Spectrophotometric Measurements of PH In-Situ: Laboratory and Field Evaluations of Instrumental Performance
Digital Object Identifier (DOI)
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.
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Citation / Publisher Attribution
Environmental Science & Technology, v. 40, issue 16, p. 5036-5044
Scholar Commons Citation
Liu, Xuewu; Wang, Zhaohui Aleck; Byrne, Robert H.; Kaltenbacher, Eric A.; and Bernstein, Renate E., "Spectrophotometric Measurements of PH In-Situ: Laboratory and Field Evaluations of Instrumental Performance" (2006). Marine Science Faculty Publications. 1636.