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

Author(s): Clayton TD, Byrne RH

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:pHT(sw) =1245.69T+ (2.11 × 10−3)(35 − S) + log(R − 0.00692.2220− R0.1330)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 + 578A/434A). The at-sea analytical precision of the technique, evaluated on two recent NOAA cruises, is approximately 0.0004 pH units.

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