Lead fluoride, solubility

Lead shows excellent resistance to phosphoric and sulfuric acid in almost all concentrations and at elevated temperatures, as well as to sulfide, sulfite, and sulfate solutions. The corrosion film is insoluble lead sulfate which rapidly reforms if it is damaged. Lead is also resistant to chlorides, fluorides, and bromates at low concentrations and low temperatures. However, because lead is soluble in nitric and acetic acids, it is not resistant to these acids. 

Lead Fluoride. Lead difluoiide, Pbp2, is a white oithorhombic salt to about 220°C where it is transformed into the cubic form some physical properties ate given in Table 1. Lead fluoride is soluble in nitric acid and insoluble in acetone and ammonia. It is formed by the action of hydrofluoric acid on lead hydroxide or carbonate, or by the reaction between potassium fluoride and lead nitrate. 

Lead chloride is freely soluble in hot aqueous solutions, but lead fluoride is almost insoluble in dilute HE solutions. When the HE concentration reaches about 40%, steel is preferred. 

Elemental composition Pb 84.50%, F 15.50%. The compound can be identified from its physical properties and x-ray measurement. Lead may be analyzed by various instrumental techniques (See Lead). Fluoride ion may be determined by dissolving a minute quantity of the compound in water (the compound is slightly soluble in water) and analyzing the solution by ion chromatography or hy fluoride-ion selective electrode. 

Fluoride, hexafiuorosilicate, and sulphate in the presence of each other The following differences in solubilities of the lead salts are utilized in this separation lead hexafiuorosilicate is soluble in water lead fluoride is insoluble in water but soluble in dilute acetic acid lead sulphate is insoluble in water and in boiling dilute acetic acid, but soluble in concentrated ammonium acetate solution. 

Wash well with cold water. Divide into two parts. (i) Smaller portion. Add excess dilute acetic acid and boil (this will dissolve any lead fluoride). White residue indicates sulphate. The residue is soluble in ammonium acetate solution. (ii) Larger portion. Treat cautiously with concentrated sulphuric acid and test with a moist glass rod. Milkiness of the water and etching of the tube indicate fluoride. Add Ba(N02)2 solution and warm gently. White crystalline precipitate indicates hexafluoro-silicate. 

Solubility product. The solubility of lead fluoride, PbF , in water is 0.064 g/100 ml solution at 20°C. Calculate its solubility product... 

Fused salt solutions may be found in which the solubility of these oxides is appreciable at high temperatures and from which crystals grow as the solution is cooled. Some of the fluxes which have been used for growth of the oxides of concern here are (a) potassium nitrate-sodium nitrate, (b) lead fluoride-bismuth oxide, (c) lead oxide-bismuth oxide, and (d) lithium hydroxide-boric acid-molybdenum oxide. Temperatures frequently are in the range of 1300°C. 

Rubidium metal alloys with the other alkaU metals, the alkaline-earth metals, antimony, bismuth, gold, and mercury. Rubidium forms double haUde salts with antimony, bismuth, cadmium, cobalt, copper, iron, lead, manganese, mercury, nickel, thorium, and 2iac. These complexes are generally water iasoluble and not hygroscopic. The soluble mbidium compounds are acetate, bromide, carbonate, chloride, chromate, fluoride, formate, hydroxide, iodide. 

Highly irritating and poisonous Very soluble in water. Liquid liberates heat as it dissolves in water. The entrapment of water in an anhydrous hydrogen fluoride cylinder can cause rapid generation of heat and pressure which can lead to an explosion. Containers should never be heated to >52°C. A liquid hydrogen fluoride spill area should not be entered unless protective clothing (impervious to the compound) and a self-contained gas mask are worn Fumes in air... 

Precipitation of fluoride compounds from solutions of hydrofluoric acid, HF, is performed by the addition of certain soluble compounds to solutions containing niobium or tantalum. Initial solutions can be prepared by dissolving metals or oxides of tantalum or niobium in HF solution. Naturally, a higher concentration of HF leads to a higher dissolution rate, but it is recommended to use a commercial 40-48% HF acid. A 70% HF solution is also available, but it is usually heavily contaminated by H2SiF6 and other impurities, and the handling of such solutions is extremely dangerous. 

In the case of a mixture of hydrofluoric and sulfuric acids, the process is more complex. It can be noted that sulfuric acid most probably interacts mainly with iron and manganese, whereas hydrofluoric acid serves mostly in the dissolution of tantalum and niobium and their conversion into soluble fluoride complexes. Nevertheless, due to the high acidity of the solution, here too the formation of hexafluorotantalate and hexafluoroniobate complex ions, TaF6" and NbF6, is expected. Hence, it is noted that the acid dissolution of tantalum-and niobium-containing raw material leads to the formation of hexafluoro-acids — HTaF6 and HNbF6. 

In both cases, the fluorination of the complex oxides of tantalum and niobium leads to the formation of the water-soluble compounds (NH4)2TaF7 and (NH4)3NbOF6, the insoluble lithium fluoride and die gaseous components H20, NH3 and HF. 

Another way of applying the selective extraction method directly on the initial solution is to produce a solution of low acidity. This can be achieved by using the hydrofluoride method for fluorination and decomposition of raw material. As was discussed in Paragraph 8.2.2, the raw material is fluorinated by molten ammonium hydrofluoride yielding soluble complex fluorides of ammonium and tantalum or niobium. The cake obtained following fluorination is dissolved in water, leading to a solution of low initial acidity that is related for the most part to the partial hydrolysis of complex fluoride compounds. The acidity of the solution is first adjusted to ensure selective tantalum extraction. In the second step, the acidity of the raffinate is increased to provide the necessary conditions for niobium extraction. 

Self-Test 11.11B The solubility product of lead(Il) fluoride, PbF, is 3.7 X 10 s. Estimate the molar solubility of the salt. 

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 chlorofiuoride by adding sodium fluoride solution to a saturated lead chloride solution (50). 

Phase-transfer techniques are widely used for the preparation of polymers. For example, potassium fluoride is used to produce poly(etherketone)s under phase-transfer conditions (Scheme 10.18). Use of this reagent allows the chloroaro-matics to be used as starting material as opposed to the more expensive flu-oroaromatics that are usually employed [23]. This method is suitable for the synthesis of high molecular weight semicrystalline poly(ether ketone)s, although the presence of excess potassium fluoride in the reaction mixture can lead to degradation reactions. The use of a phase transfer catalyst can allow the use of water-soluble radical initiators, such as potassium peroxomonosulfate used to promote the free-radical polymerization of acrylonitrile [24],... 

Nevertheless, fluoride does lead to a reduction in the solubility of hydroxyapatite in aqueous solution, even in the absence of trace levels of fluoride in solution, and hence can be seen to have an effect in the solid state as well [57], Apatites are complex and diverse materials which have the general formula Caio(P04)eX2 (X = F, Cl, OH) and they represent a crystallographic system, in which there can be considerable replacement of species. Thus, with little or no change in the dimensions of the crystal lattice, there can be exchanges of OH for F, Ca + for Sr +, and PO4 for CO and all of these are known to occur in biological systems. Natural hydroxyapatite, for example, is often partially carbonate substituted [58]. 

More recently, it has been shown that topical fluoride preparations do not lead to fluoridation of the hydroxyapatite crystal [181]. Rather they form a calcium fluoride-like substance that is deposited onto the tooth surface and dissolves when the local pH is lowered [182]. The resulting dissolution adjacent to the tooth surface provides a source of soluble fluoride that can be incorporated into the mineral structure, and thus augment remineralisation. 

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