Shipboard flow injection determination of sea water pH with spectrophotometric detection

The use of a flow injection system with spectrophotometric detection for the measurement of sea water pH is described. The acid-base absorption properties of phenol red injected into a sea water stream are measured. The performance of the technique in a shipboard environment is evaluated and shown to have a working precision of ±0.005 pH units with a sampling frequency of ca. 25 h−1. The internal consistency of carbon dioxide measurements throughout an expedition to the Southern Ocean is estimated to be commensurate with precisions of ±0.005 pH unit, ±2 μatm pCO2 and ± 1 μmol kg−1 TCO2. This level of precision, combined with the simplicity and low indicator consumption rate of the pH technique and the fact that no standard is needed for calibration make the method ideally suited for in situ pH measurement in marine waters.