Question

a scientist 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 4.0. which ph indicator should the scientist use?

a. methyl violet, which changes from violet to yellow around a ph of 2.0

b. cresol red, which changes from yellow to purple around a ph of 7.2

c. thymolphthalein, which changes from colorless to blue around a ph of 9.3

d. methyl yellow, which changes from yellow to red around a ph of 4.0

Explanation:

To determine the appropriate pH indicator, we need an indicator that changes color around the target pH (below 4.0).

• Option A: Methyl violet changes around pH 2.0, which is too low as the scientist is interested in changes below 4.0 but the starting pH is 6.0, and we need an indicator that signals the change around 4.0.
• Option B: Cresol red changes around pH 7.2, which is neutral - slightly basic, not relevant for detecting pH below 4.0.
• Option C: Thymolphthalein changes around pH 9.3 (basic range), so it's not useful here.
• Option D: Methyl yellow changes from yellow to red around pH 4.0, which matches the scientist's need to detect when the pH drops below 4.0 (since the starting pH is 6.0, when the pH reaches 4.0 and below, the indicator will change color, signaling the desired point).

Answer:

D. Methyl yellow, which changes from yellow to red around a pH of 4.0