Polarogram current-scan

In order to investigate the redox reaction between NADH in W and Q in 1,2-dichlor-oethane (DCE), a current-scan polarogram was recorded at the interface of W and DCE, for which the compositions were as in Eq. (10), by scanning the current applied between W and O and measuring iiw/o- The Q employed in this study was chloranil (CQ) ... 

The oxidation product in DCE, CQ, has a charge, and hence it may be liable to transfer to W. The transfer of CQ at the W/DCE interface was investigated by current-scan polarography. Here, CQ in DCE had been prepared by reducing CQ in DCE at a column electrode with carbon fiber working electrode [40]. The polarogram (curve 2 in Fig. 8) indicates that CQ transfers from DCE to W in the potential range more positive than -0.7 V. 

In order to clarify the reason for the coupling of the redox reaction between O2 and CQH2 with the ion transfer at the W/DCE interface in system of Eq. (25), current-scan polarograms for ion transfers at the W/DCE interface (cf. curves 3 to 5 in Fig. 5) were compared with that for the interfacial redox reaction (cf. curve 1 in Fig. 8). From the comparison, it is clear that transfers of TPenA" " and TBA+ from W to DCE proceed at potentials in Range A where the polarographic wave due to the redox reaction... 

The facilitated transfers of Na+ and K+ into the NB phase were observed by the current-scan polarography at an electrolyte-dropping electrode [12]. In the case of ion transfers into the DCE phase, cyclic voltammetry was measured at an aqueous gel electrode [9]. Both measurements were carried out under two distinctive experimental conditions. One is a N15C5 diffusion-control system where the concentration of N15C5 in the organic phase is much smaller than that of a metal ion in the aqueous phase. The other is a metal ion diffusion-control system where, conversely, the concentration of metal ion is much smaller than that of N15C5. Typical polarograms measured in the both experimental systems are shown in Fig. 2. 

Commercial polarographs are also available in which the voltage scan is carried out automatically while a chart recorder plots the current-voltage curve. A counter-current control is incorporated which applies a small opposing current to the cell which can be adjusted to compensate for the residual current this leads to polarograms which are better defined. Most of these instruments also incorporate circuits which permit the performance of alternative, more sensitive types of polarography as discussed in Section 16.9... 

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. 

For both techniques, the analyte (in the concentration range 10 -10 mol dm ) is dissolved in a still solution that also contains supporting electrolyte, so the sole form of mass transport is diffusion. Usually, the potential is scanned from a value of Ei at which the analyte is electro-inactive to a final potential f at which the current is limiting. The resultant plot of current (as y) as a function of potential (as jc) is termed a polarogram. 

Figure 6.8 (a) Polarogram of the reduction of chromate ion at a dropping-mercuiy electrode, with [Cr04 ] = 1 mmol dm in degassed 0.1 mol dm NaOH solution, and using a scan rate v of 20 mV s. The small lower trace represents the residual current obtained at the same DME but in the absence of chromate, (b) The sequence of operation and the way that drop size dictates the current note that current is negative since reduction is involved. From Bard, A. J. and Faulkner, L. R., Electrochemical Methods Fundamentals and Applications, Wiley, 1980. Reprinted by permission of John Wiley Sons, Inc. 

Polarogram — Figure. Potential program and the respective (a) direct current (DC) (staircase ramp), (b) normal pulse (NP) and (c), differential pulse (DP) polarograms of 0.1 mM Cd(NC>3)2. All measurements were in water with 0.1 M KC1, E is versus a SCE, scan rate = 2 mVs-1 and drop time = 2 s. Differential pulse height = 10 mV... 

Figure 1. Direct-current polarograms. Bottom trace supporting electrolyte alone (0.1 M [n-Bu N]-[PF ] in dimethyoxyethane). Top trace 5 x 10 M cobaltocinium hexafluoropbosphate in same electrolyte. Potentials vs. aq see scan rate 1.5 mV/s natural drop time.

Leaching with HCl, centrifugation of the slurry, dichloromethane extraction, scanning of alternative current polarograms... 

Figure 12 shows an idealised example of a polarogram for a dc DME scan of current versus applied potential. The E1/2 value is the value at the point of inflection of the S-curve, and is characteristic for different analytes. Note the legends for residual and limiting currents. The line drawn perpendicular to the potential axis from the j esidual current line to a point well into the plateau of the curve represents id. The length of this line will be proportional to the analyte concentration. Figure 13 shows the curve of a dc DME cmwe as it... 

Dc polarograms, as obtained in historic practice, are records of the current flow at a DME as the potential is scanned linearly with time, but sufficiently slowly (1-3 mV/s) that the potential remains essentially constant during the lifetime of each drop. This constancy of potential is the basis for the descriptor dc in the name of the method. In more modem practice, the potential is applied as a staircase function, such that there is a small shift in potential (normally 1-10 mV) at the birth of each drop, but the potential otherwise... 

If the electrochemical reaction is totally controlled by pure kinetics, the cyclic voltammogram has no peak and the shape resembles a polarogram [12]. The kinetic current in this case is independent of the scan rates. The current observed in chronoamperometry also shows a constant value, independent of time. The kinetic current, i, observed from both techniques is exactly the same and expressed by the following equation assuming C l ... 

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