An Automated Instrument for Measurement of Total Alkalinity in Seawater

Abstract

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 ?mol kg?1. The precision is close to that reported for in situ prototypes (±4 ?mol kg?1, Sami?alk) and the required precision for ocean acidification measurements (±1 ?mol kg?1). This represents the first demonstration of TA titration using microfluidic technology, and the first use of an RGBPD 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.

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ORCID for M. Mowlem: orcid.org/0000-0001-7613-6121

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