Determination of trace amounts of manganese in natural waters by flow injection stopped-flow catalytic kinetic spectrophotometry

Trace amounts of manganese were analyzed by flow injection catalytic kinetic spectrophotometry, based upon the catalytic effects of Mn(II) on the redox reaction between sodium periodate and 4,4ʹ-bis(dimethylamino)-diphenylmethane. The reaction could occur rapidly at room temperature in the presence of nitriloacetic acid at pH 5.4. The measurements were conducted at 602 nm of an unstable intermediate blue product of the reaction, which greatly increased the sample throughput. Flow injection technique was utilized to precisely control the timing of merging and reaction of the sample and reagents. A stopped-flow approach was used to improve the sensitivity of the system. A detection limit (3(sigma)) of 0.073 (mu)g l^-1 was achieved at a sampling frequency of 60 h^-1. The relative standard deviation was 0.5% for the determination of 10 (mu)g l^-1 Mn (n = 11). The proposed method was validated by the analysis of an estuarine water reference material, SLEW-2 and successfully applied to the analyses of several natural waters.