MEYER MELVIN MARKOWITZ New York University, New York, N. Y.
sition with all the accompanying gradations of properties as the composition is varied from X:O/P20s = 2 to X:O/P205 = 1. Investigations of the processes involved in the anionic condensation reactions of the solid hydrogen phosphates by thermal means (11) show that these reactions proceed as a transition from one state of anionic condensation characterized by hydrogen bonding (12) to another state characterized by oxygen bridging - - between the PO4tetrahedra (IS). . .
THosE phosphate anions containing one or more PO4 tetrahedra joined through an oxygen atom are known as "condensed phosphates." Concomitant with the increasing industrial importance (1) of the sodium and potassium condensed phosphates is. a continuous addition to an already extensive and contradictory literature (2,S). It is the intent of this article to summarize the results of those physicochemical investigations which have led to a clearer definition of these diverse materials. The early work of Thomas Graham (4 constitutes the first comprehensive investigation of the condensed phosphates through his studies of the thermal decompositions of the dihydrogen (X1H2POI) and monohydrogen (XiHPOa) orthophosphates. This work stands as a tribute to his observational acumen. He noted in the instance of sodium dihydrogen orthophosphate (5) that: "The salt cannot sustain the loss of any portion of this water (i. e., the water of constitution) without assuming a new train of properties." Thus, Graham found the reaction sequence on heating sodium dihydrogen orthophosphate to be: NaHzPO, N%H2P2O7 sodium metaphosphate Graham was able to identify a t least three forms of sodium metaphosphate-a soluble type, an insoluble type, and a glass--each of which differed in physical and chemical properties and the method of preparation. Clark's experiments describing the conversion of sodium monohydrogen orthophosphate into sodium pyrophosphate (6) were also verified by Graham. All condensed phosphates may be thought of as stemming from the acids formed in the course of the molecular dehydration of orthophosphoric acid (7) or as the ultimate products of the heating of hydrogen phosphate salt mixtures of appropriate H/P ratios (8). The reaction course for orthophosphoric acid may be represented generally as:
-
n
~
,
-
POLYPHOSPHATES
-
~H . ~ ,(p.o8. 4 +
+,)-'"
+ "
+ ( n + 1 )H*O
(1) (')
Superficially, then, it would appear that many distinct phosphate ions may be formulated (9). These fall int,o two groups which may be differentiated by the magnitude of n in the formula Xk +2(PnO3,+ I ) - ( " +') (10) corresponding to their salts. Those compounds for which n is a small whole number are designated as L'polyphosphates," and those substances for which n is large, such that the empirical formula approaches X'PO,, are designated as L'metaphosphates." Thus, the polyphosphates may be considered as a series of compounds approaching the metaphosphate compo-
The existence of only three polyphosphates has been firmly established. These are the pyrophosphate or diphosphate (P207-4,n = 2 ) , the triphosphate (P8010-5, n = 3), and the tetraphosphate (Pp013-6,n = 4) anions. The results of crystallographic studies (Id), conductance data (16), complexing tendencies (16), compound formation (l7), and hydrolysis studies (18) substantiate the existence of the pyrophosphate anion as a discrete species. There has been no complete crystallographic analysis of a triphosphate (19) but the phase studies of the systems potassium pyrophosphate-potassium metaphosphate (20) and sodium pyrophosphate-sodium metaphosphate (21), acid titration (294, complex-ion formation (M), and kinetic studies of the alkaline hydrolysis of sodium trimetaphosphate ( 2 4 , as well as reversion data (25) affirmthe existence of the triphosphate ion. The existence of a crystalline tetraphosphate in solid state preparations has been reported on the basis of phase and X-ray