Solvent and the Supporting Electrolyte:

The diffusion current depends on 'D' of the electroactive species that in turn depends on the viscosity of the solution. 'i_d' decreases along with increase in the viscosity of the solution as D is inversely proportional to the viscosity, η of the medium. As per to Ilkovic equation, thus id is proportional to η^1/2.  Here η varies as the aqueous solution changed partly to non-aqueous medium using solvents such as methanol, dioxane etc.

Changes within the composition of the supporting electrolyte through adding complexion ligands to the electrolyte solution changes the size of the metal complex and thus D of the ion.  i_d thus decreases along with complexation and E^1/2   shifts to more negative potentials.

It is essential to keep the concentration of every constituent of the supporting electrolyte constant within about + 10% to ensure the variation of i_d resulting from this effect (viscosity) will be negligibly small.