NaOH Pretreated Molybdate-Carbon Paste Electrode for the Determination of Phosphate in Seawater by Square Wave Voltammetry with Impedimetric Evaluation

Abstract

Phosphate (PO4 3−) is an important nutrient for phytoplankton growth and at high loadings can result in water quality deteriorations. Autonomous PO4 3− measurements are required for monitoring purposes, and are best achieved using sensitive, portable and low-cost techniques. Here we describe a new electrochemical sensor for PO4 3− detection in seawater. The electrochemical quantification of PO4 3− typically depends on the reaction between molybdate and PO4 3− under acidic conditions to form a phosphomolybdic complex, which is electrochemically active. In this work, we prepared a carbon paste electrode (CPE) modified with molybdate and pretreated in 0.1 M NaOH using cyclic voltammetry (CV). The modified CPE was employed for the determination of PO4 3− in artificial seawater (35 g l−1 NaCl) acidified with sulfuric acid to pH 0.8. The analytical conditions, including pH, waiting time for complexation, square wave amplitude and frequency, were optimized. An additional cleaning step (cyclic voltammetry (CV)) of 10 cycles in 0.1 M NaOH at −0.5 to 0.5 V was required between PO4 3− determinations to dissolve the phosphomolybdic complex formed on the surface of the working electrode. Electrochemical impedance spectroscopy (EIS) results confirmed that the molybdate-modified CPE (molybdate/CPE) exhibited a low charge-transfer resistance (Rct) toward PO4 3−, and showed an improved analytical performance for different concentrations of PO4 3−. A calibration plot in the range of 0.01–5 μM with a limit of detection (LOD) of 0.003 μM was obtained. The proposed electrode demonstrated good precision (4.3% and 5.8%) for concentrations of 5 μM and 0.2 μM, respectively. The proposed method was employed to analyze PO4 3− in seawater samples on a research cruise in the North Sea, with results in close agreement to those obtained using conventional colorimetric measurements.