Silicate electrochemical measurements in seawater: Chemical and analytical aspects towards a reagentless sensor

Abstract

International audienceFrom the study of molybdenum oxidation in aqueous solutions we developed a semi-autonomous method to detect silicate in aqueous samples. Molybdenum oxidation was used to form molybdate in acidic media. The silicomolybdic complex formed with silicate is detectable by amperometry or cyclic voltammetry. The new electrochemical method is in good agreement with the method conventionally used for environmental water silicate analysis. In the second stage, a completely reagentless method was developed using molybdate and proton produced during molybdenum oxidation. Reproducibility tests show a precision of 2.6% for a concentration of 100 [mu]mol L-1. This new method will be very suitable for the development of new autonomous silicate sensors easy to handle and without reagents. In this paper we present the analytical and chemical aspects necessary for a complete documentation of the method before the development of a new reagentless sensor

First Deployment and Validation of in Situ Silicate Electrochemical Sensor in Seawater

“…In the first compartment of 2 mL, a molybdenum electrode was oxidized at 1V in order to produce molybdates (MoO 2− 4 ) and protons (H + ) according to Equation (1). To reach the acidic pH needed (pH ≈ 1.5) to form the silicomolybdic complex (Equation 2), the counter electrode was isolated from the molybdenum electrode behind a 180 µm thick non-proton exchange membrane (N117 Du Pont TM Nafion R PFSA Membrane) to limit the reduction of formed H + (Lacombe et al, 2008). The solution was stirred using a magnetic stirrer after the molybdenum oxidation during 6 min to complex 100% of silicate in solution.…”

“…We propose to use an electrochemical sensor to detect silicate without any liquid reagent addition using an in situ oxidation of a molybdenum electrode to form the silicomolybdic complex detectable on gold working electrode, thanks to a special design of the electrochemical cell using Nafion R membrane. The limit of quantification achieved using a 2 mm diameter working electrode and commercial potentiostat was 0.5 µmol L −1 (Lacombe et al, 2008;Aguilar et al, 2015).…”

Silicon-based electrochemical microdevices for silicate detection in seawater

“…The methodology for the electrochemical determination of silicates with the microdevices is adapted from the one described in 1 DGEBA: DiGlycidyl Ether of Bisphenol A 2 IPDA: Isophorone Diamine Lacombe et al and consists of two steps [39]. First, the molybdenum metal electrode oxidation is performed in a 3 mL sample by applying a constant potential of 1.0 V until a 14 C electric charge was reached in order to form molybdates (MoO 4 2− up to a concentration of 6.5 mmol L −1 ) and protons (H + , pH ≈ 1.5).…”

“…This method requires a simple oxidation of a molybdenum electrode in order to form molybdate and protons. To achieve the needed acidic pH and form the silicomolybdic complex, a non-proton exchange membrane is added in order to isolate the counter electrode and avoid the reduction of the H + formed at the anode [39]. We are currently working on the development of miniaturized and autonomous silicate sensors using this method and in order to reduce the associated cost, the use of silicon-based technologies for the mass fabrication of electrodes and the realization of integrated microdevices is proposed.…”

Use of In‐Situ Methods

“…However, the use of reagents sets limits on the time that instruments can be deployed and adds significantly to the bulk and weight of the analytical system to be transported and deployed, and there has been a move to develop reagentless methods where possible (Lacombe et al ., 2008).…”