Performance of colorimetric methods for the analysis of low levels of manganese in water.

The present paper describes a laboratory investigation conducted with the main aim to compare two colorimetric methods (1-(2-pyridylazo)-2-naphthol (PAN) from HACH® and porphyrin ligand (T-(4CP)P)) with ICP analyses for the evaluation of low concentrations of manganese in water. The colorimetric porphyrin method was found to provide reliable results compared to those obtained by inductively coupled plasma - optical emission spectroscopy (ICP-OES) even at low Mn2+ concentrations (<10 µg L-1). The presence of MnO2 particles results in an overestimation of the dissolved Mn2+ if unfiltered samples are analyzed by ICP and PAN methods. The presence of particles does not significantly impact sample analysis by porphyrin (T-(4CP)P) colorimetry. Although ICP-OES is still the analytical approach of reference, the porphyrin colorimetric method displays a high detection range and precision and could be a suitable on-site alternative to the current widespread use of the PAN method. In all cases, for both colorimetric methods, the use of Rochelle salt is recommended to alleviate calcium cation interference.