A new spectrophotometric method to quantify dissolved manganese in marine pore waters

This study explores the use of Cadmium (II) meso-Tetrakis(4-sulfophenyl) porphyrin complex (Cd-TSPP) as an indicator for the spectrophotometric determination of dissolved manganese (II) in pore waters from marine sediments. Both single absorbance and second derivative based methods were evaluated with a multifunctional plate reader. Cd-TSPP has a maximum absorbance at 433 nm in solution, but the core Cd(II) ion in the complex can be quickly replaced by Mn(II) at pH 4–8 to form a new Mn porphyrin complex (Mn-TSPP) with maximum absorbance at 469 nm. The absorbance of Mn-TSPP at 469 nm and its second derivative spectra at the same wavelength show excellent linear relationships with Mn(II) concentrations (R2 = 0.997 and R2 = 0.999, respectively) in the 0–37.5 μM range for 200 μL samples. The detection limits using single absorbance and the second derivative are 0.4 and 0.3 μM Mn(II), respectively. A correction subtracting reference absorbance at 490 nm from 469 nm signal was required in the single absorbance approach in order to eliminate the effect of baseline fluctuations. In contrast, the second derivative approach shows higher selectivity, accuracy and precision for Mn(II) determination in pore water. The reaction is not affected by pH (4–8) or salinity but is temperature sensitive in the range of 10–45 °C with an activation energy of 22.12 kJ mol−1. No interferences from metal ions such as Pb(II), Cu(II), Fe(II), Zn(II) etc. were found. The proposed method, which is rapid and suitable for small size samples, was successfully applied to the determination of Mn(II) in pore water.