Amalgam, sodium electrode process

Galvanostatic and Potentiostatic Polarization Measurements. Electrode processes may be classified into two types reaction or charge-transfer controlled, and diffusion or mass-transfer controlled. The electrode processes in diaphragm and membrane chlor-alkali cells are charge-transfer controlled. On the other hand, the formation of sodium amalgam on a mercury cathode, is diffusion-controlled. 

Creighton An electrolytic process for reducing sugars to their corresponding polyols. Glucose is thus reduced to sorbitol, mannose to mannitol, and xylose to xylitol. The electrodes are made of amalgamated lead or zinc the electrolyte is sodium sulfate. Invented in 1926 by H. J. Creighton. 

Amalgam Cell Starting with the chlorine electrode, the reversible standard potential is +1.37 V (all potentials are given versus NHE), due to the sodium concentration of 6 mol L-1 we have to note +1.33 V. The current-overpotential curve represents an unhindered process... 

The chloralkali process, which involves the electrolysis of brine, is widely used for the production of sodium hydroxide and chlorine gas. During electrolysis it is necessary to keep the sodium hydroxide separate from the chlorine, to prevent the formation of sodium hypochlorite, NaOCl, and this determines cell design. In older processes, the cathode used was flowing mercury. At this electrode, sodium is formed, and this dissolves in the mercury to form a sodium amalgam. The sodium amalgam is removed continually from the cell and reacted with water to produce hydrogen gas and... 

It is possible to discharge sodium electrolytically from an aqueous solution if a mercury cathode is used. On the basis of the standard electrode potentials [E (Na, Na) =-2.71V], the decomposition of water to form hydrogen at the cathode would be a far easier process even when allowance is made for the high hydrogen overpotential (q.v.) at mercury. However, the fact that sodium forms intermetallic compounds with mercury which are soluble in mercury and diffuse away so reduces the activity of the sodium at the cathode surface, and its tendency to re-ionise that the discharge of sodium becomes the preferred process. This method is not used commercially because of the high cost of extraction of sodium from its amalgam, and recourse is had to fused electrolytes. 

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