INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS 581
andMnO^4 8H 5e Mn 4H O2
2 (43)the ew of Fe^3 ^ is the aw/1 and ew of MnO 4 or KMnO 4 is the
fw/5. The number of equivalence is represented by the frac-
tion. W/ew, in equation 41.(a) Potentiostatic coulometry
The working electrode is maintained at a constant, pres-
ent potential in potentiostatic coulometry. In this process the
analyte changes oxidation state reacting quantitatively with
the current: i.e., at 100% current efficiency. Redox reac-
tions with less reactive species in the analyte solution are
precluded. Maintenance of 100% current efficiency allows
the amount of analyte present to be determined unambigu-
ously by the quantity of coulombs consumed application of
Faraday’s law (see equation 41). As the redox reaction pro-
ceeds, the current and, therefore, the reaction rate decreases
because of increases in polarization effects, changes in
solution resistance, etc. Nevertheless the working electrode
potential is maintained restricting the rate of the redox reac-
tion of the analyte. The amount of current consumed usu-
ally decreases with time and is measured by an integrating
device: namely, an electronic, chemical or electromechanical
coulometer. An electronic controlled-potential coulometric
titrator developed by Kelley et al.^101 includeds a potentiostat,
which provides and maintains a constant potential, a d.c.
current source, an electrolytic cell, and an integrating cou-
lometer providing the readout in coulombs. This device has
a range of 10 microamperes to 10 milliamperes, an accuracy
of 0.01%, and a response time of 10 milliseconds.
An application of this method is the analysis of a mix-
ture of antimony(III) and antimony(IV) accomplishable
in two steps.^102 From the voltammetric data, the centered
plateau reduction voltages for the following reactions are
as follows:Sb^5 ^ 2e Sb^3 ^ , 0.21 V vs SCE (44)and
Sb^3 ^ 3e Sb^0 , –0.35 V vs SCE. (45)The supporting electrolyte (6M hydrochloric acid plus 4M
tartaric acid) is reduced at 0.35 V, before addition of the
sample to remove impurities. Upon addition of the sample
of antimony species, the solution is deaerated with nitrogen.
The potentiostat described previously^101 is set at 0.21 V
and the experimental voltage rises slowly to that value during
the reduction of Sb^5 ^. The current starts to decrease when
the set voltage is reached because of the decreasing concen-
tration of Sb^5 ^ , other polarization effects, etc. When all the
Sb^5 ^ has been reduced to Sb^3 ^ , the current will decrease to a
negligible value signaling the end to the first analytical step.
The second reduction is carried out by setting the potentio-
stat at 0.35 V and repeating the process. In calculating theSb^3 ^ content of the sample a correctin for the Sb^3 ^ gener-
ated in the first reduction must be made. When interfering
substances are present, e.g., the analysis of plutonium in the
presence of iron, an indirect approach can be used.^103
Potentiostatic coulometry has the same advantages of
controlled-potential electrogravimetry. In employing this
method rather than electrogravimetry, the attendant prob-
lems of poorly adhering electrolytic deposits are eliminated.
In addition analytes that do not yield electrolytic deposits,
but are amenable to coulometric analysis can be analyzed.
This technique requires a longer analysis time compared to
amperostatic coulometry because of the decrease in current
flow as the reaction proceeds.(b) Coulometric titrations
Coulometric titrations (amperostatic or controlled cur-
rent coulometry) are carried out at constant current (see
Figure 35). At the working electrode a reagent is generated
that reacts with the analyte in one of several types of reac-
tions namely, oxidation/reduction, acid/base, complexation
or precipitation (see Figure 36). When all the analyte has
reacted with the generated reagent, the endpoint or com-
pletion of the reaction may be detected by potentiometry,
indicator color changes, amperometry, or conductance. In
some coulometric titrations, however, part of the current
arises from direct reduction or oxidation of the analyte at the
electrode and the remainder through the generation of the
reagent. Subsequent reaction of the reagent and the remain-
ing unreacted analyte ends the titration and 100% current
efficiency is maintained. The measured quantity for a cou-
lometric titration is the number of coulombs necessary to
generate the reagent; it is comparable to the volume of titrant
in a classical titration. (A current efficiency of 100% is,
therefore, required.) Classical and coulometric titrations are
comparable in a number of ways; these two methods have
similar endpoint detection methods and stoichiometric reac-
tions between titrant and analyte must be rapid, complete,
and free of side reactions. Since constant current is used in
this technique, an accurate timer is used in the coulometric
titrator in Figure 35. The product of time and current (see
equation 41) will give the number of coulombs. An integra-
tor is not needed, as in potentiostatic coulometry, where the
current can change with time.
The generation of reagents can be internal, in the cell
containing the analyte, or external to the cell. Outside gen-
eration of reagents is often convenient for several reasons:
Electrolytic interference of substances in the sample solution
and incompatibility in conditions fostering efficient genera-
tion of the reagent can occur in internal generation.(x) Internal generation
Two modes of internal generation are utilized and are
referred to as primary and secondary coulometric titrations.
In the primary mode the analyte reacts directly with a spe-
cies generated from the electrode material. Therefore, no
other species should be present which will react electrolyti-
cally with the working electrode within about 0.5 V of theC009_005_r03.indd 581C009_005_r03.indd 581 11/23/2005 11:12:28 AM11/23/2005 11:12:28