This paper presents the method of double labelling in the study of the kinetics of homogeneous isotope exchange reactions.
This method was tested by the determination of the Sn(II)−Sn(IV) exchange rate in hydrochloric acid medium. The system was
labelled by the tracer119mSn [initially in the Sn(IV) state]; when the isotope equilibrium was established, Sn(IV) was again labelled by tracer113Sn. The separation of Sn(II) and Sn(IV) in the given time of exchange was performed by the extraction of Sn(IV)-hydroxyquinolate
into chloroform. The specific activities of the separated components were determined from the ratio of113Sn and119mSn activities. The exchange rate was calculated from the time dependence of specific activities. The advantage and possibilities
of the method of double labelling in the study of isotope exchange are discussed.