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.

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

I. M. Kolthoff

Papers

Quantitative chemical analysis