At what pH will cocaine, a basic drug with a pKa of 8.6, be essentially 100% in the un-ionized B form?

Question

Quizplus · Accepted Answer

Since cocaine is a basic drug, it will be mostly in its un-ionized form (B) at higher pH values. To determine the pH at which cocaine is essentially 100% in the B form, we need to look at the pKa value of cocaine.

The Henderson-Hasselbach equation can be used to calculate the ratio of ionized (BH+) to un-ionized (B) forms of the drug at a given pH:

pH = pKa + log([BH+]/[B])

At a pH equal to the pKa, the ratio of BH+ to B is 1:1. At pH values lower than the pKa, the ratio of BH+ to B is higher, meaning more of the drug exists in the ionized form. At pH values higher than the pKa, the ratio of BH+ to B is lower, meaning more of the drug exists in the un-ionized form.

In this case, the pKa of cocaine is 8.6. To determine the pH at which cocaine is essentially 100% un-ionized (B), we can set the equation to reflect a ratio of 1:1000 (since the log of 3 is approximately 3 and we want a ratio of 1:1000, or 0.001):

10.6 = 8.6 + log([BH+]/[B])
2 = log([BH+]/[B])
1000 = [BH+]/[B]

So, at a pH of 10.6, the ratio of BH+ to B is 1:1000, meaning that essentially all of the cocaine exists in the B form. Therefore, the correct answer is C) 10.6.

At What PH Will Cocaine, a Basic Drug with a PKa