through use of the Nernst equation with a minimum need for calibration and reference standards. The differential method described has been applied to other systems, although these will not be described in detail in this paper. In particular, the assay of serum lactic acid dehydrogenase using the stop-flow method has been done showing excellent correlations with values obtained by the standard Wroblewski method, and the non-enzymatic oxidation of ascorbic acid by ferricyanide has also been measured using the stop flow method.
ACKNOWLEDGMENT We would like to acknowledge the support and cooperation of The Ohio State University Hospital Clinical Chemistry Division in providing clinical samples.
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RECEIVEDfor review October 1, 1973. Resubmitted November 5, 1975. Accepted November 5, 1975. We gratefully acknowledge support by the National Institutes of Health, Grant GM-15821. R.I.P. was also supported in part by a National Science Foundation predoctoral fellowship.
Constant Potential Pulse Polarography Joseph H. Christie,' Larry L. Jackson, and Robert A. Osteryoung" Department of Chemistry, Colorado State University, f o r t Collins, Colo. 80523
The new technique of constant potential pulse polarography, in which all pulses are to be the same potential, is presented theoretically and evaluated experimentally. The response obtained is in the form of a faradaic current wave superimposed on a constant Capacitative component. Results obtained with a computer-controlled system exhibit a capillary response current similar to that observed in normal pulse polarography. Calibration curves for Pb obtained using a modified commercial pulse polarographic instrument are in good accord with theoretical predictions.
We have previously reported on the background capacitative current developed in pulse polarography ( 1 ) . As the drop expands a t constant potential, a charging current must flow to maintain constant surface charge density on the electrode. This charging current is not compensated in the ordinary pulse polarographic experiment and the output current difference contains a term dependent on the difference between the charging currents flowing before and after the potential step is applied to the electrode. This point has been discussed in detail (2). The technique of alternate drop pulse polarography (2) was developed to compensate for charging currents by differencing the current at a pulsed drop and a non-pulsed
drop a t the same time in drop life and a t the same potential. There is no charging current component in this difference since the charging current is a function of potential and time only. The faradaic response is of the same form as in ordinary pulse polarography, but is diminished in magnitude by the diminution factor (1 - .\/76/3(T a)), .where 6 is the pulse width and T is the delay time. In this paper, we present a simpler variant of pulse polarography in which the diminished faradaic response is superimposed on a constant, but non-zero, capacitative component.
THEORETICAL The wave-form for this variant pulse polarographic technique is shown in Figure 1. The name-constant potential pulse polarography-derives from the fact that each pulse is to the same potential Ez; the potential El during the delay time is different for each drop. This wave-form may be considered. the complement of the ordinary normal pulse polarographic wave-form. The current is measured only once in the life of each drop: a t time T + 6 while the potential is a t Ea. Using the same assumptions as in our earlier work ( 2 ) , we can write for the current 2
I = Zf(El,E2,7+ 6) - - k m 2 / 3 Q ( E a ) /+ ( ~6)'13 3
Present address, U S . Geological Survey, B r a n c h o f A n a l y t i c a l Laboratories, 345 M i d d l e f i e l d Road, M e n l o P a r k , Calif. 94025.
where k = 0.8515 cm2/g2l3,m is the rate of flow of mercury, and Q ( E )is the potential dependent charge density on the ANALYTICAL CHEMISTRY, VOL. 48, NO. 3, MARCH 1976
Table I. Extreme Pulse Currents Observed E , > Ey, E , >> Ey2