Purification and Physical–Chemical Characterization of Bromocresol Purple for Carbon System Measurements in Freshwaters, Estuaries, and Oceans

[Image: see text] 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.