The method of concentration-dependent distribution in the quantitative use of radioisotopes : Theoretical calibration curves and comparison to the “substoichiometric principle”

The method is based on the utilization of a calibration curve which shows the dependence of the distribution ratio in a two-phase system containing the substance to be determined or a substance reacting with it, on the total concentration of the substance to be determined. The relationship of the proposed method to radiometric titration and to the “substoichiometric principle” in isotopic dilution is discussed. Formulae for the theoretical calibration curves in 4 systems are derived: sorption conforming to the Langmuir isotherm or to the Freundlich isotherm, extraction with a constant quantity of an extracting agent, and sorption or extraction in the presence of a constant quantity of a chelating agent. Theoretical sensitivities are given.