Grade Level: 6th to 9th; Type: Physical Science
This science fair project consists of an investigation of several tea types (black, green, oolong and white tea) that have been commercially decaffeinated for the presences of detectable traces of caffeine using a tannic acid assay.
- What is caffeine?
- What is tannic acid and why is it used in this science fair project?
- What is the difference between caffeinated tea and decaffeinated tea?
- What is Gunpowder Green tea and why is it used as a control in this science fair project?
- Which decaffeinated tea type (black, green, oolong, white) contains the least amount of caffeine?
- Gunpowder green tea pellets
- Tannic acid Ϯ
- Decaffeinated black, green, oolong, white teas
- Distilled water
- Graduated cylinder or metric measuring cup
- Five tea cups
- Rubbing alcohol
- Masking tape
- Medicine dropper
- Permanent marker
- Five test tubes (or five empty small clear plastic pill bottles)
- Small metric food scale
Ϯ Tannic acid may be available at a local pharmacy or can be ordered online from Nasco Science Company (www.eNasco.com/science) or 1-800-558-9595.
- Label five teacups with the masking tape and permanent marker as follows: Gunpowder tea, decaffeinated black, decaffeinated green, decaffeinated oolong and decaffeinated white tea.
- Open the four teabags (if used) or place the loose tea leaves of each of the four teas into its corresponding labeled cup.
- In the fifth cup add the same amount of Gunpowder tea pellets. This tea is the control.
- Bring distilled water placed in a teapot to a rolling boil and pour it over the teas.
- Allow the teas to steep for 30 seconds to one minute. This will remove most of any caffeine if any is present. The longer the tea is brewed, the more caffeine will be removed but so will other compounds in the tea leaves.
- Carefully pour 2 ml of each tea infusion into five test tubes (or pill bottles if used instead of test tubes) and allow the tea infusions to cool.
- Dissolve 1 g of tannic acid in 1 ml of alcohol, and dilute with distilled water to 10 ml. Prepare this solution fresh.
- Using a medicine dropper add tannic acid drop-wise to the caffeinated Gunpowder tea. If a white precipitate forms, caffeine is present. Stop adding the tannic acid! If an excess amount of tannic acid is added, the precipitate will dissolve and disappear!
- Record the number of drops added until the caffeine was detected. Convert the drops to metric measurements by adding the same number of drops of tannic acid to a graduated cylinder or measuring cup.
- Using a medicine dropper add tannic acid drop-wise to the four decaffeinated teas stopping immediately when or if a white precipitate is seen.
- Record the results in a table.
- Using a sheet of graph paper or a computer equipped with Excel® visually display the data in the table by plotting a bar or line graph with the amount of tannic acid added (ml) to the teas along the vertical axis verse the name of the teas along the horizontal axis.
Terms/Concepts: gunpowder green tea, precipitate, tea infusion, Camillia sinensis plant, black tea, green tea, oolong tea, white tea, tannic acid; why was tannic acid used to detect the presence of caffeine in so-called decaffeinated teas?
References: A Laboratory Manual of Organic Chemistry for Medical Students, by Matthew Steel, pp. 143-144 (Nabu Press 2010).