Spectrophotometric Determination of Fluoride in Water

Spectrophotometry is a common method for measuring fluoride in water, typically using a dye‑metal complex. The principle involves the reaction between fluoride ions and a coloured complex (e.g., zirconium‑dye or lanthanum‑dye). Fluoride displaces the dye, forming a colourless complex and reducing the original colour intensity. The decrease in absorbance is proportional to fluoride concentration.

The procedure is straightforward. A series of fluoride standards and the water sample are mixed with a fixed volume of colour reagent and buffer (pH 4.5–5.0). After a short reaction time (15–30 min), the absorbance is measured at a specific wavelength (e.g., 570 nm or 620 nm). A calibration curve is prepared by plotting absorbance against standard concentrations, and the sample’s fluoride content is read from the curve.

Common interferences include high levels of chloride, sulphate, bicarbonate, aluminium, or iron. For clear, low‑ionic‑strength waters, direct analysis is often acceptable. For coloured, turbid, or highly contaminated samples, a pre‑distillation step with sulfuric acid is recommended to isolate fluoride. Aluminium interference can be masked by adding a complexing agent like citrate.

This method is suitable for drinking, surface, and ground water with fluoride concentrations between 0.1 and 1.5 mg/L, offering a detection limit of about 0.02 mg/L. Its advantages are simplicity and low cost; limitations include sensitivity to temperature, timing, and coloured samples. Overall, spectrophotometry provides a practical and reliable option for routine fluoride monitoring.

In summary, spectrophotometric determination of fluoride using a dye‑metal complex is a practical, reliable method for routine water quality analysis. It provides a useful balance between accuracy and operational ease, especially where ion‑selective electrodes or ion chromatography are not available.