Pulse Polarography

Normal polarography has been replaced by various forms of pulse polarography, several examples of which are shown here. Normal pulse polarography (a), for example, uses a series of potential pulses characterized by a cycle of time of τ, a pulse-time of tp, a pulse potential of ΔEp, and a change in potential per cycle of ΔEs. Typical experimental conditions for normal pulse polarography are τ ≈ 1 s, tp ≈ 50 ms, and ΔEs ≈ 2 mV. The initial value of ΔEp is ≈ 2 mV, and it increases by ≈ 2 mV with each pulse. The current is sampled at the end of each potential pulse for approximately 17 ms before returning the potential to its initial value. The shape of the resulting voltammogram is similar to that of normal polarography, but without the current oscillations. Because we apply the potential for only a small portion of the drop’s lifetime, there is less time for the analyte to undergo oxidation or reduction and a smaller diffusion layer. As a result, the faradaic current in normal pulse polarography is greater than in the polarography, resulting in better sensitivity and smaller detection limits.

Figure11.45

In differential pulse polarography (b) the current is measured twice per cycle: for approximately 17 ms before applying the pulse and for approximately 17 ms at the end of the cycle. The difference in the two currents gives rise to the peak-shaped voltammogram. Typical experimental conditions for differential pulse polarography are τ ≈ 1 s, tp ≈ 50 ms, ΔEp ≈ 50 mV, and ΔEs ≈ 2 mV.

Other forms of pulse polarography include staircase polarography (c) and square-wave polarography (d). One advantage of square-wave polarography is that we can make τ very small—perhaps as small as 5 ms, compared to 1 s for other pulse polarographies, which can significantly decrease analysis time. For example, suppose we need to scan a potential range of 400 mV. If we use normal pulse polarography with a ΔEs of 2 mV/cycle and a τ of 1 s/cycle, then we need 200 s to complete the scan. If we use square-wave polarography with a ΔEs of 2 mV/cycle and a τ of 5 ms/cycle, we can complete the scan in 1 s. At this rate, we can acquire a complete voltammogram using a single drop of Hg!

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