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

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 aspHT=−log(k2Te2)+log(R−e11−Re3e2)\\documentclass[12pt]{minimal}\t\t\t\t\\usepackage{amsmath}\t\t\t\t\\usepackage{wasysym}\t\t\t\t\\usepackage{amsfonts}\t\t\t\t\\usepackage{amssymb}\t\t\t\t\\usepackage{amsbsy}\t\t\t\t\\usepackage{mathrsfs}\t\t\t\t\\usepackage{upgreek}\t\t\t\t\\setlength{\\oddsidemargin}{-69pt}\t\t\t\t\\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}Based on the mCP characterization in these extended conditions, the temperature and salinity dependence of the molar absorptivity ratios and − {bf{log}}({{boldsymbol{k}}}_{{bf{2}}}^{{boldsymbol{T}}}{{boldsymbol{e}}}_{{bf{2}}}) 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 − {bf{log}}({{boldsymbol{k}}}_{{bf{2}}}^{{boldsymbol{T}}}{{boldsymbol{e}}}_{{bf{2}}}) = −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.