Sensor for direct measurement of carbonate ions in seawater

Abstract

A system and associated methodology to directly measure the concentration of carbonate ions in seawater by ultraviolet absorbance spectroscopy. Metal ions are added to seawater and the absorbance spectra of the added ions are measured in the ultraviolet. The spectral absorbance (light attenuation) of ions such as divalent lead or copper in seawater is predominantly determined by the carbonate ion content of seawater. Through a knowledge of (1) the strength of association between carbonate and either divalent lead or divalent copper and (2) the spectral characteristics of these cations in seawater (e.g., Pb2+ complexed solely as PbCO3 and Pb2+ complexed solely in the form of chloride complexes), it is possible to directly determine seawater carbonate ion concentrations from absorbance measurements at a variety of wavelengths in the ultraviolet. Using such procedures, carbonate ion concentrations can be directly measured, rather than calculated from other CO2 system parameters such as pH and total dissolved inorganic carbon. Measurements of carbonate ion concentrations, and measurements of solution pH that also involve spectrophotometric procedures, will allow both types of measurements in a single, small, robust instrument. Furthermore, via simple thermodynamic calculations, conjugate measurements of carbonate and pH can then be used to calculate all carbon system parameters of interest, including the total concentration of dissolved inorganic carbon in seawater.

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