+] is given below.
pH = -log{H3O+]
The H3O+ concentration in water ranges from 100 in hydrochloric acid to 10-14 in sodium
hydroxide. In pure water, which is neutral (neither acidic nor basic) the H3O+
concentration is equal to 10-7. The logarithm of the concentration is the “power of ten”
exponent in these concentration terms. For ease of understanding, the pH scale ranges
from 0-14, with 7 being neutral. Acids have pH values less than 7, while bases have pH
values greater than 7. Remember that the pH scale is logarithmic- a solution of pH 3 is
ten times more acidic than a solution of pH 4, and 100 times more acidic than a solution
of pH 5.
Purpose:
You will make a pH scale by using a set of standard acid and base solutions of known
pHs (pH 2-12). Cabbage juice will be applied to the known pH causing a color change.
Data will be recorded by using colored pencils. This chart will be used as a reference for
future lab activities.
Materials:
o Lab sheet
o Spot plate
o Known solutions ranging from 2-12 on the pH scale
o Colored pencils
o One partner
o Safety goggles
Procedure:
1. Visit station 1 (you may start at any station)
2. Place 4 drops of the known pH solution into one depression of the spot plate.
3. Place 4 drops of purple cabbage juice indicator in the same depression as the
known pH. A color change should take place.
4. Record the color on your data sheet using a colored pencil to fill in the box.
5. Rotate to another station until you have visited all ___ of the stations.
6. Go to the sink and rinse off your spot plate. Turn it upside-down to dry on the
paper towels.
7. Return to your seat and answer the questions.
Safety Precautions:
The standard acid and base solutions used in this experiment are body tissue irritants.
Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles. Wash
hands thoroughly with soap and water before leaving the laboratory.
Pre Lab Questions:
1. What should you do if a chemical is splashed in your eye? For how long?
2. What is the first thing you should do if a chemical touches your skin?
3. What is the second thing you should do if a chemical touches your skin?
4. What happens to students who are not following directions during a lab activity?
5. Where is the safety shower located?
6. Where is the eye wash located?
7. How many drops of the known pH do you put in the spot plate depression?
8. How many drops of cabbage juice do you put in the spot plate depression?
Data:
pH 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Color
Strong Acids….weak acids…….…..Neutral……...weak base...…...strong bases
Questions: (use your notes and this lab to help you find answers)
1. Describe a hydronium ion.
o Charge =
o Example using element symbols=
o Acid or base?
o Proton acceptor or donator?
2. Describe a hydroxide ion.
o Charge =
o Example using element symbols =
o Acid or base?
o Proton acceptor or donator?
3. How do we make natural indicator solutions?
4. What is a logarithm?
5. A pH on 1 is how many times stronger than a pH of 3? Show your work!
6. What is an anthocyanin?
7. What is the function of an indicator?
8. Define and give an example of extraction.
o Definition =
o Example =
9. Define corrosive and give an example (strong/weak, acid/bases) using your new
pH scale.
o Definition =
o Example =
10. A pH of 10 is how many times weaker than a pH of 14?
11. If Joann received 78 points out of 90 on her lab, what grade did she receive?
o Show work =
o Joann’s percent =
o Joann’s final grade on the lab =
12. If Keith received 92 points out of 120 on his unit packet, what was his percent and
final grade on the packet?
o Show work =
o Keith’s percent =
o Keith’s final grade on his unit packet