Calcium fluoride, dissolving

In this section we consider the equilibria associated with solids dissolving in water to form aqueous solutions. When an ionic solid dissolves in water, we typically assume that it dissociates into separate hydrated cations and anions. For example, when calcium fluoride dissolves in water, we typically represent the situation as follows ... 

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. 

The method of obtaining aluminum metal by the electrolysis of alumina dissolved in cryolite was discovered in 1886 by Hall in the U.S. and at about the same time by Heroult in France. Cryolite, a natural ore found in Greenland, is no longer widely used in commercial production, but has been replaced by an artificial mixture of sodium, aluminum, and calcium fluorides. 

Anion Interstitials The other mechanism by which a cation of higher charge may substitute for one of lower charge creates interstitial anions. This mechanism appears to be favored by the fluorite structure in certain cases. For example, calcium fluoride can dissolve small amounts of yttrium fluoride. The total number of cations remains constant with Ca +, ions disordered over the calcium sites. To retain electroneutrality, fluoride interstitials are created to give the solid solution formula... 

The results shown in table 8.2 lead to a mean value ArH° = -918.00 0.50 kJ mol-1. The uncertainty quoted is twice the standard deviation of the mean. It was assumed that CaF2 (cr) was quantitatively obtained in the final state because no enthalpy change was observed in a series of experiments where pure calcium fluoride was dissolved in HF(aq) [143],... 

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. 

Apatite, a natural calcium fluoride phosphate, can adsorb low to moderate levels of dissolved metals from soils, groundwater, and waste streams. Metals naturally chemically bind to the apatite, forming extremely stable phosphate phases of metal-substituted apatite minerals. This natural process is used by UFA Ventures, Inc., and is called phosphate-induced metals stabilization (PIMS). The PIMS material can by used in a packed bed, mixed with the contaminated media, or used as a permeable barrier. The material may be left in place, disposed of, or reused. It requires no further treatment or stabilization. Research is currently being conducted on using apatite to remediate soil and groundwater contaminated with heavy metals, and the technology may also be applicable to radionuclides. The technology is not yet commercially available. 

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 ... 

A saturated solution of calcium fluoride in contact with solid CaF2 contains constant equilibrium concentrations of Ca2+ aq) and F (aq) because at equilibrium the ions crystallize at the same rate as the solid dissolves. 

A reliable method of measuring the mineral matter content of a coal is an acid demineralization procedure. The method depends on the loss of weight of a sample when treated with 40% hydrofluoric acid at 50 to 60°C (122 to 140°F). Treatment of the sample with hydrochloric acid before and after treatment with hydrofluoric acid helps prevent the retention of insoluble calcium fluoride (CaF2) in the coal. Pyrite is not dissolved in the treatment, consequently, pyrite and a small amount of retained chloride must be determined separately. Since two-thirds of the mass of the pyrite (FeS2) is accounted for by the presence of ferric oxide (Fe203) in the residual ash, the mineral matter content is then given by the formula... 

Phosphors used in the production of fluorescent lamps are of the calcium halophosphate type, prepared by firing together calcium hydrogen phosphate, calcium carbonate and a lesser amount of calcium fluoride. The phosphor (0.25 g) can be dissolved in a small amount of concentrated... 

Oxalate in the presence of fluoride Both calcium fluoride and calcium oxalate are precipitated by ammonium oxalate solution in the presence of dilute acetic acid. The fluoride may be identified in the usual manner with concentrated sulphuric acid or as described below. The oxalate is most simply detected by dissolving a portion of the precipitate in hot dilute sulphuric acid and then adding a few drops of a very dilute solution of potassium permanganate. The latter will be decolourized if an oxalate is present. 

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). 

Solid calcium fluoride (Cap2) reacts with sulfuric acid to form solid calcium sulfate and gaseous hydrogen fluoride. The HF is then dissolved in water to form hydrofluoric acid. A source of calcium fluoride is fluorite ore containing 96.0 wt% Cap2 and 4.0% SiOi-... 

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. 

This relationship is true whenever calcium ions and fluoride ions are in equilibrium with calcium fluoride, not just when the salt dissolves. For example, if you mix solutions of calcium nitrate and sodium fluoride, calcium fluoride precipitates. The net ionic equation for this precipitation is the reverse of the dissolution. 

The procedure involves the coprecipitation of the transuranic nuclides on calcium fluoride from acid solution after reduction of the plutonium and neptunium with bisulfite. The calcium fluoride precipitate is dissolved in aluminum nitrate-nitric acid solution and the plutonium and neptunium separated on an ion-exchange resin column. The column... 

Since calcium fluoride is insoluble in water, loss of fluoride in this form is relatively benign. The fluoride present in the slag, however, may be mobilized into the water phase if the hot slag is contacted with water to fracture the material for use as fill. The fluoride not captured in the slag is converted to silicon tetrafluoride and hydrogen fluoride which leave the furnace with the phosphorus vapor and carbon monoxide. However, they are absorbed by the water stream of the phosphorus condensers (Eq. 10.11). Thus, the water from the phosphorus condenser contains dissolved and colloidal phosphorus and dissolved fluoride as the ion and as the complex fluosilicate anion. 

A solution is produced by stirring 1 gram of calcium fluoride in one liter of water at 25°C. Careful analysis shows that 0.0163 grams of calcium fluoride has dissolved. Calculate the p for calcium fluoride based on these data. 

Apatites must undergo a solid-state transition to amorphous calcium phosphate before they can dissolve and the spontaneous replacement of hydroxide with fluoride ions slows the rate at which this transition occurs (Fig. 16.6b). Conversely, as an acid environment becomes more alkaline, fluoride ions promote the precipitation and crystallization of amorphous calcium monohydrogen phosphate/calcium fluoride into fluoro- and difluoro-apatites faster than amorphous calcium phosphate would crystallize into hydroxyapatite. Thus, fluoride ions have two effects on enamel that protect from caries they slow enamel dissolution in lactic acid and promote its re-precipitation and crystallization when the lactic acid is neutralized. 

Oxidizing gas and slag-forming materials, such as silica (Si02) sand, lime (CaO), limestone (CaCOs), dolomite (CaO-MgO), or calcium fluoride (CaF2) are added to the still-molten raw silicon in a refinement step. Less noble elements than silicon are oxidized and dissolved in the slag, which is removed. To avoid excessive heat losses, some of the silica can be provided directly by oxidation of some of the silicon melt instead of sand. Carbon is removed from the melt in the form of SiC precipitates. 

When a solid ionic compound is dissolved in water it is often assumed that the compound is completely separated into an anion and a cathion. As example we look at the dissolution of solid calcium fluoride in... 

Consider dissolving slightly soluble calcium fluoride, Cap2, in H2O. 

In precipitation reactions, two soluble ionic compounds react to form an insoluble product, a precipitate. The reaction you just saw between silver nitrate and sodium chromate is an example. Precipitates form for the same reason that some ionic compounds do not dissolve the electrostatic attraction between the ions outweighs the tendency of the ions to remain solvated and move randomly throughout the solution. When solutions of such ions are mixed, the ions collide and stay together, and the resulting substance comes out of solution as a solid, as shown in Figure 4.5 for calcium fluoride. Thus, the key event in a precipitation reaction is the formation of an insoluble product through the net removal of solvated ions from solution. 

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