COMPLEXATION

• What are the types of mixtures involved in complex acid-base titrations?

  Mixtures of strong and weak acids or bases
• What types of acids can be titrated in complex acid-base titrations?

  Acids with more than one titratable group
• What conditions allow for the determination of components in a mixture of strong and weak acids or bases?

  Concentrations are similar and Ka or Kb is between $10^{-4}$ and $10^{-8}$
• What is observed during the titration of mixtures of strong and weak acids or bases?

  Two useful end points are observed
• What is necessary for obtaining an accurate titration curve for polyprotic acids?

  The ratio must be greater than $10^3$
• What happens if the ratio is smaller than $10^3$ during titration?


  Error becomes excessive, especially at first equivalence
• What occurs when $Ka1/Ka2 < 1000$?


  The first endpoint is not sharp
• What is the major constituent before the addition of titrant in a titration curve for polyprotic acid?

  H2A
• How should H2A be treated before the addition of titrant?

  Treat as a weak monoprotic acid
• What is the major constituent before the first equivalence point?

  H2A and HA-1
• How should the mixture be treated before the first equivalence point?

  Treat as a buffer
• What is the major constituent at the first equivalence point?

  NaHA
• How should NaHA be treated at the first equivalence point?

  Treat as an amphiprotic salt
• What is the major constituent before the second equivalence point?

  HA-1 and Na2A
• How should the mixture be treated before the second equivalence point?

  Treat as a buffer
• What is the major constituent at the second equivalence point?

  Na2A
• How should Na2A be treated at the second equivalence point?

  Treat as a weak monoequivalent base
• What is the major constituent post equivalence point?

  NaOH
• How should NaOH be treated post equivalence point?

  Treat as a strong base
• How do you calculate the pH before the addition of titrant in a titration of 30 mL of 0.100 M H2A?

  Use the dissociation constant Ka1
• What are the Ka values for H2A in the given titration?

  Ka1 = $8.5 \times 10^{-4}$, Ka2 = $1.6 \times 10^{-7}$
• How do you calculate the pH after the addition of 25 mL of titrant?

  Use the concentration of H2A and NaOH
• How do you calculate the pH after the addition of 30 mL of titrant?

  Use the equivalence point calculations
• How do you calculate the pH after the addition of 58 mL of titrant?

  Consider the excess NaOH in the solution
• How do you calculate the pH after the addition of 60 mL of titrant?

  Use the final endpoint calculations
• How do you calculate the pH after the addition of 65 mL of titrant?

  Account for the complete reaction with NaOH
• What are the key points in titration of polyfunctional acids and bases?

  • Mixtures can be strong/weak acids or bases
  • Accurate titration curves depend on ratios
  • Major constituents change at equivalence points
  • K values must be tested in sequence
• What are the regions and major constituents in the titration curve for polyprotic acids?

  1. Before addition: H2A (weak monoprotic)
  2. Before first equivalence: H2A and HA-1 (buffer)
  3. At first equivalence: NaHA (amphiprotic salt)
  4. Before second equivalence: HA-1 and Na2A (buffer)
  5. At second equivalence: Na2A (weak monoequivalent base)
  6. Post equivalence: NaOH (strong base)
• How do you approach calculations in titrations of polyprotic acids?

  • Calculate pH before titrant addition
  • Calculate pH after each titrant addition
  • Use Ka values for accurate calculations
  • Consider equivalence points and excess reactants