April, 19611
DEHYDRATIOY OF SODIUM TRIPIIOSPTIATE HEXAHYDRATE
2su, state, and %orthe 3da, state, the discrepancy is more than two orders of magnitude. Thus it is clear that when only the first ten members of the set are used, there is no redundancy, even though each member of the set is used twice. We conclude that using the simple 1.c.m.o. procedure, as we have done, overcompleteness is likely to become troublesome if many more than ten functions ai:e used. However, by that time, with a suitable choice of Z for each individual state, it is possible to reproduce both the energy and the wave function to a degree of accuracy m-hich will be sufficient for most purposes, and the wave functions obtained in this way will certainly be easier to manipulate than the exact functions. Should greater ,accuracy be required, we can see no obvious reason why a few more members of the set should not be included and any resulting redundancy removed by an orthogonalization procedure such as that just described; it may however be necessary to compute the basic integrals to an accuracy of better than the 1 part in lo8which we have used. We wish to acknowledge our indebtedness to Dr. B. F. Gray for his collaboration in the early stages of this work and to Mr. W. B. Brown and
645
Mr. D. J. Evans for many discussions; also to Professor D. R. Bates, F. R. S. for permission to use his published contour maps in the construction of Fig. 1 and 2. Appendix The matrix elements for the hydrogen-like set, arbitrary 2, are given below. The functions are eigenfunctions of the operator
- -1 v2
p' =
2
with E;" = Z2Ej where Thus Sii = 2(2
and Hij
2(2
ei
- Ze2 r
corresponds to Z
=
+ 2Eii)-'/2 (2 + 2€jj)-'/3 (Eii + Si])
+ 2$ii)-'/~ ( 2 + 2$ii)-'/* + + + + ( 2 - l)e2(Lii+ Kji)]
eZ/R) - eP(Jii K i j )
[(Eii
Gii)(ei"
where 6 ; j = 1 if i = j, or = 0 if i # j, and the other symbols have their previous meaning; the relation between Kij and Kji is
+ (ti; + Gii)(ei" -
Ze'Kji = Ze2Kij
~ j " )
Ldj now occurs between all pairs of 4i and having the same Lquantum number.
c$j
THE 'DEHYDRATION OF SODIUM TRIPHOSPHATE HEXAHYDRATE BY WARREN0. GROVESAND JAMES W. EDWARDS Momanto Chemical Company, Research and Engineering Division, Dayton, Ohio Receiaed October 8.1960
The mechanism of the dehydration of sodium triphosphate hexahydrate has been investigated. Two routes for the dehydration have been found, the second, in the presence of water vapor, leads to direct formation of anhydrous sodium triphosphate, a reaction commercially important in the spray-drying of synthetic detergenta. The most significant result is the influence of water vapor on the course of reaction. In the range of 100 to 150°, by controlling the pressure of water vapor, the reaction can be changed from a complete degradation to ortho- and pyrophosphates to an essentially pure dehydration to anhydrous sodium triphoephate, form 11. Under certain conditions a new crystalline orthophosphate, a second modification of the double salt, NaH~PO~.NaHPO~, appears. Other variables affecting the course of the reaction are particle size, sample purity, amount of grinding and previous history of the sample. It is concluded that two competing types of dehydration, a dilrect dehydration and a hydrolytic degradation, follow two fundamentally different mechanisms. The predominance of one or the other is controlled principally by the pressure of water vapor over the system,
Introduction The dehydration of sodium triphosphate hexahydrate has been studied by a number of investigators1-8 both because of its importance commercially and because of interest in the unusual behavior of: this hydrate. Although the triphosphate ion is normally only metastable in the presence of water,6 undergoing more or less rapid reversion to ortho- and pyrophosphates, the hexahydrate of sodiim triphosphate is remarkably stable. It may tie stored for years under ordinary conditions without undergoing appreciable de(1) P. Bonneman-BeInia, Ann. Chim., 16, 39.5 (1941). (2) J. R. Mills, Thesis, University of Illinois, 1952. (3) B. Raistrick, Roy, Coil. Sei., 19, 9 (1949). (4) E. Thilo and H. Seeman. Z. anorg. allgem. Chem.. 261, 65 (1951). (5) 0. T. Quimby, Cham. Reus., 40, 141 (1947). (8) 0. T. Quimby, J . Phys. Chem.. 68, 603 (1954).
