Methodology of measurement of ionic strength based on field-flow fractionation

In this report, we put forward an experimental method to determine the ionic strength of an aqueous solution. To this end, we have developed a theory of ionic strength I expressed in terms of the retention ratios in field-flow fractionation (FFF) as I^1/2=κ(1-R)/(1-R_o)-ε^-1/2. Here R is a measured retention ratio using an FFF technique, for instance, sedimentation FFF (SdFFF), and R_o is the sterically-corrected standard retention ratio as given by the standard retention theory (SRT) for a latex particle system of diameter d. For a standard latex system with known d (or R_o) and I, we can construct a linear calibration of I^1/2 against (1-R)/(1-R_o). Therefore, if we measure the retention ratio R of a carrier liquid of which ionic strength is of interest, then we will be able to estimate the ionic strength from the calibration curve thus built. In this paper, we have demonstrated the relation of I^1/2with respect to (1-R)/(1-R_o) for the polystyrene latex systems of which information on R, R_o, and I is available from Ref. [1].