DIFFUSION COEFFICIENTS OF HYDROLOGIC TRACERS MEASURED BY A TAYLOR DISPERSION TECHNIQUE

We developed an experimental system for measuring aqueous diffusion coefficients of hydrologic tracers based on the Taylor dispersion technique. The system utilized conventional high-performance liquid chromatography instrumentation. We determined the diffusion coefficients for alkali halides and the dye tracers uranine (fluorescein) and Brilliant Blue FCF (FD&C blue #1). We also made the first experimental determinations of the diffusion coefficients of 16 fluorobenzoates (FBAs), and 2 chlorobenzoates (CBAs) in water, and in 0.1 M potassium phosphate buffer at pH 7.0. The measured diffusion coefficients for halide salts were within 5% of literature values. The diffusion coefficients of the dye tracers ranged from 5.68 to 6.48 * 10^-10 m² s^-1, depending upon concentration. The diffusion coefficients of the benzoates ranged from 8.5 to 9.8 * 10^-10 m² s^-1 in water, and from 7.4 to 9.8 * 10^-10 m² s^-1 in buffer. The diffusion coefficients of the FBAs and CBAs in water were 17% greater on average than calculated from their molecular structures. Diffusion coefficient differences among benzoate isomers could be only partially explained by molecular geometry. We will discuss questions related to measurement of diffusion coefficients in complex matrices, the appropriateness of using a single diffusion coefficient when tracer concentration varies in time and space, and the required accuracy of diffusion coefficients for different applications.