Abstract
A new method is presented for the gravimetric determination of fluoride by precipitation as calcium fluorophosphate. The precipitation is formed by dropwise addition of a fluoride solution to a solution containing excess of calcium and phosphate ions, which is buffered with formic acid-ammonium formate at pH 3.70. The precipitate is crystalline, and confirmed to be fluoroapatite, Ca5F(PO4)3, by X-ray diffraction. It is readily filtered on a G4 sintered glass crucible and washed with the 0.1 M buffer solution and water, and then dried at 130°C to a constant weight. The gravimetric factor is 0.03768, and the determinable range is (28) mg F with the relative standard deviation of 2.4%. Sodium ion less than 100 mg dose not interfere. This method has the advantage of no interference from phosphate ion and ready filtration and washing.
