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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 ( m CP) 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  m CP and a novel thermostated spectrophotometric device, the pH on the total scale (pH T ) can be calculated from direct measurements of the absorbance ratio R of the dye in natural samples as   \documentclass[12pt]{minimal} 				\usepackage{amsmath} 				\usepackage{wasysym}  				\usepackage{amsfonts}  				\usepackage{amssymb}  				\usepackage{amsbsy} 				\usepackage{mathrsfs} 				\usepackage{upgreek} 				\setlength{\oddsidemargin}{-69pt} 				\begin{document}$${\boldsymbol{p}}{{\boldsymbol{H}}}_{{\boldsymbol{T}}}{\boldsymbol{=}}{\boldsymbol{-}}{\bf{log}}({{\boldsymbol{k}}}_{{\bf{2}}}^{{\boldsymbol{T}}}{{\boldsymbol{e}}}_{{\bf{2}}}){\boldsymbol{+}}\,{\bf{log}}(\frac{{\boldsymbol{R}}{\boldsymbol{-}}{{\boldsymbol{e}}}_{{\bf{1}}}}{{\bf{1}}{\boldsymbol{-}}{\boldsymbol{R}}\frac{{{\boldsymbol{e}}}_{{\bf{3}}}}{{{\boldsymbol{e}}}_{{\bf{2)$$\end{document}   p    H    T    =  −    log   (k    2    Te    2)   +    log   (R  −    e    11  −  Re    3e    2)Based on the  m CP characterization in these extended conditions, the temperature and salinity dependence of the molar absorptivity ratios and −    \documentclass[12pt]{minimal} 				\usepackage{amsmath} 				\usepackage{wasysym}  				\usepackage{amsfonts}  				\usepackage{amssymb}  				\usepackage{amsbsy} 				\usepackage{mathrsfs} 				\usepackage{upgreek} 				\setlength{\oddsidemargin}{-69pt} 				\begin{document}$${\bf{log}}({{\boldsymbol{k}}}_{{\bf{2}}}^{{\boldsymbol{T}}}{{\boldsymbol{e}}}_{{\bf{2}}})$$\end{document}   log   (k    2    Te    2)of purified  m CP is described by the following functions:  e   1  = −0.004363 + 3.598 × 10 −5   T ,  e   3 / e   2  = −0.016224 + 2.42851 × 10 −4   T  + 5.05663 × 10 −5 ( S  − 35), and −    \documentclass[12pt]{minimal} 				\usepackage{amsmath} 				\usepackage{wasysym}  				\usepackage{amsfonts}  				\usepackage{amssymb}  				\usepackage{amsbsy} 				\usepackage{mathrsfs} 				\usepackage{upgreek} 				\setlength{\oddsidemargin}{-69pt} 				\begin{document}$${\bf{log}}({{\boldsymbol{k}}}_{{\bf{2}}}^{{\boldsymbol{T}}}{{\boldsymbol{e}}}_{{\bf{2}}})$$\end{document}   log   (k    2    Te    2)= −319.8369 + 0.688159  S  −0.00018374  S   2  + (10508.724 − 32.9599  S  + 0.059082 S   2 ) T −1  + (55.54253 − 0.101639  S ) ln T −0.08112151 T . This work takes the characterisation of  m CP beyond the currently available ranges of 278.15 K ≤ T ≤ 308.15 K and 20 ≤ S ≤ 40 in natural seawater, thereby allowing high quality pH T  measurements in polar systems.

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