Purification and Physical-Chemical Characterization of Bromocresol Purple for Carbon System Measurements in...

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: pH_SW = -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 pH_SW 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.