Acid base indicators

The indicators in table VIII are all acid base indicators.
We also know redox-indicators to be used in redox titrations (other module).

The acid base indicator mostly is a weak organic acid with a rather complex structure. Abbreviated formula: HIn.
In water:
HIn + H2O H3O+ + In-       (equilibrium IND)
HIn has color 1         In- has color 2

Imagine this indicator in acid environment, for example in a solution of hydrochloric acid.
In this environment dominate the (H3O+)-ions and the equilibrium IND dislocates to the left. You can also say that in acid environment the indicator takes the HIn performance; color 1 dominates.

Every indicator has a zone of change that can be found in the table.
An example: Methyl orange has a change zone of 3.1 - 4.4 (red - orange)
Imagine a strong acid solution with pH=1. Add some drops of Methyl orange solution, that gets the color red.
Now we add strong base like (KOH(aq), the acid will bit by bit be neutralised, the pH-value will gradually increase. It started with pH=1, slowly this value increases.
As soon as the value passes 3.1, then you start to see a color change from red to orange. Only after reaching 4.4 the color has definitly become orange.
Within the change zone, the color is a mix of the two colors.

Again: the color of the indicator depends on the environment of the solution.
In acid environment the equilibrium IND dominates at left, as also does the color of the HIn molecules.
Note that the change zone certainly not always is close to 7!