Infrared spectra of the vapor phase over solutions approximating those used in the dissolution of plutonium, typically 10 M in nitric acid and 0.2 M in hydrofluoric acid, reveal that the hydrofluoric acid (HF) activity can be monitored in a facile and noninvasive manner. The amount of vaporized HF is strongly dependent upon the nitric acid content; the vapor-phase HF content over a series of solutions in which the HF concentration is held constant at 0.5 M increases by a factor of nearly 25 as the HNO3 concentration is increased from 0 to 14 M. This effect is due to changes in the HF activity and is not a result of changes in the heat of vaporization, which was measured (by van't Hoff plots) to be 9.4 ± 0.6 kcal/mole for 16 mock process solutions ranging from 0.01 to 0.1 M HF and 8 to 14 M HNO3. This result demonstrates that spectroscopic methods can use the vaporization of an analyte to measure the activity rather than the concentration.
Keith R. Brenneman and Robert J. Donohoe, "Monitoring Hydrofluoric Acid Activity by Vapor-Phase Infrared Spectroscopy," Appl. Spectrosc. 48, 808-812 (1994)