Which technique is approved for monitoring fluoride levels in water?

The Ion Selective Electrode (ISE) technique is specifically designed for measuring the concentration of ions in a solution, making it particularly well-suited for monitoring fluoride levels in water. The ISE for fluoride operates on the principle that it generates a potential difference that is directly related to the fluoride ion concentration, allowing for rapid and accurate measurements.

This method is favored for its specificity to fluoride, ease of use, and ability to provide real-time data without the need for extensive sample preparation. Moreover, the ISE can function over a range of concentrations, which is beneficial for drinking water applications where fluoride levels may vary.

The other methods mentioned may not be as effective or appropriate for direct measurement of fluoride in water. For instance, gas chromatography is primarily used for volatile compounds and wouldn't effectively analyze fluoride ions. Visible spectrophotometry and colorimetric analysis could be applicable under certain conditions, but they generally require more complex sample treatment and may not provide the same level of specificity and direct measurement capability as an ion selective electrode.