1120 buffer

• Buffer solutions are solutions whose pH value changes only very slightly upon the addition of small amounts of either an acid or base
• common-ion effect is the suppression of the ionization of a weak acid/base caused by adding more of an ion that is a product of this ionization to shift the equilibrium to the left (favour reactants).

  A common example is adding strong acid/base to weak acid/base OR adding conjugate acid-base pair
• Why can't strong acids and bases used to make buffer solutions?

  because they will neutralise each other
• Composition of common buffer solutions

  1. weak acid and its conjugate base
  2. weak base and its conjugate acid
• A buffer solution is an equilibrium between a weak acid and its conjugate base essentially, so Ka = [H+] for buffer solutions
• a buffer is generally good for maintaining a nearly constant pH in a range of about 2 pH units centered on the pH of the buffer solution
• Preparing buffer solutions

  1. Weak acid + titrate with strong base
  2. Weak acid + salt of weak acid
  3. Salt of weak acid + titrate with strong acid
  4. Weak base + titrate with strong acid
  5. Weak base + salt of weak base
  6. Salt of weak base + titrate with strong base
• How is buffer capacity maximised?

  1. High concentration of buffer components
  2. Equal concentration of buffer components
• HIn + H2O ⇌ H3O+ + In-

  Add acid: Increase H3O+ = equilibrium shifts to left = [HIn] increase = show acid colour
• HIn + H2O ⇌ H3O+ + In-

  Basic solution added: Decrease H3O+ = equilibrium shifts to right = [In-] increase = show base colour
• [In-] / [HIn] < 0.10 to show acid colour
• [In-] / [HIn] = 1 to show intermediate colour
• [In-] / [HIn] > 10 to show base colour
• to show acid colour: pH < pK(HIn) - 1
• to show intermediate colour: pH = pK(HIn)
• to show base colour: pH > pK(HIn) + 1
• Equivalence point in titration is the point which titrant added is enough to neutralise analyte solution
• In strong acid - strong base titration, the pH at equivalence point is 7
• In weak acid - strong base titration, the pH at equivalence point is higher than 7
• In weak acid - strong base titration, pH = pKa and [HA] = [A-] at half neutralisation point
• in the titration of weak polyprotic acid with a strong base, at the first equivalence point, the Ka2 > Kb = H+ producing reaction dominates = pH is smaller than 7
• in the titration of weak polyprotic acid with a strong base, at the second equivalence point, the Kb > Ka3 = OH- producing reaction dominates = pH is bigger than 7