Basicity of Dimethyl Sulfoxide in an Aqueous Solution as Measured by the Spectrophotometric Indicator Method

The basic nature of dimethyl sulfoxide (DMSO) has been known in water, and so has its pKa value, which varies somewhat according to the measuring methods. In the present work, a spectrophotometric method employing thymol blue and o-nitroaniline as indicators was applied; the thermodynamical pKa° value of DMSO was determined to be −1.04 at 25°C. Since there is no dependence of pKa° upon the temperature, the proton transfer from water to DMSO in water is likely to be athermal.