STUDY OF DIFFUSION-COEFFICIENTS IN MULTICOMPONENT SOLUTIONS USING THEDARESBURY SYNCHROTRON X-RAY-FLUORESCENCE MICROPROBE

In multi-component solution systems, the diffusion of one component ca n influence the diffusion of a second; in some instances, especially i f the first component is an acid, the second component may diffuse aga inst its own concentration gradient. The fundamental cause of this phe nomenon is the local electrical field gradient established when partic ular ion species in mixtures of ionic components have different transp ort numbers. In ternary, quaternary and higher order systems, determin ation of the main and cross-term diffusion coefficients is difficult, and usually indirect. The Daresbury x-ray fluorescence (DXRF) micropro be was used to measure directly the concentrations of individual eleme nts in an aqueous gel 1.45 mm from an interface with a solution of con stant composition. The solutions contained known mixtures of cobalt, c opper and zinc chlorides with and without hydrobromic acid. Similar me asurements were made every 300 s over a period of 3 h. After this, mea surements were made at 100 mum intervals from the interface of the gel to a depth of 6 mm to map the concentrations of Co, Cu and Zn. The di ffusion coefficients were estimated by comparison of the concentration -time profiles for each metal with analytical or numerical simulations . The results show a significant coupling of the Cu, Co and Zn diffusi ve flows with the diffusive flow of the acid. This work has shown that the DXRF microprobe can be used to measure concentration-distance or concentration-time profiles of many elements simultaneously, thus enab ling more direct measurements of multicomponent diffusion coefficients in solution to be made.