18.3 pH curves

Buffer solutions

  • A buffer is a solution that resists a change in pH upon addition of small amounts of a strong base, strong acid or water.
  • Buffer Is composed of weak acid and its conjugate base, or a weak base and its conjugate acid

Indicators

  • An acid-base indicator is a weak acid or a weak base where the componets of the conjugate acid-base pair have different colours
  • When choosing the best indicator for a titration, the midpoint of an indicators colour change must correspond to the equivalent point of the titration
  • Indicator - Copy
  • Equilibrium Indicator - Copy

pH curves

  • There are 4 possibilites of pH curves
    • Strong acid – Strong base
      • Starting point on pH axis is important as it gives initial pH of acid

Strong acid - strong base

  • Weak acid – Strong base
    • Starting point on pH axis is important as it gives initial pH of acid
    • Half equivalence point is the stage at which half the amount of weak acid has been neutralized
    • At this point, pKa=pH

Weak acid - strong base

  • Strong acid – Weak base
    • Starting point on pH axis is important as it gives initial pH of base
    • Half equivalence point is the stage at which half the amount of weak base has been neutralized
    • At this point, pKb = pOH

Strong acid - weak base

  • Weak acid – Weak base
    • Starting point on pH axis is important as it gives initial pH of base
    • Gradual change, equivalence point is hard to determine, graph has no purpose

Weak acid - weak base

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