This work provides an algorithm to describe the salinity (S P) and temperature (T) dependence of the equilibrium and molar absorptivity characteristics of purified bromocresol purple (BCP, a pH indicator) over a river-to-sea range of salinity (0 ≤ S P ≤ 40). Based on the data obtained in this study, the pH of water samples can be calculated on the seawater pH scale as follows: pHSW = -log(K 2 e 2) + log((R - e 1)/(1 - Re 4)) where -log(K 2 e 2) = 4.981 - 0.1710S P 0.5 + 0.09428S P + 0.3794S P 1.5 + 0.0009129S P 2 + 310.2/T - 17.33S 1.5/T - 0.05895S P 1.5 ln T - 0.0005730S P 0.5 T, e 1 = 0.00049 ± 0.00029, and e 4 = -7.101 × 10-3 + 7.674 × 10-5 T + 1.361 × 10-5 S P. The term pHSW is the negative log of the hydrogen ion concentration determined on the seawater pH scale; R is the ratio of BCP absorbances (A) at 432 and 589 nm; K 2 is the equilibrium constant for the second BCP dissociation step; and e 1, e 2, and e 4 are BCP molar absorptivity ratios. A log(K 2 e 2) equation is also presented on the total pH scale. The e 4 value determined for purified BCP in this study can be used with previously published procedures to correct BCP absorbance measurements obtained using off-the-shelf (unpurified) BCP. This work provides a method for purifying BCP, fills a critical gap in the suite of available purified sulfonephthalein indicators, enables high-quality spectrophotometric measurements of total alkalinity, and facilitates pH measurements in freshwater, estuarine, and ocean environments within the range 4.0 ≤ pH ≤ 7.5.
© 2021 The Authors. Published by American Chemical Society.