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

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 (pH(T)) can be calculated from direct measurements of the absorbance ratio R of the dye in natural samples as [Formula: see text] Based on the mCP characterization in these extended conditions, the temperature and salinity dependence of the molar absorptivity ratios and − [Formula: see text] of purified mCP 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 − [Formula: see text] = −319.8369 + 0.688159 S −0.00018374 S (2) + (10508.724 − 32.9599 S + 0.059082S (2)) 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 pH(T) measurements in polar systems.