Electrolytic discharge

In order to produce current flow through an electrolytic cell for the discharge (or electrodeposition) of any metal, a potential, at least equal to if not greater than the zero current or reversible potential must be applied. For zinc ions this would be 0.763 V [3], The potential at which continuous deposition of material (or discharge of ions) commences is called the discharge or decomposition potential (Fig. 6.6). 

Faraday s first law of electrolysis [4] states that the mass of any substance liberated by a current is proportional to the quantity of electricity which has passed . Thus any increase in current (density) ought to lead to an increase in the rate of discharge and a more economical process. However the discharge of the metal ions, to generate metal, is not as straight forward as it seems and there can be several problems associated with the discharge rate. 

The first problem is that any ion needs to be brought from the bulk solution to the electrode surface before it can be discharged. On approaching the electrode, the ion needs to cross a boundary layer called the diffusion layer. This layer is about 0.1 mm (or 100 (tm) thick and is distinguished from the electrode double layer which is 100000 times smaller (Fig. 6.7). 

On entering the diffusion layer, the ion loses its solvation molecules (all ions in solution are solvated) and approaches the metal surface, where it is adsorbed as a naked ion before the electron transfer process takes place. Obviously the wider is this diffusion layer (5), the longer it will take the ion to diffuse across it and the slower will be the overall process. Anything which can diminish or disrupt this layer (i.e. make it smaller) will improve the speed of the process. 

Stirring is one obvious solution and this method is adopted commercially [5]. However the fastest form of stirring is ultrasonic agitation. Applying ultrasound has been found to decrease the thickness of this diffusion layer, and so assist the electrode reaction (Fig. 6.8). 

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These early works have been reviewed by Fioshin (4) and well summarized by Bbeitenbach (5). Besides, Breitenbach has made a study of the polymerization mechanism using the copolymerization method and has shown that the reaction mechanism depends on the ions used in the electrolytic discharge and on the monomer present in the system. Cationic processes were also found to be initiated in a nitrobenzene solution of styrene by the anodic discharge of perchlorate and borotetrafluoride ions. The possibility that the three different mechanisms could occur simultaneously was demonstrated in the same system of acrylonitrile-styrene using a divided electrolytic cell. 

The decarboxylation of acids may take place by both radical and ionic processes. Radical processes involving the electrolytic discharge of a carboxylate anion (the Kolbe reaction ) may give rise to dimeric products. 

In contrast to [V(f/-arene)2], the isoelectronic mixed-sandwich [VCp(r/-C7H7)] is not reduced prior to electrolytic discharge ( — 2.3 V) in MeCN. However, it is easily oxidized (E° = 0.19 V) by coulometry (413) or with iodine (414) to give a stable monocation. 

Asahi Glass AZEC-BI Electrolyzer. The AZEC-Bl is a newly developed bipolar electrolyzer (Fig. 59). Metal bipolar cell frames are suspended in a steel structure similar to a filter press. A special hydraulic system presses the fi ames together. Caustic and brine are supplied by PTFE hoses to each frame, and discharge also takes place individually through PTFE hoses into metal headers. A special overflow method was developed for each frame to give smooth and stable electrolyte discharge. 

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