[Note 1:  A knowledge on the part of the student of the ionic theory as applied to aqueous solutions of electrolytes is assumed.  A brief outline of the more important applications of the theory is given in the Appendix.]

BEHAVIOR OF ORGANIC INDICATORS

The indicators in most common use for acid and alkali titrations are methyl orange, litmus, and phenolphthalein.

In the following discussion of the principles underlying the behavior of the indicators as a class, methyl orange and phenolphthalein will be taken as types.  It has just been pointed out that indicators are bodies of complicated structure.  In the case of the two indicators named, the changes which they undergo have been carefully studied by Stieglitz (!J.  Am.  Chem.  Soc.!, 25, 1112) and others, and it appears that the changes involved are of two sorts:  First, a rearrangement of the atoms within the molecule, such as often occurs in organic compounds; and, second, ionic changes.  The intermolecular changes cannot appropriately be discussed here, as they involve a somewhat detailed knowledge of the classification and general behavior of organic compounds; they will, therefore, be merely alluded to, and only the ionic changes followed.

Methyl orange is a representative of the group of indicators which, in aqueous solutions, behave as weak bases.  The yellow color which it imparts to solutions is ascribed to the presence of the undissociated base.  If an acid, such as HCl, is added to such a solution, the acid reacts with the indicator (neutralizes it) and a salt is formed, as indicated by the equation: 

(M.o.)^{+}, Oh^{-} + H^{+}, Cl^{-} —­> (M.o.)^{+} Cl^{-} + (H_{2}O).

This salt ionizes into (M.o.)^{+} (using this abbreviation for the positive complex) and Cl^{-}; but simultaneously with this ionization there appears to be an internal rearrangement of the atoms which results in the production of a cation which may be designated as (M’.o’.)^{+}, and it is this which imparts a characteristic red color to the solution.  As these changes occur in the presence of even a very small excess of acid (that is, of H^{+} ions), it serves as the desired index of their presence in the solution.  If, now, an alkali, such as NaOH, is added to this reddened solution, the reverse series of changes takes place.  As soon as the free acid present is neutralized, the slightest excess of sodium hydroxide, acting as a strong base, sets free the weak, little-dissociated base of the indicator, and at the moment of its formation it reverts, because of the rearrangement of the atoms, to the yellow form: 

Oh^{-} + (M’.o’.)^{+} —­> [M’.o’.Oh] —­> [M.o.Oh].

Phenolphthalein, on the other hand, is a very weak, little-dissociated acid, which is colorless in neutral aqueous solution or in the presence of free H^{+} ions.  When an alkali is added to such a solution, even in slight excess, the anion of the salt which has formed from the acid of the indicator undergoes a rearrangement of the atoms, and a new ion, (Ph’)^{+}, is formed, which imparts a pink color to the solution: