Pseudo-derivative d.c. and pulse polarographic methods at short drop times

Abstract

Short drop time, fast scan rate d.c. polarographic techniques, whilst being superior to conventional methods in many respects, have the disadvantage of a higher charging to faradaic current ratio. Recent theoretical studies have suggested similar considerations, as well as additional problems from faradaic distortion, would apply in normal pulse polarography. In this work derivative readout is examined as a means for providing improvement in the short drop time d.c. and pulse methods. The validity of the Ilkovic (d.c.), Cottrell (pulse) and other polarographic equations are examined at drop times down to 50 ms. At this drop time the limiting current in pulse polarography can be less than in d.c. polarography, contrary to normal expectations. In the 50 ms drop time region, maxima, inhibition and other phenomena present under conventional conditions can be eliminated. With derivative methods, the noise level at a given scan rate decreases, the shorter the drop time. Since the charging current problem is also decreased, short drop time pseudo-derivative methods coupled with fast scan rates provide an attractive analytical technique. The technique of differential pulse polarography at short drop time is also examined and results are compared with the derivative methods. For the reduction of cadmium in 1 M HCl, a limit of detection of between 5×10 −7 and 10 −6 M was found with both the pseudo-derivative and differential pulse methods at a drop time of 0.10 s. This represents an order of magnitude improvement over the d.c. method.