Page 29 - Pharmaceutical analytical chemistry |

Theory of neutralization titrations

The standard solutions employed in neutralization titrations are strong
acids or strong bases because these substances react more completely
with analytes than do their weaker counterparts. Standard acids are
prepared from hydrochloric and sulfuric acids, while standard basic
solutions are ordinarily prepared from sodium and potassium hydroxides.

Theory of indicator behavior

Acid-base indicators are weak organic acids or bases that, upon
dissociation or association, undergo internal structural changes that give
rise to color differences. Color changes for acid-base indicators can be
ascribed to such equilibria as:

HIn + H2O ↔ H3O+ + In-
In + H2O ↔ InH+ + OH-
In both cases, the color of the molecular form of the indicator is different
from the color of the ionic form.
Equilibrium constant expressions for the two dissociations are:
a. For acidic indicators:

b. For basic indicators:

The indicator changes color over a pH range. The transition range
depends on the ability of the observer to detect small color changes. With
indicators in which both forms are colored, generally only one color is
observed if the ratio of the concentration of the two forms is 10:1, only

28