W5 Solubility of weak acids and bases

Question 1

What does a strong acid do?

Answer 1

Completely dissociates in aqueous solution, at any pH e.g. HCl

Question 2

What is a weak acid?

Answer 2

Partially dissociates in aqueous solution e.g. acetic acid

Question 3

An Ionised species is more/less soluble than an unionised species?

Answer 3

An Ionised species is more soluble than the unionised species

Question 4

Examples of weakly acidic drugs?

If they are more polar, what advantage does this have?

Answer 4

Warfarin, phenobarbital, NSAIDs, phenytoin
These drugs will be more polar and therefore have a greater aqueous solubility at higher pH values

Question 5

How can the SOLUBILITY at a given pH for a weak acid be determined?

Answer 5

Henderson-Hasselbalch equation (for weak acids)
pH = pKa + log([A-] / [HA])
OR
pH = pKa + log( S –S0 / S0)

where:
S = (saturation) solubility of weakly acidic drug at a given pH
S0 = solubility of undissociated drug

Question 6

Examples of weakly basic drugs?

Answer 6

Ranitidine, Cimetidine

These drugs will be more polar and therefore have a greater aqueous solubility at lower pH values

Question 7

Henderson-Hasselbalch equation
(for weak bases)

Answer 7

pH = pKa + log( [B ]/ [BH+])
OR
pH = pKa + log (S0/ S –S0)

S = (saturation) solubility of weakly basic drug at a given pH
S0 = solubility of undissociated drug

Question 8

What are examples of amphoteric drugs? (3)

Answer 8

Oxytetracycline, Nitrazepam, Norfloxacin

Question 9

What are amphoteric drugs?
What do they have 2 values of?

Answer 9

Contain both acidic and basic groups so these drugs will
have two (or more) pKa values, e.g. “pKa1” and “pKa2”

They exhibit a solubility minimum at a point where there is no net charge, i.e. the molecule is neutral

Question 10

What is the Henderson-Hasselbach equation?

(no need to memorise)

Answer 10

pH= pKa + log [A-] / [HA]