Electrochemical Sensors for In Situ Phosphate and Nitrate Measurements in Seawater
Ivan Romanytsia1, Dancheng Chen Legrand1,2, Carole Barus1, Nicolas Striebig3 and Veronique Garcon1, (1)LEGOS/UMR 5566, TOULOUSE, France, (2)Fondation de Coopération Scientifique Sciences et Technologies pour Aéronautique et l’Espace, Toulouse, France, (3)Observatoire Midi-Pyrénées, Toulouse, France
Abstract:
Monitoring the evolution of concentrations of dissolved inorganic nutrients, like phosphate and nitrate, provides insights on the oceanic biogeochemical cycles. This long term monitoring is key to investigate how changing oceanic conditions will alter biogeochemical cycles. We report here the latest development and analytical method to measure phosphate and nitrate concentrations in seawater without any addition of liquid reagents. We propose to use a derivative electrochemical method such as Square Wave Voltammetry (SWV) to detect phosphomolybdic complex and nitrate as this method offers a higher sensitivity than classical cyclic voltammetry and avoids the need of stirring the solution like for chronoamperometry technique.
Phosphate is a non-electroactive species and its determination is performed by measuring its corresponding phosphomolybdic complex formed in situ after oxidation of two molybdenum electrodes placed into two different compartments connected with a proton-exchange membrane. [1]. All the SWV parameters such as step potential, amplitude and frequency have been determined to detect phosphomolybdic complex as fast as possible and with the lowest limit of detection. Depending on the frequency used, two calibration curves have been obtained for two phosphate concentration ranges: 0.07-1.06 µM (250 Hz) and 0.5-6 µM (2.5 Hz). We are currently working to adapt those parameters to laboratory prototype and results obtained will be presented.
On the other hand, nitrate can be detected directly on gold electrode modified with silver nanoparticles (AgNPs) where the nitrate reduction process can be easily measured at -0.97 V [2]. This method allows to obtain good calibration curves with a detection limit of 10 nM, very short measuring time (2.8 s) and long life time of the modified electrode (minimum 47 days storage in seawater).
[1] Jonca et al., Electrochimica Acta 88 (2013) 165-169
[2] Fajerwerg et al., Electrochem. Commun. 12 (2010) 1439-1441
