Reagentless and silicate interference free electrochemical phosphate determination in seawater

An electrochemical method for phosphate determination in seawater was based on the oxidation of molybdenum in order to form molybdates and protons and subsequently, to create the phosphomolybdic complex electrochemically detectable by means of amperometry at a rotating gold disk electrode [J. Jońca et al., Talanta 87 (2011) 161]. To avoid silicate interferences, the method required an appropriate ratio of protons over molybdates equal to 70. Since the ratio of protons over molybdates created during molybdenum oxidation is only 8, the previous method still needed addition of sulfuric acid and thus was not free from addition of liquid reagents. In the present work, this aspect is solved by modification of the electrochemical cell construction. The method is now totally free from addition of any liquid reagents and gives a possibility to determine phosphate by amperometry in the concentrations range found in the open ocean with a detection limit of 0.11 μM. Having in mind the energy savings for future in situ sensor development, amperometry at rotating gold disk electrode was replaced by differential pulse voltammetry at static one. Phosphate can then be determined with a detection limit of 0.19 μM. Both methods are characterized by good reproducibility with an average measurements precision of 5.7% (amperometry) and 3.8% (differential pulse voltammetry). Results also show a good accuracy with an average deviation from theoretical values of phosphate concentration of 3.1% for amperometry and 3.7% for differential pulse voltammetry.