Isotopic double tracers for the measurement of the biological half-life of an element or compound, exemplified by 125I---131I taken up in a seaweed

As an example for the measurement of biological half-life of an element using isotopic double tracers, a seaweed was cultured in sea water containing constant concentrations of 125I and 131I and the activities taken up were measured for various parts and compositions of the seaweed. Mathematical relationships between the biological half-life of iodine and the 125I/131I ratio were obtained and compared with the experimental data. The half-life in each composition was shown to increase with the resistivity to extraction or decomposition. Also, iodide in the sea water was found to be oxidized in the presence of seaweed in the light.