Evolution of compact electrochemical in-situ pH-pCO2 sensor using ISFET-pH electrode

A non-glass type of in-situ pH sensor which uses an ion-sensitive field-effect transistor (ISFET) as the pH electrode and a chloride ion selective electrode (Cl-ISE) as the reference electrode was reported in 1998. An in-situ pCO₂ sensor that the pH sensor was sealed with a gas permeable membrane filled with the inner solution was also informed in 2002. In the field tests, the in-situ response time of the devised pH sensor was as quick as 1 second or less, and the estimated measurement accuracy was ±0.005 pH units and a depth rating of 6000 m. The in-situ (3000 m depth, 1.8°C) response time of the pCO₂ sensor was less than 60 seconds. In order to adapt the in-situ pH-pCO₂ sensor to wide area mapping and/or long term monitoring of pH and pCO₂ in the ocean, next generation of electro-circuit of the sensor and ISFET-pH electrode were newly developed. Several mapping observations of pH and pCO₂ were performed using ROV or AUV, and the sensor detected distributions of low pH and high CO₂ seawater which were derived from hydrothermal vent. The long term monitoring by the sensor for one month showed daily variation of pH.