title: Spectrophotometric measurements of pH in-situ: laboratory and field evaluations of instrumental performance.
abstract: Automated in-situ instrumentation has been developed for precise and accurate measurements of a variety of analytes in natural waters. In this work we describe the use of 'SEAS' (spectrophotometric elemental analysis system) instrumentation for measurements of solution pH. SEAS-pH incorporates a CCD-based spectrophotometer, an incandescent light source, and dual pumps for mixing natural water samples with a sulfonephthalein indicator. The SEAS-pH optical cell consists of either a liquid core waveguide (LCW, Teflon AF-2400) or a custom-made PEEK cell. Long optical path lengths allow use of indicators at low concentrations, thereby precluding large indicator-induced pH perturbations. Laboratory experiments show that pH measurements obtained using LCW and PEEK optical cells are indistinguishable from measurements obtained using conventional spectrophotometric cells and high-performance spectrophotometers. Deployments in the Equatorial Pacific and the Gulf of Mexico demonstrate that SEAS-pH instruments are capable of obtaining vertical pH profiles with high spatial resolution. SEAS-pH deployments at a fixed river-site (Hillsborough River, FL) demonstrate the capability of SEAS for observations of diel pH cycles with high temporal resolution. The in-situ precision of SEAS-pH is assessed as 0.0014 pH units, and the system's measurement frequency is approximately 0.5 Hz. This work indicates that in-situ instrumentation can be used to provide accurate, precise, and highly resolved observations of carbon-system transformations in the natural environment.
authors.affiliation: College of Marine Science, University of South Florida St Petersburg, 33701-5016, USA.
authors.forename: Xuewu
authors.initials: X
authors.lastname: Liu
authors.forename: Zhaohui Aleck
authors.initials: ZA
authors.lastname: Wang
authors.forename: Robert H
authors.initials: RH
authors.lastname: Byrne
authors.forename: Eric A
authors.initials: EA
authors.lastname: Kaltenbacher
authors.forename: Renate E
authors.initials: RE
authors.lastname: Bernstein
mesh_terms: D001331:Automation; D002244:Carbon / Q000032:analysis*; D004784:Environmental Monitoring / Q000379:methods*; D004867:Equipment Design; D005618:Fresh Water / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D008027:Light; D008956:Models, Chemical; D008962:Models, Theoretical; D009792:Oceans and Seas; D045483:Rivers; D013053:Spectrophotometry / Q000295:instrumentation* / Q000379:methods*; D013696:Temperature
chemical_list: D002244:Carbon