IN-STREAM MASKING OF ALUMINUM IN THE DETERMINATION OF FLUORIDE BY FLOW-INJECTION POTENTIOMETRY

The ability of selected complexing agents to mask aluminium and release fluoride for potentiometric determination in-stream has been assessed. The experimental manifold incorporated a cascade flow cell and comprised two flow paths, the first for conventional flow-injection and the second enabling stopped-flow analyses to be performed. For solutions containing equimolar quantities of Al and F at concentrations up to 10(-3) M in each ion, better than 90% fluoride recovery was achieved in 16s using Tiron. At higher aluminium loadings, fluoride was released more slowly, with citrate and DCTA being found to be more efficient than Tiron. At an Al:F ratio of 4:1 (10(-3)M F-), relative rates for fluoride over a 10 minute period occurred in the following order: citrate > DCTA > Tiron > tartrate > EDTA Acetate and hexamine buffers, often recommended as components in TISAB formulations, were as effective as EDTA in releasing fluoride. Using an acetate buffer with citrate as the decomplexing agent slowed this release. Aged Al/F solutions responded almost as quickly as fresh solutions to Tiron and DCTA, with small pH-related effects being observed for solutions 20 days old.