Abstract: In an ammoniacal buffer solution of pH 9.5,and in the presence of 1.0 mol·L-1 NaNO2 solution,a sensitive catalytic polarographic wave of the complex formed by Co(Ⅱ) and neutral red (Co-NR) was observed at -1.19 V (vs.SCE).Linear relationship between the 2nd derivative of peak current and the concentration of Co(Ⅱ) was obtained in the range of 2.0×10-8-5.0×10-6mol·L-1 of Co(Ⅱ),with a detection limit of 1.0×10-8mol·L-1.Results of test for the composition of the complex Co-NR showed that the mole ratio between Co ion and NR was 1 to 1.As shown by the results of cyclic voltammetric study and studies by some other electrochemical methods,on the property of the polarographic wave and the mechanism of the electrode reaction,the polarographic wave was proved to be of catalytic and adsorptive behavior and was an irreversible process.The peak current was produced by reduction of cobalt ion in the Co-NR complex,and the number of electron transfer of 2 was involved in the electrode reaction.In its application to the determination of cobalt in samples of VB12 injection and natural water,RSD′s (n=6) less than 0.7% were obtained,and recoveries found were in the range of 98%-101%.