GAS-DIFFUSION WITH PRECONCENTRATION FOR THE DETERMINATION OF FLUORIDEIN WATER SAMPLES BY FLOW-INJECTION

The preconcentration of fluoride is achieved on-line by converting it to trimethylsilane which then diffuses through a gas permeable membran e to be absorbed in a stationary sodium hydroxide acceptor stream. Thi s stream is enclosed in the sample loop of an injection valve and afte r preconcentration, the fluoride sample is flushed into a flow injecti on manifold for spectrophotometric analysis by the zirconium/alizarin S procedure at 520 nm. The method is suitable for fluoride analysis in the range 0.1-10 mg/l at a sampling rate of 17/hr. Phosphate does not interfere and aluminium and iron can be tolerated at 200 and 500 time s the fluoride concentration, respectively. The LOD was calculated to be 0.055 mg/l and LOQ was found to be 0.18 mg/l.