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 R-g 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 R-g as variables is demonstrated. Except for very large R-g values (over 200), which is not an experimental interesting configuration, the error is below +/- 0.01 on normalized current. (c) 2007 Elsevier B.V. All rights reserved.