Spectrophotometric Seawater

pH Analyzer

Model AS-pH2

The Apollo SciTech® AS-pH2 Analyzer has been developed for high precision and accuracy, automated and fast determination of seawater pH. It is based on the principle of colorimetric determination of the relative color intensity using a pH indicator (m-cresol purple) at three wavelengths (434 nm, 578 nm, and 730 nm), with the first two absorptions as a known function of pH and the third one as a baseline correction (Clayton and Byrne 1993; Liu et al. 2011; user-definable). The automated analyzer is designed according to Carter et al. (2013) with improvements. A high-quality Agilent spectrophotometer (previously Agilent model 8453 or 8454 and is now replaced by model Cary 60) is used for the colorimetric determination and a high precision Tecan Cavro digital pump is used for the delivery of pH indicator and the sample. The system is automated by analyzing a set of 9 samples in about 45 minutes.
The software program can be set to measure at other wavelengths if a different pH indicator is used, but the user needs to calculate the pH using the appropriate equations manually or in a spreadsheet program.


  • Precision: better than 0.001 pH units
  • Accuracy: better than 0.003 pH units
  • Wavelengths for pH analysis: 434 nm, 578 nm, and 730 nm
  • Indicator for pH: m-cresol purple or others
  • Spectrophotometer: Agilent Cary 60
  • Digital Pump: Tecan Cavro XLP pump equipped with a 12-port valve and a 10 mL syringe
  • Spectrophotometric cell: 10 cm with thermostated water jacket
  • Precision of Temperate-control system: 0.1oC
  • Automated: run 9 samples sequentially
  • Typical sample volume: 40 mL per analysis
  • Time consumption: ~ 5 minutes per sample
  • Working environment: Both land and shipboard laboratories