Lead chlorofluoride, determination

Addition of calcium nitrate solution to a fluoride gives a white precipitate of calcium fluoride, CaFj. If the latter is precipitated slowly, it can be filtered off and weighed to estimate the fluoride. Fluoride can also be determined by the addition of sodium chloride and lead nitrate which precipitate lead chlorofluoride, PbClF. This is filtered off and weighed. 

Fluoride may be determined by precipitation as lead chlorofluoride, the precipitate being dissolved in dilute nitric acid and, after adjusting the pH to 5-6, the lead is titrated with EDTA using xylenol orange indicator.10... 

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. 

The estimation of fluoride as lead chlorofluoride, PbClF, was first described by Stark1 and developed by Hoffman and Lundell,2 Hawley, Fischer and Peisker4 and Kapfenberger.5 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. 

Here again the fluorine was determined ultimately by precipitation as lead chlorofluoride, but the breakdown of the organic compound is more difficult than with the phosphorofluoridates. The two2 methods recommended are... 

Heats of precipitation have been employed to determine the enthalpies of sparingly soluble simple and complex fluorides for example, that of calcium fluoride by adding solid calcium chloride to a solution of excess sodium fluoride saturated with calcium fluoride (88), and of lead chlorofluoride by adding sodium fluoride solution to a saturated lead chloride solution (50). 

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... 

Sodium fluoride is determined in the B.Vet.C. by precipitation as lead chlorofluoride. 

A sample placed behind a 25 jum copper beam intensity monitor is irradiated for 15-20 min with a 2 mA beam of 25 MeV He. By chemical etching in 6 M nitric acid a 25-50 Asurface layer is removed. The sample is dissolved in 14 M nitric acid to which some sodium fluoride, zinc nitrate and gallium oxide carrier are added. After addition of phosphoric acid, the temperature is raised to 125° and 150 ml of distillate are collected. Gallium oxide is added to the distillate and gallium hydroxide is precipitated and filtered off. Finally, fluorine is precipitated as lead chlorofluoride The precipitate is repeatedly measured with a Ge(Li) detector during a 10 h period to allow the control of the half-life. Afterwards the yield of the chemical separation is determined by activation with an Ac-Be isotopic neutron source using the F(n,a) N reaction. 

F is separated by steam distillation of fluorosilicic acid followed by precipitation of lead chlorofluoride. For helium-3 activation (effective incident energy = 15-18 MeV) no extraction of polonium is required. The results obtained for industrial lead were (0.99 + 0.21) /ig/g (n = 8) for He activation and (0.79 + 0.36) ng/g (n = 8) for He activation. The standard deviations of resp. 21 and 45 % compared to the 10 % obtained when roughly the same analytical technique was applied to the determination of oxygen in copper (213)(245), indicate a less homogeneous distribution of the oxygen in lead. The variance for the He results is significantly... 

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. 

---