Beitrag zur kinetik thermochemischer reaktionen von uranverbindungen in einer graphitmatrix im gleichstrombogen—Grundsätzliche ueberlegungen über die rolle thermochemischer vorgänge als partnereffekte in der spektrochemischen analyse

Abstract

A theoretical discussion shows how thermochemical investigations in the d.c. arc must be incorporated in fundamental spectrochemical research in order to arrive at significant and useful conclusions about the part played by thermochemical reactions as sources of interelement interference. This is explained in terms of a schematical classification of possible influences of thermochemical reactions on spectral-line intensities. This discussion emphasizes the fact that in this context a clear differentiation has to be made between intensity in total energy procedures, i.e. a flux integrated over the evaporation period, and intensity taken as a flux integrated over an arbitrary period during evaporation. The theoretical discussion is linked with an experimental study of thermochemical processes occurring with uranium compounds in a graphite matrix. It was established that uranium compounds such as U 3O 8 and UO 2 are converted into carbides UC and UC 2 when arced in a d.c. graphite arc. The reactions were followed during the evaporation period. This was accomplished by quantitative X-ray diffraction analysis of the reaction products present in the cavity after various arcing periods. in addition, the temporal change of the temperature in the electrode was measured by observing the fusing process with X-rays for a series of substances with known melting points. Comparison of the results of these investigations with those of spectroscopic observations revealed a close relationship between the thermochemical reactions and the evaporation behaviour of uranium compounds. Measurements of the total intensity for when samples were burnt to completion showed unambiguously, however, that this total intensity is independent of the type of uranium compound (U 3O 8) UO 2, UC, UC 2) in the electrode cavity. These results indicate that thermochemical processes may exert a strong influence on the evaporation pattern, but do not affect the total number of free uranium atoms and ions that flow through the excitation zone during the evaporation period. The absence of “themochemical interference effects” under the relevant experimental conditions was thus adequately proved.