Underway mapping of coastal seawater pH using an automated shipboard analyzer with spectrophotometric detection

The development of field-deployable methods and instruments for the measurement of pH and other carbonate parameters is important for the assessment of the marine carbon cycle, ocean acidification and marine carbon dioxide removal techniques. In this study, a high-precision fully automated integrated syringe-pump-based environmental-water analyzer for pH (iSEA-pH) was developed. The pH is determined spectrophotometrically using purified indicator dye with a high precision (better than ±0.001) and high frequency (3.5 min/sample). For the short-term analysis, the measurement frequency was 18 h^-1, which revealed pH = 7.8148 ± 0.0005 (n = 104) for aged surface seawater (S = 35) from the western Pacific. For long-term analysis, the measurement frequency was 2 h^-1 for 4 days, and the results showed that pH = 7.8148 ± 0.0010 (n = 200). Three commonly used pH indicators (meta-cresol purple, thymol blue and phenol red) were purified with improved flash chromatography procedures. The autonomous iSEA-pH can automatically correct for the influence of temperature, salinity and other factors on pH measurements to achieve rapid and accurate on-site measurements, which meet the "climate" goal of the Global Ocean Acidification Observing Network (uncertainty is ±0.003). Three identical iSEA-pH systems were developed and successfully applied in mesocosm experiments and several coastal and open ocean cruises with excellent in field performance.