A low-cost, colorimeter-based portable device, for rapid, on-spot determination of fluoride in water

xAccurate fluoride measurement in water is required for selecting safe water sources or for treatment options to lessen its impact on public health. A low-cost, portable device has been developed as a rapid fluoride measurement system. It works with an optimal concentration of alizarin red S-zirconium complex (ARS-Zr, the sensor reagent). The device is an LED-phototransistor-based assembly to detect changes in light absorbance at 527 nm, due to monotonically decreasing ARS-Zr concentration with increasing fluoride concentration in water. We have optimised reagent composition in the device, enabling us with a wider fluoride detection range (0.2-9 ppm), rapidly within 4 min; simultaneously ensuring better reagent stability for up to 34 days. Limit of detection (LOD) and limit of quantification (LOQ) of the device are 0.2 and 0.6 ppm, respectively. The device replicates an analytical spectrophotometer's performance, with a maximum error up to 5% in only a couple of cases. The hand-held device does not give any interference in the presence of even a very high 200 ppm concentration of other common heavy metals (Ca2+, Zn2+,Fe2+, Mg2+,Cu2+) and anions (Cl, NO 3, SO2 (4-), I, CH3COO-, CO2 (3-)), which can be present in water. Fluoride concentration measurement above 1 ppm is unaffected, by even high phosphate concentration up to 10 ppm (typical phosphate level being only = 2.3 ppm). Similarly, for silica concentration up to 10 ppm (typical silica level being 1-30 ppm), fluoride measurement remains unaffected. However, the presence of Fe3thorn gives significant interference with fluoride. Overall, we demonstrate the device as an on-spot method, for rapid fluoride measurement, independent of TDS (57.6-151 ppm), pH (6.6-12.5) or conductivity (111.2-283 mu S/ cm) of the samples, at a low device cost of US$ 25 and reagent cost of 0.44 cents per sample.