Polarographic circuit

Fig. 5.14 The DC and AC polarographic circuits (a) and the current-potential curves for DC and AC polarographies (b).

Figure 1 Fundamental scheme of a three-electrode polarographic circuit for /Ffdrop correction. RG, ramp generator A, current measurement device V, voltmeter WE, working electrode RE, reference electrode AE, auxiliary electrode.

Fig. 1. Schematical view of polarographic circuit. (C) capillary of the mercury dropping electrode (V) electrolysis vessel (S) solution to be analysed (R) reference electrode (Nj) nitrogen inlet and outlet (M) mercury reservoir (G) galvanometer (DF) potentiometer ...

The electronic equipment used in the study of electrode processes may vary considerably in degree of sophistication and in price. However, it can conveniently be described by referring to the simple polarographic circuit which is given in Fig. 48. This basic circuit is made up of a variable voltage... 

Fig. 48. Schematic presentation of a polarographic circuit (a) A two electrode arrangement (b) A three electrode arrangement.

Sharp variations in the current are rather inconvenient and, therefore, a damping device is introduced into every polarographic circuit. The design of a typical damping device is shown in Fig. 60. In effect all it does is to increase the time of response of the recorder by letting the sharply variable current pass through a capacitor before it reaches the recorder circuit. 

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... 

Electrical circuits for an automatic compensation of charging currents and a direct recording of the faradaic current are available in modem polarographs to reduce the influence of the charging currents. However, the accuracy of such compensation is limited, particularly at low reactant concentrations. 

In the ac circuit of the polarographic cell there is such an external ohmic resistance that via the alternating voltage (300 V) together with a superimposed dc the voltage over the cell alternates from 0 to -2V vs. an SCE within these limits oxidation of Hg and reduction of Na+ (electrolyte) to Na(Hg) remains sufficiently restricted. 

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. 

The method can successfully be used in analyses of impurities in metals and alloys, for estimation of minor elements in monomolecular films of oxide layers of Fe-Cr-Ni alloys, for detection of metal impurities in environmental pollution, for studying the depression of high-grade semiconducting materials and for analysis of the corrosion products of contact junction diodes used in microelectronic circuits. Much sophistication is desirable on the instrumental side so as to incorporate an automatic recording device to make an FR polarograph suitable for wider applications and common use. 

Fig. 5.8 Schematic diagram of polarographic (or voltammetric) circuits for two-electrode (a) and three-electrode (b) systems. WE(DME) indicator or working electrode (dropping mercury electrode in the case of polarography) RE reference electrode CE counter electrode DC voltage (V) DC voltage source Current (/) current measuring device.

Fig. 5.45 Circuit for a three-electrode polarograph. The bold lines are for positive feedback iR-drop compensation. Q is a capacitor to decrease the current fluctuation.

The electrical resistance of non-aqueous electrolytic solutions is often much higher than that of aqueous ones, and so polarographic and voltammetric measurements in non-aqueous solutions should be made with a three-electrode device. A computer-aided three-electrode instrument, equipped with a circuit for iR-drop compensa-... 

A renewal of interest in the other rate-controlling processes started in those groups who were developing the impedance method [49, 53] and the a.c. polarographic method [12, 25], probably because it was found that, in many cases, Randles equivalent circuit did not hold and also because the appropriate mathematics are more tractable in the frequency domain. Still, it is recommended that the a.c. studies are combined with the diagnostic results which can be obtained from steady-state techniques and/or cyclic voltammetry. 

Kelley, M.T., and D. J. Fisher Unique Polarographic Damping Circuit for... 

Commercial electrochemical detectors whose cells are directly connectable to the postcolumn efflux of the HPLC apparatus are available from a number of manufacturers. This method of detection is comparable in sensitivity to detection by absorption spectroscopy when the amperometric circuit is operated as a DC pulse polarograph. 

A polarographic experiment involves recording a current-voltage curve on a mercury electrode immersed in a test solution. The electrical circuit is completed by a reference electrode (and, usually, a separate auxiliary electrode), but these are of no consequence to the discussion to follow. 

Some dc polarographs have a so called derivative circuit. This, in fact, attempts to give, purely by electronic means,... 

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