Encyclopedia of Environmental Science and Engineering, Volume I and II

572 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS

issue constantly from the DME. The rate of mercury flow,

m, is 1 to 3 mg/second and is dependent on the lumen and

length of the capillary and the height of the mercury column

(the applied pressure) in classical (linear scan) polarogra-

phy. Drop diameters are between 0.1 and 1 mm. The large

excursions seen in Figure 26A on the plateau of the curve are

due to the growth of the mercury drop. In current-sampled,

and normal- and differential-pulse polarography drop time,

t, is controlled by a mechanical or electrical device causing

dislodgement of the drop at chosen drop times of 1 to 5 sec.

The excursions on the curve are diminished because drops

times are decreased as compared to classical polarography

(see Figure 26A). The DME provides a renewable surface

electrode that prevents the anomalous surface effects experi-

enced, at times, with solid electrodes.

The polarographic cell solution containing the analyte

and an indifferent electrolyte are purged with pure nitro-

gen to remove all dissolved oxygen. (A reduction wave for

oxygen occurs and interferes with the analyte wave.) The

indifferent electrolyte, e.g., potassium chloride, removes the

possible condition of electrostatic attraction or repulsion of

a charged analyte resulting in a migration current, i m. This

then allows the analyte to approach the electrode by the pro-

cess of diffusion leading to a diffusion current, i d , solely. The

voltage is scanned with a linearly increasing d.c. ramp of

1 to 5 mv/sec (mv, millivolts). In the voltage scanning pro-

cess, before reduction or oxidation of the analyte takes place,

impurities in the solution contribute to a small current known

as the residual current, i r , resulting in a baseline of slowly

increasing current as a function of potential (see Figure 27).

The electrochemical redox reaction of the analyte begins at

the decomposition potential in a heterogeneous transfer of

electrons between the analyte in solution and the electrode

surface. The current increases, sharply, until the limiting cur-

rent, i L , is reached where it remains constant due to concen-

tration polarization. Concentration polarization is a necessary

condition in some types of voltammetry. Figure 27 shows a

polarogram and the measurement of the diffusion current of a

polarographic wave for a substance i d.

A number of scanning and measuring modes, i.e., linear

scan (classic), current-sampled (tast), normal and differen-

tial pulse, and fast-linear sweep, are available. Figure 26A

and B illustrates the various polarographic curves for each

of these scanning modes. The classical “S” shaped curve

is obtained with linear (classic), current-sampled (tast),

normal pulse techniques. The fast linear scan gives a dis-

torted “S,” while the differential pulse produces a differ-

ential curve. The height of the wave, the diffusion current,

Polarogram

Polarography

voltage

Drop

surface

Differential pulse polarography Direct-current polarography

DP 10 DP 50 DC DCTe s t

FIGURE 26A Direct-current, DC, and differential pulse, DP, polarographic methods. (Last and current-sampled

polarography are synonymous.) The following are detection limits and E1/2 resolution values i.e., 2  10 ^6 M and

0.2 V, 1  10 ^6 M and 0.2 V, and 1  10 ^7 M and 0.05 V for DC, DC tast, and DP polarography, respectively.

(Courtesy of Brinkmann Instruments, Inc.)

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