An accurate water activity model for sulfuric acid solutions and its implementation on moisture sorption isotherm determination

Because of a lack of reliable models to predict the water activity (aw) of H2SO4 solutions, the standard sulfuric acid solution (SSL) method requires a periodic check and adjustment of the acid concentration to ensure that equilibrium will be reached at a predetermined aw point. To establish a reliable equation describing the aw of H2SO4 solutions, five molecular activity models, i.e. nonrandom two-liquid (NRTL), Wilson, universal quasi-chemical (UNIQUAC), modified NRTL-1 (M-NRTL 1) and modified NRTL-2 (M-NRTL 2), were introduced and compared in this study. Each of the models was applied to the 542 literature vapor-liquid equilibrium data points of H2SO4 solutions. The results showed that the NRTL model was the best with respect to its accuracy and simplicity. This NRTL model significantly simplified the implementation of the sulfuric acid solution method for MSI measurements. Excellent agreement was found between this simplified SSL method and the conventional saturated salt solution (SSS) method for MSI measurements of both microcrystalline cellulose (MCC) and apples at 25 °C. Therefore, this newly improved SSL method, based on the NRTL model, could be an inexpensive, convenient and reliable alternative to the conventional SSS method in food MSI measurements in the future.HighlightsThe NRTL model can accurately predict the aw of H2SO4 solutions.The NRTL model can significantly simplify the known sulfuric acid solution (SSL) method.This improved SSL method is valid and reliable for use in MCC and apples.A working table of the aw of H2SO4 solutions was developed for ease of use.