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 ( i SEA-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 i SEA-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 i SEA-pH systems were developed and successfully applied in mesocosm experiments and several coastal and open ocean cruises with excellent in field performance.