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University of Southampton Research Repository Eprints Soton University of Southampton Research Repository ePrints Soton Copyright © and Moral Rights for this thesis are retained by the author and/or other copyright owners. A copy can be downloaded for personal non-commercial research or study, without prior permission or charge. This thesis cannot be reproduced or quoted extensively from without first obtaining permission in writing from the copyright holder/s. The content must not be changed in any way or sold commercially in any format or medium without the formal permission of the copyright holders. When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given e.g. AUTHOR (year of submission) "Full thesis title", University of Southampton, name of the University School or Department, PhD Thesis, pagination http://eprints.soton.ac.uk UNIVERSITY OF SOUTHAMPTON An Automated Instrument for Measurement of Total Alkalinity in Seawater David Robert Owsianka Thesis for the degree of Doctor of Philosophy FACULTY OF NATURAL AND ENVIRONMENTAL SCIENCES Ocean and Earth Sciences 24 April 2014 UNIVERSITY OF SOUTHAMPTON ABSTRACT FACULTY OF NATURAL AND ENVIRONMENTAL SCIENCES Ocean and Earth Sciences Doctor of Philosophy An Automated Instrument for Measurement of Total Alkalinity in Seawater by David Robert Owsianka Understanding the effects of increased CO2 uptake on the marine environment is a high priority for scientific study, as this leads to acidification. Precise means of measuring the degree of acidification, and doing so regularly over long time periods is a key requirement in separating natural from man‐made variation. This project examines new technologies for development of an instrument to measure one of the four main carbonate system variables, total alkalinity (TA), which is determined by acid/base titration. A red green and blue photodiode (RGB‐PD) is examined as a novel photodetector for spectrophotometric measurements. This offers ≈ 400 times reduction in size (footprint) and cost compared to a conventional charge coupled device (CCD) spectrophotometer. Using bromocresol green (BCG) indicator, spectrophotometric pH measurements with the RGB‐PD give a precision of <0.007 pH, and agree to within ≈0.01 pH units between pH 3.0 and 5.0 with measurements made using a conventional spectrophotometer. pH measurements are made by performing simultaneous photometry on two absorption bands in the BCG visible spectrum. The RGB‐PD is also examined for TA determination. A fully automated prototype instrument utilising microfluidic technology achieved a precision of between ±8 ‐ 19 μmol kg‐1. The precision is close to that reported for in situ prototypes (±4 μmol kg‐1, Sami‐alk) and the required precision for ocean acidification measurements (±1 μmol kg‐1). This represents the first demonstration of TA titration using microfluidic technology, and the first use of an RGB‐ PD for high precision multi‐wavelength spectrophotometry for chemical analysis. These are significant steps towards development of small, cheap, and rugged automated instruments for TA measurement. These contributions advance the realisation of extensive, long‐term measurements in challenging environments. iii Contents Contents .................................................................................................................................................... v List of Figures ....................................................................................................................................... xi List of Tables ....................................................................................................................................... xiii Authors Declaration .......................................................................................................................... xv Acknowledgements ........................................................................................................................... 17 Chapter 1 ............................................................................................................................................... 18 Introduction ......................................................................................................................................... 18 1.1 “The Other CO2 Problem” ............................................................................................................. 18 1.2 Definitions of uncertainty, and handling of errors ............................................................ 19 1.2.1 Uncertainty .............................................................................................................................. 19 1.2.2 Error............................................................................................................................................ 20 1.2.3 Accuracy and Precision ....................................................................................................... 20 1.2.4 Discrepancy ............................................................................................................................. 21 1.3 Carbonate Chemistry ..................................................................................................................... 21 1.3.1 Chemical Equilibrium– The Seawater Carbonate System .................................... 21 1.3.2 The Big 4: Carbonate System Parameters ................................................................... 23 1.4 Alkalinity Theory ............................................................................................................................. 26 1.4.1 An Expression for Total Alkalinity ................................................................................. 26 1.4.2 pH Scales – NBS, Free Hydrogen, Total Hydrogen, Seawater .............................. 28 1.4.3 Acid/Base Titration ‐ The Equivalence Point and proton condition ................ 29 1.5 Total Alkalinity Determination .................................................................................................. 30 v 1.5.2 Non‐linear least squares ..................................................................................................... 33 1.5.3 Open vs. Closed Cell Titration .......................................................................................... 34 1.6 Aims and Objectives ....................................................................................................................... 35 1.6.1 Chapter 2 ................................................................................................................................... 36 1.6.2 Chapter 3 ................................................................................................................................... 37 1.6.3 Chapter 4 ................................................................................................................................... 37 Chapter 2 ............................................................................................................................................... 39 Literature Review .............................................................................................................................. 39 2.1 Total Alkalinity Measurements ................................................................................................. 39 2.2 Components of a Titration System ........................................................................................... 41 2.2.1 The Titration Vessel ............................................................................................................. 41 2.2.2 Sample Quantification and Titrant Additions ............................................................ 42 2.2.3 pH Monitoring ......................................................................................................................... 43 2.2.4 Titrant Composition ............................................................................................................. 45 2.2.5 Evaluation of Titration Data .............................................................................................. 45 2.3 Performance of Existing Systems ............................................................................................. 46 2.3.1 Single step additions – the excess acid method ........................................................ 52 2.3.2 Conclusions about existing systems .............................................................................. 54 2.4 Spectrophotometric pH Determination for Total Alkalinity Titrations ................... 55 2.4.1 Introduction ............................................................................................................................. 55 2.4.2 Method ....................................................................................................................................... 55 2.4.3 Tracer monitored titrations .............................................................................................. 59 2.4.4 Conclusions of the Spectrophotometric Method ...................................................... 60 2.5 Conclusions Drawn from Reviewed Literature .................................................................. 60 Chapter 3 ............................................................................................................................................... 63 A novel optical system for spectrophotometric pH and titration measurements .....
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