A unified new analytical approximation for negative feedback currents with a microdisk SECM tip

Analytical approximations for steady-state negative feedback currents in scanning electrochemical microscopy (SECM) experiments using a microdisk are discussed. A precise study of the limit behavior for small tip–substrate distances and all microdisks leads to an analytical expression without any fit. From this analytical limit a new mathematical function is proposed which is the first expression that accurately fit any negative feedback approach curve with only three adjustable parameters (for a given Rg value, characterizing the insulator thickness of the microdisk electrode, between 1.001 and 1000). Finally a unified new analytical approximation of all negative feedback approach curves with tip–substrate distances and Rg as variables is demonstrated. Except for very large Rg values (over 200), which is not an experimental interesting configuration, the error is below ±0.01 on normalized current.