Investigation of Isotope Effects with the Aid of Kinetic and CatalyticPolarographic Currents

Abstract

Two types of chemical reactions, used to investigate isotope effects, are considered: reactions preceding the electrochemicalstage proper and reactions accompanying this stage. The results of studies on the dehydration of formaldehyde are presentedas examples of reactions of the first type and those on the oxidation of titanium(III) to titanium(IV) by hydroxylamine andthe decomposition of hydrogen peroxide by catalase are presented as reactions of the second type. The results of a polarographicdetermination of the rate constants for reactions involving isotopic molecules, on the basis of which it is possible todetermine the isotope effect for the corresponding reaction, are described together with the results of the determination ofthe equilibrium constants with the aid of both classical polarography and oscillographic polarography with rapid potentialscanning. The equilibrium constants of the acid dissociation and dehydration reactions involving formaldehyde were determinedand were employed to determine the corresponding thermodynamic isotope effects. It is shown that a direct isotopeeffect obtains in the acid dissociation reaction and the inverse effect obtains in the dehydration reaction. The possibility of using the results of the study of isotope substitution in relation to catalytic currents in the cobalt–sodiumnitrohydroxylaminate–cysteine system in an ammonia buffer for the determination of the isotopic composition of water isdemonstrated. It is concluded that the polarographic method has many possibilities for the investigation of both kinetic andthermodynamic isotope effects and the isotope effects of the solvent. The bibliography includes 44 references.