International Geologiical Congress - Oslo 2008

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OSP-01 General contributions to marine geoscience & paleoceanography

 

Novel high temperature and pressure chemical sensor assemblage for investigation on deep sea and mid-ocean-ridge

 

Ronghua Zhang, Institute of Mineral Resources, Chinese Academy of Geological Sciences (China)
Xuetong Zhang, Institute of Mineral Resources, Chinese Academy of Geological Sciences (China)
Shumin Hu, Institute of Mineral Resources, Chinese Academy of Geological Sciences (China)
 

 

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 AgCl Cl-, H+, H2O ZrO2 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 Ag2S H2S, H+, H2O Zr O2 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.

 

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