Toward an in Situ Electrochemical Sensor Measuring Phosphate Concentration in Seawater

Abstract

Dissolved orthophosphate (phosphate) is one of the major nutrient elements in natural waters (lakes, rivers, oceans) easily uptaken by primary producers. Monitoring its concentration in seawater will allow to better understand the biogeochemical cycles and the ocean role in climate evolution. Phosphate is an electroinactive specie that is previously complexed in situ at acidic pH with molybdates. In order to avoid silicate interference a ratio protons over molybdates equal to 70 is required. The reagents are formed in situ by oxidation of 2 different molybdenum electrodes. The first oxidation allows to form H+ that cross a proton exchange membrane to obtain the pH conditions needed in the complexation cell. Then, the second molybdenum electrode is oxidized to reach the ratio (H+/MoO4 2-) = 70 and form the phosphomolybdic complex. The counter electrode is isolated behind a Nafion® membrane to limit the reduction of H+ into H2. Because phosphate concentration in seawater is quite low, i.e. few nanomolar in the surface ocean, up to 5 µmol.L-1 at deepest waters, electrochemical detection method requires high sensitivity. Therefore, cyclic voltammetry cannot be used neither chronoamperometry as a rotating electrode is needed to control the convection and it is not suitable for an in situintegrated sensor. Square Wave Voltammetry (SVW) is proposed to detect phosphomolybdic complex in seawater as it offers an optimal combination of potential modulation (potential ramp combined with short term potential pulses) and a specific current sampling reducing the effect of capacitive current. The mechanism of reduction of phosphomolybdic complex is still not fully elucidated. Depending on the pH and the phosphate concentration, different types of complex are formed, and polymer structures have also been observed [1]. Therefore, the reduction of Mo(VI) into a mix of Mo(V)/Mo(IV) observed on the square wave voltammograms showed different behaviours depending on experimental conditions. Indeed, at 250 Hz frequency, a saturation of the signal is quickly observed when the phosphate concentration increased (> 1 µmol.L-1) compared to 2.5 Hz frequency that allowing to obtain a linear behaviour on the whole concentration range (0-6 µmol.L-1) [2]. All the experimental parameters such as molybdenum oxidation charges, time of complexation and SWV parameters, have been optimized using a laboratory prototype in order to detect the smallest phosphate concentration possible, as fast as possible. Those parameters are currently on adaptation for the in situ version of the sensor as the design of the electrochemical cells has a strong influence on the diffusion time (homogenisation of the solution) and so on the detection signal. The electronics of the sensor is also in development and a comparison of its performance compared with the commercial Metrohm® potentiostat will be presented. The in situsensor will then be deployed in the ocean for inter-comparison tests. [1] Jonca et al.,Int. J. Electrochem. Sci. 7 (2012) 7325-7348 [2] Barus/Romanytsia et al., Talanta 160 (2016) 417-424