Alkali Metal Orthophosphates

Lithium orthophosphate, Li3P04, has recently become of interest in connection with its electrical properties (Section 12.19). It is reported [9] to exist in three (possibly four) different forms  

Some of the above salts dissolve incongruently, that is, to say if they are dissolved in water and evaporated, another species may be obtained. 

The general method (used commercially) of making alkali metal orthophosphates is to neutralise orthophosphoric acid with alkali metal hydroxide or carbonate. Crystals of a specific hydrate can then be obtained by evaporation of a solution within the temperature range over which the hydrate is stable. Many of the sodium phosphates are commercially available chemicals and they have 

Slight hydrolysis of Na3P04 always occurs in aqueous solution (5.19) and for this reason it finds use in detergent compositions as a mild source of alkali (Table 5.14). It breaks up fats and grease into water-soluble compounds (5.33). It can be used as a water softner, although for this purpose it has been largely superceded by polyphosphates (Section 5.4). Insoluble phosphates are precipitated from the compounds which give rise to water hardness (5.34, 5.35). 

The purest form of anhydrous trisodium phosphate can be prepared by heating a dry mix of sodium carbonate and sodium pyrophosphate (5.37). The anhydrous salt can also be prepared directly from some mineral phosphates by fusion with sodium carbonate at 900°C (5.38). 

---

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. 

Volume 31 J. Eysseltova and T. P. Dirkse, Alkali Metal Orthophosphates... 

Lithium orthophosphates are unimportant and differ from the other alkali metal phosphates in being insoluble. At least 10 crystalline hydrated or anhydrous sodium orthophosphates are known and these can be grouped into three series ... 

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

Orthophosphates. Expls characterized by the evolution of N on heating may be effectively stabilized by the addition of approx 1% of a neutral mixt of alkali metal dihydrogen orthophosphate (ie, NaH2PC>4) and dialkali metal hydrogen orthophosphate (ie, Na2 HPO4)... 

Orthophosphates. Aside from their use as reagents in the chemical laboratory, most of the possible metal salts of orthophosphoric acid have not found extensive application in the chemical industries. There are, however, a few notable exceptions. The normal phosphates of the alkali metals are readily soluble in water and are used to a considerable extent as cleansing agents and water softeners. 

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... 

Although alkali metal and ammonium orthophosphates are water soluble, most orthophosphates of other metals are insoluble or nearly so (Table 5.12). Melting points are often in excess of 1000°C (Table 5.13), although many of these salts under normal atmospheric conditions tend to lose P2O5 before their melting point is reached. 

Some beryllium phosphates also crystallise with partly covalent structures analogous to those of known polymorphic varieties of silica. Thus, the orthophosphates MBeP04 (M = K, Rb, Cs) have tridymite-type networks of alternating Be04 and PO4 tetrahedra, with the alkali metal cations situated in the cavities formed in the structure (long-chain beryllium polyphosphates also form silicalike structures (Section 5.4)). 

Heavy metal orthophosphates (M = Cr, Mn, Fe, Co, Ni, Zn, Hg, Pb, Ag) can be prepared in hydrated form by simple double decomposition involving aqueous solutions of an alkali hydrogen phosphate and the appropriate metal salt. Cobalt phosphate octahydrate, for example, is obtainable as a beautiful lavender-coloured precipitate from cobalt chloride and potassium dihydrogen phosphate. 

Syuthesis of mixed-metal orthophosphates (double salts) cau ofteu be effected by straightforward laboratory methods such as mixiug aqueous solutious (5.94), or heatiug a metal ammouium phosphate with alkali carbouate to 800°C (5.95, 5.96). 

Soluble varieties of oligopolyphosphates (n = 10-50) give solutions which are neutral or very slightly acidic, in contrast to the shorter-chain compounds (e.g. n = 2, 3, 4) which give an alkaline reaction. Linear polyphosphates are reasonably stable in neutral or alkaline solution at room temperature. Their hydrolysis is strongly acid-catalysed however, and, like all condensed phosphates, they can be eventually converted to orthophosphates by boiling. Alkali metal pyrophosphates are very stable in alkaline or neutral solution at normal temperature. Although pyrophosphates are hydrolysed under... 

The soluble cyclic metaphosphates all undergo cleavage on alkaline hydrolysis to produce, initially, the corresponding linear polyphosphate (5.183), which will then undergo further splitting until eventually only orthophosphate anions are left in solution. Both alkali metal triphosphates and tetraphosphates can nevertheless be isolated by this method, but octametaphosphate is more resistant to hydrolysis. The linear octaphosphate, when it is produced, is quickly split into smaller units before significant amounts can be collected (Figure 5.28). 

Hypophosphates are extremely stable to alkali hydroxides. No decomposition occurs in 80% NaOH at 200°C after 1 h, but with fused caustic soda at 320°C there is rapid conversion to orthophosphate (5.244). If silver, mercury or copper hypophosphates are heated below 200°C in nitrogen, decomposition to metal, orthophosphate and a mixture of condensed phosphates occurs. Strong heating gives a mixture of metaphosphate and metal (5.245). 

The various systems are treated in the order in which the alkali metals are listed in Group I of the Periodic Table. Most of the available solubility data are for the orthophosphates of sodium and potassium, and for these two systems an introductory chapter on the M0H-HjP0 -H20 (M = Na or K) system is given. Each of these chapters (chapters 2 and 7) also refers to compounds to be considered in later chapters. Following each of these introductory chapters there are chapters dealing with the solubility data for individual orthophosphates having different M/P ratios, and the ternary and multicomponent systems in which these orthophosphates are components. Only one chapter is devoted to each of the orthophosphates of lithium, rubidium and cesium. 

---