In this project you will investigate the basic chemistry and light requirements
ID: 59553 • Letter: I
Question
In this project you will investigate the basic chemistry and light requirements of photosynthesis. Phenol red is a pH indicator dye that is red to magenta in color at pH 7 or higher (basic or alkaline) and a pale yellow to orange color at below pH 7 (acidic). Pour 20 ml. of the dilute phenol red solution into a 50-ml. beaker. Take a deep breath and hold it for at least 20 seconds. Gently breathe out through a straw placed in the phenol red solution. Repeat this process until the solution changes color from reddish-orange to yellow. The carbon dioxide in your breath partially dissolves in water to form carbonic acid. This acid lowers the pH of the phenol red solution and causes the color change. CO_2 + H_2o rightarrow H_2co3 rightarrow H^+ + HCO3-; Take an initial absorption reading of the phenol solution with the spectrophotometer at 582 nm .Use R/O water as the blank. Record this on Table 4. You need two pieces of Elodea that are each 2.5 cm in length. After you have cut your two pieces, place both into one test tube; this test tube will be the treatment tube. Place half of the phenol red solution from the 50-mL beaker (it's yellow now!) into the test tube with the Elodea plants and pour the remainder of the phenol red solution into the other (empty) test tube (control tube). Place both test tubes under the fluorescent lamp for 60 minutes. Gently swirl both test tubes every few minutes. The color should eventually change in one of the tubes. Mix the solution in your tubes, take them to the spectrophotometer, and measure the absorbance at 582 nm of each of them. Use water as the blank. Complete Table 4 and answer questions on Data Sheet. Explain why the phenol red solution changed color when yon breathed into it. Why does the color change in one tube but not the other tube?Explanation / Answer
Phenol red pH indicator changes color to yellow under acidic pH, while it changes color to red, under alkaline pH. When phenol red solution (which is yellow in color) is subjected to fluorescent light, there is no change in the color. Hence, the absorbance remains more or less similar even after exposure to light.
When carbon dioxide is introduced in water, it produces carbonic acid, which reduces the pH of water, making it acidic. Conversely, when carbon dioxide is removed from water, the water turns less acidic, and hence pH of water increases.
In the presence of Elodea plant (an aquatic plant), when exposed to fluorescent light, the plant carries out photosynthesis, which utilizes dissolved carbon dioxide in water. This decreases the amount of carbon dioxide in water, and thus making water less acidic, and resulting in pH increase. Due to higher pH (alkaline), the color of phenol red changes to red.
Thus, in the control tube, there is no change in pH, hence the color of phenol red remains yellow, however in the test tube containing the Elodea plant, the color of phenol red changes due to increase in the pH of the solution.
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