Requirement for Supporting Electrolyte

The charge placed on the electrode to drive a potential difference with respect to the solution is compensated by a double layer in which there is an excess of ions of opposite charge, as is discussed in more detail in Chapter 10. 

This double layer is narrower for a more conductive solution, allowing electron tunnelling to take place from outside the double layer, and thus ensuring that the full potential difference applied between the working and reference electrodes is felt by a molecule approaching the working electrode. The driven Faradaic current then reflects the applied potential. 

The screening of bulk electric fields by the addition of supporting electrolyte is useful since it prevents migration due to electrostatic attractions from influencing the voltammetric current and allows the use of a simpler diffusion-only theory based on Fick s laws (see Chapter 3). What is more, passing current through a resistive solution generates a potential difference known as ohmic drop , since its value can be understood from Ohm s law  

Lastly, the uniformity of ionic strength provided to a solution by the presence of ample supporting electrolyte limits effects due to the non-ideality of the solution. According to the Debye-Htickel theory, the presence of electrostatic interactions between ions causes solution non-ideality because these forces are on average stabihsing. Therefore, activity, the quantity appearing in the Nernst equation, differs from concentration by a factor known as the activity coefficient, y, in a maimer which for a dilute ( 0.01 M) solution is given by a simplified formula  

The addition of supporting electrolyte generally elevates concentrations to regimes where the Debye-Htickel formula does not apply, but it also ensures that non-ideahty is uniform across the solution and therefore gradients of non-ideality do not contribute to mass transport. Note that because a well-supported solution is non-ideal, the exact value of the formal potential, E, of a redox couple is sensitive to the concentration of supporting electrolyte. 

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