The Level of Impurity That Can Be Tolerated

For the former group, the allowed level of impurity is relatively easy to assess. It should be several orders of magnitude less than the concentration of the material being studied, so that the mass-transport-limited current density at which the impurity can react will be smaller than the smallest current we wish to 

This figure shows that reducing the concentration of oxygen by a factor of 250, should extend the range over which the hydrogen-evolution reaction (HER) can be studied by about 0.28 V. 

The other aspect of allowed impurity level relates to the maximum rate of adsorption, as compared to the duration of the experiment, or rather to the time interval between successive renewals of the surface. We shall explain this in relation to studies on the dropping-mercury electrode, which is typically renewed every second, although the argument can be applied also to solid electrodes, under certain favorable conditions, as we shall see. 

In the case of the dropping-mercury electrode, the limiting current for a one-electron reduction is on the order of 3 xA mM , or a current density of about 100 jaA cm mM for a typical surface area of 0.03 cm. The flux of the impurity reaching the surface, in units of molcm s is equal to the diffusion-limited current density, divided by the charge per mole, nF. Assuming, as before, an impurity concentration of 1.0 (aM, we obtain a flux of 

Considering that a monolayer amounts to about 2 n mol cm , it would take about 2000 s to deposit a monolayer of impurity. Since the drop is renewed every second, the maximum coverage by an impurity that exists in solution at a concentration of 1.0 jaM cannot exceed 0 = 5 x lO .  

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