Seawater pH measured with a dye, not an electrode — a new edition of the standard
ISO 18191:2026 supersedes the 2015 edition. The spectrophotometric method with m-cresol purple covers pH on the total hydrogen ion scale from 7.4 to 8.2 at practical salinity from 20 to 40.
- The second edition appeared in July 2026; the measurand is pH on the total hydrogen ion concentration scale, expressed in moles per kilogram of seawater.
- The measurement consists of adding an indicator dye and reading absorbance — the precision of this route is markedly better than potentiometry, which struggles to hold its reference in water of high ionic strength.
- The scope is deliberately narrow: typical oceanic waters, excluding brackish waters of low salinity and brines.
- The standard serves monitoring of ocean acidification and changes in the carbonate system — it requires a spectrophotometer with stable temperature control and a pure dye of known characteristics.