Abstract
Fluoride ion is precipitated as lead chlorofluoride in the presence of some kinds of metal chloride and can be determined from the excess of the standard lead nitrate solution found in the filtrate which can be obtained by a complexometric titration. We have observed for the first time that any one of the chloride of potassium, ammonium, barium and strontium can be used for this purpose instead of sodium chloride which is usually employed. The use of magnesium chloride was found to be a little less preferable, while chloride of calcium was quite unfavorable. It is recommended to use a standard lead nitrate solution containing 100 mg of lead ion in order to precipitate fluoride ion in a 200 ml solution at 1015°C and pH 4 followed by setting aside overnight.
To determine small amounts of fluoride ion (0.56 mg), take the sample solution in a 300 ml beaker, add 13g of sodium chloride, potassium chloride, ammonium chloride, barium chloride or strontium chloride, then add a standard lead nitrate solution (containing 100 mg of lead ion), dilute the mixture to nearly 200 ml with water, adjust the pH of the solution to approximately 4, allow to stand overnight to settle the precipitate at 10 to 15°C, transfer the solution together with the precipitate into a 200 ml volumetric flask and dilute it to the mark with a small amount of water (1015°C). After mixing, filter through a dry filter paper into a dry vessel, discard the first small portion of the filtrate, to a subsequent 100 ml portion, add solid hexamine or an acetateacetic acid buffer solution to adjust the pH to 5, titrate the excess of lead ion in the filtrate with a 0.01M EDTA solution using Xylenol Orange as indicator.
Alternatively, another recommendable method of titration is as follows : when sodium, potassium or ammonium chloride is used, add an excess of 0.01M EDTA solution to a 100 ml portion of the filtrate and add an ammonia-ammonium chloride buffer solution to adjust the pH to 10, and then titrate the excess of EDTA with a 0.01M zinc nitrate or magnesium chloride solution using Eriochrome Black T as indicator.
A successive determination of sulphate and fluoride is also possible by the method mentioned above; by an addition of barium chloride and lead nitrate, barium sulphate and lead chlorofluoride are precipitated, respectively. Then it is possible to determine fluoride ion in the presence of some of diverse ions without masking them, if a modification of this method is employed.
