Spectrophotometric pH measurements from river to sea: Calibration of mCP for 0 ≤ S ≤ 40 and 278.15 ≤ T ≤ 308.15 K

The indicator meta-cresol purple (mCP) has been widely used for spectrophotometric pH measurements in seawater and has been recently used in freshwater as well. Previous works have not, however, provided the comprehensive characterization of purified mCP (equilibrium and spectral behavior) required for pH measurements across the full ranges of temperature (T) and salinity (S) found in temperate estuaries. This work provides, for the first time, a comprehensive S- and T-dependent model for spectrophotometric pH measurements appropriate to freshwater, estuarine water, and seawater. Our model combines previous characterizations of the behavior of (a) purified mCP in pure water (S=0), (b) purified mCP in seawater (20≤S≤40), and (c) unpurified mCP at 298.15K and 0≤S≤40, herein corrected for the effects of impurities. Using the ratio (R) of mCP absorbances at 578nm and 434nm, the summary equations for calculations of pH on the total proton concentration scale for the conditions of 0≤S≤40 and 278.15≤T≤308.15K are as follows:pHT=pKIe2+logR−e11−Re3e2wheree1=-0.007762+4.5174x10-5Te3e2=−0.020813+2.60262×10−4T+1.0436×10−4S-35pKIe2=5.561224−0.547716S0.5+0.123791S−0.0280156S1.5+0.00344940S2−0.000167297S2.5+52.640726S0.5T−1+815.984591T−1