Ascientist wants to test how much of an acid can be added to a solution before the ph of the solution changes to below a certain point. the starting ph of the solution is 6.0, and the scientist wants to know when the ph of the solution changes to below 5.0.

which ph indicator should the scientist use?

a. phenolphthalein, which changes from colorless to purple around a ph of 8.3
b. cresol red, which changes from yellow to purple around a ph of 7.2
c. congo red, which changes from red to violet around a ph of 5.0
d. thymolphthalein, which changes from colorless to blue around a ph

The correct answer is option C, that is, congo red that changes from red to violet around a pH of 5.0.

Explanation:

Congo red refers to the sodium salt of benzidinediazo-bis-1-naphthylamine-4 sulfonic acid. It is a diazo dye, which is blue in acid solution and red in alkaline solution and is used mainly as a biological stain and as an indicator. Congo red is an indicator, which is red at pH 5.0, and blue-violet at pH 3.0. In microscopy and histology, Congo red has an application for staining in amyloidosis, for the outer membrane of Gram-negative bacteria, and for the cell walls of plants and fungi.