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W h e n sodium tripho5phdte tiyclrol: i e s , one mole each of ortho- and pyrophosphate if formed. In aqueous solutions of hexametaphosphate tFc o reaction5 take p1ace simultaneously ; part is hydrolyr,ed directlj to orthophosphate, and part is depol? meriLed to trimetaphosphate which then hydrolgzes sloitlj to orthophosphate. 1x1 the hydrolysiq of triniela- to orthophosphate, triphosphate is formed as an intermediate. In the pIeseiice of an exces5 of alkali, trimetaphosphate is coni erted entire13 t o triphosphate. P>rophosphates hydro1:se directly to ortho phosphates. Hydrolysis data confirm the fapi that tetraand septaphosphateb art' mixtures.
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dehydrated phosphates, in the treatment' of water for various purposes, are too numerous to be tabulated here. The useful life of these compounds is dependent on their ability to with-. stand hydrolysis iri aqueous solutions. Many attempts ha1.c. been made to measure the rate a t which these compounds rehydrate (hydrolyze), but' the analytical methods used were inad(,quate to establish either the nature or the exact amount of products formed. Xorgen and Swoope (8) used the titratioir methods of Gerber and Miles (4),rThich do not determine pol).phosphate as such. Germain ( 5 ) determined the hexametapho+ phate by precipitating as barium hexametaphosphate and the. pyro- and orthophosphate by acidimetric titrations using a scLTies of indicators. Katzcl (10) also used titration methods. Noiic. of these methods gave sufficient, information on the produrtq formed or the mechanics of the rehydration. Recently developed met,hods for determining triphospharv and pyrophosphato ( 2 ) in the presence of each other and in tht. pre%enceof the other phosphates have made possible a more coniplete study of the hydrolysis of the dehydrated phosphatc.5. Supplementing t,he above methods, the orthophosphate x a s d(,iwmined colorimetrically by the molybdenum blue method ( 3 , and the hexametaphosphate precipitated as barium hexametwphosphate ( 7 ) . S o satisfactory method was found for determitiing trimetaphosphate in the presence of large amounts of ot1it.i. phosphates, and it was therefore determined by difference. l ? f b sulk found by analysis of pyrophosphate, polyphosphate, ailti hexametaphosphate, using the above methods, agree with t l i c findings of Andrese and Kust (1) and Partridge, Hicks, arid Smith .(9)-namely, t'hat only one true polyphosphate, the triphosphate, exists. The so-called tetraphosphate and septaphoiphate were found to be mixtures of triphosphate and niet,aphoprcsmt,
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El?EIIEK:CIerininedmaterial as trimetaphosphate is based on the following points: first, its reaction with excess alkali to form triphosphate (Figures 4-4 and 4B); second, its rate of hydrolysis as shown by the slope of the curves after the hexametaphosphate has disappeared (4.4 and 3 A ) ; finally its similarity to trimetaphosphate in forming triphosphate as an intermediate step in the hydrolysis to Orthophosphate. Triphosphate is relatively stablc in 1% sodium hydroxide solution, and pyrophosphate is entirely stable for the time of the test. Any triphosphate formed, therefore, should hydrolyze slowly to ortho- and pyrophosphat'es, both of which are stable. If hesametaphosphate hydrolyzes as trimetaphosphate does, by first, forming triphosphate, the rate of formation of orthophosphate should be the same as for triphosphate in 1%sodium hydroxide solution (Figure 2C). Comparison shows that this js not truc. Considerably more orthophosphate is formed during the period ahile hexametaphosphat,e is present, (approximately 4 hours) ; then the curve levels off to a slope corresponding to the hydrolysis rate for triphosphate. No pyrophosphate is found a t the end of 2 hours, and only a small amount is present after 4 hours. This latter is formed as the result of the hydrolysis of the triphosphate. From