Physical–chemical characterization of purified phenol red for spectrophotometric pH measurements in riverine, estuarine, and oceanic waters

Phenol red (PR) is one of several sulfonephthalein indicators used to provide rapid and precise spectrophotometric pH measurements of seawater and similar solutions. With an approximate pH-indicating range of 5.9 to 7.7, this dye is well suited to fill a critical gap in spectrophotometric pH-measurement capabilities – e.g., the slightly acidic waters of environments low in oxygen or high in carbon dioxide. For highest-quality measurements, the salinity and temperature dependence of indicator behavior must be established, but previous characterizations of PR were for impure indicator powder or for low-salinity solutions only. This work is the first to comprehensively characterize purified phenol red over wide ranges of temperature (T; absolute temperature in K) and salinity (S_P; practical scale). Measurements of spectrophotometric pH_T (total hydrogen ion concentration scale) are given by:${\text{pH}}_T=-\text{log}\left(K_2^T{e}_2\right)+\text{log}((R-e_1)/(1-R{e}_4))$where K₂^T is the second dissociation constant of fully protonated PR, and e₁, e₂, and e₄ are PR molar absorption coefficient ratios. The term R is the ratio of absorbances measured in the sample of interest at 558 and 433 nm. In this work, we derived a simplified method for determining the parameter e₁ of any sulfonephthalein indicator and also fully characterized PR physical–chemical characteristics for 275.15 ≤ T ≤ 308.15 K and 0 ≤ S_P ≤ 40, yielding:$\begin{array}{c}{e}_1=-2.12261\times {10}^{-3}+1.37448\times {10}^{-5}T+3.061\times {10}^{-10}{S}_P^{0.5}{T}^2\\ e_2=3.6429426-2.8139\times {10}^{-3}T\\ e_4=8.0884775\times {10}^{-2}+6.2187\times {10}^{-5}{S}_P-14.093126T^{-1}-5.005\times {10}^{-12}{S}_P^2{T}^2\\ -\text{log}\left(K_2^T{e}_2\right)=6.0900807-2.6700911S_P^{0.5}+0.116252996S_P-2.5437592\times {10}^{-2}{S}_P^{1.5}\\ +3.0176155\times {10}^{-3}{S}_P^2-1.396307\times {10}^{-4}{S}_P^{2.5}+7802.66T^{-1.5}+0.7402604S_P^{0.5}\text{ln}\left(T\right)-0.110614654S_P^{0.5}{T}^{0.5}\end{array}$

To test the performance of this characterization, we measured pH at sea using both PR and meta-cresol purple (the standard indicator for measuring surface-to-deep open-ocean profiles) and found substantial agreement over the entire water column. The PR-based equation for measuring pH_T can be combined with the parameterizations of other indicators to provide high-quality measurements over pH 4 to 9 for a wide range of solutions. This seamless continuity can be especially important in monitoring long-term change (e.g., ocean acidification) that may drive the pH of some waters of interest from the indicating range of one dye to another.