A new optical sensor for PCO2 measurements in seawater

A carbon dioxide sensor for continuous use on unattended ocean platforms is described. Carbon dioxide partial pressure was measured from colour changes in thymol blue, a pH-sensitive dye. A small quantity of dye solution was enclosed between 2 gas-permeable silicone membranes. Measurements at 596 nm were made by a spectrophotometer connected to the sensor by silica fibre optics. Baseline drift was assessed at 730 nm, and dye deterioration at the isosbestic point of 492 nm. The light source was a 10 W tungsten lamp. Two calibration experiments of 3 and 4 d were made with carbon dioxide fugacities in the range 340-590 uatmospheres (uatm). A variation of 1 uatm could be detected in this range with standard errors of the fit of the calibration curves being 3 and 5 uatm. There are 35 references.