Electrochemical Methods for Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 292

mole of reactant, and F is the Faraday constant, which represents the number of
coulombs in one mole of electrons (96,485 coulombs mol 1 mol).

The standard potential (E 0 ) in the Nernst equation can be calculated from the
tabulated E 0 values of the two half-cell reactions. For example, since the E 0 values
for Eqs 11.2 and 11.3 are 0.763 and 0.337 V, respectively, the E 0 value of the
standard potential of the overall reaction (Eq. 11.4) is 0

¼ 94 ; 485 J = V =

10 V under
standard conditions. The positive E value indicates the redox reaction in Eq. 11.4 is
spontaneous as written from left to right. In Eq. 11.5, note the negative sign and by
convention the concentrations of all products are in the numerator and
concentrations of the reactants are in the denominator. The concentrations of metal
Cu (s) and Zn (s) are omitted because, by convention, solids are assumed to be
pure with a unity concentration. Note also that, in essence, the concentrations in
the Nernst equation should be activities. This unique feature of electrochemistry
may be an advantage or a disadvantage depending on the applications. In this text,
we assume that they are equal in dilute solutions.

: 763þ0 : 337 ¼ 1 :

11.1.2 General Principles of Electroanalytical
Methods

Potentiometry

There are two types of potentiometric techniques: indirect potentiometry
(potentiometric titration) and direct potentiometry. With indirect potentiometry,an
abrupt potential change ( E) in a chemical reaction is used as the end point during

titration. The major advantages of potentiometric titration over the manual titration
(Chapter 6) include the automation and its ability to titrate in colored and turbid
solutions. For direct potentiometry, the underlying principle for the measurement of
chemical concentrations is the relationship between potential (E) and analyte's
concentration as described in the Nernst equation (Eq. 11.5). Note that the potential
developed between two electrochemical cells is measured in the absence of
appreciable currents. The potential of the indicator electrode with respect to the
reference electrode responds to changes in the concentration of the chemical species
of interest (Again, strictly speaking it responds to changes in activity). The most
common devices employed for the measurement of potential include voltmeters,
potentiometers, or pH meters. Electrodes of various types will be detailed in the
following section.

Coulometry

Instead of measuring potential (E), coulometry measures the quantity of electricity
or the charge (the root word ''coul'' denotes charge) generated by the complete
reaction of an analyte at an electrode. In contrast to potentiometry, an electrical
voltage is applied in coulometric measurement to drive the nonspontaneous
redox reaction (hence electrolytic cell). The charge (Q, unit

in coulombs;