Answer:
A Weak acid indicator has general formulae HIn- and base has In-.
When constructed a plot determining wavelength equilibrium reached according to the equation HIn(aq) + H2O(l) ↔ In-(aq) + H3O+(aq)
HIn(aq) is the acid, that has a different color from the base In-(aq). When the pH is low, the concentration of the hydronium ion H3O+ is high and equilibrium is toward the left, producing the color A.
At high pH, the concentration of H3O+ is low, so equilibrium tends toward the right side of the equation and color B is displayed
