Encyclopedia of Environmental Science and Engineering, Volume I and II

580 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS

method. The limitation is an uncontrolled, working electrode

potential. As the reaction progresses, the voltage must be

increased to provide a constant current because of polariza-

tion effects. This can lead to the deposition of other species.

However, this method is advantageously applicable when the

species is alone in the solution, more readily reduced than

hydrogen ion, and therefore, not requiring a controlled work-

ing electrode potential. Some of the cations amenable to this

technique are cadmium(II), cobalt(II), copper(II), iron(III),

lead(II), nickel(II), silver(I), tin(II), and zinc(II). If interfering

ions are present they may be complexed in order to prevent

electrodeposition or removed by prior chemical precipitation.

Another application of this technique is the removal of a spe-

cies which would interfere in a subsequent analytical operation.

A mercury electrode is frequently used in this last application.

Controlled, working electrode potential electrolysis has

the greatest applicability to mixtures of analytes. The con-

trol of the working electrode permits selective deposition of

metals with standard potentials that differ by several tenths

of a volt. A schematic diagram in Figure 34 embodies the

elements of the apparatus. One example is the analysis of

a sample containing copper, bismuth, lead, cadmium, zinc,

and tin. The first three metallic ions are deposited at selec-

tive potentials; tin is held in solution as the tartrate complex.

Cadmium and zinc are selectively deposited from ammonia-

cal solutions. Finally, the solution is acidified to decompose

the tin-tartrate complex and tin is deposited.^99 Table 10 lists

analyses amenable to this technique.^100 Controlled, work-

ing electrode potential electrolysis is a slower process than

the constant current method, since the current flow tends to

decrease in the controlled-potential technique due to polar-

ization and other effects.

(2) Coulometry

In coulometry one measures the quantity of electricity

(number of coulombs) that is required to carry out a redox

reaction of the analyte or generate a reagent that reacts with

the analyte. Two general methods are used in coulometry,

namely controlled-potential (potentiostatic) and controlled-

current (amperostatic), commonly known as coulometric

titrations. All the current must be used, solely, either directly

or indirectly, for the reaction concerned with the analyte,

that is, the current efficiency must be 100% for a quantita-

tively accurate, analytical result. In these methods standards

are not needed; the proportionality constant (Faraday’s con-

stant) relating the number of coulombs and the amount of

the analyte is derivable from known physical constants. In

the cell the effects of polarization are decreased by using

electrodes with large surface areas and by vigorous stirring

of the solution during electrolysis.

Faraday’s law relates the amount of electricity in cou-

lombs to the number of equivalents of reactant in a redox

reaction. Faradays’s constant, 96,487 coulombs/equivalent,

is the stoichiometric factor relating electrical charge passing

in an electrolysis and the equivalence of substance reduced

or oxidized. Since one ampere is the rate of flow of one

coulomb per second, the time integrated flow of the current

during a redox reaction will yield the number of coulombs,

Q. When the current varies over the reaction time period an

integration is necessary. With constant current the number of

coulombs can be calculated by the equation,

it  Q  96,487 W/ew. (41)

The quantities are i, amperes; t, seconds; W, sample weight

in grams, and ew, equivalent weight in grams per equivalent.

The equivalent weight is the atomic weight, aw, or formula

weight, fw, divided by the number of electrons in the redox

or half cell reaction. For example in the reactions,

Fe^3 ^  e  Fe^2 ^ (42)

Counter or auxiliary

electrode

Reference electrode

Potential

control

Working electrode

Potentiostat

Var iable

voltage

source

Potential

measuring

device

FIGURE 34 Schematic for a controlled-potential electrolysis

apparatus.

TABLE 10

Controlled-potential electrolysis for metal separation and determination

Metal Separated Metal Matrix

Sb Pb, Sn

Bi Sb, Cd, Cu, Pb, Sn, Zn

Cd Zn

Cu Sb, Bi, Cd, Pb, Ni, Sn, Zn

Pb Al, Cd, Fe, Mn, Ni, Sn, Zn

Ni Al, Fe, Zn

Ag Cu and more active metals

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