at the end point by about 6 units. For titrations of strong acids and weak bases, we
need an indicator, such as methyl orange, that changes color between 3.1 and 4.4
in the acid region. In the titration of a weak acid and a strong base, we should use
an indicator that changes in the basic range. Phenolphthalein is a suitable choice
for this type of titration because it changes color in the pH 8.3 to 10.0 range.
TITRATION—VOLUMETRIC ANALYSIS
Knowledge of the concentrations of solutions and the reactions they take part in
can be used to determine the concentrations of “unknown” solutions or solids. The
use of volume measurement in solving these problems is called titration.
A common example of a titration uses acid–base reactions. If you are given a
base of known concentration, that is, a standard solution, let us say 0.10 M NaOH,
and you want to determine the concentration of an HCl solution, you could titrate
the solutions in the following manner.
First, introduce a measured quantity, 25.0 milliliters, of the NaOH into a
flask by using a pipet or burette in a setup like the one in the accompanying
diagram. Next, introduce 2 drops of a suitable indicator. Because NaOH and HCl
are considered a strong base and a strong acid, respectively, an indicator that
changes color in the middle pH range would be appropriate. Litmus solution
would be one choice. It is blue in a basic solution but changes to red when the
solution becomes acidic. Slowly introduce the HCl until the color change occurs.
This point is called the end point. The point at which enough acid is added to
neutralize all the standard solution in the flask is called the equivalence point.
Suppose 21.5 milliliters of HCl was needed to produce the color change.
The reaction that occurred was