Sampled current polarogram

The product Cd(0) is dissolved in the liquid Hg drop. A solution of anything in Hg is called an amalgam. We call Figure 17-13a a sampled current polarogram because current is measured only at the end of each drop life. 

Figure 17-19 Comparison of polarograms of 5 mM Cd2 in 1 M HCI. Waveforms are shown in Figures 17-15 and 17-18. Sampled current drop time = 1 s, step height = 4 mV, sampling time = 17 ms. Square wave drop time = 1 s, step height = 4 mV, pulse period = 67 ms, pulse height = 25 mV. sampling time = 17 ms.

G. Consider the cyclic voltammogram of the Co3+ compound Co(B9C2Hn). Suggest a chemical reaction to account for each wave. Are the reactions reversible How many electrons are involved in each step Sketch the sampled current and square wave polarograms expected for this compound. 

The equality of the anodic and cathodic peak heights suggests that the reactions are reversible. The expected sampled current (panel a) and square wave (panel b) polarograms are sketched below. 

Figure 11. Direct current polarograms of ferricytochrome c at various concentrations. Sample contained Tris-H2S04 buffer, pH 7.2, drop time = 5.6 s, mercury flow rate = 0.813 mg/s. Concentrations of cytochrome c are (1) 40 fiM, (2) SO (jlM, (3) 120 (nM, (4) 160 /JL.M, (5) 320 fiM. Adapted from Reference (98) with permission.

Fig.l. Sampled currents dc polarograms for 0.2 mM PrThz and Cys. Drop times indicated at the curves. 

Example 3-5 Polarogram A was obtained for a lOmL lead-containing sample. The limiting current increased (to B) after adding 100 pL of a 0.10 m lead standard to the 10 ml sample. Calculate the original lead concentration in the sample. 

Further, the operator must be able to choose the drop lifetime and the scan parameters, viz., the starting potential, direction (cathodic or anodic), rate and end potential, together with the sensitivity of the current measurement and the amplification in the ohmic cell resistance compensation circuit. Convenient additional facilities are (a) display of the polarogram on an oscilloscope, (b) delivery of hard copy of the polarograms on a chart recorder and (c) repeated recording of the polarographic curve for the same sample. 

In last polarography, only the current at the end of each drop is sampled and recorded on the polarogram. Then, the current oscillation as in Fig. 5.10 does not occur. 

Figure 3.30 (A) Current-time curves illustrating the sampling sequence for current-sampled polarography if, diffusion-controlled faradaic current id), double-layer charging current it = if + idl. (B) Current-sampled polarogram.

Fig. 10.8. Tast polarography showing (a) Variation of total current (/c + /f) and sampling just before drop fall at time t = r (b) The resulting polarogram.

Current-sampled polarography offers only a marginal improvement on the sensitivity of the classic method because there is a more favorable faradaic-to-charging-current ratio at the end of the drop time when the drop is almost stationary. Thus, its only real benefit over the classic method is the clearer polarogram obtained. 

Fig. 6 A typical current-sampled polarogram. Note the staircase pattern indicating the individual current sampling periods.

The special case of square-wave voltammetry (SWV) is worth noting separately from other alternating current techniques because it is both more rapid and more sensitive than DPP/DPV. In SWV, the applied potential waveform is a staircase with constant step height on which is superimposed an asymmetrical forward and reverse voltage pulse of constant amplitude and very short duration, typically less than 10 ms. Thus, the entire polarogram may be run in about approximately 1 s, with the enhanced sensitivity of the method owing to sampling of the current at the end of both the forward and reverse directions of the pulse. 

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