Fluoride salts, solubility

Although the data for the silver halides suggest that silver(I) fluoride is likely to be more soluble than the other silver halides (which is in fact the case), the hydration enthalpies for the sodium halides almost exactly balance the lattice energies. What then is the driving force which makes these salts soluble, and which indeed must be responsible for the solution process where this is endothermic We have seen on p. 66 the relationship AG = — TAS and... 

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. 

Ce(III) forms a water-insoluble hydroxide, carbonate, oxalate, phosphate, and fluoride sparingly soluble sulfate and acetate and soluble nitrate and chloride (and bromide). In solution the salts are only slightly hydrolyzed. The carbonate is readily prepared and is a convenient precursor for the preparation of other derivatives. The sparingly soluble sulfate and acetate decrease in solubihty with an increase in temperature. Calcination of most Ce(III) salts results in Ce02. 

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. 

The elements have remarkably low specific gravity, and a high atomic volume (q.v.). The oxides and hydroxides are markedly basic they do not exhibit acidic qualities. The physical properties of the salts—solubility in water, molecular volume, optical properties, and the variation in the form of the crystals show the same order of variation as the atomic weights of the elements. Lithium differs in mafiy respects from the other members of the family. The salts of the alkali metals —nitrates, chlorides, sulphides, sulphates, phosphates, carbonates, etc.—are nearly all soluble in water, although lithium, carbonate, phosphate, and fluoride are very... 

The element is difficult to maintain in aqueous solution in the form of simple salts. Solubility data seem to indicate that such amounts as can be dissolved probably do so entirely by formation of complex ions. Fluoride ion strongly complexes protactinium, and it is due to this that protactinium compounds are in general soluble in hydrofluoric acid. 

The toxic nature of fluoride ion, F, is not confined to its presence in HF. It is toxic in soluble fluoride salts, such as NaF. At relatively low levels, such as about 1 ppm, used in some drinking water supplies, fluoride prevents tooth decay. At excessive levels, fluoride causes fluorosis, a condition characterized by bone abnormalities and mottled, soft teeth. Livestock are especially susceptible to poisoning from fluoride fallout on grazing land as a result of industrial pollution. In severe cases, the animals become lame and even die. 

Typically, the ammonium or hydronium form of the zeolite is slurried in water and reacted with the requisite amount of the ammonium fluoride salt of either iron or titanium. Given the sparingly soluble nature of ammonium fluotitanate, the (NH )2TiF6 can be added to the zeolite slurry in slurry form, or directly as salt. Alternatively, the iron may be added to the zeolite slurry as a slurry of the ammonium fluoride salt crystals in water, or the FeF3 crystals can be mixed with a water solution of ammonium fluoride or ammonium bifluoride, such that the composition of the treatment solution contains the stoichiometry of the ammonium iron fluoride salt, (NH4)3FeF6. The (NH )3FeF6 crystals can also be added directly to the zeolite slurry. The amount of either metal ammonium fluoride salt added during the reaction is determined by the desired product composition and the anticipated completeness of the reaction. 

A number of different fluoride salts have been used as electrolytes for electropolishing silicon. Not only must the fluoride salt be sufficiently water soluble but the anode reaction product, the corresponding fiuosilicate, must also be readily soluble in water. Potassium fluoride is highly water soluble but the fiuosilicate is not. At a critical anodic etch rate silicon will passivate in KF solutions due to the precipitation of SiFg on the surface. Ammonium fluoride and ammonium fiuosilicate are both sufficiently water soluble. The acids HF and SiFg are even more water soluble. 

The solubilities of salts in water (clearly of importance in aquatic, analytical, and geochemistry) can be fairly well predicted and explained using two principles later we shall return to the more familiar of these, the hard soft acid base (HSAB) principle (see Hard Soft Acids and Bases). To control this principle, we must first consider oifly salts of anions that are hard bases, that is, salts of 0x0 and fluoro anions, and oxides, hydroxides, and fluorides. The solubilities of salts of these anions can be fairly well predicted and explained on the basis of the acidity classification of the cation and the basicity classification of the anion, that is, on a principle of acid base strength. The numerous solubility rules taught in General Chemistry could be replaced with four solubility principles (Table 4), two of which are quite reliable and two of which are less reliable, for known reasons. 

