The Relation of Current Density to Reaction Rate

The current density is proportional to the rate of the heterogeneous reaction taking place at the interface. The relationship 

Consider the following example, showing the high sensitivity that can be achieved by measurement of the current. The charge required to form a monolayer of hydrogen atoms adsorbed on platinum in the reaction 

Physical Bcctrochemistry Fundamentals, Techniques and Applications. Eliezer Gileadi Copyright 2011 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-31970 1 

This is an order-of-magnitude calculation. The correct number, obtained experimentally for hydrogen atoms adsorbed on platinum, is 0.22mCcm . Thus, it is necessary to pass a current of, say, lOpAcm for 22 s to form a monolayer. As far as the electrical measurement is concerned, one could easily measure a very small fraction of a monolayer. Now, 0.22 mC cm equals about 2 x 10 molcm (for n = 1). In the case of hydrogen atoms, this amounts to 2 ng of added weight. Thus, measurement of the current and time makes it possible to determine quantities of adsorbed hydrogen well below 1 ng cm , which is better than the sensitivity achieved by the electrochemical quartz-crystal microbalance, itself by far the most sensitive method of weighing small amounts of materials adsorbed from solution on a surface (cf Chapter 17). 

The same high sensitivity that makes measurements so convenient causes great difficulty in the electrolytic industry. It takes (9.65 x 10 x n) C = (26.80 x n) Ahto generate Imol of a product formed by electrolysis. A cylinder of compressed hydrogen contains about 0.5 kg of the gas. It would take a water electrolyzer running at a total current of 1000 A about 13 h to produce this small amount of hydrogen Incidentally, the same amount of electric charge would produce 35.5 times as much CI2 in the electrolysis of a solution of NaCl. 

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