Determination of manganese with triethanolamine and o-tolidine by conventional and long-path photometry

A very sensitive method for the spectrophotometric determination of manganese is reported. To the sample is added triethanolamine (TEA) and sodium hydroxide to give a pH above 11; after atmospheric oxidation of manganese(II) to the green manganese(III)—TEA complex, sodium pyrophosphate is added and the solution is acidified. Manganese(III) thus forms a complex with pyrophosphate. Then o-tolidine is added and is oxidized in a 2e step to the intensely yellow quinonediimine, while manganese(III) is reduced to manganese(II). The absorbance is measured at 440 nm. The calibration curve is linear up to 1.6 μg Mn ml^-1 in the final solution; the limit of determination is 0.2 μg Mn ml^-1. For the 20-cm path cell, the respective data are 45 ng Mn ml^-1 and 2 ng Mn ml^-1. The only severe interferences are strong oxidants like dichromate or cerium(IV), which are readily reduced with sulfurous acid. Vanadium in amounts up to 2–3 times that of manganese can be dealt with by an appropriate blank solution; larger amounts of vanadium must be removed e.g. by a cupferron extraction.