An improved barium-rhodizonate method for determination of sulfate ion in biological fluids.

Simple and direct determination of sulfate ion concentrations has many important applications, such as analysis of sulfohydrolase activities in biological fluids. Unfortunately, a reported barium-rhodizonate spectrophotometric method with many advantages faces a solubility challenge. To overcome this problem, solvation of rhodizonate complexes in its metathesis reaction was systematically investigated by 46 solvents/compounds using curvilinear regression methods to fit sulfate calibration intervals for signal linearity and color stability. The results revealed solvent structure-activity relationships to the color formation and provided optimal solvent formulae that enable this colorimetry in the stoichiometric way. The limit of water content in the colorimetric matrix increased from 20 to 45% and color formed for reading was stable for 45-135 min. The rhodizonate reagents were stable at -70 °C for >6 months. This established the robustness of the assay, which can now measure the sulfate ion concentration at 0.18 nmol, in comparison to 1 nmol of the early reports. The method provides a simple and direct analysis of sulfate ion, suitable for kinetics studies of sulfohydrolase activity in biological fluids.