Indicators

Equivalence point

  • You use and indicator to tell you when you reach the endpoint of the reaction
  • This occurs when the indicator changes colour

However, in reality, you should only stop when the solutions are mixed in exactly the right proportions

i.e. all H+and OH ions from the reagent solutions have reacted to form H2O

  • This is called the molar equivalence point
  • In an ideal world, the endpoint and the equivalence point are the same, or near to the same

This point is not necessarily the neutral point (pH 7); due to hydrolysis, the resultant solution may have different a pH.

How indicators work

Indicators are usually weak acids that ionise. For example, litmus, or phenolphthalein.

Litmus is a weak acid – the ¬†formula can be simplified to HLit. The “H” is the proton which can be donated. The “Lit” is the rest of the ¬†molecule. This is the equilibrium established when litmus is dissolved in water:
litmus
The un-ionised litmus is red, whereas the ion is blue. Using LCP, you can predict what colour the solution will be if added to acidic/alkaline solutions.

 

Indicators

  • Indicators do not change colour at any one particular pH.
  • Instead, they change over a narrow range of pH.
  • There is a gradual smooth change from one colour to another

Choosing indicators

  • You need to choose an indicator that changes colour as close as possible to the equivalence point
  • Since in most cases, the graph around the EP is quite steep, it usually does not matter as long as the colour change occurs at the steep point
Strong acid added to strong base

indicator sasb

  • You could use either of the two in this experiment, since they both change at the steep point
  • There will be virtually no difference in the volume of acid added, so any one would be fine
Strong acid added to weak base
indicator sawb
  • Phenolphthalein does not change colour near the EP, so you must use methyl orange
Weak acid added to strong base

indicator wasb

  • You would use phenolphthalein, since it changes colour near the EP
  • You would not use methyl orange, as it obviously does not change colour near the equivalence point
Weak acid added to weak base

indicator wawb

  • Neither indicator would work in this case
  • This is why titrating weak acids/bases against each other is not advisable

 

 

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