Spectrophotometric high-precision seawater pH determination for use in underway measuring systems.

Aßmann, S., Frank, C. and Körtzinger, Arne (2011) Spectrophotometric high-precision seawater pH determination for use in underway measuring systems. Open Access Ocean Science, 7 (5). pp. 597-607. DOI 10.5194/os-7-597-2011.

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Abstract

Autonomous sensors are required for a comprehensive documentation of the changes in the marine carbon system and thus to differentiate between its natural variability and anthropogenic impacts. Spectrophotometric determination of pH – a key variable of the seawater carbon system – is particularly suited to achieve precise and drift-free measurements. However, available spectrophotometric instruments are not suitable for integration into automated measurement systems (e.g. FerryBox) since they do not meet the major requirements of reliability, stability, robustness and moderate cost. Here we report on the development and testing of a~new indicator-based pH sensor that meets all of these requirements. This sensor can withstand the rough conditions during long-term deployments on ships of opportunity and is applicable to the open ocean as well as to coastal waters with a complex matrix and highly variable conditions. The sensor uses a high resolution CCD spectrometer as detector connected via optical fibers to a custom-made cuvette designed to reduce the impact of air bubbles. The sample temperature can be precisely adjusted (25 °C ± 0.006 °C) using computer-controlled power supplies and Peltier elements thus avoiding the widely used water bath. The overall setup achieves a measurement frequency of 1 min−1 with a precision of ±0.0007 pH units, an average offset of +0.0005 pH units to a reference system, and an offset of +0.0081 pH units to a certified standard buffer. Application of this sensor allows monitoring of seawater pH in autonomous underway systems, providing a key variable for characterization and understanding of the marine carbon system.

Document Type: Article
Keywords: Marine chemistry; marine carbon system; indicator-based pH sensor; high resolution CCD spectrometer; Spectrophotometric high-precision seawater pH determination
Research affiliation: OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-CH Chemical Oceanography
OceanRep > The Future Ocean - Cluster of Excellence > FO-R10
OceanRep > The Future Ocean - Cluster of Excellence
Refereed: Yes
Open Access Journal?: Yes
DOI etc.: 10.5194/os-7-597-2011
ISSN: 1812-0784
Projects: Future Ocean
Date Deposited: 01 Nov 2011 08:45
Last Modified: 10 Apr 2017 11:11
URI: http://oceanrep.geomar.de/id/eprint/12422

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