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Wize University Chemistry Textbook > Buffers and Titrations

Indicators

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Indicators

An indicator is a weak acid or base added in a very small quantity to the titration solution before the experiment begins. Usually they are weak acids.

The indicator has a special property: its acid and conjugate base forms appear as different colours in solution

Example:
HIn + H2O ⇌ In- + H3O+
(red) (blue)

Wize Concept
The pKa value of the chosen indicator should be close to the pH of the solution at the equivalence point (ideally within 1 pH unit of the equivalence point)

We want the indicator to signal a colour change close to the equivalence point.

When the pH of the solution=pKa of the indicator, the indicator's acid and conjugate base components will have an equal concentration [HIn]=[In-] and we will see a mixture of the two colors!

Example:
In our example above, we'd see a
purple
colour

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Equivalence Point vs Endpoint

Equivalence point: moles of H+=moles of OH-

End point: when we see a colour change because of the indicator

Example:
Look at methyl orange in the table below. Indicate an endpoint and a possible equivalence point.
Photo by OpenStax / CC BY
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HIn + H2O ⇌ In- + H3O+
(red) (blue)


1) When pH=pKa of indicator:

  • [HIn]=[In-]
  • red + blue → we'd see
    purple
    !

2) When pH < pKa

  • [HIn] > [In-]
  • we'd see
    red
    !

3) When pH > pKa

  • [HIn] < [In-]
  • we'd see
    blue
    !

Practice: Indicators

The indicator cresol red has Ka = 5.0 x 10-9. Over what approximate pH range does it change colour?
Extra Practice
The indicator cresol red has Ka = 5.0 x 10-9. Over what approximate pH range does it change colour?
Phenolphthlein is used as an indicator in acid base titrations because it is halochromic, it changes color based on pH. Shown below are the two forms of phenolphthalein at low pH and high pH. One of the forms is colorless and one is bright purple. Which one is colored? Explain your answer.
Henderson Hasselbalch: Buffers and Indicators
The indicator cresol red has Ka = 5.0 x 10-9. Over what approximate pH range does it change colour?
Sketch the titration curve for a weak acid (Ka = 3.4 x 10-6) being titrated by a strong base. What is the pH at the equivalence point? What would be the ideal pKa value for an indicator in this titration?
For which type of titration will the pH be basic at the equivalence point?
Henderson Hasselbalch: Titrations
Suppose you performed a potentiometric titration on the solutions in flasks A, B, and C by a gradual addition of HCℓ (aq) to each flask shown.