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Feature Article

In-Situ pH Measurement And Real-Time Calibration
Applications to Ocean Observatories at Deep-Sea Vents

AUTHORS:


Dr. Chunyang Tan

Dr. Kang Ding

Dr. William E. Seyfried, Jr.



Signal drifting can be compensated by in-situ calibration during the measurement cycle.
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The vast biogeochemical processes in the ocean operate on temporal and spatial scales, from seconds near hydrothermal-seawater mixing zones at midocean ridges to years for water masses of different density in the ocean as a whole. In-situ chemical sensors that can be applied to this dynamic ocean system represent an alternative strategy to direct measurement of seawater samples using more conventional oceanographic techniques. It has long been recognized that pH is a key parameter of seawater and can provide information fundamental to a wide range of chemical, physical and biogeochemical processes. The high pressure and corrosive conditions associated with deep-ocean environments, however, underscore the need for pH sensors that can tolerate these conditions.

Thus, we have developed a solid-state pH sensor that makes use of an iridium-oxide cell, which, together with a similarly solid state silver-chloride reference electrode, provides the robust functionality that is needed to make oceanographic measurements, even at extreme temperatures. This is especially relevant to biogeochemical applications in diffuse flow hydrothermal vents where temperatures as high as 75° C are typical in the course of mixing between hot end-member hydrothermal fluids and cold ocean bottom seawater. The diffuse flow hydrothermal environment supports a rich community of chemoautotrophic microbes, many types of which exist in symbiosis with more complex fauna. Indeed, microbial communities at deep-sea vents may provide important clues to the origin of life on Earth and other planetary bodies where water is/was available in the evolutionary history of the planet. The redox reactions inherent to microbial metabolism often involve sulfur and iron species, which are highly sensitive to pH variability, underscoring the need for pH measurement if the temporal and spatial evolution of microbial communities is to be understood.

Experiments performed with the iridium-oxide-based pH electrodes have been conducted at temperatures as high as 150° C, in addition to ocean bottom pressures, and have revealed excellent Nernstian response. These experiments have also confirmed that the pH response is rapid and characterized further by an excellent linear dynamic range, which facilitates applications in a wider range of ecological settings than previously thought possible. These data, however, also revealed some variability of the standard state potential (E°) due to irreversible compositional/structural alteration of the surface properties of iridium metal. Accordingly, as with most pH electrodes, the long-term deployments will necessarily require in-situ calibration capabilities. Calibration of pH and similar chemical sensors on the ocean floor represents a serious challenge. Indeed, to our knowledge, this has not been satisfactorily accomplished for any electrochemical measurements performed to date.

Using a novel experimental approach, however, we have completed a series of critical improvements in this area and have completed the development of an in-situ calibration system. The concept of coupled in-situ pH measurement and calibration has been our main objective related to seafloor observatory efforts. Thus, a new device, which we refer to as “pH Calibrator,” was developed as the next generation of in-situ sensor instrumentation. This self-calibrating sensor system can be operated on the seafloor or in the water column to carry out the long-term monitoring of pH of seawater or diffuse hydrothermal fluid.

During the development of the instrument, several technical criteria were considered: reliability, power consumption, range and consumption rate of pH buffers, remote operation, and environmental factors that could affect length of deployment. The resulting self-calibrating pH system can be deployed by various methods, such as Lander system or ROVs. The unit is especially important with the parallel development of seafloor cabled observatories, which can provide the power needs for calibration inherent to the long-term acquisition of high-quality chemical sensor data in challenging chemical and physical environments.


Instrumentation
In-situ calibration of the iridium-oxide electrode is generally similar to that used for lab calibration of the well-known glass electrode. Two pH buffers of known and verified values are used to obtain the liner function between the pH and cell potential, thus the pH of the sample can be derived from the calibration data. With the in-situ calibration method, inaccuracy caused by signal drift from the pH electrode (iridium oxide) is explicitly addressed. It is apparent that the practical determination of pH requires the electrode to interact with the target sample and the buffers in series under the same pressure-temperature condition, while simultaneously recording the cell potential. To achieve this, a computer-controlled and automated flow-through strategy for fluid delivery and transfer is required.

The dimension of the pH Calibrator is 70 by 28 by 28 centimeters without the sensor probe. The system weighs about 20 kilograms in seawater, with a depth rating of 4,500 meters. The instrument is constructed with two main modules. The sensor probe module is composed of four solid-state electrodes, a pH electrode (iridium oxide), dissolved hydrogen electrode (Pt), dissolved hydrogen-sulfide electrode (silver sulfide) and reference electrode (silver chloride), and a temperature sensor (thermistor). The sensor unit allows the simultaneous measurement of multiple parameters. The sensing elements of the electrodes are sealed in a small sensor cell, with a volume of about 1 milliliter, by a precompressed Viton sealant immersed in silicone oil of a pressure-balanced damp. This design obtains a very high rate of buffer displacement and the reliable seal of the electrodes under pressure. The sensing tip of the electrodes and thermistor are set on the same axial plane in the sensor cell to limit the effects of temperature and chemical gradients. The sensor probe is tethered with communication cable and flexible flow tubing to the hydraulic module of the system to permit fine-scale measurement of the environment by a submersible manipulator arm. To continue this article please click here.



Dr. Chunyang Tan is an engineer at Sanya Institute of Deep-sea Science and Engineering, Chinese Academy of Sciences, and is currently working at the University of Minnesota as a research fellow. He is responsible for the mechanical and electrical design of ocean observation instruments. His expertise is in deep ocean sensor system design and development of in-situ instruments for cabled ocean observatories.

Dr. Kang Ding is a senior researcher at the University of Minnesota. His field of research involves hydrothermal chemistry through experimental efforts at elevated temperatures and pressures. He has been involved extensively with deep submergence science for developing and applying in-situ chemical sensors to study hydrothermal fluids at midocean ridges.

Dr. William E. Seyfried, Jr. is a professor at the University of Minnesota. At present, he is involved in experimental, field and theoretically based studies related to hydrothermal processes at a wide range of temperatures and pressures, with application specifically to midocean ridge hydrothermal systems.







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