Dataset: Water column GeoFish and bottle pH data from Leg 2 (Hilo, HI to Papeete, French Polynesia) of the US GEOTRACES Pacific Meridional Transect (PMT) cruise (GP15, RR1815) on R/V Roger Revelle from October to November 2018

Methodology:
60 mL of unfiltered water was subsampled from 12L GO-Flo bottles on the GTC carousel for each pH measurement. They were analyzed at sea in primarily single analyses using an automated Ocean Optics UV-VIS spectrophotometric system that was modified from Carter et al. (2013) with pure m-cresol purple (mCP) indicator dye. They were usually analyzed within 3 hours of collection and the salinity and temperature dependent pH equation from Clayton and Byrne (1993) was applied.

Sampling and analytical procedures:
Sampling followed the GEOTRACES cookbook sampling method. 60 mL of unfiltered water was subsampled from 12L GO-Flo bottles, mounted on the GTC carousel. The 60 mL polypropylene syringes were fitted with polycarbonate 3-way valves to remove air bubbles and prevent exchange of CO2 prior to analysis and rinsed 3 times with unfiltered water. The sample syringes were then kept at room temperature in the dark until placed in a circulating water bath at 25°C for a minimum of 20 minutes to bring the samples close to analysis temperature.

Once at analysis temperature, each syringe was attached to one of the ports on the automated syringe pump that delivered sample and dye to a 10 cm cell with an inner volume of 10 mL for sample seawater. The cell is placed in a thermostated-holder that is temperature controlled by water continuously pumped from a circulating water temperature bath, so the sample remains at 25±0.1°C during analysis. The samples were analyzed at sea within 3 hours of collection and the automated measurement sequence was initiated in LabVIEW and required a total processing time of 5 minutes.

Instruments:
The temperature of the absorbance cell is controlled using deionized water pumped from a VWR circulating water bath (Cat. No. 89202-966) and through the water-jacketed CUV-10 cuvette holder with a 10 cm pathlength, 10 mL volume cuvette with quartz windows. QP400-1-UV-VIS premium fiber optic cables are used from the HL-2000-FHSA light source to the CUV UV 10 cm cuvette holder and then to the Optics STS-VIS-L-25-400-SMA miniature spectrophotometer. The automation and data processing are controlled from a computer program written in LabVIEW. A Norgren Kloehn Versa Pump 6 syringe pump with a 4-way valve and 48000 step resolution was controlled by the LabVIEW program that delivers the sample, then dye to the cuvette.