In situ pH probing of hydrothermal solutions by NMR

An NMR method for pH probing of hydrothermal solutions is described. NMR tu bes withstanding autogenous pressures at temperatures up to 200 degrees C a re used in conventional NMR probe heads. After selection, several molecules stable in aqueous solutions up to 200 degrees C are shown usable as pH pro bes. Two amines, imidazole and 1,4-diazabicyclo[2.2.2]octane, are complemen tary 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 (d issociation constants) and delta(i) (chemical shifts of the different proto nation 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 succe ssfully applied to follow the pH evolution during hydrothermal synthesis of aluminum hydroxide, and of AlPO4-CJ2.