In-situ pH measurement and real-time calibration

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. (…)

Tanm C., Ding K. & Seyfried W.E., 2014. In-situ pH measurement and real-time calibration. Sea Technology Magazine. Article.

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