Development of a new submersible colorimetric sensor for in situ detection of pHT on profiling floats

The international BioGeoChemical-Argo program recommends the measurement of 6 essential ocean variables (EOVs), one of which being the inorganic carbon. Estimates of pH variation in the open ocean using traditional sampling methods show a decrease of 0.002 units per year [1]. Understanding and predicting pH variations using precise, certified in situ detection over oceans around the world over the long term is currently essential. In the frame of the PIANO (Plan d’Investissement Argo Nouvelles Observations, Ifremer) project we designed and developed a new submersible pH_T colorimetric sensor deployable on profiling floats sensible enough to be suitable for the acidification monitoring. This development is aimed on the use of the referenced colorimetric method ([2]–[4]), applied to microfluidic channels with in situ calibration using on-board standards. The first phase of the project was to determine the miniaturized actuators able of working under 350 bars and 4°C. Then, a 3D printed manifold was fabricated using stereolithography. It was optimized to integrate a micromixing area, an optical detection and a temperature sensor (pt100). Once the final architecture of the manifold was fixed, the system's repeatability and accuracy were then tested. Electronic and mechanic design were established considering minimum energy consumption and space. In parallel, tests were set up with the metrology laboratory to study the ageing of on-board pH_T standards over time (2-years test). An overall protocol was devised, focusing on the procedure for rinsing the bags to minimize bacterial contamination and purchasing stable standards. The first results of this future submersible pH_T sensor against robust benchtop spectroscopic measurements of referenced waters will be discussed.

Figure 1: 3D printing of the optical cell

Figure 2: 3D printing mixing unit (based on tesla design)

 

[1]         F. J. Millero, “The marine inorganic carbon cycle.,” Chem. Rev., vol. 107, no. Table 1, pp. 308–341, 2007.

[2]         X. Liu, M. C. Patsavas, and R. H. Byrne, “Purification and characterization of meta-cresol purple for spectrophotometric seawater ph measurements,” Environ. Sci. Technol., vol. 45, no. 11, pp. 4862–4868, 2011.

[3]         J. D. Müller et al., “Metrology for pH measurements in brackish waters-part 1: Extending electrochemical pHT measurements of TRIS buffers to salinities 5-20,” Front. Mar. Sci., vol. 5, no. JUL, pp. 1–12, 2018.

[4]         J. D. Müller and G. Rehder, “Metrology of pH measurements in brackish waters-part 2: Experimental characterization of purified meta-cresol purple for spectrophotometric pHT measurements,” Front. Mar. Sci., vol. 5, no. JUL, pp. 1–9, 2018.