You Have a Summer Job as an Intern in an Analytical

Multiple Choice

You have a summer job as an intern in an analytical chemistry laboratory. You are about to titrate a solution of nicotine (pK_b  6.0) , which is a base similar to ammonia, with a standard hydrochloric acid solution. Two pH indicators are available for you to use: thymol blue (pK_a  3.47) and phenolphthalein (pK_b  9.4) . Which indicator do you select as being the more appropriate one to use?

A) Both indicators would work equally well because at the equivalence point the pH of the solution changes very sharply from strongly basic to strongly acidic.

B) Phenolphthalein is the better choice because it is one of the most commonly used pH indicators.
C) Neither of these indicators would work because their pK_a values are not close to 7.00, which is the pH of the solution at the equivalence point.
D) Phenolphthalein is the better choice because the sample pH at the equivalence point will be close to the pK_a value of 9.4.
E) Thymol blue is the better choice because the sample pH at the equivalence point will be close to the pK_a value of 3.47.