Calcium fluoride, solubility

In the more difficult calcium fluoride solubility problem, ion pairing prevented us from reducing the equations to one unknown. Instead, [H+] and [F ] were both unknown. However, for a fixed value of [H+], we could use SOLVER to solve for [F . The correct pH is the one at which the charge balance is satisfied. 

Many ionic halides dissolve in water to give hydrated ions. The solubility of a given halide depends on several factors, and generalisations are difficult. Ionic fluorides, however, often differ from other halides in solubility. For example, calcium fluoride is insoluble but the other halides of calcium are highly soluble silver fluoride. AgF, is very soluble but the other silver halides are insoluble. 

In the geochemistry of fluorine, the close match in the ionic radii of fluoride (0.136 nm), hydroxide (0.140 nm), and oxide ion (0.140 nm) allows a sequential replacement of oxygen by fluorine in a wide variety of minerals. This accounts for the wide dissemination of the element in nature. The ready formation of volatile silicon tetrafluoride, the pyrohydrolysis of fluorides to hydrogen fluoride, and the low solubility of calcium fluoride and of calcium fluorophosphates, have provided a geochemical cycle in which fluorine may be stripped from solution by limestone and by apatite to form the deposits of fluorspar and of phosphate rock (fluoroapatite [1306-01 -0]) approximately CaF2 3Ca2(P0 2 which ate the world s main resources of fluorine (1). 

Although stable at ambieat temperature, calcium fluoride is slowly hydrolyzed by moist air at about 1200°C, presumably to CaO and HF. Calcium fluoride is not attacked by alkahes or by reactive fluorine compounds, but is decomposed by hot, high boiling acids, as ia the reactioa with coaceatrated sulfuric acid which is the process used to produce hydrogea fluoride. Calcium fluoride is slightly soluble ia cold dilute acids, and somewhat more soluble ia solutioas of alumiaum hahdes. 

Properties. Lithium fluoride [7789-24-4] LiF, is a white nonhygroscopic crystaUine material that does not form a hydrate. The properties of lithium fluoride are similar to the aLkaline-earth fluorides. The solubility in water is quite low and chemical reactivity is low, similar to that of calcium fluoride and magnesium fluoride. Several chemical and physical properties of lithium fluoride are listed in Table 1. At high temperatures, lithium fluoride hydroly2es to hydrogen fluoride when heated in the presence of moisture. A bifluoride [12159-92-17, LiF HF, which forms on reaction of LiF with hydrofluoric acid, is unstable to loss of HF in the solid form. 

The S3Tithesis of calcium fluoride requires soluble sources of Ca ions and F ions. We can choose any pair of appropriate salts that happen to be available in the laboratory. Once we have selected suitable salts, we use stoichiometry to calculate the masses of each one. 

CCP in milk is mentioned in connection with casein above (Section VI.C). Fluorapatite is a major constituent of phosphate rocks, and a constituent, probably important, of human tooth enamel for those whose drinking water contains significant amounts of naturally occurring or added fluoride. Fluorapatite is significantly less soluble than hydroxyapatite - the relationship between the solubilities of fluorapatite and hydroxyapatite parallels (but is much less extreme than) that between calcium fluoride (Ksp — 3.9 x 10 11 mol3 dm-9) and calcium hydroxide (Ksp = 7.9 x 10 6 mol3 dm 9). Calcium diphosphate, Ca2P207, is believed to be the least soluble of the calcium phosphates. 

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). 

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. 

The overall mode of action of fluoride varnishes is the same as gels, namely that they deposit soluble fluoride as calcium fluoride. When local pH adjacent to this deposit falls, fluoride is solubilised and then proceeds to become incorporated into the tooth mineral as fluorapatite [196]. 

Think of a question that is answered by the following answer Calcium fluoride is much less soluble in water than is sodium fluoride. ... 

Calcium Fluoride. [CAS 7789-75-51. CaF.-. white precipitate, formed by reaction of soluble calcium salt solution and sodium fluoride solution. pK = 10.40. See also Fluorite. 

Let s consider the solubility equilibrium in a saturated solution of calcium fluoride in contact with an excess of solid calcium fluoride. Like most sparingly soluble ionic solutes, calcium fluoride is a strong electrolyte in water and exists in the aqueous phase as dissociated hydrated ions, Ca2+(aq) and F (aq). At equilibrium, the ion concentrations remain constant because the rate at which solid CaF2 dissolves to give Ca2+(aq) and F aq) exactly equals the rate at which the ions crystallize to form solid CaF2 ... 

Ca(OH)2.Aq + 2HBr.Aq = CaBr2.Aq + 2HaO. These salts are also white and soluble in water. There is, however, one exception, namely, calcium fluoride, CaF2, which occurs native as fluor- or Derbyshire spar. It forms colourless cubical crystals, and is the chief compound of... 

