Lecture 9 - Acid-base theory, pH, pKa 2

Question 1

What do we use Henderson-Hasselbach equations for?

Answer 1

Used to understand the degree of drug ionisation- the ratio of ionised to unionised drug to be calculated at any pH.

Question 2

Give the general Henderson- Hasselbach equation for both weak acids and bases.

Answer 2

Question 3

Weak acid Henderson-Hasselbalch equation

Answer 3

Question 4

Weak base Henderson-Hasselbalch equation

Answer 4

Question 5

How can the Henderson-Hasselbalch equation be rearranged to work out the molar ratio of the conjugate acid/base species (the ratio of ionised/unionised species)

Answer 5

[conj base]/ [conj acid] = 10^-(pH-pKa)

Question 6

Molar fractions can be converted into a …

Answer 6

percentage of ionised/unionised species.

Question 7

Draw a pH dissociation profile for weak acids and bases

Answer 7

Question 8

At a low pH…

Answer 8

bases are in ionised form and acids are in unionised form

Question 9

At a high pH…

Answer 9

bases are in unionised form and acids are in ionised form

Question 10

In a pH dissociation profile, the pKa…

Answer 10

is the pH at which 50% of species are ionised.

Question 11

At =/- 2 pH units away from pKa…

Answer 11

weak acids/bases are completely dissociated/undissociated.

Question 12

Determine the molar ratio and percentage of conjugate acid-base species of the weakly acidic drug, Aspirin- RCOOH (pKa = 3.5) in the stomach (pH=1).

Answer 12

0.3% of ionised acid and 99.7% of unionised acid