Sensors for CO2 Partial Pressure (pCO2), Total Alkalinity (TA) and pH – Recent Developments and Field data

Chair: Libby Jewett
Peer Fietzek (1,2), Carsten Frank (1), Steffen Aßmann (1)

1 Kongsberg Maritime Contros GmbH, Kiel, 24148, Germany

2 GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, 24105, Germany

Background
The motivation for CO 2 determination in water is manifold and extends from scientific applications (i.e. ocean acidification studies) to industrial usage (i.e. CCS). Due to the chemical properties of CO2, i.e. formation and dissociation of carbonic acid, a complete determination of the marine carbonate system is required for many studies. Therefore, the measurement of TA and pH experiences great attention, both as individual measurement quantities as well as within CO2 system determination exercises.

Methods
A proven method to determine CO 2 concentrations in gas is by means of infrared absorption spectrometry, i.e. NDIR detectors. Semi-permeable membranes enable sensor designs, in which a gas headspace is equilibrated and allows for usage of NDIR detectors in e.g. CO2 underwater sensors.
TA is determined by acidic titration of seawater samples with subsequent CO 2 stripping and pH determination. Recent progress in the field of automated fluid analysis now enables realization of autonomous analysers that unattendedly carry out this task at high precision. Likewise dedicated sensor designs can be realized that directly determine the pH of discrete water samples or in a semi-continuous fashion at high quality.

Findings
CONTROS HydroC ® CO2 sensors have been used within multiple applications and on various platforms over the last years. Recent application include coastal deployments that enabled analysis of carbonate system dynamics.
The CONTROS HydroFIA ® TA is a new commercially available analyser for total alkalinity. First field measurements highlight its potential further enhanced through ongoing developments within high-class international research projects (i.e. AtlantOS, TAACT). Similar projects target the development and assessment of state of the art analysers for pH (i.e. BONUS PINBAL).

Conclusions
Numerous applications within aquatic sciences benefit from the autonomous determination of carbonate system parameters pCO2, TA and pH. This contribution presents the development status of and results obtained with state of the art sensor technology.