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
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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

• 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, pK_a=pH

• 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, pK_b = pOH

• 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