pKa and lidocaine

[Dharma]

BR Bio passage 1-4, question 22.

Which of the following is true question.

Answer: The higher the pKa of the local anesthetic the lower the lower the concentration of base in the tissue.

In the passage it states that "The concentration of base or cation in the solution depends on the pKa of the local anesthetic. A decrease in the amount of base facilitates removal of the local anesthetic, resulting in a shorter duration of action.

In the explanations it states:

Look at the structure of lidocaine. When it is protonated, it carries a positive charge. In other words, the cation is in the acid form. Therefore, when we have a high pKa, we have a small Ka. This, of course, means the reaction does not go very far to the right. In other words, we have a large concentration of acid and a small concentration of base in the tissue."

It's that last sentence that gets me. If we have a high pKa, wouldn't that mean there is a low concentration of acid? I know I'm missing something here.

Help?

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[Dharma]

maybe it is referring to the likelihood that the high pKa means that the acid is less likely to dissociate, hence its conjugate base will also be present in lower concentration?

 

[tartrate]

If we have a high pKa, wouldn't that mean there is a low concentration of acid?

False

maybe it is referring to the likelihood that the high pKa means that the acid is less likely to dissociate, hence its conjugate base will also be present in lower concentration?

True

 

[Dharma]

thanks tartrate!

 

[BerkReviewTeach]

It's that last sentence that gets me. If we have a high pKa, wouldn't that mean there is a low concentration of acid? I know I'm missing something here.

Help?

When the pKa is high, there is a good chance that the environmental pH will be less than the pKa. According to the HH equation, when pH is less than pKa, the species will exist more in its protonated form than its deprotonated form.