Encyclopedia of Environmental Science and Engineering, Volume I and II

582 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS

is generated at a potential between that of the potential of

the analyte redox reaction and the potential of an interfering,

unwanted reaction. The interfering reaction can be due to

electrolysis of the supporting electrolyte or another electro-

active substance in the sample. An illustration of a secondary

titration is as follows: The coulometric titration of ferrous

ion uses the Ce^3 ^ /Ce^4 ^ system as the intermediate reactant.

At the onset of the titration the ferrous ion is directly oxi-

dized at the platinum working electrode. Since the current

is constant, the potential increases as the concentration of

ferrous decreases. At the oxidation potential of cerous ion

and ceric ion is generated and in turn oxidizes the ferrous

ion. The summation of number of coulombs consumed by

each electrolytic reaction, direct and ceric oxidation, is that

needed to titrate the ferrous ion present. The endpoint chosen

is the presence of excess ceric ion indicating the complete

oxidation of ferrous ion. The ceric ion can be detected pho-

tometrically or amperometrically. Since an excess of cerous

ion is present, the working electrode is maintained at the

unique potential of the cerous/ceric couple. The potential of

the working electrode is prevented from increasing to a value

where the interfering substances would be electrolyzed.

In secondary coulometric titrations reagents can be gen-

erated that are inconvenient to use in classical titrimetry.

Some examples are the generation of hydroxide ion free of

contamination by carbonate and the reactive titrants bro-

mine, chlorine and titanous. More on this subject is given in

the section on external generation of titrants.

(y) External generation

External generation of reagents can be necessary because

of the presence of interfering substances in the sample. In

Figure 37 a double-arm apparatus for the outside generation

of reagents is shown.^104 Specifically the equations for the

generation of acid (hydrogen ion) or base (hydroxide ion)

Stirrer

Sintered glass frit

Auxiliary electrode

Electrolyte solution

Generator electrode

FIGURE 36 Coulometric titration cell.

Titration

vessel

Working or generating

electrode

Ganged

switch Clock

Potentiometer

for current

measurement

ED

Constant

current

source

Counter or auxiliary electrode

(in separator)

Detection

circuit

Detection electrodes

FIGURE 35 Coulometric titration apparatus.

potential of the analyte reactivity. An example of this mode

is the reaction of ionic halide ions, mercaptans, or sulfhy-

dryl groups with electrolytically generated silver ion from a

silver working electrode. The endpoint may be determined

amperometrically.

In the secondary mode an active intermediate is gener-

ated which reacts directly with the analyte. The intermediate

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