Development of in situ CO2 and pH sensor (Hybrid CO2-pH sensor: HCS) for AUV

We have been developing new in situ CO₂ and pH sensor (Hybrid CO₂-pH sensor: HCS) for the cruising AUV. The measurement principle for the CO₂ sensor is based on spectrophotometry. The CO₂ in the surrounding seawater equilibrates with the indicator solution across the gas permeable membranes. The equilibration process causes a change of pH in the indicator solution, which results in the change of optical absorbance. The CO₂ is calculated from the optical absorbance of the pH indicator solution equilibrated with CO₂ in seawater through a gas permeable membrane. In our analytical system, we used an amorphous fluoropolymer tubing form of AF-2400 by DuPont for the gas permeable membrane due to its high gas permeability coefficients. On the other hand, we adopt potentiometric analysis using original glass and reference electrodes as a pH sensor for deep sea because of the most commonly used technique for sea water pH measurements.