Oxygen Waves
Two waves caused by the reduction of dissolved oxygen can interfere with the waves of other
electroactive species unless the solution is purged with nitrogen prior to obtaining the polarogram
The reactions have been utilized for the determination of dissolved oxygen or hydrogen peroxide.
Quantitative Analysis
Either calibration graphs prepared from standards or the method of standard addition (p. 30) can be
used. For the former, the standards should be as similar as possible in overall chemical composition to
that of the samples so as to minimize errors caused by the reduction of other species or by variation in
diffusion rates. Often, the limiting factor for quantitative work is the level of impurities present in the
reagents used.
Modes of Operation Used in Polarography
The earliest types of polarograph involved the manual changing of a dc applied potential stepwise and
plotting it against corresponding current values. Later versions automated this process so that a chart
recording could be obtained. More recently, oscillographic or rapid scan polarography, pulse
polarography, stripping voltammetry and ac polarography were introduced primarily to increase
sensitivity and to facilitate the resolution of closely spaced polarographic waves. The first three of these
techniques have found more application than the last.
Linear Sweep Oscillographic Polarography
A repetitive dc potential sweep of 0.5 to 1.0 V is synchronized with the growth of each mercury drop
and the resulting current-potential curve displayed on the screen of a cathode-ray oscilloscope. Because
the current increases with the surface area of the drop, the potential sweep is timed to occupy the last 2 s
of a 7 s drop-life where the relative change of surface area is minimal. Exact synchronization is
achieved mechanically by tapping the capillary every 7 s to detach the drop. Figure 6.11(a) shows the
variation of the drop area and applied potential as a function of time, and Figure 6.11(b) the current–
potential curve as seen on the oscilloscope screen. The peak current ip is not a polarographic maximum
but is caused by the rapid potential sweep; it is however directly proportional to the concentration of the
electroactive species. A twin-cell version of this type of instrument enables the polarogram of a blank
solution to be automatically subtracted