Theory/Discussion

When any acid is dissolved in water, it reacts with the water according to the equation:

HA(aq) + H2O(1) H3O+(aq) + A-(aq) (1)

When the acid is a weak acid, there is an equilibrium created between the acid, and its

conjugate base.

HA(aq) + H2O(1) Eq H3O+(aq) + A-(aq) (2)

where HA is any acid and Ais the conjugate base of HA.

When the reaction is reversible, because the acid is a weak acid, an equilibrium is

established, based on the relationship:

Ka = [H3O+] [A-] (3)[HA]

From this the Henderson-Hasselbalch Equation is derived:

pH = pKa + log [A-] (4)[HA]

During the course of a titration, an acid in solution reacts with a base in solution, and

when a strong acid or strong base is involved, this reaction always goes to completion.

HA(aq) + OH-(aq) → A-(aq) + H2O (1) (5)

At the equivalence point (which is the place where moles of acid = moles of base) the

titration is complete, no HA remains; and the only substances in the beaker are water, the

conjugate base, A-, and a spectator ion. The end point is the place at which the indicator

changes color. The endpoint and the equivalence point are not always identical, but they

are always very close.