Abstract: In a supporting electrolyte containing 0.04-0.lM ethylene diamine, 0.4-l.0M NaNO₂ and at pH11, we obtained a sensitive polarographic catalytic wave of cobalt. The catalytic wave is in a peak shape withpeak potential between -1.2-1.3V (SCE). The limits of detection ofdc and single-sweep polarography for cobalt are 5×10^-9M and 5×10^-10M respectively.The method has been applied to the determination of cobalt in nickel salts.It is confirmed that in the compact layer of electrical double layer the associated ion pair [(Co(en)³)²⁺·NO₂^-]+ results in the formation of catalytic wave. The overall electrode reaction may be represented as follows: where E₁＞E₂