Spectrophotometric determination of a barium-metal ion indicator complex formation constant

Metal ion indicators are frequently used in the titrimetric determination of solution concentrations of various metals. The determination of aqueous barium concentration is of interest in both environmental and geochemical fields. Barium is of particular interest in the energy industry as it has been used in the extraction of petroleum and could be present in fracking wastewater. A titrimetric method utilizing a metal-ion indicator along with EDTA may be a more cost-effective approach than atomic spectroscopy. Methylthymol blue (MTB) has been utlized in the titrimetric determination of barium previously, however, endpoint detection remains a challenge. We hypothesize that the formation constant between barium and MTB may be insufficiently large to generate a distinct endpoint. Here, a spectrophotometric titration of barium and MTB was performed in aqueous solution to determine the complex formed and the equilibrium constant of the reaction. The appearance of a sharp isosbestic point in the titration indicates the formation of a single dominant complex between Ba and MTB. Based on equilibrium, mass, and charge balance equations, the barium cation concentration was determined by the quadratic formula. Using Beer’s law, a theoretical fit was determined for the experimental data with equilibrium constant and molar absorptivity coefficients determined by least squares minimization. The equilibrium constant for formation of the Ba-MTB complex was determined along with the molar absorptivity coefficients of MTB and Ba-MTB.