}~+~$H$_2$O$_{(l)}~\rightleftharpoons~$Ind$^-_{(aq)}~+~$H$_3$O$^+_{(aq)})
When the solution is added to an acid, the high hydronium concentration causes the equilibrium to shift to the left, as per Le Chatelier's Principle. This increases the concentration of HInd and thus makes the solution red. When the solution is added to a base, the hydroxide ions and hydronium ions react, increasing the concentration of water. According to Le Chatelier's Principle, this causes the equilibrium to shift to the right, increasing the concentration of Ind
- and thus making the solution green. The intensity of the colour is based on the degree of ionisation of the acid or base. In stronger acids or bases, the intensity of the colour will be greater. Therefore the solution can be used as an indicator as it is changes colour in acids and bases.
