Spectrophotometric Determination of Carbonate Ion Concentrations: Elimination of Instrument-Dependent Offsets and Calculation of In Situ Saturation States

Abstract

This work describes an improved algorithm for spectrophotometric determinations of seawater carbonate ion concentrations ([CO]) derived from observations of ultraviolet absorbance spectra in lead-enriched seawater. Quality-control assessments of [CO] data obtained on two NOAA research cruises (2012 and 2016) revealed a substantial intercruise difference in average Δ[CO] (the difference between a sample's [CO] value and the corresponding [CO] value calculated from paired measurements of pH and dissolved inorganic carbon). Follow-up investigation determined that this discordance was due to the use of two different spectrophotometers, even though both had been properly calibrated. Here we present an essential methodological refinement to correct [CO] absorbance data for small but significant instrumental differences. After applying the correction (which, notably, is not necessary for pH determinations from sulfonephthalein dye absorbances) to the shipboard absorbance data, we fit the combined-cruise data set to produce empirically updated parameters for use in processing future (and historical) [CO] absorbance measurements. With the new procedure, the average Δ[CO] offset between the two aforementioned cruises was reduced from 3.7 μmol kg to 0.7 μmol kg, which is well within the standard deviation of the measurements (1.9 μmol kg). We also introduce an empirical model to calculate in situ carbonate ion concentrations from [CO]. We demonstrate that these in situ values can be used to determine calcium carbonate saturation states that are in good agreement with those determined by more laborious and expensive conventional methods.

Observing Changes in Ocean Carbonate Chemistry: Our Autonomous Future

“…Recently, a method for accurate [CO 3 2− ] measurement based on spectrophotometry has been developed [124][125][126].…”

Spectrophotometric Measurement of Carbonate Ion in Seawater over a Decade: Dealing with Inconsistencies

“…as reported in all the works of the methodology, 24,25,[28][29][30]44 regardless of the Pb(II) reagent, the spectrophotometer, or the approach used (Figure 3). The approaches by BY08 and EAS13 yield Δ[CO 3 2− ] that mostly scatter around zero (CAIBOX, MOC2, and OVIDE) except for Mediterranean Sea cruises (i.e., high-salinity waters; HOTMIX, MSM72, MEDWAVES, and TALPRO) and for the iFADO cruise, where Δ[CO 3…”

“…The Δλ 241.1 term is supposed to remain constant as long as the equipment is not recalibrated. 30 However, 15 Δλ 241.1 measurements were averaged to obtain a stable value (section 2.1). Our experience quantifying Δλ 241.1 indicates that (i) according to SHA17, shifts in Δλ 241.1 on the order of 0.05 nm are supposed to be significant, which would imply that the holmium standard should be measured each time that the spectrophotometer is used, not only when recalibrated, because differences of this order were found for measurements of the same holmium oxide standard in different measurement exercises; (ii) the average Δλ 241.1 found for SHI2600 (0.20 nm ± 0.06 nm) is lower than the wavelength accuracy of the spectrophotometer (± 0.3 nm; Table S3), and (iii) the average Δλ 241.1 equals the uncertainty of the certified holmium peak position (241.15 nm ± 0.20 nm).…”

Fabrication of an All-Solid-State Carbonate Ion-Selective Electrode with Carbon Film as Transducer and Its Preliminary Application in Deep-Sea Hydrothermal Field Exploration

“…infrared spectrophotometry and coulometric titration [11][12][13][14][15][16][17]. These analytical methods require sample pre-treatment, which is a complex, time-consuming and insensitive process, cannot meet the demands of long-term field monitoring.…”

“…Therefore, accurate knowledge of carbonate levels in seawater is necessary to maintain the balance of marine ecosystems, while the implementation of realtime monitoring of carbonate ion concentrations in hydrothermal fluids is a major advance in the detection of hydrothermal fields [2,3,[8][9][10]. Most of the commonly used methods for determining carbonate concentrations rely on laboratory analytical instruments such as gravimetric methods, colour change, equilibrium pressure, gas chromatography, infrared spectrophotometry and coulometric titration [11][12][13][14][15][16][17]. These analytical methods require sample pre-treatment, which is a complex, time-consuming and insensitive process, cannot 2 of 13 meet the demands of long-term field monitoring.…”