ISO 18191:2015

Overview - ISO 18191:2015 (Water quality, pHt by m‑cresol purple)

ISO 18191:2015 specifies a spectrophotometric method for determining the pH of sea water on the total hydrogen ion concentration (pHt) scale using the indicator dye m‑cresol purple. The procedure reports [H+]t as moles per kilogram of sea water and is validated for oceanic pHt values from 7.4 to 8.2 for normal sea water with practical salinity 20–40. The method targets high‑precision oceanographic monitoring with repeatability better than 0.003 pHt units.

Key topics and technical requirements

• Principle: pHt is calculated from the absorbance ratio of the two indicator forms (acid HI and base I) after adding m‑cresol purple and applying Beer–Lambert relations.
• Wavelengths: Measure absorbances at the base and acid peaks (typical: 578 nm and 434 nm) and at a non‑absorbing reference (e.g., 730 nm).
• Instrumentation: High‑quality spectrophotometer (double‑beam recommended); 10 cm pathlength optical cells or flow‑through cuvettes; accurate micropipetting for indicator addition.
• Temperature control: Thermostated cell compartment to ±0.1 °C (thermostat bath to ±0.05 °C) and measurement typically at 25.0 °C.
• Indicator and reagents: Pure m‑cresol purple solution (≥2 mmol·L−1) adjusted to ~pH 7.9; ultrapure deionized water for reagent preparation.
• Sampling and handling: Rapid filling and minimal headspace to limit CO2 exchange; analysis preferred immediately, with samples generally stable up to 24 h under refrigeration.
• Quality control: Background and baseline corrections using the reference wavelength; target absorbances 0.4–1.0 at peak wavelengths; consistency in cell positioning and alignment.

Applications and who uses it

• Oceanographers and marine chemists performing high‑precision measurements of seawater pH for:
  • Long‑term ocean acidification monitoring and research
  • Carbonate system studies (DIC/TA exchange and monitoring)
  • Environmental impact assessments and carbon capture/storage site monitoring
• National and international observation programs requiring intercomparable, standardized pH datasets.
• Laboratories conducting research cruises, coastal monitoring, and process studies that require traceable spectrophotometric pHt determinations.

Related standards and context

• Prepared under ISO/TC 147 (Water quality), this method complements other water quality and carbonate system methods (e.g., measurements of total alkalinity and dissolved inorganic carbon) used in oceanographic monitoring programs. Users should consult other ISO water‑quality documents and local regulations for integrated monitoring protocols.

Keywords: ISO 18191:2015, pH in sea water, pHt, m‑cresol purple, spectrophotometric determination, oceanic pH monitoring, sea water salinity, ocean acidification.