Question

Acid/base indicator with a $ p{{K}_{a}} $ of $ 6.8 $ is Bromothymol blue. During an acid base titration, at what approximate pH, will bromothymol blue undergo a color change?
(A) $ 1 $
(B) $ 3 $
(C) $ 5 $
(D) $ 7 $
(E) $ 13 $

Answer 1

Hint :We know that the bromothymol blue is the indicator that acts as a weak acid in a solution. Thus, it can be either in a deprotonated form or protonated form, by appearing blue or yellow, respectively. It is also a bright aquamarine by itself and greenish-blue in a neutral solution.

Complete Step By Step Answer:
Bromothymol blue is also called bromothymol sulfonephthalein, and BTB, which is a pH indicator. It is mostly used in applications that hold measuring substances that would have a neutral pH (nearly seven) relatively. As a common use, we can say it for measuring the presence of carbonic acid in a liquid. It is typically sold in its solid form because of the sodium salt of the acid indicator. The other names of Bromothymol Blue can be given as 3,3-Dibromothymolsulfonphthalein and Dibromothymolsulfophthalein. This neutral form deprotonation results in a structure highly conjugated, considering the color difference.
A deprotonation intermediate mechanism is responsible for the greenish color in the neutral solution. Besides, the Bromothymol blue is highly acidic and is magenta in color. The bromothymol blue's general carbon skeleton is common to most of the indicators, including thymol blue, bromocresol green, and chlorophenol red. Bromothymol blue indicator is sparingly soluble in oil but soluble in ether, water, and alkalis' aqueous solutions. It is also less soluble in nonpolar solvents, including toluene, benzene, and xylene, and it is practically insoluble in the petroleum ether. Bromothymol blue indicator has been used in conjunction with phenol red in monitoring the fungal asparaginase enzyme activity, with the phenol red turning pink Acid/base indicator with a $ p{{K}_{a}} $ of $ 6.8 $ is Bromothymol blue. During an acid base titration, the approximately pH at which bromothymol blue will undergo a color change is $ 7. $ The pH range for colour change is $ p{{K}_{a}}\pm 1. $ It is around $ 5.8 $ to $ 7.8. $
Therefore, the correct answer is option B.

Note :
Remember that the bromothymol blue can be used either to observe photosynthetic activities or as a respiratory indicator (which turns yellow as $ C{{O}_{2}} $ is added). A common demonstration of the pH indicator properties of BTB involves exhaling through a tube into the neutral solution of Bromothymol blue.