SPECTROPHOMETRIC MEASUREMENTS OF PH IN-SITU

摘要

Automated in-situ instrumentation has been developed for sensitive, precise and accurate measurements of a variety of analytes in natural waters In this work we describe the use of 'SEAS' (Spectrophotometry 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 custom-made PEEK tubing Long optical pathlengths allow use of indicators at low concentrations, thereby precluding 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 the SEAS-pH instrument is 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 better than 0 002 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 unique capabilities for observations of carbon-system transformations in the natural environment.