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

In multi-component solution systems, the diffusion of one component can influence the diffusion of a second; in some instances, especially if the first component is an acid, the second component may diffuse against its own concentration gradient. The fundamental cause of this phenomenon is the local electrical field gradient established when particular ion species in mixtures of ionic components have different transport numbers. In ternary, quaternary and higher order systems, determination of the main and cross-term diffusion coefficients is difficult, and usually indirect. The Daresbury x-ray fluorescence (DXRF) microprobe was used to measure directly the concentrations of individual elements in an aqueous gel 1.45 mm from an interface with a solution of constant composition. The solutions contained known mixtures of cobalt, copper and zinc chlorides with and without hydrobromic acid. Similar measurements were made every 300 s over a period of 3 h. After this, measurements 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 diffusion 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 diffusive 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 enabling more direct measurements of multicomponent diffusion coefficients in solution to be made.