A New Colorimetric Method for the Determination of Soluble Fluorides

Abstract

Soluble fluorides in neutral solution react with Fe⁺⁺⁺ to give a complex which does not develop a color with the various reagents for iron. Guyot, 1 Greef 2 and Treadwell and Köhl 3 have developed a colorimetric titration method for fluoride based on this fact. Our efforts have been directed towards the application of this principle as a method for the determination of fluorides colorimetrically. We have used thiocyanate, salicylic acid, 8-hydroxy quinoline, and acetylacetone as reagents for the development of color with the excess iron added to a fluoride solution. We have found acetylacetone to be the superior reagent because the others either give colors which are not stable to light, or the amount of fading induced by varying concentrations of fluoride is erratic. Our present method of procedure is as follows: The solution containing the fluoride is rendered just acid to phenolphthalin and diluted to 100 cc. To each of two 25 cc. volumetric flasks is added 1 cc. of a freshly prepared solution of FeCl₃ containing 0.3 mg. Fe/cc. and 1 cc. of a 0.05% solution of acetylacetone in water. The contents of the flasks are mixed and the red color of ferric acetylacetone develops. To one of the flasks is added 10 cc. of the fluoride solution. (If this aliquot assays over 0.3 mg. fluoride the assay is incorrect and must be repeated with a smaller aliquot; if less than 0.01 mg. a larger aliquot is taken.) The flasks are diluted to volume and the contents mixed The color of the contents of the flask to which the fluoride was added fades as a consequence of the action of the F^-. The solution containing the fluoride is placed in the right cup of the colorimeter, while the solution containing no fluoride is placed in the left cup and set at 20 mm.