Calcium carbonate solid-state reactions with

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. 

For high purity calcium aluminate compositions, solid-state synthesis is still the norm [33, 34], Most CAC compounds are made by solid-state reactions between ground powders of calcium carbonate and purified alumina. The sintering temperatures depend on alumina content. More recently attempts have been made to synthesize CA compounds using processes with temperatures less than 900°C. These latter methods include sol-gel synthesis and precipitation and are important for production of high-purity homogenous powders with small grain size. 

One of the major uses of DTA has been to follow solid-state reactions as they occur. AU decomposition reactions (loss of hydrates, water of constitution, decomposition of inorganic anions, e.g.- carbonate to carbon dioxide gas, etc.) are endothermic and irreversible. Likewise are the synthesis reactions such as CaO reacting with AI2O3 to form calcium... 

The periodates 0 the alkaline earths.—C. F. Rammelsberg13 obtained calcium metaperiodate, Ca(I04)2, by the action of periodic acid on a soln. of calcium hydroxide or dimesoperiodate, the solid separates on concentrating the acid soln. similarly, when a soln. of strontium carbonate in an excess of periodic acid is cone, in a desiccator, large, probably triclinic, crystals of hexahydrated strontium metaperiodate, Sr(I04)2.6H2O, are formed. They lose 12-36 per cent, of water when cone, over cone, sulphuric acid, and the remainder at 100°. This salt explodes when heated the aq. soln. has an acid reaction, and gives a precipitate when treated with ammonia. When the attempt is made to concentrate the soln. of barium metaperiodate, Ba(I04)2, prepared in a similar manner, the dimesoperiodate is deposited when the soln. is cone., so that the metaperiodate has not been isolated in a solid state. 

Molecular theory of caustification.—An excess of solid calcium hydroxide is supposed to be present at the start, so that as fast as calcium hydroxide is removed from the soln. by reacting with the potassium carbonate, more passes into soln. Thus the cone, of the calcium hydroxide in the soln. is kept constant. The. solubility of calcium carbonate is very small, and, in consequence, any calcium carbonate in excess of the solubility will be precipitated as fast as it is formed. The reaction proceeds steadily from right to left because, all the time, calcium hydroxide steadily passes into soln., and calcium carbonate is steadily precipitated but the solubility of calcium carbonate steadily increases with increasing cone, of potassium hydroxide. There is a steady transformation of the potassium carbonate into potassium hydroxide in progress The cone, of the potassium carbonate is steadily decreasing, while the cone, of the potassium hydroxide is steadily increasing. Consequently, when the potassium hydroxide has attained a certain cone, so much calcium carbonate will be present in the soln. that the reaction will cease. Hence the cone, of the potassium carbonate should be such that it is all exhausted before the state of equilibrium is reached. If the cone, of the potassium hydroxide should exceed this critical value, the reaction will be reversed, and calcium carbonate will be transformed into calcium hydroxide. 

For a reaction with positive gas mole change, Eq. (47) indicates that Kx decreases with pressure. Because ce is a monotonically increasing function of Kx, the equilibrium extent of a reaction with positive Avgas always decreases as pressure is increased. This is an example of Le Chatelier s principle, which states that a reaction at equilibrium shifts in response to a change in external conditions in a way that moderates the change. In this case, because the reaction increases the number of moles of gas and thus the pressure, the reaction shifts back to reactants. The isothermal compressibility of a reactive system can, therefore, be much greater than that of a nonreactive system. This effect can be dramatic in systems with condensed phases. For example, in the calcium carbonate dissociation discussed in Example 12, if the external pressure is raised above the dissociation pressure of C02, the system will compress down to the volume of the solid. Of course, a similar effect is observed in simple vaporization or sublimation equilibrium. As the pressure on water at 100°C is increased above 1.0 atm, all vapor is removed from the system. 

Ca—O = 2.30 A). The closeness of the molecules help explains why under irradiation with 7-rays from °Co, a solid-state cyclodimerization reaction in induced, producing cis,trans-TO.epe. ic acid, one of four possible diastereomers. Related radiation-induced chemistry is displayed by aquated (3-butenoato)calcium, Ca(CH2=CHCH2C02)2(H20), synthesized from 3-butenoic acid and calcium carbonate. The carboxylate is a two-dimensional coordination polymer with nearly parallel vinyl groups and short -C=C—C=C- contacts of 3.73 A and 3.90 A (see Figure 63). ... 

Discussion Of the various methods for the formation of carbon dioxide, which will be taken up later in Experiment 40, the most convenient for laboratory preparation is the action of an acid on a carbonate. The progress of the reaction depends upon the instability of carbonic acid, which breaks up into water and carbon dioxide. Substances are chosen, therefore, which will react to form carbonic acid by double decomposition and these are, as just stated, a carbonate and an acid. If a slow steady stream of gas is desired, an insoluble carbonate, usually calcium carbonate, is selected, since a soluble carbonate reacts too rapidly. The acid employed must be one which will form a soluble compound with the metal of the carbonate, for if an insoluble compound were formed, it would coat over the solid carbonate and prevent further access of the acid. 

After the mixture is cooled to 75°, 200 ml. of benzene is added (Note 5). The mixture is stirred briefly, and the liquid is decanted from the solid into a 5-1. separatory funnel. The aqueous layer is separated and extracted again with 200 ml. of benzene. The aqueous layer is then returned to the separatory funnel, and the solid in the reaction flask is washed in with it by means of 2 1. of water. The mixture is shaken with 200-ml. portions of benzene until all solid has dissolved (usually three portions of benzene suffice). All the benzene extracts are combined and dried with 20 g. of anhydrous calcium chloride. The drying agent is removed by filtration and washed with two 20-ml. portions of benzene. Benzene is removed from the filtrate by distillation under reduced pressure (Note 6), and the solid which separates is dried further at about 10 mm. and room temperature to constant weight. The yield (Note 2) of methyl -tolyl sulfone is 298-317 g. (69-73%), m.p. 83-87.5°. Further purification is generally unnecessary, but, if desired, the product may be recrystallized from carbon tetrachloride or ethanol-water (1 1). The submitters state that the method may be extended to the preparation of methyl phenyl sulfone and, presumably, of methyl aryl sulfones generally (Note 7). 

The fact that ArG° is positive means that the reaction as written will proceed to the left (calcite precipitates), if all products and reactants are in their standard states. Unfortunately, although pure solid calcite is in its standard state (and therefore has an activity of 1.0), the calcium and carbonate ions are never in their standard states, because this has been chosen to be a hypothetical ideal solution with a concentration of 1 molal (it is not surprising that calcite would be calculated to precipitate from a solution with the ions at 1 molal ). Therefore, this result is not very useful, except in the next step. 

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