Characterization of meta-Cresol Purple for spectrophotometric pH measurements in saline and hypersaline media at sub-zero temperatures.

Loucaides, Socratis, Rèrolle, Victoire M. C., Papadimitriou, Stathys, Kennedy, Hilary, Mowlem, Matthew C., Dickson, Andrew G., Gledhill, Martha and Achterberg, Eric P. (2017) Characterization of meta-Cresol Purple for spectrophotometric pH measurements in saline and hypersaline media at sub-zero temperatures. Open Access Scientific Reports, 7 (2481). DOI 10.1038/s41598-017-02624-0.

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Abstract

Accurate pH measurements in polar waters and sea ice brines require pH indicator dyes characterized at near-zero and below-zero temperatures and high salinities. We present experimentally determined physical and chemical characteristics of purified meta-Cresol Purple (mCP) pH indicator dye suitable for pH measurements in seawater and conservative seawater-derived brines at salinities (S) between 35 and 100 and temperatures (T) between their freezing point and 298.15 K (25 °C). Within this temperature and salinity range, using purified mCP and a novel thermostated spectrophotometric device, the pH on the total scale (pHT) can be calculated from direct measurements of the absorbance ratio R of the dye in natural samples as

pHT=−log(kT2e2)+log(R−e11−Re3e2)

Based on the mCP characterization in these extended conditions, the temperature and salinity dependence of the molar absorptivity ratios and − log(kT2e2)
of purified mCP is described by the following functions: e1 = −0.004363 + 3.598 × 10−5T, e3/e2 = −0.016224 + 2.42851 × 10−4T + 5.05663 × 10−5(S − 35), and − log(kT2e2) = −319.8369 + 0.688159 S −0.00018374 S2 + (10508.724 − 32.9599 S + 0.059082S2) T−1 + (55.54253 − 0.101639 S) ln T −0.08112151T. This work takes the characterisation of mCP beyond the currently available ranges of 278.15 K ≤ T ≤ 308.15 K and 20 ≤ S ≤ 40 in natural seawater, thereby allowing high quality pHT measurements in polar systems.

Document Type: Article
Keywords: pH measurements, polar seawater, sea ice, salinity
Research affiliation: OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-CH Chemical Oceanography
Kiel University
Refereed: Yes
Open Access Journal?: Yes
DOI etc.: 10.1038/s41598-017-02624-0
ISSN: 2045-2322
Date Deposited: 07 Jun 2017 10:21
Last Modified: 01 Feb 2019 15:15
URI: http://oceanrep.geomar.de/id/eprint/38224

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