Kinetic determination of trace cobalt(II) by visual autocatalytic indication

A highly sensitive and simple visual autocatalytic method has been developed for the determination of trace cobalt. The cobalt ion released by the oxidative decomposition of inert bis[2-(5-bromo-2-pyridylazo)-5-( N-propyl- N-sulfopropyl-amino-phenolato] cobaltate (Co(III)-5-Br-PAPS) with peroxomonosulfate acts as a catalyst for the oxidative degradation of the complex. Thus a definite time lapse of degradation is observed by the sudden disappearance of colored Co(III) complexes. The degradation time varies inversely with the logarithm of the initial concentration of cobalt(II). The determination range of cobalt(II) was from 3×10 −9 to 2×10 −7 M in the presence of 5×10 −6 M of 5-Br-PAPS. The relative standard deviation of the spot size method (10 μl) was 3.5% at 1×10 −7 M cobalt(II). This autocatalytic indicator reaction system has been successfully applied for the visual determination of urinary cobalt.