Analytica Chimica Acta 596 (2007) 23–36 Simultaneous spectrophotometric flow-through measurements of pH, carbon dioxide fugacity, and total inorganic carbon in seawater Zhaohui Aleck Wang a , Xuewu Liu a , Robert H. Byrne a,∗ , Rik Wanninkhof b , Renate E. Bernstein a , Eric A. Kaltenbacher a , James Patten a a College of Marine Science, University of South Florida, 140 7th Ave. South, Saint Petersburg, FL 33701, United States b NOAA Atlantic Oceanographic and Meteorological Laboratory, Ocean Chemistry Division, 4301 Rickenbacker Causeway, Miami, FL 33149, United States Received 14 February 2007; received in revised form 1 May 2007; accepted 21 May 2007 Available online 31 May 2007 Abstract An autonomous multi-parameter flow-through CO 2 system has been developed to simultaneously measure surface seawater pH, carbon dioxide fugacity (fCO 2 ), 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 fCO 2 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. fCO 2 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 2 . 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, fCO 2 , and DIC during the CLIVAR/CO 2 A16S cruise in the South Atlantic Ocean in 2005. The field precisions were evaluated to be 0.0008 units for pH, 0.9 atm for fCO 2 , and 2.4 mol 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 atm for fCO 2 , and 2.2 ± 6.0 mol kg -1 for DIC. This system integrates spectrophotometric measurements of multiple CO 2 parameters into a single package suitable for observations of both seawater and freshwater. © 2007 Elsevier B.V. All rights reserved. Keywords: Autonomous carbon measurements; Spectrophotometric; Carbon dioxide; pH; fugacity; Dissolved inorganic carbon 1. Introduction Measurements of four seawater inorganic carbon system parameters – pH, carbon dioxide fugacity (fCO 2 ) or partial pres- sure of CO 2 (pCO 2 ), total dissolved inorganic carbon (DIC), and total alkalinity (TA) – are essential for carbon cycle inves- tigations on both global and local scales. Both observational and modeling efforts rely on high quality inorganic carbon data from field measurements. Extensive efforts have been devoted ∗ Corresponding author. Tel.: +1 727 553 1508; fax: +1 727 553 1189. E-mail address: byrne@marine.usf.edu (R.H. Byrne). to improving methodologies and instruments for determination of carbon parameters in seawater. In standard conventional methodologies [1], the four core parameters of the seawater inorganic carbon system are measured using diverse instrumentation (e.g. potentiome- try, spectrophotometry, gas chromatography, non-dispersive infrared analysis, and coulometry). Recent advances in tech- nology and materials have prompted many researchers to adapt these diverse methodologies for use in unattended in-situ devices and autonomous underway systems without compro- mising precision and accuracy relative to standard methods (e.g. DeGrandpre et al. [2] for in-situ pCO 2 ; Feely et al. [3] for under- way pCO 2 ; Tapp et al. [4] for underway pH; Martz et al. [5] for 0003-2670/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2007.05.048