(7) A. E. Zettlemoyer, C. H. Schneider, H. V. Anderson and R. J. Fuchs, ibid.. 61, 991 (1957). (8) C.Y.Shen, J. S. Metcalf and E. V. O'Gradv. Ind. E n p . Chem., 51, 717 (1959).
composition. Only when an attempt is made to separate the water from the crystalline hydrate does degradation of the triphosphate ion by water occur. Previous workers have shown that hydrolytic degradation occurs on either thermal or vacuum decomposition of the hydrate a t temperatures below about 150'. Below about 80°, water can only be removed under vacuum and with difficulty. The mechanism of this low temperature vacuum dehydration involves initially an amorphous phase containing triphosphate ions.s These triphosphate ions subsequently degrade with the crystallization of tetrasodium pyrophosphate, the only crystalline product detectable by X-ray diffraction. The products obtained on thermal dehydration between 80 and 120' are similar to those from vacuum dehydration,? suggesting a similar mechanism; however, an alternative mechanism involving primary cleavage of a triphosphate ion by the water with which it is associated in the hexahydrate lattice
WARREN0. GROVES AND JAMES W. EDWARDS
646 100 80 60 40
20
0 1
2
3
4
100 80
0
I
2
100 80
60 40
-20
5
6
Yol. 65
Several experiments were conducted using large crystals, and also with samples deliberately contaminated with either excess acidic or basic components. Details of these experiments are included in the section on Results and Discussion following. Analyses.-Semiquantitative X-ray diffraction analyses were carried out using the General Electric XRD-3 X-ray Spectrogoniometer. Quantitative analyses by paper chromatography using the technique of Karl-@oupag were carried out m the research laboratories of the Inorganic Chemicals Division of Monsanto Chemical Company. Apparatus.-For the rate studies sample weights were obtained continuously under vacuum, or controlled pressure of water vapor, by utiliiing a quartz spiral spring. To avoid condensation of water vapor within the system all exposed tubing was wound with nichrome heating ribbon, and insulated; the Pyrex tube serving as the spring housing wm contained within a larger heated tube. The sample tube, long enough to accommodate the maximum spring extension under full load and attached to the spring housing tube with a standard taper joint, was immersed in a bath of “FiFcher Bath Wax” whose temperature was controlled automatically to =k0.05”. Attached to the sample tube just below the bath surface was a mercury “U-tube” manometer, the other arm of which was attached by rubber tubing to a convex?tional manometer system. The water vapor reservoir consisted of a bulb loosely packed with glass wool on which water was condensed, immersed in a small heated waterbath whose temperature was controlled manually. The quartz spring, obtained from the Houston Technical Laboratories, was calibrated at a series of temperatures from 25 to 175’ for loads of 0.7 to 1.0 g. Weights calculated from the observed extension were accurate to zt0.5 mg. The sample, 0.8 g., was contained in a small aluminum foil bucket suspended by a fine platinum wire from the quartz spring. Two aluminum foil radiation shields threaded on the wire minimized radiation to or from the upper parts of the apparatus. In a typical run the sample was weighed into the bucket, the bucket suspended in the system, and the system evacuated to 10-6 mm. of Hg. In the meanwhile the water vapor reservoir bath was brought to temperature and the waxbath (removed from the system) heated to a temperature 3 or 4’ higher than that of the run. After closing the main stopcock to the vacuum system, the stopcock to the water vapor reservoir was opened slightly and the bath quickly raised around the sample tube. Equilibrium was attained within two to five minutes; weight readings were made at convenient intervals, usually of five minutes.
