A colorimetric method for fluoride determination in aqueous samples based on the hydroxyl deprotection reaction of a cyanine dye

A new highly sensitive and selective colorimetric method for fluoride determination in water is described. The novel reagent for this method is a cyanine dye (C1) on which the hydroxyl group has been protected by reaction with tert-butyldimethylsilane (TBS) to form the silanated dye, C2. C2 is selectively attacked by fluoride ions to reform C1. C1 has an absorption maximum at 600 nm with a molar absorptivity of about 200,000. Under optimized conditions, absorbance at 600 nm is proportional to fluoride concentration up to about 1 × 10−4 mol/L with a detection limit of 1.0 × 10−7 mol/L. Because of the specific affinity of fluoride for the TBS functional group on C2, there is little interference by other ions. The method has been successfully applied for fluoride determinations in local rainwater samples. Results determined by the proposed method agree favorably with those determined by a fluoride ion selective electrode method.