Application of colorimetric sensor in monitoring dissolved CO₂ in natural waters

Dissolved CO₂ originating from underground structures at high concentrations may pose a threat to public and environmental health. Therefore, a convenient monitoring technique that allows fast detection of dissolved CO₂ needs to be developed. In this study, a low-cost colorimetric CO₂ sensor was applied for monitoring dissolved CO₂. The sensor is composed of an acrylic reactor, cresol red pH indicator solution, and a gas-permeable membrane, and the performance of the sensor was tested for the detection of dissolved CO₂ at the range of 2-800 mg CO₂ L^-1. Color change of the detection solution within the sensor was mainly dependent on CO₂ dissolved in the water sample. This was analyzed using an RGB program that extracts the red, green, and blue intensity of a target color on a scale of 0-255. ΔGB, an index of CO₂ concentration corresponding to the change in intensity of green (G) and blue (B) extracted by the RGB program, exhibited a linear relationship with dissolved CO₂ concentrations (r² > 0.95, p < 0.005). In the field, the sensor was able to measure dissolved CO₂ between 10 and 411 mg CO₂ L^-1 within 1 min. Overall, our CO₂ sensor has high potential to be used in detection of dissolved CO₂ in groundwater and surface waters.