2,6-Dimethyl-γ-pyrone as a 13C NMR indicator for estimating the acidity of heteropolyacid solutions

13C NMR of solutions containing 2,6-dimethyl-y-pyrone as a 1 3 C NMR indicator were used to estimate the acidity of aqueous solutions of ordinary acids (HC1, HNO 3 , HClO 4 ) and heteropolyacids (HPA) (13 HPA of five structural types were used). In the concentration range [H + ] = 0-1.7 mol/l, the change in the signal positions in the spectra reaches 10 ppm, which enables the use of this method to compare the solution acidities of ordinary acids. In HPA solutions, indicator-HPA anion and, probably, indicator-H + -HPA anion interactions were detected. It was concluded that the indicator method is inapplicable for acidity determinations in HPA solutions.