Concentrated sulfuric acid Calcium chloride Fluorides, F" Evolution of hydrogen fluoride dimer White, slimy precipitate of calcium fluoride slightly soluble in dilute hydrochloric acid... 

The methods of synthesis of fluorapatite have been widely dis cussed (J ). It is for example possible to obtain fluorapatite by substituting the hydroxyl ion for the fluoride ion, either in a-queous solution at room temperature, or through a solid state reaction at 800°C. It can also be prepared by the action of 6-tricalcium phosphate on calcium fluoride at about 800°C. Its solubility and thermal stability have already been established. While much is known about fluorapatite, many questions still exist concerning the mechanism of their formation, their composition and the structure of some of them. Two of these problems are dealt with here. First, we discuss the formation mechanism of fluorapatite by a solid state reaction between calcium fluoride and apa-titic tricalcium phosphate. Then we present the preparation and the structure of a carbonated apatite rich in fluoride ions. 

The F content in recent bone or dentine apatite is normally less than 0.1 wt.%. For ancient specimen, F is known to diffuse during burial into bone material. Its enrichment is generally a part of many complex diagenetic changes of bone and tooth, which remains after their deposit. Fluorine can react with the bone and dentine mineral phase to form calcium fluoride compounds. It usually substitutes for hydroxyl ions in hydroxyapatite, leading to the less soluble fluorapatite compound (Ca10(PO4)6(F)2, FAP). 

Alternatively, antisealants can be used to control calcium carbonate scale at LSI values as high as 2.0-2.5, depending on the specific antisealants. Calcium also forms scales with fluoride, sulfate, and phosphate. The LSI will not help predict these scales analysis of water quality, using the ion product and solubility constants, is required to determine the potential for scaling with calcium fluoride or calcium phosphate. Antisealants currently available can address calcium fluoride and calcium sulfate scale they do not address calcium phosphate scale (although newer antisealants will be available in the near future to address this scale). 

Calcium fluoride scale can form when the concentration of fluoride is as low as 0.1 ppm if the concentration of calcium is high. Scaling will occur when the ion product exceeds the solubility constant. Antisealants or sodium softening can be used to control calcium fluoride scale. 

In soils, F can be found in four major fractions (1) dissolved in soil solution (2) sorbed to Al, Fe, and Mn oxides and hydroxides and carbonates (3) solid phases, such as fluorite and fluorophlogopite and (4) associated with organic compounds. The solubility of F in soil solution is variable and is affected by pH, speciation, adsorption and desorption reactions, and dissolution and precipitation reactions (Luther et al., 1996). Acidic conditions and low calcium carbonate content are favorable to F solubility and can therefore enhance both root uptake (Weinstein and Alscher-Herman, 1982) and migration to surface and ground water (Smith, 1983). These conditions can lead to human, plant, and animal health issues. Soils that do contain appreciable amounts of calcium carbonate and are neutral to slightly alkaline conditions can fix F as insoluble calcium fluoride (CaF2), and reduce its bioavailability and mobility (Kubota et al., 1982 Tracy et al., 1984 Reddy et al., 1993 Poulsen and Dudas, 1998). 

EXAMPLE 7-2 The solubility product of calcium fluoride is 4 x 10 Calculate its solubility S, neglecting hydrolysis of fluoride and assuming 5 equal to zero. 

N aFd-Si02+2H2S. The hydrogen sulphide is collected and utilized, and the mixture of silica and sodium fluoride is treated with calcium hydroxide or carbonate to form respectively soluble sodium hydroxide or carbonate, and insoluble calcium fluoride mixed with silica. The insoluble residue is mixed with hydrochloric acid and then with sodium chloride and sodium fluosilicate is regenerated. 

The maximum concentration of a salt in an aqueous solution is called the solubility of the salt in water. Solubilities can be expressed in moles of solute per liter of solution (mol/L or M). For example, the solubility of calcium fluoride in water is 3.4 x 10 " mol/L. So, 0.00034 mol (or less than 0.03 g) of CaF2 will dissolve in 1 L of water to give a saturated solution. If you try to dissolve 0.00100 mol of CaF2 in 1 L of water, 0.00066 mol of CaF2 will remain undissolved. 

Like most salts, calcium fluoride is an ionic compound that dissociates into ions when it dissolves in water. Calcium fluoride is also one of a large class of salts that are said to be slightly soluble in water. The ions in solution and any solid salt are at equilibrium, as the following equation for CaF2 and water shows. 

Metal hydroxides can be described as salts of a strong base, the hydroxide ion. The solubility of salts in which the anion is a different weak or strong base is also affected by pH. For example, consider a solution of a slightly soluble fluoride, such as calcium fluoride. The solubility equilibrium is... 

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