X-ray radiography applied to an investigation of thermal diffusion

A laboratory-built x-ray radiography device (combining an x-ray tube as source and a CCD camera as detector) that is simple and easy to use is presented as a complementary analysis tool for the analysis of chemical processes. It allows the in situ acquisition of x-ray absorption images of solid or liquid samples with a lateral resolution of 50 mu m and a sensitivity of 10(-3) atom per atom. Its main application is in the analysis of liquid ionic solutions such as those met in electrochemical reactions where heavy ions move in a less absorbing medium. Despite a low resolution, a good absorption contrast in images of moving species in a liquid is obtained within a few seconds and the digital format of these images allows quantitative measurements of local concentration changes in solutions. For instance, the system has here been applied to investigate thermal diffusion in a binary solution of potassium iodide KI. X-ray imaging of the solution leads to the determination of the Soret coefficient and to its evolution as a function of the muzzle concentration. These measurements complement existing data on this thermodynamic effect in a scarcely explored concentration field.