studies of the system lead pyrophosphate-lead metaphosphate (26). Thilo and coworkers (27) in their reports of the preparation of sodium tetraphosphate by the hydrolysis of sodium tetrametaphosphate obtained only an uncrystallizable, viscous liquid containing much water; however, they were able to prepare amorphous materials corresponding to anhydrous silver tetraphosphate and calciunl tetraphosphate octahydrate. More recent work by Quimby (28) has resulted in the isolation of essentially pure, crystalline acridinium and guanidininm tetraphosphates. Dilute tetraphosphoric acid, obtained by ion exchange of a solution of the sodium salt. was neutralisedaith the bases. Further evidence (28) for the existence of the tetraphosphate ion is based upon the characteristic X-ray patterns of the salts, paper chromatography studies (29, SO), reversion rates, chemical behavior with the tris-(ethy1enediamine)cobalt (111) ion, and chain-length determinations. Grunze and Thilo (Sf), by modifying Ebel's paperchromatography techniques for the separation of
36
37
VOLUME 33, NO. 1, JANUARY 1956
condensed phosphate mixtures (SO), report the separation and semiquantitative determination of polyphosphates of n values t o eight. The size of the polyphosphate chain was gaged by a relation between the chromatographic position constant and the solvent characteristics. Further work, however, is necessary for the establishment of the tme nature of these materials. Thus, the preparation of polyphosphates of n greater than four in a characterizable state still awaits accomplishment. The nonappearance of the higher polyphosphates (n greater than three) in the course of the phase investigations of the potassium pyrophosphate-potassium metaphosphate and sodium pyrophosphatesodium metaphosphate systems cannot be taken as proof for their nonexistence. Their absence may very well be due to the stringent thermal conditions upon which their formation and crystallization may hinge (3.9). METAPHOSPHATES
The metaphosphates, the second group of condensed phosphate anions, are those possessing or closely approaching the empirical formula X1PO3. Because of the tetracovalency of phosphoms to oxygen, the stable existence of monomeric metaphosphates is very unlikely; such a particle would have a high degree of coordinative unsaturation and would consequently be very reactive chemically (52). Thus, the metaphosphates exist in polymeric groupings and their formulas are correctly expressed as (XrP03),,, where n' indicates the extent of anion polymerization. The monometaphosphates cited in the literature (35) have proved to be mixtures of various condensed phosphates (54, 55). The case for the existence of dimetaphosphates is denied on the grounds of the structural strain (56) and of the strong coulombic repulsion arising from two PO, tetrahedra linked by an edge (58), as well as by conductivity data (59). A dimetaphosphate has been reported as an intermediate in the thermal decomposition of sodium dihydrogen orthophosphate (42), but further confirmatory evidence is lacking. Trimetaphosphates, as cyclic six-membered structures (PaOp-a), are well substantiated by cryoscopic (4S), conductance (40), potentiometric-titration (S7), and compound-formation studies One form of sodium trimetaphosphate is commonly known as "Knorre's salt" (45). The determination of crystal structure for an aluminum (46) and for an ammonium (47) metaphosphate support the existence of discrete cyclic tetrametaphosphate ( P 4 O c 4 )units, as do conductance (41), acid titration (@), and compoundformation investigations (49). Cyclic metaphosphates of n' greater than four are not known. It is the linear metaphosphates that exhibit the greatest variety and complexity of interrelationships. It has been shown that these long-chain polymetaphosphates are linear aggregates of POa units. Stoichiometrically, the linear metaphosphates in aqueous solution are not t m e metaphosphates like the cyclic
(a).
