Similarly, in the last part of the titration curve (when an excess of base has been added), the
addition of small amounts of base will not change the [OH−] significantly, and the pH remains
relatively constant. The addition of base most alters the concentrations of H+ and OH− near the
equivalence point, and thus the pH changes most drastically in that region.
As described above, titration is used to determine the concentration of an acid or a base. Imagine
that we have an acidic solution of volume VA and unknown normality NA. We add to this acidic
solution a basic solution of known normality NB a little bit at a time (a drop at a time), keeping track
of the amount of base we have added. The equivalence point, as defined above, is the point at which
the amount of acid equals the amount of base, or:
VANA = VBNBwhere VB is the volume of the base we have added so far when the equivalence point is reached.
Using this equation we can determine the normality or concentration of the original acid:
The question is, of course, how do we know we have reached the equivalence point? We can use a
pH meter and monitor the pH as a function of base added. More commonly, however, we use a
couple of drops of an indicator and watch for a color change. Indicators are weak organic acids or
bases that have different colors in their undissociated and dissociated states. If the solution in which
it finds itself is below a certain pH, it will be of one color; if the solution pH is above that, it will be of
a different color. The indicator most commonly encountered in Introductory Chemistry labs is
probably phenolphthalein, which is colorless at low pH but becomes red around pH = 8, and is most
often used in strong acid/strong base titrations. If we add a few drops of this to the acid at the
beginning, then at a certain point in the titration the solution will take on a pale reddish hue (pale
because it is so dilute). This point is the endpoint, signifying the end of the titration. The volume of
base added at that point is used in the equation above as VB. You may be thinking, this is not the
same as the equivalence point! Indeed: The equivalence point is expected to occur at a pH of 7 for
this kind of titration, yet phenolphthalein does not change color until pH ~ 8. The reason we can get
away with this is because the only piece of information we need from the equivalence point is the
volume of base added by the time it is reached. Since the pH is rising so sharply near the