The Ilkovic equation is one of the fundamental relationships in quantitative polarography. It establishes a direct relationship between the diffusion current obtained at a dropping mercury electrode (DME) and the concentration of the electroactive species in solution. Since the diffusion current is proportional to the concentration of the analyte, the equation provides the theoretical basis for quantitative determination by polarographic methods. The equation also shows the dependence of diffusion current on the number of electrons involved in the electrode reaction, diffusion coefficient of the analyte, rate of growth of the mercury drop, and lifetime of the drop. In polarography, the current flowing through the electrochemical cell is measured while the applied potential is varied. When the potential becomes sufficiently negative or positive to reduce or oxidize an electroactive substance at the dropping mercury electrode, the electrode reaction occurs. Under suitable conditions, the rate of this electrode reaction is controlled by the diffusion of the analyte from the bulk solution toward the electrode surface. The resulting current is called the diffusion current. It is this current that is used for quantitative analysis. Consider the reduction of an electroactive species, represented by O, at the mercury electrode: O + ne⁻ → R where O is the oxidized form, R is the reduced form, and n is the number of electrons transferred. When an appropriate potential is applied, the concentration of O at the electrode surface becomes very small compared with its concentration in the bulk solution. Consequently, a concentration gradient is established between the bulk solution and the electrode surface. The electroactive species therefore diffuses toward the electrode and is reduced. The rate of diffusion determines the rate of the electrode reaction and hence determines the measured current. According to Fick's first law of diffusion, the diffusion flux is proportional to the concentration gradient: J = −D(dC/dx) where J is the diffusion flux, D is the diffusion coefficient of the analyte, C is its concentration, and x is the distance measured from the electrode surface.