Question

A student poured a solution of bromothymol blue (BMB) into three test tubes. (BMB turns yellow in the presence of CO2.) Then he placed an aquatic plant in two of the test tubes, as shown in the diagram (you will see the diagram when you are taking the test). He placed a stopper on each test tube and placed them all in the dark for two days. Predict what change of color, if any, would occur in each of the test tubes. Explain your answer. (2 Points Extra Credit): Describe what color change would occur in each tube, if any, if the experiment were done in the light. Explain your answer.

Answer 1

Bromothymol blue is an indicator dye that turns yellow in presence of an acid. It remains blue if the pH of the given solution is greater than 7.6 and yellow when the pH is less than 6. The yellow coloration is due to the formation of carbonic acid in the from the carbon dioxide present in the solution.

When aquatic plants are added to two test tubes containing bromothymol blue, stoppered and kept in dark for two days, no color change is observed. It is because the carbon dioxide present is utilized in the process of photosynthesis by plants. If the experiment is conducted in presence if the light, the color of bromothymol blue would be yellow due to the presence of carbon dioxide.

Answer 2

• Aquatic plants are placed in two test tubes with bromothymol blue dye and kept in the dark for two days; then, the colour does not change because carbon dioxide present in the tube is used by the plant in the process of photosynthesis.
• If a plant is placed in a BTB tube in the presence of light, then the color changes into yellow.

Further Explanation:

Bromothymol blue (BTB) is also referred to as bromothymol sulfone phthalein, is a pH indicator that helps in measuring the neutral pH of a substance. A very common use of this is determining the occurrence of carbonic acid in a liquid. It acts as a very weak acid. It can be present in a deprotonated or protonated form and appear in blue or yellow colour, respectively. In a neutral solution, this dye appears in green color and appears yellow in acidic solution.

On the other hand, the deprotonated form of BTB transmits blue colour in a basic solution. The donating group (alkyl substituent) and electron-withdrawing group (bromine atom) are accountable for the activity of bromothymol blue. The substituted alkyl group is mainly accountable for the active range of indicators.

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Answer Details:

Grade: High School

Subject: Biology

Topic: Bromothymol Blue

Keywords:

Protonated, deprotonated, electron-withdrawing, bromothymol, sulfone, substituted, green, blue, yellow, deprotonated, protonated.