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Ocean Science An interactive open-access journal of the European Geosciences Union
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Discussion papers
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 09 Jan 2020

Submitted as: research article | 09 Jan 2020

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This discussion paper is a preprint. It is a manuscript under review for the journal Ocean Science (OS).

Impact of impurities in bromocresol green indicator dye on spectrophotometric total alkalinity measurements

Katharina Seelmann1, Martha Gledhill1, Steffen Aßmann2, and Arne Körtzinger1,3 Katharina Seelmann et al.
  • 1GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
  • 2Kongsberg Maritime Contros GmbH, Kiel, Germany
  • 3Christian-Albrechts-Universität zu Kiel, Kiel, Germany

Abstract. Due to its accurate and precise character, the spectrophotometric pH detection is a common technique applied in measurement methods for carbonate system parameters. However, impurities in the used pH indicator dyes can influence the measurements quality. The work described here focuses on influences from impurities in the pH indicator dye bromocresol green (BCG) on spectrophotometric seawater total alkalinity (AT) measurements. First, a high-performance liquid chromatography (HPLC) purification method for BCG was developed. A subsequent analysis of BCG dye from four different vendors with this method revealed different types and quantities of impurities. After successful purification, AT measurements with purified and unpurified BCG were carried out using the novel autonomous analyzer CONTROS HydroFIA® TA. Long-term measurements in the laboratory revealed a direct influence of impurity types and quantities on the drift behavior of the analyzer. The purer the BCG, the smaller was the drift increment per measurement. Furthermore, we could show that a certain impurity in some indicator dyes changed the drift pattern from linear to non-linear, which can impair the AT measurements during a long-term deployment of the system. Laboratory performance characterization experiments revealed no improvement of the measurement quality (precision and accuracy) by using purified BCG as long as the impurities of the unpurified dye do not exceed a quantity of 2 % (relationship of peak areas in the chromatogram). However, BCG with impurity quantities higher than 6 % provided AT values, which failed fundamental quality requirements. Concluding, to gain optimal AT measurements, an indicator purification is not necessarily required as long as the purchased dye has a purity level of at least 98 %.

Katharina Seelmann et al.
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Katharina Seelmann et al.
Katharina Seelmann et al.
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Publications Copernicus
Short summary
We investigated the impact of indicator dye impurities on spectrophotometric seawater measurements, especially how they may influence the drift behavior and the measurement quality of an autonomous analyzer using this indicator. These measurements are important for ocean carbon observations. This work revealed that impurities up to a quantity of 2 % do not influence the measurement drift and quality. Higher impurity levels lead to a changing drift behavior and quality deteriorations.
We investigated the impact of indicator dye impurities on spectrophotometric seawater...