L8/9 - Acid-base Theory, pH, pKa & pKb

• Lowry-Brønsted theory

  Acid = proton donor, Base = proton acceptor
• Extent of ionisation(What is it dependent on? What does it affect?)

  pH dependentAffects absorption, distribution, elimination
• pH

  Measure of H+ ion concentrationpH = -log [H+]
• pKa(equation, relationship between pKa/Ka and acid strength, relation to pH)

  -log(ka).The smaller the pKa, the stronger the acidThe smaller the Ka, the weaker the acidThe pH at which 50% of the species are ionised.After +/- 2 units away from pKa, the acid/base will be fully ionised/unionised
• pKb


  -log(kb)The smaller the pkb, the stonger the baseThe smaller the Kb, the weaker the base
• Zwitterions


  Molecules with both positive and negative charges. Can act as an acid or a base
• Amphoteric species/compounds


  Can function as both acids and bases eg water
• Equilibrium constant (Ka) expression

  [H3O+][A-] / [HA]
• Equilibrium constant (Kb) expression


  [OH-][BH+] / [B]
• pKw and Kw


  -log(kw)Kw = ionic product of water (14 at 25degrees)pKa + pKb = pKw
• Drugs can be classed as...

  Drugs can be weak organic acids (eg aspirin, ibuprofen, diclofenac), weak organic bases (eg morphine, procaine), OR their salt (eg ephedrine HCl)
• Strong acids/Bases

  fully dissociate/ionise in solution
• weak acids/bases

  partially dissociate/ionise in water
• when pH = pKa

  Acid/base is 50% ionised

  
• Henderson-Hasselbalch equation(What is it? What does it show? What does it allow us to calculate? How does it rearrange?)

  pH = pKa + log [conj. base]/[conj. acid]]Shows the relationship between pH and pKa. Allows us to calculate ratio of ionised to unionised drug at any pH[base]/[acid] = 10^(pH-pKa)

  
• Low pH (effect on acids and bases)

  acids are fully unionisedbases are fully ionised
• High pH (effect on acids and bases)

  acids are fully ionisedbases are fully unionised
• pH dissociation profile

  Shows ionised and unionised forms of a drug. pH = pKa

  
• isoelectric point


  pH at which net charge is 0