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Address : Institute of Mineral Resources, Chinese Academy of Geological Sciences, China
Celebration date : 3 Wednesday September 2008
Author name : Zhang, Ronghua; Zhang, Xuetong; Hu, Shumin

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  • Article title : Novel high temperature and pressure chemical sensor assemblage for investigation on deep sea and mid-ocean-ridge
    Article type : Marine Geology
    Location : International Geological Congress,oslo 2008

    Fulltext :

    It is necessary to develop novel compositive sensors in order to directly in situ measure pH, H2, H2S, Eh of deep sea water and hydrothermal fluids at mid-ocean-ridge (MOR) in a wide temperature range (2-400°C). It was found that temperature from inside of vents to outside varied from about 408°C to 2°C. YSZ/HgO/Hg (YSZ: Yttria Stabilized Zirconia, 9%Y2O3) ceramic-based chemical sensors have been used to measure chemical parameters of hydrothermal vent fluids at MOR, which possesses good electrochemical properties at high temperatures but not sensitive below 200°C. Melt of NaCO3 can favor oxidation of Zr to form ZrO2 thin films on Zr surface. This method is suitable to make Zr/ZrO2 oxidation/reduction electrode companied with Ag/AgCl electrode (or Ag/Ag2S electrode, or Au electrode) to composite pH sensor (or H2S sensor, or H2 sensor) for in situ measurement of chemical parameters of high temperature aqueous solutions. The Zr/ZrO2 electrode has a chemical stability in measuring pH of high-temperature aqueous solutions, which is combined with a Ag/AgCl reference electrode. Measurement of the Zr/ZrO2 sensor potential against a Ag/AgCl reference electrode indicated that the sensor potential varies linearly with pH over a wide pH range, as prepared by various NaCl-HCl-H2O (NaOH-NaCl-H2O for basic), at temperatures of 2°C and 200°C. The electrochemical cell for pH determination can be illustrated as Ag ι AgCl ι Cl-, H+, H2O ι ZrO2 ι Zr. Cell potential ΔE (v) T,P as function of pH can be described as ΔE(V)T,P = ΔE° + 2.3026RT/F [ log a (Cl-1) - 1/2 log a (H2O) ] - 2.3026RT/F pHT,P . Where ΔE° is the cell potential at standard state, which can be calculated from the standard state potential of E°(Ag/AgCl) and E°(Zr/ZrO2). In order to measure fugacity of H2S dissolved in fluids, Ag/Ag2S electrode was used in association with Zr/ZrO2 electrode. Thus, we construct the following cell as a dissolved H2S sensor: Ag ι Ag2S ι H2S, H+, H2O ι Zr O2 ι Zr We also put Au electrode in the sensor assemblage. The sensors of coupling of Au and Ag/AgCl, and coupling of Au and Zr/ZrO2 are used to measure fugacity of H2 and Eh of the fluids. Therefore the Zr/ZrO2 electrode can be used to measure pH, H2, H2S, Eh of hydrothermal fluids in the temperature range of 2 to 200°C, which would alternate the YSZ sensors in the environment below 200°C, according to authors’ experimental study. YSZ sensors have been calibrated at high temperatures up to 400°C. Thus, we have constructed compositive sensor assemblages, which are gathered of the YSZ sensors and Zr/ZrO2 sensors. They consist of multi-electrodes, YSZ/HgO/Hg, Ag/AgCl, Ag/Ag2S, Au, and Zr/ZrO2. The first try of using Zr/ZrO2 sensors and compositive sensor assemblages to measure chemical parameters of deep-sea water were successful in South China Sea in 2004.