Comparison of three colorimetric methods for the determination of manganese in freshwaters

Three calorimetric methods for the determination of manganese in freshwaters have been investigated and compared with the atomic-absorption spectrometry (AAS) method. Two of the methods, viz. the formaldoxime (FAD) and pyridylazonaphthol (PAN) methods were found to yield poor calibration curves upon standardization, and to have high average errors for the estimation of manganese concentrations in standard solutions. The FAD method, when applied to freshwater samples, was found to be subject to an interfering UV absorbance which could not be negated successfully by a reagent blank; the method thus tended to give high estimations at low concentrations of manganese, but the results obtained also appear to be very sensitive to the nature of the freshwater matrix. On the other hand the PAN method was found to give very high estimations for the concentration of manganese, in both standard solutions and in freshwater samples, owing to the nature of the absorbance of the ligand used for manganese determination. This method also appears to be affected by the freshwater matrix, leading to gross overestimation of manganese levels. A third method, based on a porphyrin ligand [T-(4-CP)P], was found to give results comparable with those of the AAS method in estimating the soluble manganese content in freshwaters.