these observations it is evident that the portion of hexametaphosphate n-hich hydrolyzes goes directly to orthophosphate. The t'riphosphate is formed from that part which depolymerizes to trimetaphosphate; in the presence of excess alkali the latter converts to t,riphosphate. From t,he ratio of depolymerized to hydrolyzed phosphorus Eqmtion A, Table 11, is believed to represent the hydrolysis of hexametaphosphate in water. Commercial sodium hexametaphosphate contains approximately 30% of triphosphate and 70% hexametaphosphate. The analysis is shown a t zero time in Figure 5A, which indicates the rate of hydrolysis of a 1% ' solution of commercial hexametaphosphate in water a t 100" C. Hexametaphosphate is slightly less stable in 10% than in 1% concentration (Figure 3 3 ) . Otherwise the curves are the same. Figure 5C presents the hydrolysis of a 1% solution of hexametaphosphate in 1% sodium hydroxide solution a t 100' C. As would be expected, a large amount of triphosphate is found, but no trimetaphosphate. The curves are similar to those found for the laboratory-prepared product in excess alkali. At 70" C. a 1% solution of commercial hexametaphosphate hydrolyzes as shown (Figure 3)).As was the case with the laboratory-prepared product, the commercial hexametaphosphate is much more stable at 70" than a t 100" C. POLYPHOSPHATES. Figure 6 A shows the hydrolysis of a 1% solut,ion of com'mercial tetraphosphate in water a t 100" C. I t
hydrolyzes in the manner expected of such a mixture. Proof of the existence of trimetaphosphate in this product was found by hydrolyzing it-in a 1% sodium hydroxide solution. At the end of 2 hours a t 100' C. an increased amount of triphosphate was found, no hexametaphosphate was present, and the analysis lcft no difference to be calculated to trimetaphosphate. Scptaphosphate as shown a t zero time in Figure 6B Tvas found to be a mixture of about equal quantities of hexametaphosphate and triphosphate. A 1% solution a t 100" C. hydrolyzes as indicated on the curve. CO;YCLUSIONS
Temperature greatly affected the rate of hydrolysis of the molecularly dehydrated phosphates. The hydrolysis rates were much slower a t 70" than a t 100" C. Triphosphate hydrolyzed more slovly t'han hexametaphosphate and was noticeably more stable in the presence of excess alkali. TThen triphosphate hydrolyzed, one mole each of ortho- and pyrophosphate was formed. Hexamet'aphosphate hydrolyzed to orthophosphate and depoIymerized to trimetaphosphate in aqueous solutions. The reactions were simultaneous. KOpyroor triphosphate was formed directly from the hydrolysis of the hexametaphosphate. Some triphosphate was found as a result of the hydrolysis of the trimetaphosphate formed. Trimetaphosphate hydrolyzed first to an acid triphosphate which then hydrolyzed to orthophosphate. If any pyrophosphate was formed it was hydrolyzed to orthophosphate and the amount present was not sufficient to be detected. I n the presence of an excess of alkali, trimetaphosphate was converted to triphosphate, the reaction being very rapid at 100" C. ACKNOWLEDGMENT
The author wishes to thank Howard Adler and \V. H. Woodstjock for many helpful suggestions and criticism. LITERATURE CITED
(1) Andress, K. R., and Wust, K. Z., anorg. allgem. Chem., 237,
113-21 (1938).
( 2 ) Bell, R. N., IND. ENG.CHEM., ANAL.ED.,in press. (3) Fiske, C . H., and Subbarow, Y., J . Bid. Chem., 66,375 (1925). (4) Gerber, A. B., and Miles, F. T., IXD.ENG.CHEX.,ANAL.En.,
IO,519 (1938). (5) Germain, Louis, Chimie & industrie, 35,22-6 (1936). (6) Hatch, G. B., U. S. Patent 2,365,190 (Dec. 19, 1944). (7) Jones, L. T., IND.ENG.CHEM., ANAL.ED.,14, 536 (1942). (8) Morgen, R. A., and Swoope, R. L., IND. EKG.CHEM.,35, 821-4 (1943). (9) Partridge, E. P., Hicks, Victor, and Smith, G. W., J . Am. Chem., SOC.,63,454-66 (1941). (10) Watzel, R., Die Chemie, 55, 356-9 (1942). PRESENTED before the Division of Physical and Inorganic Chemistry a t the 109th Meeting of the A M E R I C A S CHEMICAL SOCIETY, Atlantic City, N. J.
