Apparatus for continuous-flow underway spectrophotometric measurement of surface water pH

A continuous-flow method for spectrophotometric measurement of seawater pH providing high temporal resolution has been developed incorporating a CCD-based spectrophotometer and dual pumps for seawater and indicator dye solution. The method showed good laboratory precision (±0.0007 pH units). The accuracy of the measurement system was assessed by comparing in situ measured pCO2 values calculated from measured pH and alkalinity. An average difference between the results of ca. ±8 μatm was obtained. Some difficulties was encountered during longer-term deployment on offshore cruises, including particulate matter in the surface seawater line, bio-fouling of the optical cell, and air-bubbles in the surface seawater line. Protocols were adopted to either prevent these problems or to recognise and remove data affected by them. Accuracy was also assessed by comparing the measured pH with values obtained by conventional discrete spectrophotometric measurements. On the first cruise, poor accuracy was found during portion of the cruise where pH <7.80 was recorded by the continuous-flow apparatus, probably because of fouling of the pump and connector tubing, which was very evident on this cruise. In other regions, the agreement between the two methods was within ±0.02 pH units. On the second offshore cruise, accuracy was assessed by analysing both a continuous flow of surface seawater and an isolated seawater sample that had been atmospherically equilibrated. In the first case, an accuracy of ±0.005 pH units was achieved, while the second case gave a better result of ±0.002 pH units. We conclude that after minor improvements, the measurement system should be well suited to high-frequency pH measurements of surface waters.