Tetrametaphosphate

Although reasonably stable at room temperature under neutral conditions, tri- and tetrametaphosphate ions readily hydrolyze in strongly acidic or basic solution via polyphosphate intermediates. The hydrolysis is first-order under constant pH. Small cycHc phosphates, in particular trimetaphosphate, undergo hydrolysis via nucleophilic attack by hydroxide ion to yield tripolyphosphate. The ring strain also makes these stmctures susceptible to nucleophilic ring opening by other nucleophiles. 

Sodium tetrametaphosphate [13396-41-3] M 429.9, pKj 2.60, pKj 6.4, pKj 8.22, pK 11.4 (tetrametaphosphoric acid, H4P4O12). Crystd twice from water at room temperature by adding EtOH (300g of Na4P40i2,H20, 2L of water, and IL of EtOH), washed first with 20% EtOH then with 50% EtOH and air dried [Quimby J Phys Chem 58 603 1954]. 

Gouy method, 32 4, 10 Graham s salt, 4 4, 5, 6, 7, 10-11, 39 glasslike, 4 41-50 tetrametaphosphates from, 4 20 trimetaphosphate from, 4 44-45 ultraphosphates from, 4 56 Grain coarsening, 31 14 Graphite... 

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. 

In addition, Pauling and Sherman (222) had proposed a ring structure for the anion of aluminium tetrametaphosphate on the basis of a crystal structure determination. The triphosphate NaBP3Oi0 must, then, have the chain structure already proposed by Schwarz (263), for Rudy and Schloesser (250) showed that the acid corresponding to the triphosphate has three strongly and two weakly acidic OH groups with the weakly acidic groups... 

A comparative study of the products of dehydration of the dihydrogen monophosphates of polyvalent cations showed that the stable end-products for cations with ionic radii between 0.57 and 1.03 A. (Cu++, Mg++, Ni++, Co++, Fe++, Mn++, Zn"1-1", Cd++, A1+++) are tetrametaphosphates. When the cations are either larger or smaller the end-products of dehydration are crystalline high-molecular polyphosphates (Li+, Be++, K+, Rb+, Cs+, Ag+, Zn++, Cd++, Hg++, Ca++, Sr++, Ba++ Pb++, Cr+++, Fe+++, Bi+++). In the case of the alkali salts only sodium trimetaphosphate occurs as a condensed phosphate with a cyclic anion (304, 305). Up to the present, an alkali tctrametaphosphate has not been observed as the dehydration product of a dihydrogen monophosphate. Consequently, alkali tetrametaphosphates arc obtainable only indirectly. Reference is made later (Section IV,C,4) to the fact that the tetraphosphates of barium, lead, and bismuth are formed as crystalline phases from melts of the corresponding composition. There are also reports of various forms of several condensed phosphates of tervalent iron and aluminum (31, 242, 369). 

As described in Section VII, only chromatography is dependable for determining the (P309)3- anion quantitatively in presence of other condensed phosphates. However, since trimetaphosphate does not rapidly form a sparingly soluble precipitate with any other cation, especially in acid solution, an approximately quantitative determination is possible after precipitation of all other phosphates with barium (71, 180), provided tetrametaphosphate is not present. It can also be determined by quantitative evaluation of infrared spectra (58) and, when only crystalline substances are present, by Debye X-ray photographs (178). 

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). 

The infrared and ltaman spectra of sodium tetrametaphosphate differ little from those of the trimetaphosphates (57, 139). The Raman spectra of crystalline tetrametaphosphates vary with the nature of the cation (275) and, from the infrared spectra, different symmetries are assigned to the anions of different salts. 

Potassium tetrametaphosphate crystallizes with two molecules of water and becomes anhydrous at 100°C. Above this temperature the anhydride takes up water from the atmosphere and goes over to the high-molecular... 

Measurements of the electrical conductivity and solubility of difficultly soluble salts in sodium tetrametaphosphate solutions 62, 151, 196, 197) and potentiometric titrations lead to the assumption of the existence of complexes of the type of Na+(P40i2)3 Ba"1-1", Sr, Ca++, Mg++, Mn++, Ni++, Cu++(PiO]2) - La(P40i2) Cu(P40i2)26- and Ni(P40i2)26. No comparable dissociation constants for these have so far been given, though in any case they will be smaller than for the corresponding ion pairs of the trimetaphosphate anion 145). 

The alkali tetrametaphosphates are stable in aqueous solution at or below room temperature provided the pH is between approximately 5 and 10 61). Temperatures of about 40°C and higher, or lower pH values, bring about hydrolysis to monophosphate through several intermediate stages... 

In strongly alkaline solution hydrolysis of sodium tetraphosphate takes place so much more slowly than that of the tetrametaphosphate that tetraphosphate accumulates in the solution in a practically pure state... 

Decomposition Half-Lives for Sodium Trimeta- and Tetrametaphosphate Anions in Aqueous Solution at 60°C as a Function of pH... 

Pure or very nearly pure sodium tetraphosphate is obtained in the bimolecular reaction involving alkaline cleavage of the cyclic anion of sodium tetrametaphosphate in aqueous solution at temperatures up to 40°C (233, 312, 355, 356). The tetraphosphate Na6P40i3-aq... 

The tetraphosphate anion is practically stable in neutral and alkaline solution at room temperature but at 65.9°C it is about half a power of ten less stable than the triphosphate anion over the whole pH range (61). Tetraphosphate has its maximum stability at 65.5°C at pH 10 and this is why it can be prepared from tetrametaphosphate at such temperatures. Its stability becomes smaller with decreasing pH. Apparently during the hydrolysis of an aqueous solution, which follows a first order law at constant pH, the tetraphosphate is cleaved initially exclusively at the end of the chain, to form mono- and tri-phosphate. 

Perhaps the formation of trimetaphosphate and small quantities of tetrametaphosphate from dissolved polyphosphates, which can amount to 70% in presence of Mg++ ions, is an indication of the form in which the anion chains are present in the solution. In any case this formation of trimetaphosphates is not, contrary to what was proposed initially (293, 326), an argument for the assumption that trimetaphosphate rings constitute a structural unit in the polyphosphates. Thus in solutions of Maddrell s salt, the anions of which are known to be linear chains from crystal structure determination (78) (see Section IV,E,2), up to 50% yields of trimeta-... 

A further condensation occurs in the metaphosphates, 4P03, with a pseudo cation anion ratio 1 1. In these compounds, phosphor has still the same coordination number, but the POf " ions are either condensed to rings, e.g. in the tetrametaphosphate (NH4)4(P4012), or to chains of infinite length, the structure of the phosphates in a tetrametaphosphate being... 

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

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