Electrochemical Methods for Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 299

complex organic molecule that acts as an ion-carrier. Two ISEs with successful
environmental applications are the fluoride electrode and nitrate electrode (Fifield
and Haines, 2000). The ISE methods for these two ions are particularly important
because of their apparent advantages over other methods, including low detection
limits and the elimination of tedious sample preparation procedures. These are
further described below.
The fluoride electrode is of the first type, that is, the solid-state membrane.
Direct measurements of the free ion are reliable down to 1 g/L. The membrane

consists of a single lanthanum trifluoride (LaF 3 ) crystal; within this hexagonal
lattice the fluoride ions are relatively mobile. This crystal is doped with europium
fluoride (EuF 2 ) to reduce the bulk resistivity of the crystal. The potential across the
membrane is analogous to that of pH electrode (Eq. 11.1). That is,

m

RT
F ln a F ¼ constant

2 : 303RT

F

E cell ¼ constant þ pF unknown ð11 : 12Þ

where a F is the activity of F . This ISE is extremely selective for F ions and is
only interfered by OH that reacts with the lanthanum to form lanthanum hydroxide,
with the consequent release of extra F ions.
Interference is a common problem with ISEs. Other limitations include the
effects of ionic strengths and the potential formation of complex compounds that are
not responsive to ISEs. Many of these problems, however, can be solved by adding a
total ionic-strength adjustment buffer (TISAB). The commercially available TISAB
for fluoride electrode is a mixture of an acetate buffer (pH 5.0-5.5), 1 M NaCl, and
cyclohexylenedinitrilo tetraacetic acid (CDTA). The pH in this range will ensure a low
OH concentration in the solutions while also avoiding the formation of appreciable
HF, which is not responsive to the membrane. The CDTA is a chelating agent, which
can release F from possible complex compounds such as Al 3þ ,Fe 3þ ,andSi 4þ .
The nitrate electrode is a good example of the second type, that is, the liquid
membrane incorporating an ion exchanger. The nitrate electrode method is useful in
environmental analysis because it compares favorably with colorimetric methods for
the determination of nitrates in soils and water. A method detection limit (MDL) of
2 mg/L and a linear calibration range 5-200 mg/L can be obtained using the U.S.
EPA method 9210A. The membrane of commercial nitrate electrode is usually
polyvinyl chloride (PVC). As the nitrate ion is a strongly hydrophilic anion, the anion
exchanger should be a strongly hydrophobic cation such as tetraalkylammonium salt.
For the same reason, when divalent cations, such as Ca 2þ and Mg 2þ are analyzed, a
cation exchange is used (e.g., calcium organiophosphorous compound for Ca 2þ ).

11.2.3 Potentiometric Titration (Indirect
Potentiometry)

Potentiometric titration is a volumetric method in which the potential (E) between
two electrodes is measured as a function of the added volume of titrant (V). The
indicator electrode can be selected to respond to either a reactant or a product.