In situ pH monitoring of deep-sea water in Southern China Sea using high temperature and pressure sensors

摘要

Development of new sensors is essential to measure in-situ pH and other chemical parameters of deep sea water and hydrothermal fluids in a wide temperature range (2-400°C) at mid-ocean ridge vents.The YSZ (Yttria Stabilized Zirconia, 9%Y2O3) ceramic-based (HgO/Hg) chemical sensors possess excellent electrochemical properties at high temperatures, although they are not sensitive below 200°C. A Zr/ZrO2 oxidation/reduction electrode was constructed by oxidation of Zr wire in Na2CO3 melt. This Zr/ZrO2 electrode has chemical stability while measuring pH of high-temperature aqueous solutions, combined with a Ag/AgCl reference electrode. Comparison of the Zr/ZrO2 sensor potential to a Ag/AgCl reference electrode shows that the sensor potential varies linearly with pH over a wide range, as tested by various NaCl-HCl-H2O solutions (NaOH-NaCl-H2O for basic solutions), at temperatures of 20-200°C. Thus, the Zr/ZrO2 electrode can be used in the temperature range below that of the YSZ sensors. The Zr/ZrO2 sensor has been used to measure pH of high-temperature fluids in the laboratory and to monitor pH of deep sea water in Southern China sea.