Innovative Sensing Technology of Seawater pH by ISFET

Measurement of two variables among the four measurable ones: pH, CO₂, dissolved inorganic carbon (DIC) and total alkalinity (TA) is necessary to determine the carbonate system in the ocean, fundamental to evaluate Ocean Acidification and carbon absorption capacity. pH is measured routinely, and CO₂ analyzer is deployed in some buoy systems, but a combination of pH-CO₂ leads to a large error propagation. A combination of pH-alkalinity provides the most accurate results, both of which need direct measurement or detection of acid-titrated change in pH of seawater. pH is conventionally measured by a combination glass electrode with Ag/AgCl both for internal and reference electrodes immersed in internal solution of KCl, which is connected to sample solution (seawater) through a junction. Recently, an ion sensitive field effect transistor (ISFET) is applied to measure pH, which is more robust and compact. At present, ISFET pH sensor for sale commercially is united with a solid transistor and a gate membrane, and thus the electronic circuit of transistor should be molded not to be exposed to seawater. The reference electrode is Ag/AgCl immersed in KCl applied from the glass electrode, and thus it is responded to chloride ion in seawater and modified. However, the gate membrane needs not be united with the solid transistor, and Ag/AgCl in reference electrode needs not be exposed to the seawater as far as electronic current can flow between them. The present ISFET pH sensor does not use up its advantages. Here we have developed an innovative pH measurement ISFET: an ion sensitive membrane is separated from the transistor connected with a wire, and reference electrode is Ag/AgCl in saturated KCl covered with ionic liquid gel. The ion sensitive membrane can be small enough to measure millimeter-order change in pH in seawater or in sediments. The transistor is separated from seawater unaffected by change in temperature and pressure. The ionic liquid gel in the reference electrode prevents Ag/AgCl from responding to chloride ion while keeping electrical connection with sample solution. This pH sensor will be applied to alkalinity by measurement of pH before and after addition of acid in a flow system, which will open the way for continuous measurement of both pH and alkalinity with a compact instrument.