Titration curves (pH)

Titration curves can be described as what happens before, after and during the equivalence point.

  1. Titration curves for strong acid v strong base

Running acid into the alkali
  • The pH drops very little until it reaches the equivalence point, where it drops steeply
  • Then, it continues to fall by small amounts

sasb1 graph

Running alkali into the acid
  • This is very similar to the previous curve except the pH starts off low and increases as you add more sodium hydroxide solution.

sasb2 graph

  1. Titration curves for strong acid v weak base

Running acid into the alkali
  • The initial gradient is steeper than before, since a strong acid is being added to a weak base
  • However, after some acid is added, it slows and looks similar to before
  • This is because it forms a buffer solution
  • Note that, whilst the steep point is still steep, it is smaller than before
  • Hence, the pH range that you need to use is smaller

sawb1 graph

Running alkali into the acid
  • The shape is still the same as #1, due to an excess of HCl
  • After the equivalence point, there is no large change in pH due to a buffer solution being formed
  • Note that the equivalence point occurs when the solution is acidic

sawb2 graph

  1. Titration curves for weak acid v strong base

Running acid into the alkali
  • Before, the curve is the same as #1, as you have an excess of NaOH
  • Again, the equivalence point occurs during a smaller pH range
  • Past the MEP, a buffer solution is created, so pH change is minimal
  • Note that the MEP occurs when the solution is basic

wasb1 graph

 Running alkali into the acid
  • The start of the graph shows a relatively steep curve, as you are adding a strong base to a weak acid
  • This however slows as a buffer is established
  • After, the curve is the same as #1

wasb2 graph

  1. Titration curves for weak acid v weak base

Running alkali into the acid

wawb graph

  • It so happens that these two are both about equally weak – in that case, the equivalence point is approximately pH 7.
  • THIS IS NEVER DONE IN PRACTISE, as the endpoint would likely miss the MEP, since the pH range is so small.
Running acid into the alkali
  • This is just a combination of graphs.
  • Up to equivalence point, it is just like #2
  • Equivalence point has a very small pH range
  • Lack of a steep section means it is difficult to measure exactly when MEP has been reached

 

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