Calibration-free Spectrophotometric Measurements of Carbonate Saturation States in Seawater

Author

Erin E. Cuyler, University of South FloridaFollow

Graduation Year

2018

Document Type

Thesis

Degree

M.S.

Degree Name

Master of Science (M.S.)

Degree Granting Department

Marine Science

Major Professor

Robert H. Byrne, Ph.D.

Committee Member

Lisa Robbins, Ph.D.

Committee Member

Pamela Hallock Muller, Ph.D.

Keywords

Carbon Dioxide, Ocean Acidification, pH, Coastal Monitoring, Spectrophotometry, Marine Chemistry

Abstract

This work describes efforts to improve methodologies and instrumentation for investigation of the marine CO₂ system. In the first section of my thesis, a method was developed that provides simple, calibration-free measurements of seawater carbonate saturation states (Ω_spec) based solely on the use of a laboratory spectrophotometer. Measurements of pH are made in paired optical cells, one with and one without added nitric acid. The amount of added nitric acid is determined through the direct proportionality between nitrate concentration and UV absorbance. After an initial calibration, the method is calibration-free and requires no volumetric or gravimetric analyses thereafter. Saturation state measurements can be obtained in twelve minutes and attain Global Ocean Acidification Observing Network accuracy goals over a wide range of conditions. This simple one-step measurement protocol is ideal for monitoring ocean acidification conditions in coastal areas, fish hatcheries and shellfish farms.

The second section of my thesis outlines the development of a next-generation handheld photometer for field measurements of seawater pH. The hand-held instrument is simple to use, inexpensive, and has a pH measurement accuracy of ±0.01. Each photometer is calibrated by relating pH-indicator absorbance ratios (R_B) obtained with the broadband photometer to indicator absorbance ratios (R_N) obtained with a high quality, narrowband scanning spectrophotometer. The R_B vs R_N relationship for each photometer is entered into the instrument’s software, providing discrete, real-time pH observations from measurements of R_B, salinity and temperature. Measurement accuracy is assessed through comparisons of photometer pH with pH observations obtained using standard measurement procedures and high-quality spectrophotometers. The targeted user-groups for these instruments include middle and high school students, citizen scientists, and resource managers

Scholar Commons Citation

Cuyler, Erin E., "Calibration-free Spectrophotometric Measurements of Carbonate Saturation States in Seawater" (2017). USF Tampa Graduate Theses and Dissertations.
https://digitalcommons.usf.edu/etd/7011