The lead chlorofluoride method

The estimation of fluoride as lead chlorofluoride, PbClF, was first described by Stark and developed by Hoffman and Lundell, Hawley, Fischer and Peisker and Kapfenberger. Hoffman and Lundell determined the correct pH and chloride-ion concentration for the precipitation. The time for complete precipitation and the effect of varying amounts of fluoride on the composition of the precipitate were examined by Hawley, and these factors were reexamined by us in greater detail. 

A weight of compound, containing about 0-05 g. of fluorine, was dissolved in 10 ml. of dry alcohol and metallic sodium (about 0-5 g., i.e. at least 5 equivalents) was added. After the sodium had dissolved, the mixture was gently heated under reflux for 5 min., and then washed out with about 100 ml. of water into a beaker, made acid to bromophenol blue with dilute nitric acid and then just alkaline with 10 per cent sodium hydroxide solution. Three ml. of 10 per cent sodium chloride solution were added and the solution was diluted to 250 ml. One ml. of concentrated hydrochloric acid was added, and the solution heated on a water-bath to about 80°. Then 5-0 g. of flnely powdered A.R. lead nitrate were added with stirring (still at 80°). As soon as all the lead nitrate had dissolved, 5-0 g. of crystalline sodium acetate were added, with vigorous stirring. The product was then heated on the water-bath for 15 min. and Cooled in ice, and the precipitate was filtered off on a Swedish filter paper. It was washed once with water, foin- times with satin-ated PbClF solution 

Here again the fluorine was determined ultimately by precipitation as lead chlorofluoride, but the breakdown of the organic compound is more difiicult than with the phosphorofluoridates. The two methods recommended are 

Details of both these methods as used by us are given below in specimen analyses. With regard to the lx mb method, the lead washer supplied by the manufacturers was useless the lead melted and disintegrated at the temperature of the reaction. Copper was also found to be attacked and was otherwise unsatisfactory. Gold, on the other hand, was found to be most suitable. It is soft, will withstand the required temperature without melting or disint at-ing, and is not attacked by fluorine, fluoride or alkali. 

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Fluorides are usually determined by the lead chlorofluoride method. In the presence of an excess of chloride the double halide is precipitated from an acetate-buffered solution by addition of lead nitrate and heating on a water-bath for thirty minutes. After ageing overnight the precipitate is... 

Discussion. This method is based upon the precipitation of lead chlorofluoride, in which the chlorine is determined by Volhard s method, and from this result the fluorine content can be calculated. The advantages of the method are, the precipitate is granular, settles readily, and is easily filtered the factor for conversion to fluorine is low the procedure is carried out at pH 3.6-5.6, so that substances which might be co-predpitated, such as phosphates, sulphates, chromates, and carbonates, do not interfere. Aluminium must be entirely absent, since even very small quantities cause low results a similar effect is produced by boron ( >0.05 g), ammonium (>0.5 g), and sodium or potassium ( > 10g) in the presence of about 0.1 g of fluoride. Iron must be removed, but zinc is without effect. Silica does not vitiate the method, but causes difficulties in filtration. 

In each class the problem may be resolved into two essential parts (i) the breakdown of the organic compound under appropriate conditions to give a quantitative yield of fluoride ions in aqueous solution, and (ii) the determination of the concentration of these fluoride ions. Methods of breaking down the organic compounds were examined and the procedure adopted for the phosphorofluoridate was different from that used for the fluoroacetate series. From both, however, sodium fluoride was obtained as the breakdown product containing all the fluorine present. After numerous preliminary experiments we came to the conclusion that on the macro-scale a very convenient method of determining the quantity of fluoride ions in the products was by precipitation as lead chlorofluoride,2 PbCIF, which was then dissolved in dilute nitric acid and the chloride was determined by the Volhard method and calculated to the equivalent amount of fluorine. We determined carefully the conditions for the quantitative precipitation of lead chlorofluoride. 

A more rapid method, based upon titration of the lead in the precipitated lead chlorofluoride with EDTA at pH 10 using solochrome black as indicator, was suggested by Sakharova and Shishkina. This procedure had a number of shortcomings and the following improved method was developed ... 

Lead(II) chlorofluoride is sparingly soluble in water, and is the precipitate deposited when a lead salt is added to a solution containing both fluoride and chloride ions. This precipitation of PbFCl is the basis of an important analytical method for the determination of fluoride. PbBrF is also only very sparingly soluble in water. 

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