Ocean Acidification

• Need for a spectrophotometric model using purified mCP across the estuarine salinity range is described.

• Estuarine pH data using unpurified mCP are corrected for impurity absorbances.

• A new model is presented to characterize behavior of purified mCP across the range of temperatures and salinities for temperate estuaries.

• New estuarine model corroborates models of purified mCP in pure water and seawater and bridges the salinity gap between them.

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 298.15 K and 0 ≤ S ≤ 40, herein corrected for the effects of impurities. Using the ratio (R) of mCP absorbances at 578 nm and 434 nm, the summary equations for calculations of pH on the total proton concentration scale for the conditions of 0 ≤ S ≤ 40 and 278.15 ≤ T ≤ 308.15 K are as follows:

where e₁ = − 0.007762 + 4.5174^∗ 10^− 5 T

This new model, appropriate for use with purified mCP, produces pH values that are within ± 0.004 of those obtained using previously published data and purified-mCP models for pure water and seawater.