A preliminary study of a new chromogen derived from phenolphthalein for the trace analysis of fluoride

Abstract

This study describes a simple, new, rapid, and sensitive method for the detection and determination of trace amounts of fluoride using a novel chromogen, phenolphthalein diacetate abbreviated as PHPDA. The method is based on the reaction of fluoride with the colorless PHPDA to produce pink colour dianion with a λmax of 571 nm. Previously unknown PHPDA was synthesized from phenolphthalein and characterized by 1H NMR, 13C NMR, ESI-MS and UV-vis spectroscopy. PHPDA is selective for fluoride ions in giving pink color species. The effect of the medium on deacetylation of PHPDA with fluoride was studied in H2O, dioxane, THF, DMF, DMSO and tBuOH and found that DMSO and tBuOH were the only solvents which could facilitate the removal of the acetate group to generate pink colour. However, in DMSO the pink colour slowly fades and the solution becomes colorless while in tBuOH the colour remains stable. The absorbance of the pink dianion formed at 571 nm increases linearly with the concentration of fluoride in both solvents DMSO and tBuOH. Hence, a spectrophotometric method for the assay of fluoride based on an increase of the colour intensity of the phenolphthalein dianion, in DMSO and tBuOH was designed. The interference study shows that the determination is free from interference of other halides and some cations. Under optimum conditions calibration graphs were linear with the correlation coefficients of (r2 = 0.979 and 0.992) in DMSO and tBuOH respectively. This approach was successfully applied for determination of fluoride in commercial tea bags and toothpaste. To best of our knowledge, this is the first study of a fluoride trigged colour enhancing reaction which is used in photometric determination of fluoride.