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 degrees 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 pH(T) = -log(k(2)(T)e(2)) + log(R - e(1)/1 - R-e3/(e2)) Based on the mCP characterization in these extended conditions, the temperature and salinity dependence of the molar absorptivity ratios and - log( k(2)(T)e(2)) of purified mCP is described by the following functions: e(1) = -0.004363 + 3.598 x 10(-5)T, e(3)/e(2) = -0.016224 + 2.42851 x 10(-4)T + 5.05663 x 10(-5)(S-35), and -log( k(2)(T)e(2)) = - 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 pHT measurements in polar systems.