Results and Discussion The most significant result of this work has been Time, hours. the discovery of the influence of water vapor presFig. 1.-Rate of water loss of NasP301o.6H20at: (a) 110’; sure on the course of the dehydration of sodium (b) 120’; (c) 150’. triphosphate hexahydrate. I n the range from 100 might also explain the results. Above about 120’ to 150°, by controlling the pressure of water vapor, the increasing amounts of crystalline anhydrous the reaction can be changed from a complete degsodium triphosphate found in the dehydration radation to ortho- and pyrophosphate to an esproducts raise the question of direct dehydration sentially pure dehydration to anhydrous sodium us. recombination of initial degradation products. triphosphate. That at least two distinctly difIn an attempt to answer some of these unresolved ferent routes with radically different mechanisms questions we undertook to study the effect of are involved in these reactions is established by the mater vapor pressure on the rates of dehydration. experimental results here described. The routes This investigation very shortly led to a very sur- will be designated as degradative dehydration and prising and unexpected result. By maintaining direct dehydration, respectively. a certain pressure of water vapor over the dehyThe rates of water loss of the hexahydrate a t drating system, nearly pure anhydrous crystalline different temperatures and under various pressures sodium triphosphate could be obtained. This of water vapor are shown in Figs. 1 (a)-(c). At paper is a report on the results of this study. 110’ and 108 mm., Fig. la, the degradative dehydration yielded principally crystalline tetraExperimental Materials.-Sodium triphosphate hexahydrate was pre- sodium pyrophosphate. The runs at 211 and 384 pared from commercial sodium triphosphate by recrystal- mm. are examples of direct dehydration, 87 and lization several times from the aqueous solution by slow 92% triphosphate being obtained. The principal addition of ethyl alcohol. Analysis: 19.8% P; pH of 1% differences to be noted are: (1) difference in the solution, 10.02; % of P as triphosphate, 99.1%; as pyrophosphate, 0.9%; ignition loes, 22.71y0’,.
(9) E. Karl-Kroupa, A n d . Cham., 38, 1091 (1966).
April, 1961
DEHYDRATION OF SODIUM TRIPHOSPHATE HEXAHYDRATE
induction period, and (2) difference in the shape of the curve. The degradative reaction starts slowly after an induction period of about 20 minutes, while the direct dehydrations set in abruptly after a period of 3 to 7 minutes. The degradative dehydrations show a long acceleration period following induction and before a gradual decay. The direct dehydrations start abruptly and decay slowly, actually following very closely fractional order rate equations. Orders have been found to range between 0.31 and 0.58 and hold from 0 to 90% completion based on final total water loss. The reaction during the run at 535 mm. may be classed as a direct dehydration based on the short induction period' and adherence to the fractional order rate law. However, the low final weight loss and analysis of product show that extensive degradation has taken place. The effect of excessive water pressure is threefold. It slows the dehydration by tending to reverse the reaction; it causes hydrolytic degradation of the initially formed anhydrous triphosphate; and it induces the crystallization of trisodium acid pyrophosphate monohydrate. I n Fig. l b similar data for runs at 120' are presented. Here the degradative dehydration at 149 mm. produced a crystalline orthophosphate double salt as well as tetrasodium pyrophosphate. The slight inflection in the curve is real and probably corresponds to initiation of formation of the crystalline orthophosphate. Degradation of triphosphate and pyrophosphate ions to form the orthophosphate ions consumes water which would otherwise be evolved. The difference between induction periods for the two reactions is less than a t 110' but still evident. The run at 716 mm. again shows the effect of excessive water vapor pressure. The sharp difl'erence between degradative and direct dehydration in this temperature region is further emphasized by the very critical dependence on water vapor pressure. The effect of decreasing water vapor pressure is illustrated in Fig. la. After initiating a, direct dehydration at 212 mm., the pressure was gradually reduced. The water loss proceeded a t the normal rate for a direct dehydration until a pressure of 147 mm. was reached. At this point as evidenced by the sharp break in the curve, direct dehydration abruptly ceased and the independent degradative dehydration with its long acceleration period took over. The effect of increasing water vapor pressure is illustrated in Fig. lb. A degradative dehydration initiated a t 150 mm. increased slightly in rate as the pressure was increased until a t 194 mm. there was an abrupt rise in rate as direct dehydration commenced. At 150°, Fig. I C , the rates of water loss a t 94, 189 and 530 mm. are almost equal but a gradual transition from degradative to direct dehydration is shown by the increase in total water loss. These results indicate that a t this high temperature the degradative and direct mechanisms can operate simultaneously over a range of water vapor pressures, or that some anhydrous triphosphate may be formed by the mechanism of the degradative dehydrations.
