I developed a general approach to the analyzing of the first passage problem for stochastic diffusion in equilibrium electrochemical systems. I found Fokker–Planck equation that controls the process of stochastic diffusion in any electrochemical system at the equilibrium. I found the link between the above-mentioned Fokker–Planck equation and electrochemical impedance. On the basis of the Fokker–Planck equation, I derived analytical expression for the characteristic function of random time passage by the process of electrochemical stochastic diffusion. The developed theory is useful for the working out of a method for electrochemical noise diagnostics based on the information on the process of electrochemical stochastic diffusion in the bounded limits.