Abstract
An NMR method for pH probing of hydrothermal solutions is described. NMR tubes withstanding autogenous pressures at temperatures up to 200 °C are used in conventional NMR probe heads. After selection, several molecules stable in aqueous solutions up to 200 °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 14N chemical shift (δ) versus pH is established. The dependence on temperature of the amine parameters, Kai (dissociation constants) and δi (chemical shifts of the different protonation states), is investigated. Both amines present high NMR sensitivities to protonation changes, expressed as d[δ(14N)]/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.
Published Version
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