DIFFERENTIAL-PULSE CATHODIC STRIPPING VOLTAMMETRIC DETERMINATION OF MANGANESE(II) AND MANGANESE(VII) AT THE 1-(2-PYRIDYLAZO)-2-NAPHTHOL-MODIFIED CARBON-PASTE ELECTRODE

A carbon paste electrode modified with 1-(2-pyridylazo)-2-naphthol was fabricated and used to preferentially accumulate Mn-(II) at open circ uit. After medium exchange, the accumulated Mn-(II) at the electrode s urface was electrochemically oxidized. The hydrated MnO2 thus formed w as then cathodically stripped using differential pulse voltammetry. Va rious factors influencing the accumulation and stripping were studied. An optimized procedure was then developed for the determination of Mn -(II). Mn-(VII) could also be determined by chemical conversion to Mn- (II) using acidified H2O2. Following this, the Mn-(VII) concentration could be obtained by subtraction. Under optimum conditions, a detectio n limit of 6.9 x 10(-9) M Mn-(II) (0.38 ppb) was found for 200 s accum ulation. For 8 determinations, each of Mn-(II) concentrations of 1.00 x 10(-6) M, 1.00 x 10(-7) M and 1.00 x 10(-8) M, relative standard dev iations of 2.90%, 4.46% and 7.20% were obtained, respectively. Selecte d metal ions were tested for interferences and it was found that, when determining 1.00 x 10(-6) M Mn-(II) (150 s accumulation), Co-(II), Hg -(II), and Fe-(II) at 1.00 x 10(-5) M interfered. These interferences were readily eliminated by masking with complexing agents such as sodi um citrate and sodium diethyldithiocarbamate. The developed method was tested using a certified sample and then applied to the determination of manganese in seawater.