IB DP Chemistry: HL复习笔记18.1.3 Indicators-翰林国际教育

IB DP Chemistry: HL复习笔记18.1.3 Indicators

Category: 教材笔记, 福利干货 Date: 2022年7月29日下午12:26

An acid-base indicator is a weak acid which dissociates to give an anion of a different colour
Consider a weak acid, HIn:

HIn (aq) + H₂O (l) ⇌ H₃O⁺ (aq) + In^–(aq)

colour 1 ⇌ colour 2

HIn and its conjugate base In^– are different colours
The colour of the solution depends on the relative concentrations of the two species
If the solution is acidic, the above equilibrium will be shifted to the left and more HIn will be present
- Colour 1 will dominate
If the solution is alkaline, the above equilibrium will shift to the right and more In^– will be present
- Colour 2 will dominate
The colour of the indicator depends on the pH of the solution
- The colour does not change suddenly at a certain pH, but changes gradually over a pH range
- The colour of the indicator depends on the ratio of [HIn] to [In^–]
The pH at which these transitions will occur depends on the K_aof the indicator
The endpoint of the reaction is where there is a balance between [HIn] and [In^–]. At this point these two concentrations are equal:
Taking negative logs of both sides:

pK_a= pH

This means the pK_aof an indicator is the same as the pH of its endpoint
The colour change for most indicators takes place over a range of pH = pK_a± 1

Weak bases as indicators

An indicator can also be a weak base:

BOH (aq) ⇌ B⁺ (aq) + OH^- (aq)

colour 1 ⇌ colour 2

For such indicators:
- Colour 1 is observed in alkaline conditions
- Colour 2 is observed in acidic conditions

Choosing a suitable indicator

Around the equivalence point of a titration, the pH changes very rapidly
Indicators change colour over a narrow pH range, approximately centred around the pK_a of the indicator
An indicator will be appropriate for a titration if the pH range of the indicator falls within the rapid pH change for that titration

Common Indicators and their colours table

5.6.4-Indicators-Table

Strong acid - strong base

In strong acid – strong base titrations, the pH changes from 4 to 10 at the end-point so a suitable indicator must change colour within this range
- Methyl red and phenolphthalein are suitable indicators for these titrations
- Methyl orange is not ideal but it shows a significant enough colour change at the end point so is widely used

Weak acid - strong base

In weak acid – strong base titrations, the pH changes from 7 to 10 at the end-point so a suitable indicator must change colour within this range
- Phenolphthalein is the only suitable indicator for weak acid – strong base titrations that is widely available

Strong acid - weak base

In strong acid – weak base titrations, the pH changes from 4 to 7 at the end-point so a suitable indicator must change colour within this range
- Methyl red is the most suitable indicator for these titrations
- However, methyl orange is often used since it shows a significant enough colour change at the end-point and is more widely available than methyl red

Weak acid - weak base

In weak acid - weak alkali titrations, there is no sudden pH change at the end-point and thus there are no suitable indicators for these titrations
- The end-points of these titrations cannot be easily determined

The overlay on the graph shows that both phenolphthalein and methyl orange would change colour outside the point of inflection in a weak acid-weak base titration so they would not be able to show the equivalence point of the titration

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