H^{+}, (Ph)^{-} + Na^{+}, Oh^{-} —­> (H_{2}O) + Na^{+}, (Ph)^{-} —­> Na^{+}, (Ph’)^{-}

The addition of the slightest excess of an acid to this solution, on the other hand, occasions first the reversion to the colorless ion and then the setting free of the undissociated acid of the indicator: 

H^{+}, (Ph’)^{-} —­> H^{+}, (Ph)^{-} —­> (HPh).

Of the common indicators methyl orange is the most sensitive toward alkalies and phenolphthalein toward acids; the others occupy intermediate positions.  That methyl orange should be most sensitive toward alkalies is evident from the following considerations:  Methyl orange is a weak base and, therefore, but little dissociated.  It should, then, be formed in the undissociated condition as soon as even a slight excess of Oh^{-} ions is present in the solution, and there should be a prompt change from red to yellow as outlined above.  On the other hand, it should be an unsatisfactory indicator for use with weak acids (acetic acid, for example) because the salts which it forms with such acids are, like all salts of that type, hydrolyzed to a considerable extent.  This hydrolytic change is illustrated by the equation: 

(M.o.)^{+} C_{2}H_{3}O_{2}^{-} + H^{+}, Oh^{-} —­> [M.o.Oh] + H^{+}, C_{2}H_{3}O_{2}^{-}.

Comparison of this equation with that on page 30 will make it plain that hydrolysis is just the reverse of neutralization and must, accordingly, interfere with it.  Salts of methyl orange with weak acids are so far hydrolyzed that the end-point is uncertain, and methyl orange cannot be used in the titration of such acids, while with the very weak acids, such as carbonic acid or hydrogen sulphide (hydrosulphuric acid), the salts formed with methyl orange are, in effect, completely hydrolyzed (i.e., no neutralization occurs), and methyl orange is accordingly scarcely affected by these acids.  This explains its usefulness, as referred to later, for the titration of strong acids, such as hydrochloric acid, even in the presence of carbonates or sulphides in solution.

Phenolphthalein, on the other hand, should be, as it is, the best of the common indicators for use with weak acids.  For, since it is itself a weak acid, it is very little dissociated, and its nearly undissociated, colorless molecules are promptly formed as soon as there is any free acid (that is, free H^{+} ions) in the solution.  This indicator cannot, however, be successfully used with weak bases, even ammonium hydroxide; for, since it is weak acid, the salts which it forms with weak alkalies are easily hydrolyzed, and as a consequence of this hydrolysis the change of color is not sharp.  This indicator can, however, be successfully used with strong bases, because the salts which it forms with such bases are much less hydrolyzed and because the excess of Oh^{-} ions from these bases also diminishes the hydrolytic action of water.

This indicator is affected by even so weak an acid as carbonic acid, which must be removed by boiling the solution before titration.  It is the indicator most generally employed for the titration of organic acids.