Chemistry Reference
In-Depth Information
small fraction is dissolved. Hence, when calcium ion is titrated with EDTA,
the concentration of calcium ion controls the equilibrium concentration of
the EDTA anion in solution, which in turn directly controls the free con-
centration of Hg(II). The latter is monitored by a Hg electrode system to
give a direct measure of the calcium ion concentration. This type of system
can be applied to most of the divalent ions that form moderately strong
complexes with EDTA.
One of the most important and extensively used indicator electrode
systems is the glass-membrane electrode that is used to monitor hydronium
ion activity. Although developed in 1909, it did not become popular until
reliable electrometer amplifiers were developed in the 1930s. When the
outside surface of the glass membrane is exposed to an ionic solution, a
response for the hydronium ion activity meets with the Nicholsky equation,
which is similar to the Nernst expression. In view of the importance and
widespread use of the hydronium or pH electrode, this system is discussed
in a separate chapter.
Other potentiometric electrode systems are ion-selective electrodes such
as fluoride, calcium, magnesium, sodium, potassium and chloride, selective
gas electrodes based on membranes such as O
2
,CO
2
,CO,NO,NO
2
and SO
2
,
and enzyme electrodes. These electrodes fall beyond the scope of this topic
and are not discussed further.
For most potentiometric measurements, either the saturated calomel ref-
erence electrode or the silver/silver chloride reference electrode are used.
These electrodes can be made compact, are easily produced, and provide
reference potentials that do not vary more than a few mV. The silver/silver
chloride electrode also finds application in non-aqueous solutions, although
some solvents cause the silver chloride film to become soluble. Some exper-
iments have utilised reference electrodes in non-aqueous solvents that are
based on zinc or silver couples. From our own experience, aqueous refer-
ence electrodes are as convenient for non-aqueous systems as are any of
the prototypes that have been developed to date. When there is a need to
exclude water rigorously, double-salt bridges (aqueous/non-aqueous) are a
convenient solution. This is true even though they involve a liquid junction
between the aqueous electrolyte system and the non-aqueous solvent
system of the sample solution. The use of conventional reference electrodes
does cause some difficulties if the electrolyte of the reference electrode is
insoluble in the sample solution. Hence, the use of a calomel electrode sat-
urated with potassium chloride in conjunction with a sample solution that
contains perchlorate ion can cause dramatic measurements due to the pre-
cipitation of potassium perchlorate at the junction. Such difficulties nor-
mally can be eliminated by using a double junction that inserts another inert
electrolyte solution between the reference electrode and the sample solu-
tion (e.g., a sodium chloride solution).
