Dataset: Open cell and single-step method for total alkalinity titrations from samples collected on R/V Mirai cruises MR23-06C and MR23-07 in the North Pacific and Western Arctic from September to November 2023

Samples were collected from the CTD rosette following best practices (Dickson et al., 2007). Samples for single-step total alkalinity (TA) titrations were collected in 100-milliliter (mL) borosilicate glass bottles, and samples for open cell titrations were collected in 250-mL borosilicate glass bottles. Samples were poisoned with 0.04% HgCl2 and a precise headspace of ~1% was created using a pipette. Bottles were sealed with ground glass stopper, and apiezon L grease and a rubber band and plastic hose clamp were used to keep the stopper in place. The TA measured on board was measured by single-step titration with spectrophotometric endpoint detection using bromocresol green following the methods of Yao and Byrne (1998). The duplicate samples were shipped to the laboratory at Massachusetts Institute of Technology (MIT) in Cambridge, MA, USA for analysis using an open cell titration and non-linear least squares fitting as described in Dickson et al. (2003). DIC for both shipboard and land-based analysis was done by acidification and coulometric detection following best practices methods (Dickson et al. 2007).

Land-based pH on the total scale (pHt) at 20 degrees Celsius was done using a custom-built instrument similar to Carter et al. (2013) and purified meta-cresol purple indicator.

Instrumentation:
The shipboard TA analysis by single step titration was done using spectrophotometry with a single acid addition procedure using a custom-made system (Nippon ANS, Inc., Japan).

Land-based TA was performed with a custom-designed titration designed and built by the laboratory of Andrew G. Dickson (University of California, San Diego) as described in Dickson et al. (2003).

Shipboard DIC was determined using an automated TCO2 analyzer (Nippon ANS, Inc., Japan) with coulometric detection.

Land-based DIC was determined by a custom dissolved inorganic carbon extraction with coulometric detection built by the National Oceanographic and Atmospheric Administration Pacific Marine Laboratory and is a modern version of the original SOMMA system (Johnson et al. 1992).

Land-based pHt was measured at 20 degrees Celsius with a custom system designed and built by the Woosley laboratory consisting of a Kloehn syringe pump and Agilent 8453 spectrophotometer and controlled using LabView Software.