Seawater pH

Seawater pH sensors measure the total concentration of hydrogen ions in a solution. The OOI pH sensor (Sunburst SAMI-pH) measures pH (total hydrogen scale) in the marine pH range of 7-9. For tens of millions of years, Earth’s oceans have maintained a relatively stable pH level. However, since the beginning of the industrial revolution in the 1800s, the surface ocean pH is estimated to have decreased from approximately 8.25 to 8.14. While this may seem to be a very small change in pH, it represents about a 30% increase in H+ ion concentration, and is already impacting marine organisms and the shellfish industry.

(text courtesy of Interactive Oceans)

Series update

Starting in 2025, certain PHSEN-A/D instruments (Sunburst SAMI-pH) have been replaced by PHSEN-G/H series (Sea-Bird Deep SeapHOx) instruments. The Deep SeapHOx V2 sensor uses ion-sensitive field effect transistor (ISFET) technology to measure pH in marine environments at depths up to 2000 meters. The primary sensor element of the Deep SeapHOx V2 is a special deep-packaged ISFET, a solid-state sensor that senses pH in marine environments. The external reference electrode has a Ag/AgCl reference electrode in direct contact with seawater. The potential of this electrode varies with pH and chloride concentration, so unless the chloride concentration is known, the external reference is not stable. To correct this, salinity is used as an approximation of chloride concentration. The salinity data that is calculated by the attached CTD instrument (SBE-37-SMP-ODO) is applied to the pH external data and significantly reduces measurement errors, and gives the most accurate and stable pH data.

Data Products

This instrument measures the following data products. Select a data product's name to learn more.

The algorithm code used to generate data products for this instrument is also available in the ion-functions GitHub repository.

Instrument Models & Deployed Locations

The OOI includes the following instrument makes and models for this instrument type. Follow the links below to find out where in the OOI this instrument has been deployed. You'll also find quick links for each instrument to Data Explorer, where you can plot and access data.