In situ pH probing of hydrothermal solutions by NMR

An NMR method for pH probing of hydrothermal solutions is described. NMR tubes withstanding autogenous pressures at temperatures up to 200 degrees C are used in conventional NMR probe heads. After selection, several molecules stable in aqueous solutions up to 200 degrees C are shown usable as pH probes. Two amines, imidazole and 1,4-diazabicyclo[2.2.2]octane, are complementary pH indicators. Both together permit a range of about 9 pH units to be covered. Calibration of the amine N-14 chemical shift (delta) versus pH is established. The dependence on temperature of the amine parameters, K-ai (dissociation constants) and delta(i) (chemical shifts of the different protonation states), is investigated. Both amines present high NMR sensitivities to protonation changes, expressed as d[delta(N-14)]/d[pH], and have a very small sensitivity to medium effects. This pH determination method is successfully applied to follow the pH evolution during hydrothermal synthesis of aluminum hydroxide, and of AlPO4-CJ2.