The main points of this theory are as follows :

(i)    The acid-base indicators are organic aromatic compounds which can exist in at least two tautomeric forms.

(ii)    The taotomeric forms of the indicator exist in equilibrium with one another.

(iii)    One of the tautomeric forms of the indicator can exist in an acid solution while the other form can exist in alkaline solution.

(iv)    The two tautomeric forms possess different colours depending upon pH  value. of the solution. Therefore, as the   of the solution changes, the colour of the solution changes due to conversion of one form into the other.

(v)    The two tautomeric forms possess different structures known as bezenoid (benzene-like) form and quinonoid (quione-like) form.

(vi) The quinonoid form is usually deeper in colour than the benzenoid form.

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