UC San Diego

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.