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

The ability of selected complexing agents to mask aluminium and releas e fluoride for potentiometric determination in-stream has been assesse d. The experimental manifold incorporated a cascade flow cell and comp rised two flow paths, the first for conventional flow-injection and th e second enabling stopped-flow analyses to be performed. For solutions containing equimolar quantities of Al and F at concentrations up to 1 0(-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 th an Tiron. At an Al:F ratio of 4:1 (10(-3)M F-), relative rates for flu oride 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 th e decomplexing agent slowed this release. Aged Al/F solutions responde d almost as quickly as fresh solutions to Tiron and DCTA, with small p H-related effects being observed for solutions 20 days old.