Neutral Reagents in Solutions with Low Content of Supporting Electrolyte:How To Determine the Steady-State Conditions

摘要

Cyclic voltammograms obtained at ultramicroelectrodes in the electrochemical systems where an uncharged reactant is significantly more concentrated than the supporting electrolyte show an unusual feature.The forward and the backward waves cross over,forming a hysteresis loop.The width of the hysteresis increases with the relative concentration of the reactant,with the electrode size,and with the scan rate.We show that the reason for this hysteresis is the slow transport of supporting electrolyte ions that are necessary to compensate the charge of the reaction product.As a result,the steady-state concentration profile of counterions is reached significantly slower than the steady-state concentration gradient of the reactant,and the counterion transport determines how rapidly the steady state for the whole system is approached.The scan rate yielding near-steady-state voltammograms can differ by more than 1 order of magnitude for systems with high and low concentrations of supporting electrolyte.Experimental evidence for this,supported by digital simulation results,is presented.The appropriate criterion for assessing the steady state in such systems is thus the identity of the forward and backward scans,without hysteresis.If this condition is not fulfilled,the formal potentials and the related parameters determined from the obtained voltammograms may be erroneous.