The kinetics of extraction of nitric acid by tributyl phosphate-n-hexane solvents has been investigated in a stirred vessel transfer cell. Elucidation of a possible slow step in the extraction process due to the complexing reaction · TBP was of particular interest. The criteria used to determine the mechanism were:

  1. The effect of temperature.
  2. The effect of stirrer speed.
  3. Comparison of the mass transfer coefficients to those of nitric acid in a nonreacting solvent.
  4. Comparison of the mass transfer coefficients to those predicted by a previously developed correlation for two-component, two-phase systems.
On the basis of these criteria, the kinetics were found to be controlled by purely diffusional processes. The mechanism proposed involves mass transfer of the nitric acid through the aqueous film, rapid and complete conversion to the complex at the interface, and transfer of the complex to the organic phase. This last step constitutes the major portion of the over-all resistance, although resistance in the aqueous phase also constitutes a small fraction of the total. No effect of a slow chemical conversion was found. At high acid levels, the transfer process was greatly accelerated (by a factor of 2–3) by the phenomenon of interfacial turbulence, arising probably from the Marangoni effect.