Quantitative Infrared Absorption Measurements on Tri- n -Butyl Phosphate

Abstract

The molar absorptivity, ε(ν̄) of three intense infrared absorption bands in tri-n-butyl phosphate has been measured as a function of concentration. For all three bands, ε(ν̄) is independent of concentration for dilute solutions, then decreases uniformly with increasing concentration, and finally becomes independent of concentration again as one approaches the pure liquid. A saturation effect is found at about 1.0 M for all cases. Deviations from Beer's law behavior are observed at concentrations which depend upon the absorption band and/or the choice of nonpolar solvent. Self-association of the solute molecules is considered to be the cause of the decrease in ε(ν̄) with increasing concentration. From the measured data, it is possible to estimate the number of solvent molecules required to prevent this self-association. For CCl₄, we find this value to be 25 molecules of solvent per solute molecule, a value in agreement with elementary geometric consideration.