Spectrophotometric seawater pH measurements: total hydrogen ion concentration scale calibration of m-cresol purple and at-sea results

Abstract

Spectrophotometric open-ocean seawater pH measurements are simple, fast and precise. The sulfonephthalein indicator m-cresol purple ( mCP) is recommended for open-ocean surface-to-deep pH measurements. The salinity- and temperature-dependence of such measurements (293⩽ T⩽303 and 30⩽ S⩽37) is given as: pH T (sw) = 1245.69 T + (2.11 × 10 −3)(35 − S) + log( R − 0.0069 2.222 0 − R0.133 0 ) on the total hydrogen ion concentration scale ([H +] T = [H +] f + [HSO 4 −]), in units of mol·(kg-soln) −1. R is the ratio of indicator absorbances at molar absorptivity maxima (i.e. R + 578 A/ 434 A). The at-sea analytical precision of the technique, evaluated on two recent NOAA cruises, is approximately 0.0004 pH units.