metaphosphates, but rather the sizes of the chains are such as to make the effects of the oxygen-saturated ends negligible in the over-all composition. Thus, the metaphosphate formula, X1PO8. is closely approached. It has been proposed that there is present in each chain a cyclic unit which would allow for the correct metaphosphate composition as well as for complete oxygen coordination of the chain ends (50). However, there is no experimental evidence t o support this contention. The methods of preparation of these substances must be carefully controlled because the extent of aggregation or chain length is a function of such factors as the humidity, temperature, length and rate of heating and cooling, and the state of subdivision of the starting materials. The sodium polymetaphosphates present a good illustration of the complexities that may be encountered. From a compilation by Liddell (51), the appropriate thermal treatment of sodium dihydrogen pyrophosphate may lead to the formation of any one of six crystalline metaphosphates plus a glass. It should be noted that the designation of the glassy sodium metaphosphate (Graham's salt, Na20/PzOs = 1) as "sodium hexametaphosphate" is unjustified. The appellation stemsfrom the early work of Fleitmann (52) and the later studies of Tammann (55) in the supposed preparation of mixed metaphosphates of the form X:-,Y:(PO&. LENGTH OF PHOSPHATE CHAINS
Graham's salt is to he regarded as unique only in the respect that it represents the end member of a continuous series of phosphate glasses which may be formed by the rapid quenching of melts within the composition range 1 L Na20/P20s< 2 (54, 55). Van Wazer, utilizing pH titration data, has shown that the chain-length population in aqueous solutions of these glasses is dependent upon the NazO/P20sratio and may be expressed by appropriate distribution functions (54, 55). It is postulated that a dynamic equilibrium is maintained within the melt by a random making and breaking of chains (the random reorganization process). Strong support of these views is to be found in solubility fractionation experiments (55), and less strongly in the theoretical interpretations of the surface tensions (56) and viscosities (57) of melts of various NazO/Pz06ratios. Van Wazer predicts chain branching in glasses with N%O/P20S ratios below one. However, evidence recently obtained from pH and viscosity measurements (58, 69) indicates the presence of chain branch points in freshly prepared solutions of glassy phosphates within the composition range of 1 L NarO/PnOs L 1.01. End-group or pH titrations have found wide application in determining the length of phosphate chains (60, 55, 5). The utility of the method is based upon the experimental facts that for all the phosphoric acids there is one strong, titratahle hydrogen ion (Kdirs. about lo-' to per phosphorus atom in the molecule, and that the residual hydrogens are weak (Kae,,