Depending on the structure of silyl ethers, they can be deprotected by H2O, aqueous acids, and fluoride salts. Since silicon has a strong affinity for fluoride ion (bond energy, kcal/mol Si-F, 143 Si-0, 111), the O-SiRgbond is especially prone to cleavage by fluoride salts, such as n-Bu4N F , which is soluble in organic solvents such as THF and CH Clj. 

TABLE 2.6. Solubility (mol/liter) of Various Fluoride Salts in Water at Room Temperature ... 

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... 

Sodium fluoride is soluble. The sodium fluosilicate may be purified and marketed directly for purposes such as fluoridation of water supplies, or it may be used to prepare other salts, for example, synthetic cryolite (NasAlFg), valuable for aluminum smelting. 

The test identifies the substance to be examined as a salt of silver (Ag+). At the present silver is referenced in only one monograph, silver nitrate. This salt is sold in sticks, called Lapis or Lapis lunaris, and is used for the treatment of warts. Silver can exist both as silver(l) and silver(ll), but since the latter is less stable, silver(I) dominates. It is a noble metal meaning that it is most stable in oxidation state 0, its metallic form. So, contrary to most metals, it does not have at tendency to be oxidized to its ionic form and thereby corrode. Silver nitrate and fluoride are soluble, while silver nitrate, acetate, and sulfate have limited solubility. All other salts are insoluble but are, however, capable of forming many soluble complexes. The insolubility of the chloride salt and its ability to form a soluble nitrate complex are useful in identification. 

Fluoride-liberating chemicals are used in some automobile wheel cleaners, glass etching solutions, insecticides, rodenticides, aluminum production, vitamins or dietary supplements, and products to prevent dental caries. Most toothpaste contains up to 5 mg fluoride per teaspoon. Fluoride is commonly added to community drinking water. It is also found in hydrofluoric acid (see p 221), which is used for etching glass and cleaning silicon chip products. Soluble fluoride salts are rapidly absorbed and are more acutely toxic (Table 11-25). 

Coolant. Most fission products (excluding primary krypton and xenon) and all actinides escaping the fuel are soluble in the molten salt and will remain in the molten salt at very high temperatures. Cesium and iodine remain in the salt. Fluoride salts were chosen for the liquid-fueled molten salt reactor, in part because actinides and fission products dissolve in the molten salt at very high temperatures. ... 

Most salts of Ba are stable in air, but the acetate is efflorescent. The acetate, eyanide, ehloride, ehlorate, perchlorate, bromide and iodide are readily soluble the nitrate and the hexacyanoferrate(4-) moderately soluble the fluoride shghtly soluble and the earbonate, oxalate, phosphate, sulfite, sulfate, iodate and chromate insoluble. 

Table 5.2 Molar compositions, melting temperatures (°C) [23] and solubility of plutonium trifluoride (mol%) at 600°C in different molten fluoride salts considered as candidates for the fuel and the coolant circuits...

Unlike the MSR, the FHR use solid fuel and a clean liquid salt as a coolant (i.e., a coolant with no dissolved fissile materials or fission products). For the MSR, a major constraint was the requirement for high solubility of fissile materials and fission products in the salt another one for suitable salt reprocessing. For the FHR, these requirements do not exist. The requirements mainly include (1) a good coolant, (2) low coolant freezing points, (3) stability under irradiation, and (4) application-specific requirements. As a result, a wider choice of fluoride salts can be considered. In all cases, binary or more complex fluoride salt mixtures are preferred because the melting points of fluoride salt mixtures are much lower than those of single-component salts. 

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