ISO 18191:2026

Overview

ISO 18191:2026 is an international standard that outlines a spectrophotometric method for the determination of pHT (pH on the total hydrogen ion concentration scale) in seawater using the indicator dye m-cresol purple. Developed by ISO Technical Committee 147 on Water Quality, this standard provides a reference methodology for assaying seawater pHT in the range of 7.4 to 8.2, covering salinity from 20 to 40. Precise pH measurement in seawater is essential for global studies on ocean acidification, marine chemistry monitoring, and carbon cycle assessments.

Key Topics

• Measurement Principle: The method is based on a spectrophotometric technique where m-cresol purple is used as an indicator dye. The absorbance changes at specific wavelengths (578 nm, 434 nm, and 750 nm) are used to calculate the pHT of seawater.
• Hydrogen Ion Concentration Scale: The standard utilizes the total hydrogen ion concentration [H+]T, expressed as moles per kilogram of seawater, which is important for ensuring stability and comparability in pH measurements worldwide.
• Sample Handling & Storage: Guidance is provided on correct seawater sampling, immediate analysis, and storage conditions to minimize pH shifts due to CO₂ exchange and biological activity.
• Correction Procedures: Special attention is given to correcting absorbance data for background shifts and adjustments for the slight pH change caused by dye addition.
• Quality Assurance: Interlaboratory comparisons and specific measurement protocols for m-cresol purple help ensure method robustness, reproducibility, and accuracy of results.

Applications

The method specified in ISO 18191:2026 for determining pHT in seawater has practical applications in:

• Ocean Acidification Monitoring: Precise, standardized pH data support research and policy responses concerning the impact of increasing atmospheric CO₂ on ocean chemistry and marine ecosystems.
• Marine Carbonate System Studies: The method enables operational programs and research projects to monitor carbonate chemistry, a critical factor for understanding buffering capacity and biogeochemical cycles in the ocean.
• Environmental Impact Assessment: Application in monitoring sites considered for carbon capture and storage (CCS), where tracking pH is necessary for risk assessment and environmental protection.
• Global Observing Networks: The standard supports data harmonization across international programs such as GOOS (Global Ocean Observing System) and regional marine chemistry initiatives.
• Reference and Calibration: Laboratories use ISO 18191:2026 to calibrate and validate other field and laboratory pH sensors, ensuring traceability and quality control in large-scale oceanographic campaigns.

Related Standards

• ISO 5667-3: Guidance on the preservation and handling of water samples for quality analysis.
• ISO 10390: Determination of pH in soil, providing similar guidance for soil environments.
• ISO/IEC 17025: General requirements for the competence of testing and calibration laboratories, supporting robust implementation of ISO 18191:2026.
• Guide to Best Practices for Ocean CO₂ Measurements (PICES SP3): Frequently cited in marine chemistry, complements the standard by outlining best practices in sample collection and analysis.
• Standard Operating Procedures (SOPs) for laboratory and field pH measurements, often based on or aligned with ISO 18191 methodology.

ISO 18191:2026 enables marine scientists, environmental monitoring agencies, and laboratories worldwide to reliably determine seawater pHT, contributing to a better understanding of marine water quality and supporting informed management of ocean resources.