Alkaline Earth Orthophosphates

The phosphates of alkaline earth metals are generally much less soluble than those of the alkali metals (Tables 5.16 and 5.17). Among the magnesium salts are 

A crystalline precipitate of magnesium ammonium phosphate hexahydrate (struvite) is produced when other magnesium salts are added with ammonia to weakly basic or neutral phosphate solutions (5.39). The ignition of this salt to give the pyrophosphate forms the basis of a gravimetric method of analysis for P (Chapter 14.1). 

Na2HP04 + MgClj + NHj -I- 6H2O MgNH4P04 6H2O + 2NaCl 

Quick-setting cements based on magnesium orthophosphates have been developed (Chapter 12.10). 

The calcium orthophosphates, all of which are white when pure, are the most important salts of orthophosphoric acid [12-23]. They include the most widespread and abundant source of phosphorus that is, apatite they are essential components of bones and teeth and they form a much-used group of fertilisers. The animal metabolism of phosphorus is in large part related to that of calcium, and both elements are required by plants and animals (Table 5.18). 

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Next to the acidic alkali phosphates, alkaline earth orthophosphates are of interest, because they represent many CBPC products, such as CaH2P04-H20 and MgH2P04 2H20. They are also the result of an acid-base reaction between an acid phosphate and an oxide containing an alkaline earth metal, such as Ca and Mg. They exhibit the same structure... 

Figure 7.20. Schematic diagram of the range of P chemical shifts in crystalline phosphate and aluminophosphate phases. The Q° range refers to the alkali and alkaline earth orthophosphates, Q denotes the end groups, the middle and ring groups and the branching groups in these compounds. The upper three bands refer to aluminophosphates, including those of the alkali and alkaline earth metals. From data of Turner et al. (1986a).

Dimova M, Panczer G, Gaft M (2006) Spectroscopic study of barite (Kremikovtsi deposit, Bulgaria) with implication for its raigin. Ann Geol de la Peninsnle Balkanique 67 101-108 Donker H, Smit W, Blasse G (1989) On the luminesctaice of some tin-activated alkaline-earth orthophosphates. J Electrochem Soc 136 3130-3135 Dorenbos P (2003) Anomalous luminescence of Eu and Yb in inorganic compoimds. J Phys Condens Matter 15 2645-2665... 

Orthophosphate salts are generally prepared by the partial or total neutralization of orthophosphoric acid. Phase equiUbrium diagrams are particularly usehil in identifying conditions for the preparation of particular phosphate salts. The solution properties of orthophosphate salts of monovalent cations are distincdy different from those of the polyvalent cations, the latter exhibiting incongment solubiUty in most cases. The commercial phosphates include alkah metal, alkaline-earth, heavy metal, mixed metal, and ammonium salts of phosphoric acid. Sodium phosphates are the most important, followed by calcium, ammonium, and potassium salts. 

Alkali Meta.IPhospha.tes, A significant proportion of the phosphoric acid consumed in the manufacture of industrial, food, and pharmaceutical phosphates in the United States is used for the production of sodium salts. Alkali metal orthophosphates generally exhibit congment solubility and are therefore usually manufactured by either crystallisation from solution or drying of the entire reaction mass. Alkaline-earth and other phosphate salts of polyvalent cations typically exhibit incongment solubility and are prepared either by precipitation from solution having a metal oxide/P20 ratio considerably lower than that of the product, or by drying a solution or slurry with the proper metal oxide/P20 ratio. 

In all of these alkali-metal and alkaline earth-metal orthophosphates there are discrete, approximately regular tetrahedral PO4 units in... 

Lowenstein reacted ATP with orthophosphate (33) in the presence of metal ions, and obtained ADP and pyrophosphate as products. The most active metals in this reaction were, rather surprisingly for a nonenzymatic reaction, the alkaline earths, Cd+2 and Mn+2 the members of the first transition series exhibited low7 activity. The reactive intermediate was formulated as follows ... 

The metal ion specificity for the reaction with acetate was different from that in the reaction with phosphate in the former beryllium was most active, followed by nickel. The alkaline earths that were so effective with phosphate did not catalyze the reaction with acetate at all. The difference in metal specificity in the two reactions was explained by assuming that complexation with the orthophosphate and acetate constitutes an important function in the reaction. 

Orthophosphates.—Solubility.—The tribasic phosphates of the alkali metals and ammonia are soluble, while those of the alkaline earth metals and the common metals are insoluble. They are usually prepared by double decomposition between disodium hydrogen phosphate and a salt of the required metal, thus... 

Formation of the yellow precipitate of Ag3P04 is a common test for orthophosphates. On account of the acid which is liberated precipitation is not complete (see p. 181). Acid phosphates of the alkaline earth metals, e.g. CaHP04, are precipitated from solutions which are nearly neutral. Monoammonium phosphates, MNH4P04, which are so much used in quantitative analysis, are precipitated from neutral or slightly acid solution then made ammoniacal (MgNH4P04),3 or neutral or slightly acid solution (ZnNH4P04 in presence of sodium acetate and acetic acid, pH = 6-l 6-9).4... 

The decomposition reactions of rare earth phosphate minerals promoted by sintering with alkali, alkaline earth, and other compounds have been widely studied (Kizilyalli and Welch, 1980). When the minerals are sintered with alkali compounds the double orthophosphates are formed. With CaO they decompose above 700°C to form calcium phosphate. RPO4 does not react with SiOj, and with AI2O3 the reaction does not begin until 1800°C (Hikichi et al., 1980a). 

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