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 HNO<sub>3</sub> 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 HNO<sub>3</sub>. This result demonstrates that spectroscopic methods can use the vaporization of an analyte to measure the activity rather than the concentration.

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