Voltamperometria

Evaluation of Analytical Pulse Polarography
E.

P. PARRY

and R. A. OSTERYOUNG

North American Aviation Science Center, Thousand Oaks, Calif. A study has been made to evaluate parameters which affect the analytical utility of pulse polarography. The effect of pulse amplitude on peak height and peak half width, the effect of capillary parameters, the relationship between peak potential andhalfwave potential, and the effect of supporting electrolyte concentration have been evaluated and compared with the experimentally determined values. A discussion of normal pulse polarography compares the sensitivity increase over normal polarography without consideration of capacitance current differences. The sensitivity increase is near sevenfold.

I I

concept of pulse polarography wasdeveloped originally in England by G. C. Barker as an outgrowth of his work on square wave polarography. Several communications by Barker and coworkers discuss the concept of pulse polarography, adapt some of the theoretical aspects of square wave polarography to pulse polarographic techniques, and present the instrumentation in a general manner ( I , 2 ) . Essentially nothing has been published interms of experimental verification of theory, analytical usage, or optimization of important analytical parameters. Because pulse polarography appears to have considerable potential as an analytical tool (IO),we constructed a pulse polarographic instrument from solid-state components. This instrument also incorporates other electroanalytical functions, such as normal polarography andoscillographic polarography. A description of the use of this instrument for the determination of mixtures using the rapid linear sweep voltammetric mode has been published (7). An initial publication indicated the capability of our pulse polarographic instrument (8). This paper evaluates some important parameters which are necessary to optimize the analytical use of pulse polarography.
HE PRINCIPLES O FPULSE POLAROGRAPHY

T

I 1
'1 '2

/ I
'3 '4

*T

Figure 1 . Potential-time behavior on application of voltage pulse and resulting current-time behavior
b.
c.

d.
fl,
tl. f4.

Charging current only Faradaic current only Both charging and Faradaic current fa. Times at which current is measured Time at whlch pulse is applied Time a t which pulse is removed

purposes.) If, now, asampling of the current is taken a t tl, prior to pulse application, and again a t t 3 , 50 milliseconds after the first measurement and some 40 milliseconds after the pulse has been applied, the capacity current will be insignificant and the difference in currents between tl and t3 will depend only on the Faradaic cunent. (In the instrument described by Barker, the current is sampled after theapplication of the pulse only and the current differences are obtained between successive pulses.) The effect of taking the current measurement in this manner is to increase the sensitivity and to eliminate the unwanted contribution of the charging current. The shape of the curve obtained is essentially a derivative of the normal polarographic curve. In normal pulse polarography, pulses of successivelyincreasing amplitude are applied to the electrode from an initially preset potential, No ramp is employed, so the potential, except during the duration of the pulse, is maintained at the initially preset value. The curve obtained in this mode will resemble that obtained in ordinary polarographic measurements, but the sensitivity will be greater.
EXPERIMENTAL

While the principles of thistechnique have been published (1, a), a brief description of the important concepts is presented as an aid to understanding the general function of the instrument. Some of the description given below
1634
ANALYTICAL CHEMISTRY

refers to our instrument, but the comments apply to any pulse polarographic instrument. In the derivative pulse mode a voltage ramp is applied to an electrode, such as the...