Weak Acids/Bases 1 Flashcards
What is a weak acid?
An acid that is partially deprotonated/dissociates
What is a weak base?
A base that is partially protonated
What is an acid?
A proton donor
What is a base?
A proton acceptor
What are most drugs?
Weak acids/bases
Equation for the acid constant (Ka)
Ka = [A− ][H+] / [HA]
Equation for the base constant (Kb)
Kb = [OH− ][BH+ ] / [B]
What are the acid/base contants a measure of?
A measure of how much of a drug and its conjugate exist in solution.
How can acid constant be used?
By comparison to pKa
pKa = -log(Ka)
What is pKa?
The point at which 50% of the drug is ionised.
What are the general rules of pKa?
Stronger acid (lower pH) will have a smaller pKa
Stronger base (higher pH) will have a larger pKa
Value of pKa is NOT an indication of whether a drug is an acid or a base - consider functional groups - which will ionise?
What does Ka tell us?
- pH of solution
- Ionisation of drug at given pH
- Solubility of drug at given pH
- pH of buffer
For a drug to be soluble in aqueous conditions it must be…
Polar/charged/ionised
Weak acid/base solubility equations (S)
WEAK ACID: pH − pKa = log((S−So)/So)
WEAK BASE: pH − pKa = log(So/(S −So))
S = solubility So = Solubility of uncharged species
To be insolution, MUST be ionised
S is measured - does not indicate whether charged/uncharged
What is an amphoteric drug?
Drug molecule which has positive and negative charges depending on the pH
They can be zwitterionic
E.g. Doxycycline, Sulfadiazine, Acyclovir
At the isoelectric point, the overall charge is 0
What is the relevance of pH : pI (isoelectric point)?
pH < pI (drug is behaving as a base) - a more acidic environment
pH > pI (drug is behaving as an acid) - a more basic environment
Discuss salts and solubility.
Drugs are often formulated as salts to improve solubility
Must be chosen carefully as it influences physicochemical properties
Commonly used anions (-ve) for basic drugs?
hydrochloride, sulphate, bromide, chloride, phosphate, maleate, citrate, acetate
Commonly used cations (+ve) for acidic drugs?
sodium, potassium, calcium, zinc
What is the salting in/out effect?
The presence of a high amount of salt forces some of the drug out of solution (precipitation)
Observed for inorganic salts e.g. AgCl
Ksp = [Ag+][Cl-]
- Ag is poorly soluble in water
- NaCl is very soluble and dissociates to [Na+][Cl-]
- Concentration of [Cl-]/Chloride increases
- To maintain equilibrium/ksp constant, [Ag+] must reduce
- Ag+ is removed from solution and appears as a solid
This is used in purification
