Simultaneous Spectrophotometric Flow-through Measurements of PH, Carbon Dioxide Fugacity, and Total Inorganic Carbon in Seawater

Authors

Zhaohui Aleck Wang, University of South Florida
Xuewu Liu, University of South FloridaFollow
Robert H. Byrne, University of South FloridaFollow
Rik Wanninkhof, NOAA Atlantic Oceanographic and Meteorological Laboratory, Ocean Chemistry Division
Renate E. Bernstein, University of South Florida
Eric A. Kaltenbacher, University of South Florida
James Patten, University of South Florida

Document Type

Article

Publication Date

2007

Keywords

Autonomous carbon measurements, Spectrophotometric, Carbon dioxide, pH, fugacity, Dissolved inorganic carbon

Digital Object Identifier (DOI)

https://doi.org/10.1016/j.aca.2007.05.048

Abstract

An autonomous multi-parameter flow-through CO₂ system has been developed to simultaneously measure surface seawater pH, carbon dioxide fugacity (fCO₂), and total dissolved inorganic carbon (DIC). All three measurements are based on spectrophotometric determinations of solution pH at multiple wavelengths using sulfonephthalein indicators. The pH optical cell is machined from a PEEK polymer rod bearing a bore-hole with an optical pathlength of ∼15 cm. The fCO₂ optical cell consists of Teflon AF 2400 (DuPont) capillary tubing sealed within the bore-hole of a PEEK rod. This Teflon AF tubing is filled with a standard indicator solution with a fixed total alkalinity, and forms a liquid core waveguide (LCW). The LCW functions as both a long pathlength (∼15 cm) optical cell and a membrane that equilibrates the internal standard solution with external seawater. fCO₂ is then determined by measuring the pH of the internal solution. DIC is measured by determining the pH of standard internal solutions in equilibrium with seawater that has been acidified to convert all forms of DIC to CO₂. The system runs repetitive measurement cycles with a sampling frequency of ∼7 samples (21 measurements) per hour. The system was used for underway measurements of sea surface pH, fCO₂, and DIC during the CLIVAR/CO₂ A16S cruise in the South Atlantic Ocean in 2005. The field precisions were evaluated to be 0.0008 units for pH, 0.9 μatm for fCO₂, and 2.4 μmol kg⁻¹ for DIC. These field precisions are close to those obtained in the laboratory. Direct comparison of our measurements and measurements obtained using established standard methods revealed that the system achieved field agreements of 0.0012 ± 0.0042 units for pH, 1.0 ± 2.5 μatm for fCO₂, and 2.2 ± 6.0 μmol kg⁻¹ for DIC. This system integrates spectrophotometric measurements of multiple CO₂ parameters into a single package suitable for observations of both seawater and freshwater.

Was this content written or created while at USF?

Yes

Citation / Publisher Attribution

Analytica Chimica Acta, v. 596, issue 1, p. 23-36

Scholar Commons Citation

Wang, Zhaohui Aleck; Liu, Xuewu; Byrne, Robert H.; Wanninkhof, Rik; Bernstein, Renate E.; Kaltenbacher, Eric A.; and Patten, James, "Simultaneous Spectrophotometric Flow-through Measurements of PH, Carbon Dioxide Fugacity, and Total Inorganic Carbon in Seawater" (2007). Marine Science Faculty Publications. 1752.
https://digitalcommons.usf.edu/msc_facpub/1752