Monitoring Acid-Base Titrations with a pH Meter
Titration is one of the techniques used in performing chemical analysis. The apparatus used for
titration is called, a buret. The buret is a long tube marked for volumes, usually in increments of
0.10 mL. At the end of the tube there is a valve called a stopcock. The amount and rate of addition
of the solution, titrant, is controlled by manipulating the stopcock. In a typical titration a solution
containing an unknown amount of the substance to be analyzed is placed in a receiving flask. The
next step is to add precisely measured volumes of a solution of known concentration, a standard
solution, from the buret. The standard solution is added until the end of the reaction is reached.
Since I can't see when the end of the reaction is reached I have to use a special dye called an
indicator or an instrument called a pH meter to tell me when this point is reached. Once the end
of the reaction is reached, the known amount of reactant added using the buret is used to
stoichiometrically calculate the amount in the initial solution.
Indicators
An acid-base indicator is one color in acidic solution and another in basic solution. Phenolphthlein
is an example of an indicator; it is colorless in acid and pink in base. The indicator changes color
when the reaction has reached completion, for example the acid has been neutralized by the base.
At this moment the titration is stopped. The equivalence point in the titration occurs when all the
moles of acid present in the original solution have been reacted with an equivalent amount of base,
moles acid = moles base. The end point of the titration occurs when a tiny excess of base, one drop
past the equivalence point, is added. This last drop causes the solution which was neutral at the
equivalence point to now be basic which causes the change in color of the indicator dye. In
calculations we assume that this tiny excess of base is insignificant. Therefore the amount of base
required to reach the equivalence point is essentially the same as the amount required to reach the
end point.
Phenolphthalein in acid solution Phenolphthalein in basic solution
pH meters
The instrument that can be used to measure pH is called a pH meter. This is the most accurate and
precise method, with pH being determined to within +/- 0.01 pH units. Another advantage of a pH
meter is that it can be used to follow the change in the pH during the entire titration. Unlike
indicators which only tell you when a transition has taken place in the solution, the pH meter can
give you the pH continuously. The ability to follow the pH of the solution continuously allows the
equivalence point of the titration to be determined rather than just the end point.
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