574 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS
the drop is dislodged. At that point the charging current is at
a very low level and doesn’t cause much error on amplifica-
tion to increase sensitivity.
In the normal pulse technique, an increasing d.c. pulse
is imposed on the linearly increasing d.c. ramp on each suc-
cessive drop. A 20-to-100 mv pulse is imposed on each mer-
cury drop in the differential pulse mode. The voltage pulse
leads to an increased surge in current by the electroacive
species undergoing reaction at the electrode surface due to
the change in voltage. This enhanced current occurs in both
normal- and differential-pulse. There is also a decrease in
non-faradaic charging current because the measurement is
made at the end of the drop life-time about 40 msec after
the pulse is applied to allow for the exponential decay of
the capacitive current. In the normal pulse technique the
current is measured as in the current-sampled method, in
the last 5–20 msec; whereas in the differential pulse mode
the current is the difference between the diffusion current
before the pulse and the current at the end of the pulse
on each drop. The enhancement of the polarographic cur-
rent in normal and differential pulse techniques is due
to two effects, enhancement of the faradaic current and
a decrease in the nonfaradaic charging current. The dif-
ferential method is less sensitive than the normal pulse
method, however its resolution is higher. The pulse tech-
niques are more sensitive than classical polarography (see
Figures 26A and B).
Fast linear sweep, LS, polarography carries out a fast
sweep of the voltage range on one drop of mercury.^12 A dif-
ferential curve is obtained whose summit or peak current is
composed of the surge in current previously mentioned in
pulse techniques and the i d. The peak current, i p , is given by
the equationip(2.69)() 105 n ArD Cv3/2 1/2s 1/2. (34)Here Ar is the area of the electrode, v is the voltage scan rate
in volts/sec, and the remaining terms are those given in equa-
tion 33. E p for a reversible reaction is writtenE p E 1/2 0.065/n. (35)This procedure has the advantage of good resolution as in
the pulse methods, speed, and increased sensitivity (see the
discussion in the following paragraph on LSV and Figures
26A and B).
Alternating current polarography employs 1 to 35
millivolts of a.c. of 10 to 60 Hz superimposed on a d.c.
ramp of 5 mv/sec used in the previous methods. The a.c.
faradaic current responsible for the redox reactions of the
analyte is recorded as a function of the d.c. ramp volt-
age and provides a maximum peak voltage, E p , similar to
the E 1/2 of the analyte. The concentration of the analyte is
proportional to the a.c. faradaic current. Species undergo-
ing reversible redox reactions are sensitive to the a.c. tech-
nique. The detection limit is quite low and the a.c. peakaccords high resolution when several analytes are present.
Therefore, a low concentration of a reducible species can
be detected in the presence of a high concentration of a
less easily reducible species. This method responds to a
kinetic factor. Very slow reactions do not give peaks but
intermediate ones give a.c. currents limited by the reaction
rate. A.c. polarography finds advantageous use in analysis
and the study of electrode kinetics.(b) Other votammetric methods 50,75,80,81
A number of procedures use electrodes other than the
DME employing stripping, and linear sweep, LSV, and
cyclic, CV, voltammetry. These electrodes include stationary
hanging mercury drop, HMDE, and mercury film electrodes,
solid electrodes of noble metals such as platinum, palladium,
silver, and gold in the form of cylindrical (wires) or flat sur-
faces, and various types of carbon electrodes, such as treated
spectroscopic amorphous carbon, glassy carbon, and carbon
paste.^81 These electrodes can be used in the quiescent solu-
tion or as a rapidly rotating wire or disk electrode. The wide
variety of electrodes offer unique advantages for dealing
with different analytes, concentration ranges, and interfer-
ences present in sample solutions.
Linear sweep and cyclic voltammetry are related. For
LSV a rapid scan at rates of several mv/sec to several hundred
mv/sec is used. However, in cyclic voltammetry a triangular
(cyclic) voltage scan is impressed on the electrode amounting
to two linear sweeps, forward and reverse. The process allows
the selective reduction of the analyte followed by the oxida-
tion of the reduced analyte, if the redox process is reversible.
However, the order of the scan can be reversed, oxidation fol-
lowed by reduction. The cyclic voltammagram permits one
to interpret redox reaction mechanisms for complicated elec-
trode reactions. The HMDE and solid electrodes, referred to
previously, are used in quiescent solutions in this techniques.
Stripping analysis^82 is a method that concentrates the
analyte on or near the electrode (HMDE or solid) surface
in a deposition (redox electrolytic) step. The voltamma-
gram is obtained by the oxidation or reduction of the elec-
trolytic product by a fast linear sweep scan in the stripping
step. Stripping analysis is of two kinds: Anodic (ASV) and
cathodic (CSV) stripping voltammetry where cation and
anion analysis occurs, respectively.
For ASV in the deposition step a stirred dilute cupric ion
solution ( 10 ^9 M) is subjected to electrolysis at an electrode
for a predetermined time while stirring the solution. (The
lower the concentration of the analyte, the longer the deposi-
tion time.) The fixed plating voltage is more positive than the
reduction potential of the ion. For example, the cupric ion is
reduced to copper metal and the metal forms an amalgam on
the surface of the HMDE or a film on a solid electrode. At the
end of the electrolysis step the stirring is ended. In the strip-
ping step a fast linear voltage sweep in the negative direction
causes the copper to be oxidized. The recorded voltammagram
of this oxidation yields a peak whose height is proportional to
the concentration of analyte cation (see Figure 26B). In CSV
halides are deposited on the mercury anode as the mercury(I)
halides. The remainder of the procedure is similar to ASV.C009_005_r03.indd 574C009_005_r03.indd 574 11/23/2005 11:12:27 AM11/23/2005 11:12:27