Anomalous salting-out, self-association and pKa effects in the practically-insoluble bromothymol blue.

Background and purpose: The widely-used and practically insoluble diprotic acidic dye, bromothymol blue (BTB), is a neutral molecule in strongly acidic aqueous solutions. The Schill (1964) extensive solubility-pH measurement of bromothymol blue in 0.1 and 1.0 M NaCl solutions, with pH adjusted with HCl from 0.0 to 5.4, featured several unusual findings. The data suggest that the difference in solubility of the neutral-form molecule in 1M NaCl is more than 0.7 log unit lower than the solubility in pure water. This could be considered as uncharacteristically high for a salting-out effect. Also, the study reported two apparent values of pK_a1, 1.48 and 1.00, in 0.1 M and 1.0 M NaCl solutions, respectively. The only other measured value found for pK_a1 in the literature is -0.66 (Gupta and Cadwallader, 1968).

Experimental approach: It was reasoned that the there can be only a single pK_a1 for BTB. Also, it was hypothesized that salting-out alone might not account for such a large difference in solubility observed at the two levels of salt. A generalized mass action approach incorporating activity corrections for charged species using the Stokes-Robinson hydration equation and for neutral species using the Setschenow equation, was selected to analyze the Schill solubility-pH data to seek a rationalization of these unusual results.

Key results: BTB reveals complex speciation chemistry in saturated aqueous solutions which had been poorly understood for many years. The appearance of two different values of pK_a1 at different levels of NaCl and the anomalously high value of the empirical salting-out constant could be rationalized to normal values by invoking the formation of a very stable neutral dimer (log K₂ = 10.0 ± 0.1 M^-1). A 'normal' salting-out constant, 0.25 M^-1 was then derived. It was also possible to estimate the 'self-interaction' constant. The data analysis in the present study critically depended on the pK_a1 = -0.66 reported by Gupta and Cadwallader.

Conclusion: A more reasonable salting-out constant and a consistent single value for pK_a1 have been determined by considering a self-interacting (aggregation) model involving an uncharged form of the molecule, which is likely a zwitterion, as suggested by literature spectrophotometric studies.