ABSTRACT
The hydrodynamic radius is the radius of the ion as it migrates through solution. At the low applied fields typically used for conductivity measurements, this closely mirrors the radius of the solvated ion (see Topic E1), as the forces between ion and solvent in the solvation shell are sufficiently strong to ensure the ion moves with its solvation shell. This means that the hydrodynamic radius is typically much larger than the ion radius in the gas phase. The smallest, most highly charged ions (such as Li+, Al3+ and F−) before solvation have the largest solvation shells (see Topic E1). Since the overall radius of the solvated ion is the sum of the ionic radius plus the solvation shell radius, the smallest unsolvated cations often have the largest radii when solvated and move slowest through solution. Singly-charged anions often have similar hydrodynamic radii, as they tend to be larger than singly-charged cations, have smaller solvation shells, and the effect of a change in the size of the ion is often counterbalanced by the change in the solvation shell radius.