647
I 0
1
'l i " 'L#'
100 I
0
I/
I
200 300
I
I
I
700 800 Pressure, mm. Fig. 2.-Products of N a J ? 3 0 ~ 6 H ~dehydration 0 as funcNabPaOlotion of temperature and water vapor pressure. 11, only crystalline component (region A) ; (3 NaSPaOio-11, major component with Na4P207 (region C); 0 NasP&-II, major component with NarP207 and Na3HPZO7.Hz0 (region B) ; 0 Na4P207,only major crystalline component (region D) ; e Na4P207,major component with Na2HPO*.NaH2POa* (region E). 100
400
500 600
The most convincing proof of the radically different nature of the two routes of dehydration comes from experiments using large crystals in which sites of initiation and growth of reaction zone in the two types of reaction were observed to differ completely. The crystals, about 10 X 15 X 2 mm., were prepared by allowing acetone to diffuse slowly into a nearly saturated aqueous solution of the hexahydrate.8 Two runs were made a t 120°, one a t 175, the other a t 230 mm. of water vapor pressure. At 175 mm., after a long induction period during which the surface of the crystal remained clear, small opaque spots began to appear, concentrated a t first a t the corners and edges of the crystal. Simultaneously with the appearance of the spots, weight loss began. The spots, probably corresponding to slowly growing dehydration nuclei, spread diffusely over the surface of the crystal until it was completely covered. At this time, two hours after the start of the run, only one mole of hydration water had been evolved. I n contrast, a t 230 mm., change was first observed after a much shorter time as a white line approximately bisecting the crystal. This line may have corresponded to a twinning plane of the crystal. Later another line appeared roughly perpendicular to the first. The dense opaque regions spread slowly through the crystal, maintaining sharp boundaries. Disappearance of the last trace of clear surface area coincided with cessation of weight loss indicating completion of the reaction. The products of the run a t 175 mm., as determined by semiquantitative XRD analysis, included about 22% Na4P20,and a large amount of orthophosphate double salt while those of the run a t higher pressure contained 7040% triphosphate and only 3-7% Na4P207. The results on the effect of water vapor pressure on the course of the reaction are summarized in Fig. 2. Here, the temperature of each run is plotted as ordinate, the water vapor pressure as abscissa. Lines of constant per cent. saturated
648
WARREN
w.EDW.4RDS
Vol. 65
0. GROVESAND JAMES
TABLE I ANALYSISOF PRODUCTS OF DEHYDRATION OF NaQ3010.6H20 No.
31 20 24
Temp., OC.
Pressure. mm.
100 100 110
153 273 108
r
%NaaPaOio(II)
N5 60-75
....
XRD analysis NarPzOi
17-27 8-13 20-30
Chromatographio analysis of P as-wroortho
7%
tri-
Other
..
..
..
71.0
24.5
4.5
Na3HS(PO4jz*O major
...............
NasH3(P04)~*~ trace, NasP3010.6H20
65-80 ... ................ 29 110 211 65-80 ... ................ 21 110 384 50-65 13-23 Na3HP20:*H20,9-19% 110 32 535 ..... 17-27 Na3H3(P0&*"major 27 120 149 ... ................ 65-80 120 28 298 38-48 15-20 Na3HP2OvHz0, 10-30% 17 120 716
