Spectrophotometric Measurements of pH in-Situ:  Laboratory and Field Evaluations of Instrumental Performance

• Published in: Environmental science & technology Vol. 40; no. 16; pp. 5036 - 5044
• Main Authors: Liu, Xuewu, Wang, Zhaohui Aleck, Byrne, Robert H, Kaltenbacher, Eric A, Bernstein, Renate E
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 15.08.2006
• Subjects: ASW, Mexico Gulf; Automation; Carbon; Carbon - analysis; Earth sciences; Earth, ocean, space; Environmental Monitoring - methods; Environmental science; Equipment Design; Exact sciences and technology; External geophysics; Fresh Water - chemistry; Geochemistry; Hydrogen-Ion Concentration; Hydrology; Hydrology. Hydrogeology; IS, Equatorial Pacific; Laboratories; Light; light sources; Measuring instruments; Mineralogy; Models, Chemical; Models, Theoretical; Oceans and Seas; Physical and chemical properties of sea water; Physics of the oceans; Q1; Q3; Rivers; Silicates; spectrophotometers; Spectrophotometry - instrumentation; Spectrophotometry - methods; Temperature; Water; Water geochemistry; Water sampling; United States; Equatorial Pacific; North American Atlantic; North Atlantic; Gulf of Mexico; Florida; Northwest Atlantic; sea water; experimental studies; rivers; hydrochemistry; fresh-water environment; marine environment; Atlantic Ocean; pH; Pacific Ocean; North America; fresh water; laboratory studies
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/es0601843

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.