SPECTROPHOTOMETRIC SEAWATER PH MEASUREMENTS - TOTAL HYDROGEN-ION CONCENTRATION SCALE CALIBRATION OF M-CRESOL PURPLE AND AT-SEA RESULTS

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 less than or equal to T less than or equal to 303 and 30 less than or equal to S less than or equal to 37) is given as: [GRAPHICS] on the total hydrogen ion concentration scale ([H+](T) = [H+](f) + [HSO4-]), in units of mol.(kg-soln)(-1). R is the ratio of indicator absorbances at molar absorptivity maxima (i.e. R = (578)Al(434)A). The at-sea analytical precision of the technique, evaluated on two recent NOAA cruises, is approximately 0.0004 pH units.