VOLUME 33, NO. 1, JANUARY 1956 3. R., J. Am. Chem. Soe., 72, 644-7 (1950). Berlin, K1. Math. u. allgem. Naturw., 1953, No. 5, 1-26; (54) VANWAZER, (55) Ibid., pp. 647-55. Chem. Zentr., 126, 4659 (1955). (56) A N D HILL,Op. C k , pp. 82-3. (32) AUDX~ETH . . CALLIS.C. F.. 3. R. VANWAZER.AND 3. 5. METCALF.J . Am. hem. h e . , 77, 1468-70 (1955). (33) BEANS,H. T., AND S. 3. KIEHL,J. Am. Chem. Soc., 49, Bull. soc. chim. France, 33, (57) Ibid., pp. 1471-3. 1878-91 (1927). P. PASCAL, U. P., E. H. SMITH,AND P. L. WINEMAN, J. Am. (58) STRAUSS, 1611-27 (1923). Chem. Soc., 75, 3935-40 (1953). (34) KARBEAND SANDER, op. n't., pp. 4-9. (35) DAVIES,C. W., AND C. B. MONK,J. chem. Soe., 1949, 421. (59) STRAUSS,U. P., AND T. L. TREITLER,ibid., 77, 1473-6 (1955). . I . R., A N D K. A. HOLST,J. Am. Chem. Soc., (36) VANWAZER, O., Suemk Kem. Tidskr., 56, 3 4 3 4 (1944); (60) SAMUELSON, 72, 640 (1950). C. A,, 40, 4613' (1946). R. P. PFAN~TIEL A N D R. K. (37) Ihidi, pp. 64144. ILER,J . Am. Ckem. Soe., 74, 605944 (1952). (38) PAULING, L., "The Nature of the Chemical Band," 2nd ed., op. eit., p. 171. Cornell University Press, Ithaca, New Yark, 1945, p. (61) QUIMBY, 396. E. THILOA N D H. SEEMAN, Z. anorg. s. allgem. (62) VANWAZER,op. cil., p. 649. R., Ann., 61, 63 (1847). (63) MADDRELL, Ckem., 267,66 (1951). (39) DAVIES,C. W., A N D C. B. MONK,J. Chem. Soc., 1949, (64) PASCAL,P., Bull. soc. chim. France, 35, 1127-30, 1140 ( I Wd) 420-1. (65) MALMGREN, H., Acta Chem. Scand., 2, 14745 (1948). E. (40) Ihid., pp. 414-6. THILOAND I. PLAETSCHKE, Z. anmg. Chem., 260,297-309 (41) Ibid., pp. 417-9. a7107g. U . allgern. (1949). K. PLIETHAND C. WITRSTER,Z. D., Ann. chim. (Paris), 5 , 83942 (42) LAFORGUE-KANTZER, Chem., 267, 4 9 4 1 (1951). (1950); Compt. rend., 228, 66 (1951). AND E. FARBER, J. Am. P., Ann. chim. (Paris), 16,451-2 (1941). (66) VAXWAZER,3. R., M. GOLDSTEIN, (43) BONNEMAN-BEMIA, Chem. Soe., 75, 1563-7 (1953). P. NYLEN,Z. anorg. u. allgem. Ckem., 229,334 (1936). (67) VANWAZER,3. R., J . Am. Chem. Soe., 72, 906-8 (1950). P., Compt. rend., 204, 4 3 3 4 (1937). (44) BONNEMAN, R. K. ILER, J. Phys. Chem., 56, 1086+ (1952). C. G. v., Z. anorg. Ckem., 24, 369401 (1900). (45) KNORRE, DRUCKER,Acta Chem. Scnnd., 1, 221-9 (1947). R. L., AND J . SHERKAN, Z. Kriat., 96,481-7 (1937). (46) PAULING, INGELMAN AND H. MALMGREN, {bid., 422-32. C. J., A. A. KETELAAR, A N D C. H. MACGILLAVRY, (47) ROMERS, 2.anorg. u . allgem. Chem., (68) LAMM,O., AND H. MALMGREN, Nature, 164, 96W1 (1949). 245, 103-20 (1940). H. MALMGREN AND 0.LAMM, Z. A N D 0. F. HILL, Ind. Eng. (48) BELL,R. N., L. F. AUDRIETA, anwg. Ckem., 252,256-71 (1944). Chem., 44, 568-72 (1952). op. cit., pp. 80-90. K. R., W. GEHRING, AND K. FISCRER, 2. anorg. (69) KARBEAND JANDER, (49) ANDRESS, J . Am. (70) HILL, W. L., G. T. FAUST,A N D S. B. HENDRICKS, Chem., 260, 331-6 (1949). Chem. Soe., 65, 794-802 (1943). C. H. MACGILLAV~Y, op. eit., p. 350. (50) TOPLEY, AND L. M. NIJLAND, Nature, 164, H. C. 3. DE DECKER, (51) LIDDELL,R. W., J . Am. Chem. Soe., 71, 207-9 (1940). ALL0 11460> -- -" ,. T., Ann., 72, 246 (1849). (52) FLEITMANN, G. C., AND A. 3. STOSICK, J. Am. Chem. Soc., 60, G., J . prakt. Chem., 45, 43245 (1892); Z. (71) HAMPSON, (53) TAMMANN, 1814-22 (1938). phrpik. Chem., 45, 1 3 W (1890). \
THE IDA AND HENRY SCHUMAN PRIZE IN THE HISTORY OF SCIENCE AN ANNUAL award of $250 has been established by Henry and Ida Sohuman of New York City for an original prize essay in the history of science and its cultural influences. This competition is open to undergraduate and graduate students in any American or Canadian college, university, or institute of teohnoloev. P a ~ e r ssubmitted for the nriee comoetition should he nnnroximatelv , ,;On0 wmli in irtrgrli, ~ . \ ~ I u s iofw fool no^
