IB Chemistry: Indicators

IB Chemistry: Indicators

Describe qualitatively the action of an acid–base indicator.

The chemical formula HIn(aq) simply represents any indicator.

(Colour A) HIn(aq) ↔ H+(aq) + In-(aq) (Colour B)

An indicator is an weak acid or base which has a different colour in its dissociated and undissociated form.

An indicator will be colour A in an acidic solution. It will not want to dissociate as there is a high concentration of H+ ions present in the solution.

However, it will be colour B in an alkali solution as the weak acid will dissociate due to the presence of OH- ions.

State and explain how the pH range of an acid–base indicator relates to its pKin value

Similar to weak acids and bases, indicators have a dissociation constant KIn:

KIn = ([H+] x [In-]) / [HIn]

We can assume that if [In-] ≈ [HIn] in a solution, then a colour change from A to B or vice versa will occur as they are almost equal in concentration.

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Therefore, if [In-] ≈ [HIn], then from the equation we can also say that the endpoint of an indicator, i.e. when it changes colour is when KIn ≈ [H+].

We can rewrite this as pH ≈ pKIn.

Basically, when the pH of a solution is close to the pKin value of an indicator, it will experience a colour change from A to B or vice versa.

Identify an appropriate indicator for a titration, given the equivalence point of the titration and the pH range of the indicator.

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Look at the data booklet for values of pKin for indicators.

When you are given the equivalence point of a titration, you want to choose an indicator with a pKIn value close to the equivalence point, making sure that the pH of the equivalence point falls between the pH range of the indicator.

For example, if you have a titration with a strong acid and a equivalence point of 3.8, then you would want an indicator like methyl orange as it has a pH range of 3.1 – 4.4.