Acid - Base Indicators Phenolphthalein, Methyl Orange, Methyl Red, ThymolBlue-1

Acid - Base indicators (also known as pH indicators) are substances which change colour with pH.
They are divalen weak acids involved in protolytic acid-base equilibrium.

Presented simple Acid <->Base equilibrium
Presented complete Phenolphthalein equilibrium, 
Presented complete Methyl Orange equilibrium, 
Presented complete Methyl Red equilibrium, 
Presented complete Thymol Blue - 1^st equilibrium,