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

Cards (19)

  • 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