Page 286 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

10 TlTRlMETRlC ANALYSIS

For use in titrimetric analysis a reaction must fulfil the following conditions.
1. There must be a simple reaction which can be expressed by a chemical
equation; the substance to be determined should react completely with the
reagent in stoichiometric or equivalent proportions.
2. The reaction should be relatively fast. (Most ionic reactions satisfy this
condition.) In some cases the addition of a catalyst may be necessary to
increase the speed of a reaction.
3. There must be an alteration in some physical or chemical property of the
solution at the equivalence point.
4. An indicator should be available which, by a change in physical properties
(colour or formation of a precipitate), should sharply define the end point
of the reaction. [If no visible indicator is available, the detection of the
equivalence point can often be achieved by following the course of the
titration by measuring (a) the potential between an indicator electrode and
a reference electrode (potentiornetric titration, see Chapter 15); (b) the
change in electrical conductivity of the solution (conductirnetric titration, see
Chapter 13); (c) the current which passes through the titration ce11 between
an indicator electrode and a depolarised reference electrode at a suitable
applied e.m.f. (arnperometric titration, see Chapter 16); or (d) the change in
absorbance of the solution (spectrophotornetric titration, see Section 17.48).]
Titrimetric methods are normally capable of high precision (1 part in 1000)
and wherever applicable possess obvious advantages over gravimetric methods.
They need simpler apparatus, and are, generally, quickly performed; tedious
and difficult separations can often be avoided. The following apparatus is
required for titrimetric analysis: (i) calibrated measuring vessels, including
burettes, pipettes, and measuring flasks (see Chapter 3); (ii) substances of known
purity for the preparation of standard solutions; (iii) a visual indicator or an
instrumental method for detecting the completion of the reaction.

10.2 CLASSIFICATION OF REACTIONS IN TlTRlMETRlC ANALYSIS
The reactions employed in titrimetric analysis fa11 into four main classes. The
first three of these involve no change in oxidation state as they are dependent
upon the combination of ions. But the fourth class, oxidation-reduction
reactions, involves a change of oxidation state or, expressed another way, a
transfer of electrons.

1. Neutralisation reactions, or acidirnetry and alkalirnetry. These include the
titration of free bases, or those formed from salts of weak acids by hydrolysis,
with a standard acid (acidirnetry), and the titration of free acids, or those formed
by the hydrolysis of salts of weak bases, with a standard base (alkalirnetry).
The reactions involve the combination of hydrogen and hydroxide ions to form
water.
Also under this heading must be included titrations in non-aqueous solvents,
most of which involve organic compounds.
2. Cornplex formation reactions. These depend upon the combination of ions,
other than hydrogen or hydroxide ions, to form a soluble, slightly dissociated
ion or compound, as in the titration of a solution of a cyanide with silver nitrate