ISO 18191:2015 specifies a spectrophotometric determination of the pH_t of sea water on the total hydrogen ion concentration pH scale using the indicator dye m-cresol purple. The total hydrogen ion concentration, [H⁺]_t, is expressed as moles per kilogram of sea water. The method is suitable for assaying oceanic levels of pH_t 7,4 to 8,2 for normal sea water of practical salinity ranging from 20 to 40.