This document specifies a spectrophotometric determination of the pHT of seawater on the total hydrogen ion concentration pH scale. The total hydrogen ion concentration, [H+]T, is expressed as moles per kilogram of seawater. The method is suitable for assaying oceanic levels of pHT from 7.4 to 8.2 for normal seawater of practical salinity ranging from 20 to 40.
ISO 18191:2026 – Water quality — Determination of pHT in seawater — Method using the indicator dye m-cresol purple, 2nd Edition. Technical Committee ISO/TC 147, Water quality, Subcommittee SC 2, Physical, chemical and biochemical methods. ISO Standard.
