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

Zhaohui Aleck Wang, Xuewu Liu, Robert H. Byrne, Rik Wanninkhof, Renate E. Bernstein, Eric A. Kaltenbacher, James Patten

Research output: Contribution to journalArticlepeer-review

Abstract

<p> <p id="x-x-simple-para.0065"> An autonomous multi-parameter flow-through CO <sub> 2 </sub> system has been developed to simultaneously measure surface <a title="Learn more about Seawater from ScienceDirect's AI-generated Topic Pages"> seawater </a> pH, carbon dioxide fugacity ( <em> f </em> CO <sub> 2 </sub> ), and total <a title="Learn more about Dissolved Inorganic Carbon from ScienceDirect's AI-generated Topic Pages"> dissolved inorganic carbon </a> (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 &sim;15 cm. The <em> f </em> CO <sub> 2 </sub> optical cell consists of <a title="Learn more about Poly(tetrafluoroethylene) from ScienceDirect's AI-generated Topic Pages"> Teflon </a> 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 <a title="Learn more about Waveguide from ScienceDirect's AI-generated Topic Pages"> waveguide </a> (LCW). The LCW functions as both a long pathlength (&sim;15 cm) optical cell and a membrane that equilibrates the internal standard solution with external seawater. <em> f </em> CO <sub> 2 </sub> 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 CO <sub> 2 </sub> . The system runs repetitive measurement cycles with a sampling frequency of &sim;7 samples (21 measurements) per hour. The system was used for underway measurements of sea surface pH, <em> f </em> CO <sub> 2 </sub> , and DIC during the CLIVAR/CO <sub> 2 </sub> A16S cruise in the South Atlantic Ocean in 2005. The field precisions were evaluated to be 0.0008 units for pH, 0.9 &mu;atm for <em> f </em> CO <sub> 2 </sub> , and 2.4 &mu;mol kg <sup> &minus;1 </sup> 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 &plusmn; 0.0042 units for pH, 1.0 &plusmn; 2.5 &mu;atm for <em> f </em> CO <sub> 2 </sub> , and 2.2 &plusmn; 6.0 &mu;mol kg <sup> &minus;1 </sup> for DIC. This system integrates spectrophotometric measurements of multiple CO <sub> 2 </sub> parameters into a single package suitable for observations of both seawater and freshwater. </p></p>
Original languageAmerican English
JournalAnalytica Chimica Acta
Volume596
DOIs
StatePublished - Jan 1 2007

Keywords

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

Disciplines

  • Life Sciences

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