Encyclopedia of Environmental Science and Engineering, Volume I and II

INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS 573

FIGURE 26B Other polarographic and voltammetric methods. (Pulse polarography refers to normal pulse as differentiated from dif-

ferential pulse polarography. Inverse and stripping voltammetry are synonymous.) The following are detection limits and E1/2 resolution

values i.e., 5  10 ^7 M and 0.05 V, 5  10 ^7 M and 0.2 V, and 10^9 M and 0.1 V for AC and normal pulse polarography, and stripping

voltammetry, respectively. (Courtesy of Brinkmann Instruments, Inc.)

A

A

A

A

t

t

t

t

MEASURINGTIME

MEASURINGTIME

MEASURINGTIME = 20ms

DROP LIFE

DROP LIFE

DROP LIFE DROP LIFE

DROP LIFE

DROP LIFE

20 ms

20 ms

AC1rapid

AC2rapid

–v

–v

–v

–v

–v

–v

–v

–v

–I

–I

–I

–I

t

t

PULSE t

DURATION

Vstart

time

Vstart

Vstart

Vstart

4x10–3xmm/tdropx∆V

4x10–3xmm/tdropx∆V

INVERSE VOLTAMMETRY

PULSE POLAROGRAPHY

P

RAPID

PHASE SENSITIVE

AC POLAROGRAPHY

(2nd Harmonic)

AC 2

NORMAL, RAPID OR TAST

PHASE SENSITIVE AC

POLAROGRAPHY

(1st Harmonic)

AC 1

NORMAL, RAPID OR TAST

Drop Surface Waveform Polarogram

i d , is proportional to the concentration of the electroactive

analyte, while the half wave potential, E 1/2 in volts, is the

qualitative factor identifying the analyte (see Figure 27).

For the differential pulse curve, the height of the peak, i p ,

is proportional to the concentration of the analyte and the

peak potential, E p , is almost equal to the E 1/2. The standard

reduction potential, E^0 , and the E 1/2 value are approximately

equal (see equation 11). The Ilkovic equation relates the i d ,

in amperes, with the diffusion coefficient of species S, D s in

cm^2 /seconds; the concentration of the analyte, C in moles/L;

the number of electrons transferred in the redox reaction,

n; the flow rate of mercury, m in mg/seconds; and the drop

time, t in seconds as follows:

ids708nD Cm t

 12/ 2/3 1/6. (33)

In linear scan or classic polarography an impediment to low

level detection is the changing non-faradaic charging current

that electrostatically charges the mercury drop. (Consider

the drop as a condenser.) As the area of the mercury drop

increases with time, the rate of area increase decreases with

time. Thus, the non-faradaic charging current decreases with

time and is quite low at the end of the drop time. The total

polarographic current is a sum of the diffusion current, i d , the

charging current and the residual current, i r. (The i r , due to

electroactive contaminants, is rather small and invariant with

time.) Since the charging current is an unknown quantity

the signal can not be amplified to increase the sensitivity of

the method without introducing a considerable but variable

error. To obviate the charging current problem, normal-and

differential-pulse^79 and current-sampled modes were devel-

oped (see Figure 26). In each of these modes the current is

measured during the last 5 to 20 msec of each drop before

Applied voltage

Current

N P

L M

0

E1/2

1/2 id

id

FIGURE 27 Typical polarogram. The line LM repre-

sents the residual current, ir and id the diffusion current.

(Reprinted from Ref. (176), p. 394 by permission of

Prentice-Hall, Inc., Englewood Cliffs, NJ.)

C009_005_r03.indd 573C009_005_r03.indd 573 11/23/2005 11:12:26 AM11/23/2005 11:12:26