The use of electrodes, particularly the glass electrode for pH measurements and
the wide range of other ion selective electrodes (ISE) described in Topic C3,
enables titrations to be studied throughout the addition of titrant, so that small
changes may be detected. It also allows automationof the titration.
To compare potentiometric titrations with those where the end point is
detected visually, it is useful to think of the ion selective electrode as an indi-
cator electrode responding to the ion to be detected, and to remember that it
must be combined with a reference electrode, the potential of which must not
be affected by the titration reactions.
In general, the titration will produce a graph such as Figure 2(a) where pH,
pIon or E(cell) is plotted against the volume of titrant added. The sharp change
at the end point is readily observed. Derivative plots such as Figure 2(b)are an
aid to finding the end point.
Three types of potentiometric titration may be recognized. Detailed discus-
sion of the reactions is given in Topics C6 and C7, but the principles will be
discussed here.
● R, orreagent sensed. A reagent is added, for example, some copper(II)
EDTA complex, which is sensed by a copper ISE. Addition of EDTA to a
solution containing Ca^2 +(or Mg^2 +, Ni^2 +etc.) ions gives an abrupt change at the
end point where the excess EDTA reduces the concentration of Cu^2 +ions.
● S, orsample sensed.The ISE senses the ion in the solution being titrated. pH
titrations using a glass electrode belong to this type, as do titrations of
fluoride with La^3 +.
● T, ortitrant sensed. The titrant added to the sample is sensed by an ISE
responsive to that ion; for example a silver ion ISE for the titration of halide
ions by silver nitrate.
Potentiometric titrations for complex, precipitation and redox systems are
discussed in Topic C7.Applications There are many applications for acid-base titrations, several of which are
routinely used analytical methods described in the appropriate topics.
● The determination of the concentration of acid in foods and pharmaceuticals.
● The measurement of acid number (or base number) during the course of a
reaction. For example, in the production of polyester resins by the reaction of
a glycol with maleic and phthalic acids, the total acid remaining is deter-
mined by titration of a weighed sample with potassium hydroxide using
phenolphthalein as indicator.
● The Kjeldahl method for nitrogen determination is a good example of a back
titration. The sample (for example, a food product) is oxidized by concen-
trated sulfuric acid to remove carbonaceous matter. Excess sodium
hydroxide solution is then added, and the ammonia released is carefully
distilled off into a known volume of standard acid, such as 0.1 M boric acid.
The excess acid is then titrated with standard alkali.
Automated titrations are important in producing rapid, reproducible results
in commercial and research laboratories. Samples may be prepared and loaded
using mechanical pipets or direct weighing and dissolution methods. Titrant is
added to the sample titrand solution using peristaltic pumps, or burets driven
by pressure or piston systems. The addition is very reproducible after accurate
calibration. The progress of the titration is most often followed by potentio-
metric measurements, as outlined above (see also Topic H2).
Potentiometric
titrations
84 Section C – Analytical reactions in solution