Data Set (knb-lter-sbc.75.3)

SBC LTER: pH time series: Water-sample pH and CO2 system chemistry, ongoing since 2011

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These methods, instrumentation and/or protocols apply to all data in this dataset:

Protocols and/or Procedures

For subtidal sites, discrete bottles samples were collected by a SCUBA diver using a Niskin bottle, firing it manually. For intertidal sites, discrete bottle samples were collected in a bottle, manually. These data are intended to be benchmark samples for continuously recording pH sensors (e.g., SeaFET), and so water samples were collected as close to the pH sensor as possible and approximately in concurrence with the sensor's voltage measurement. Water samples were poisoned with saturated mercuric chloride immediately after collection (following SOP1, Dickson et al. 2007), sealed, and stored in a cool, dark place until the seawater was analyzed.

Salinity, pH, and total alkalinity in the water samples were measured in the laboratory. Salinity was measured using a benchtop conductivity meter (YSI 3100). Seawater pH was measured using a spectrophotometric method with indicator dye, unpurified m-cresol purple (Sigma-Aldrich, SOP 6b, Dickson et al. 2007). Total alkalinity (AT) was measured using an automated, open-cell potentiometric titration (SOP 3b, Dickson et al. 2007) with a Mettler-Toledo T50 titrator and a DG115-SC pH probe (Mettler-Toledo). The probe was calibrated using a Tris buffer (A. Dickson Laboratory, Scripps Institution of Oceanography). Titrations were performed using certified acid titrant (~0.1M HCl, 0.6 M NaCl; A. Dickson Laboratory, Scripps Institution of Oceanography). Certified reference materials (CRMs) from A. Dickson Laboratory, Scripps Institution of Oceanography, were used to determine the accuracy and precision of the titrations daily before experimental samples were measured. Analyzed CRMs were accurate within 10 micromol kg-1. pH at 25°C, AT, temperature, and salinity were used to calculate the pH and pCO2 of the treatments using CO2calc (Robbins et al. 2010), with CO­2 constants K1, K2 from Mehrbach et al. 1973 refit by Dickson and Millero, 1987 and pH expressed on the total scale (mol kg-SW-1).


Robbins, L.L., Hansen, M.E., Kleypas, J.A., and Meylan, S.C., 2010, CO2calc: A user-friendly seawater carbon calculator for Windows, Max OS X, and iOS (iPhone): U.S. Geological Survey Open-File Report 2010-1280, 17 pp.

Rivest, E. B., M. C. O'Brien, L. Kapsenberg, C. Gotschalk, C. A. Blanchette, U. Hoshijima and G. E. Hofmann. 2016. Beyond the benchtop and the benthos: dataset management planning and design for time series of ocean carbonate chemistry associated with Durafet®-based pH sensors. Ecological Informatics, 36: 209-220.

Dickson, A.G., C.L. Sabine, and J.R. Christian (Eds.). 2007. Guide to Best Practices for Ocean CO2 Measurements, PICES Spec. ed.

Author: Hofmann
Title: SBC LTER Protocol: Collection and processing of benchmark water samples for DuraFET type pH sensors
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