Determination of excess sulfuric acid in toluenesulfonic acid by nuclear magnetic resonance

method for the determination of excess sulfuric acid in toluene sulfonic acid (TSH) by means of the variation in the chemical shift of OH proton of nuclear magnetic resonance was investigated. TSH is very hygroscopic and usually contains water up to 10% (by weight). The OH proton resonance in the NMR spectra of TSH dissolved in anhydrous dioxane gives one absorption peak, because the OH proton of sulfonic acid forms hydrogen bond with water in TSH and exchange of hydrogen between SO3H and H2O takes place. The resonance position of OH proton did not depend on the composition of the three isomers of TSH, but depended on the water content. Furthermore, when TSH contained sulfuric acid, the resonance of the OH proton was appreciably shifted to a low field. In preparing the samples for constructing the calibration curve, various amount of sulfuric acid were added to TSH. The water contents of the mixtures were titrated by the Karl Fischer method, then the mass fractions of water were adjusted to 30.0% by adding an appropriate amount of water. Each of these mixtures was diluted by adding a 95-fold weight of dioxane. Linear relation was observed between the sulfuric acid content and the chemical shift of OH proton peak from the methyl proton peak of p-TSH at a given temperature. The OH proton peak was also linearly shifted to a high field with increasing temperature. The following empirical equation was obtained experimentally: δ=0.0314m-0.0288t /°C+4.23 where δ is the chemical shift of OH proton from the methyl proton peak of p-TSH, m is the mass fraction of sulfuric acid expressed by percentage in the prepared samples before dissolving in dioxane and t is the Celsius temperature at which the NMR measurement is carried out. By using the above formula the content of excess sulfuric acid in TSH can be determined with an average error of about 1%, and the present method is more rapid and simpler than other methods.