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 circuit. After medium exchange, the accumulated Mn(^II) at the electrode surface was electrochemically oxidized. The hydrated MnO₂ thus formed was then cathodically stripped using differential pulse voltammetry. Various 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 detection limit of 6.9 × 10⁻⁹ M Mn(^II) (0.38 ppb) was found for 200s accumulation. For 8 determinations, each of Mn(^II) concentrations of 1.00 × 10⁻⁶ M, 1.00 × 10⁻⁷ M and 1.00 × 10⁻⁸ M, relative standard deviations of 2.90%, 4.46% and 7.20% were obtained, respectively. Selected metal ions were tested for interferences and it was found that, when determining 1.00 × 10⁻⁶ M Mn(^II) (150s accumulation), Co(^II), Hg(^II) and Fe(^II) at 1.00 × 10⁻⁵M interfered. These interferences were readily eliminated by masking with complexing agents such as sodium citrate and sodium diethyldithiocarbamate. The developed method was tested using a certified sample and then applied to the determination of manganese in seawater.