Spectrophotometric determinations of ∆pH/∆T
Description
This data supports the findings in the manuscript titled "Predicting pH at in-situ temperature for aquatic environments". Experiments were performed to analyze how the pH of seawater changes with changing temperature conditions (∆pH/∆T) across a range of salinities. Data was collected by measuring pH spectrophotometrically with simultaneous temperature measurements via a mini surface temperature probe. The total alkalinity (AT) of the seawater sample and salinity were measured for each sample to fully define the marine CO2 system. ∆pH/∆T was determined for a pH (at 25°C) range of 7.2–8.2, a temperature range of 15–40°C and salinities 10.1, 20.2, 30.2, and 36.1.
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Methods are further described in the associated manuscript Bartoloni et al. (In prep). Materials: Seawater was collected offshore on the Southwest Florida Shelf, in the Gulf of Mexico. Seawater pH was adjusted using aliquots of 1 N HCl (Fisher Chemical, CAS 7647-01-0) or 1 N NaOH (Fisher Chemical, CAS 1310-73-2). Meta-cresol purple (mCP) (10mM in 0.7 M NaCl), purified at the University of South Florida according to Liu et al. (2011), was used in all spectrophotometric pH measurements. Equipment: All experimental pH measurements were conducted using an Agilent Cary 60 UV-Vis spectrophotometer and 10 cm optical glass spectrophotometric cells. Salinity was measured with a Guildline Portasal salinometer (Model 8410). AT was measured using a USB4000 Fiber Optic Spectrometer (Ocean Optics) with the indicator dye bromocresol purple (BCP) and methods from Liu et al. (2015). AT measurements were calibrated daily with certified reference materials (CRM; batch numbers 187 & 201) prepared at the Scripps Institution of Oceanography of the University of California, San Diego. A circulating water bath was used to control the temperature of the water-jacketed dder of the spectrophotometric cell (Thermo Scientific NESLAB RTE-7). Additions of mCP were made using a 2 mL Gilmont micrometer buret (GS-1200). A hole was drilled through the center of a Teflon cap to fit a mini surface temperature sensor (Vernier STS-BTA). Method: pH was measured spectrophotometrically following methods developed by Clayton & Byrne (1993). One deviation from these methods was required; 700 nm was used as the non-absorbing wavelength of mCP instead of 730 nm. Absorbance measurements (collected at 434 and 578 nm) were first obtained at approximately 25°C. Purified mCP (10 µL) was then added to the cell containing seawater, and absorbances were recorded. The temperature of the system was then decreased to ~15°C and subsequently raised to ~40°C. Temperature was recorded once every second and the absorbance was obtained every 30 seconds. Small baseline absorbance corrections were obtained using the non-absorbing wavelength. Seawater pH (total scale) was calculated using the ratio of absorbances at 578 and 434 nm and the mCP parametrization of Müller & Rehder (2018). The change in pH with changing temperature was determined spectrophotometrically for a pH25 range of 7.2–8.2, a temperature range of 15–40°C and salinities 10.1, 20.2, 30.2, and 36.1.