Controlled generating electrode

Steam or HW is generated according to design as current flows from each of the three electrodes to the neutral walls of the shields or cylindrical balance compartments that contain and control the electrodes. 

Controlled-current coulometry is also called coulometric titration. An apparatus for controlled-current coulometry is shown in Fig. 5.35 for the case of determination of an acid. It consists of a constant current source, a timer, an end-point detector (pH meter), and a titration cell, which contains a generating electrode, a counter electrode in a diaphragm, and two electrodes for pH detection. The timer... 

Fig. 5.35 Apparatus for controlled-current coulometry. The case for neutralization titration of an acid by internal cathodic generation of OhT. GE generating electrode CE counter electrode.

The electrolysis current (not to be confused with the detector current) for the Br2-generating electrodes can be controlled by a hand-operated switch. As the detector current approaches 20.0 p,A. you close the switch for shorter and shorter intervals. This practice is analogous to adding titrant dropwise from a buret near the end of a titration. The switch in the coulometer circuit serves as a stopcock for addition of Br2 to the reaction. 

In situ eiectrolysis-EPR methods usually employ a wire or grid electrode contained in a conventional flat or tube EPR cell. The constraints on the geometric configuration are such that secondary and reference electrodes are usually remote from the generating electrode, which often leads to problems in the control of the potential nevertheless it is a valuable technique for recording spectra of EPR active intermediates. These and related spectroelectrochemical techniques have been reviewed by Robinson.5... 

To use Faraday s law for the purpose at hand, the generator electrode reaction must be limited to the one of interest to assure that the charge consumed by the desired electron transfer reaction is essentially equal to the charge applied to the cell. Difficulties arise because it is not possible to control independently both the current and the electrode potential, and because only the current applied to the cell can be known directly in the experiment. A knowledge of the current-voltage curves for the precursor and the supporting electrolyte will provide valuable information for the design of practical experiments. 

Experimentally, the generator electrode current, /gen, is controlled (galvanostatic control), usually being slowly increased from zero in an anodic or cathodic direction depending on the electrode reaction, and the detector electrode is held at a potential such that all the B reaching it is converted into C, i.e. a potential in the limiting current region for B and C, and passes current /det. 

Experimentally, as for the steady-state collection efficiency, the generator electrode current is controlled and the potential of the detector electrode is held at a value corresponding to mass-transport-limited conversion of B to C. 

In this section, the discussion centres on the application of double channel electrodes in the study of electrode reaction mechanisms under conditions of laminar flow. (The modifications necessary to what follows when turbulent flow operates can be found in Sect. 6.2.) When employed in this way, the upstream (generator) electrode produces the species of interest, which is then detected on the downstream electrode. This procedure is illustrated schematically in Fig. 37. In general, the detector electrode is held at a potential at which the destruction of the species produced upstream is diffusion-controlled. Kinetic and mechanistic information about the electrogenerated species is then available from "collection efficiency , N, measurements, given by... 

The oxidation step is diffusion-controlled at about + 0.4 V versus the saturated calomel electrode (SCE). On setting the generator electrode to this potential and scanning the detector electrode in the range +0.6 to -0.7 V, two reduction waves were apparent. The first reduction wave, around 0.0 V versus SCE, was assigned to the reduction of the oxidized ADMA, whilst the... 

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