Biochemistry

• Strong acid


  Dissociates completely in water (100%)
• Strong acids


  • HClO4, HCl, HNO3, H2SO4
• Weak acid


  Do not dissociate completely in water
• Weak acids


  • CH3COOH, HCOOH, HF
• Weak acids


  Release protons, but only a small fraction of their molecules dissociate (ionize)
• 1 M HCl is pH = 0


  1 M acetic acid is only pH = 2.4
• Weak acids


  In equilibrium with their ionized species
• Equilibrium constant

  Governed by the Law of Mass Action, and characterized by: Ka = [H+][A-] / [HA]
• Acid Dissociation Constant (Ka)


  Describes the extent of ionization of an acid
• Water


  A very weak acid, with [H+] = [OH-] = 10-7 M = pH 7
• Only two of every 10^9 (1 billion) molecules in pure water are ionized at any instant
• Of every 1000 acetic acid molecules in a 1 M solution, only 4 are ionized
• Acid


  Proton (H+) donor
• Base


  Hydroxide (OH-) donor
• pH


  Power of Hydrogen, describes the concentration of H+ in solution
• pH


  pH = -log[H+]
• Most living cells have a very narrow range of tolerance for pH, i.e. [H+]
• [H+] is controlled in all biological organisms, and in virtually all biochemical experiments
• Litmus Paper


  Used to measure pH
• pH Paper


  Used to measure pH
• pH Meter


  Used to measure pH
• Buffer solution


  A solution composed of a weak acid and its conjugate base, or a weak base and its conjugate acid, that resists a drastic change in pH
• For biological systems, ionization of a strong acid is TOO BIG, ionization of water itself is way TOO LITTLE, ionization of a weak acid is JUST RIGHT
• Maximum Buffering Capacity


  pH = pKa ± 1
• Acid Dissociation Constant (Ka)


  Describes the extent of ionization of an acid, equilibrium constant Ka = [CH3COO-][H+] / [CH3COOH]
• Acid


  Should contain (H+) ion
• Base


  Should contain (OH-) ion
• Bronsted-Lowry Theory


  Proton (H+) donor<|>Proton acceptor
• Lewis Theory


  Electron pair acceptor<|>Electron pair donor
• Strong acid


  Dissociates completely in water (100%)
• Strong acids


  • HClO4, HCl, HNO3, H2SO4
• Weak acid


  Do not dissociate completely in water
• Weak acids


  • CH3COOH, HCOOH, HF
• While 1 M HCl is pH = 0, 1 M acetic acid is only pH = 2.4
• Weak acids


  In equilibrium with their ionized species
• Acid Dissociation Constant (Ka)


  Describes the extent of ionization of an acid
• In pure water, only two of every 10^9 (1 billion) molecules are ionized at any instant</b>
• Of every 1000 acetic acid molecules in a 1 M solution, only 4 are ionized
• pH


  Power of Hydrogen, describes the concentration of H+ in solution
• Most living cells have a very narrow range of tolerance for pH