FLUOROMETRIC-DETERMINATION OF MANGANESE(II) BY THE CATALYTIC-OXIDATION OF 2,3-DIHYDROXYNAPHTHALENE IN THE PRESENCE OF ETHYLENEDIAMINE AND HYDROGEN-PEROXIDE

The determination of manganese(II), which catalyzed a reaction between 2,3-dihydroxynaphthalene (DHN) and ethylenediamine (en), has been investigated in the presence of hydrogen peroxide. A change in the reactants was detected by absorption and fluorometric methods. Two kinds of the intermediates, obtained from a mixture of DHN, en and H2O2, were confirmed by changes in the fluorometric spectra. One of them was used for the determination of manganese(II). The fluorescence intensity of the intermediate was measured at 500 nm with an excitation wavelength of 400 nm. The mechanism of the reaction between DHN and en is discussed based on the change in the fluorescence spectra with the reaction time. The formation of 2,3-naphthoquinone was estimated by the reactivity of DHN and its related compounds. The 2,3-naphthoquinone reacted with en to form a quinoxaline derivative. The role of manganese(II), which forms the DHN-Mn complex in the presence of H2O2, was only to accelerate the formation reaction of 2,3-naphthoquinone. The reactivity of DHN to amine is discussed based on the distribution of the DHN electron density calculated by ZINDO. The limit of detection of Mn(II) by the proposed method was 3 ppt. The determination range of Mn(II) was from 10 to 140 ppt. The conditions for the determination of Mn(II) were as follows: DHN concentration, 3.6X10(-4) M; H2O2, 1.2%; PH, 10; en concentration, 1.2 M; reaction time, 5 min; measurement wavelength, 400 nm/500 nm; temperature, 30 degrees C.