Ammonium tetrametaphosphate

Crystalline polyammonium catena-polyphosphate, (NH4) H2P 03 + 1, has been prepared by heating urea and monoammonium orthophosphate under ammonia vapor for 16 hours,1 by ammoniation of superphosphoric acid,2 by thermal condensation of urea phosphate,3 and by heating various ammonium phosphates in a current of ammonia.4,5 The procedure given below, in which crude ammonium tetrametaphosphate is reorganized and condensed to a long-chain polymer in a stream of ammonia, is straightforward and permits the use of common laboratory equipment and supplies. 

Phosphorus(V) oxide is very hygroscopic and the bottle must be closed except when an addition is made. The reactor is cooled with ice water so that the reaction temperature is maintained at 5-10°. Methanol (125 mL)is added drop-wise to the stirred solution to precipitate the crude ammonium tetrametaphos-phate, which is collected by suction filtration, washed with methanol, and dried in vacuum over anhydrous CaS04. The product weighs 33 g and contains about 65% of its phosphorus as tetrametaphosphate. The remainder is a mixture of ortho-, trimeta- and short-chain phosphates. Any lumps are crushed and the crude ammonium tetrametaphosphate is spread in a no. 1 porcelain evaporating dish and heated for 2 hours at 240° in a slow stream,of anhydrous ammonia (about 100 mL/min or 3-5 g/hr). Care is taken to avoid absorption of moisture before or during heating, as this adversely affects the formation of the product. 

The tetrametaphosphate anion is transformed into the amidotetraphos-phate, (P4O12NH2)5 , by concentrated ammonia solution just as the anion ring of trimetaphosphate is cleaved by ammonia to amidotriphosphate. Acidification of the solution results in partial reconversion to the tetrametaphosphate anion, but there is also partial hydrolysis to NH4+ and tetraphosphate ions as well as to amidomonophosphate and triphosphate ions, with simultaneous formation of trimetaphosphate (800). A complete structure determination has been made for ammonium tetrametaphosphate (1) from which the cyclic structure of the anion is evident (245). The diacid salt Na2H2(P40i2) (108) (see Section V,A) is also undoubtedly a tetrametaphosphate (150) (for structural data, see Section IV,E,3). 

P03] J -groups, the metaphosphates, in which, however, not only ring-shaped complex anions, such as are found in (NH4)4(P03)4 ammonium tetrametaphosphate (Romers), but also very long chains occur as in glassy meta-phosphoric acid itself. 

Salts of the amidotriphosphate anion are obtained by the action of concentrated ammonia on the trimetaphosphate ion. With acids, the latter is regenerated with the elimination of ammonia (7.271). If ammonium trimetaphosphate is used, ammonium amidotriphosphate can be crystallised out. The monoamido tetraphosphate anion can be obtained in a similar way from ammonium tetrametaphosphate. 

K2P20 Potassium dimetaphosphate. Na2P209, Sodium trimetapnosphate. Pb2P4012, Lead tetrametaphosphate. (NH4)6PsP15, Ammonium pentametaphosphate. Na4P4019, Sodium hexametaphosphate. 

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