Dehydration of
Sodium Tripolyphosphate Hexahydrate C. Y. SHEN, J. S. METCALF, and E. V. O'GRADY Inorganic Chemicals Division, Monsanto Chemical Co., St. Louis, Mo.
Reversible dehydration of sodium tripolyphosphate hexahydrate is possible through the formation of an amorphous, anhydrous sodium tripolyphosphate
B m o w 1200 c.:removal of water from sodium tripolyphosphate hexahydrate results in hydrolysis of the tripolyphosphate anion ( 7 , 3, 5-7, 9 ) . Equimolar tetrasodium pyrophosphate and monosodium orthophosphate may be formed first (7) :
+ + 5Hz0
Na6P,010.6Hz0 + Na4PZO7 NaH2PO;
(1)
At 95' to 125' C. and bed depths of '/8 to 8 inches the pyrophosphate-orthophosphate ratio in the decomposed product is greater than 1 and the water lost can exceed five molecules (5). Thus, besides straight hydrolytic degradation, other reactions are possible :
+ +
2NajPd010.6H20 + NalP20, 2Na,HPz07 l l H 2 0 (2) Trisodium acid pyrophosphate is present in the decomposed product above 100' C. ( 9 ) . T h e low temperature form of sodium tripolyphosphate (Form 11) could be formed directly from the hexahydrate above 120' C. (5, 7). None of these reports indicated possible dehydration to anhydrous tripolyphosphate below looo C . This complex reaction a t lower temperatures was studied to identify intermediate and final products and measure the vapor pressure of sodium tripolyphosphate hexahydrate, partly because it was assumed that degradation products would not recombine to Form 11.
Preparation of Pure Sodium Tripolyphosphate Hexahydrate. Commercial sodium tripolyphosphate was
Results Dehydration of Hexahydrate. The rate of water loss (Figure 1) depends on the surface area exposed; an uncrushed crystal dehydrates much more slowly than crushed 50-mesh crystals. Nuclei did no1 appear at different stages of dehydration under a high-power microscope. In most hydrate crystals nuclei formation is important in controlling dehydrdtion rate ( 2 ) . T h e crystals remained clear until more than one molecule of water was lost, but there was n o evidence of new crystalline phases. Quantitative x-ray analysis showed that when one sixth of the water was lost. the hexahydrate was the only crystalline species detected, but there was 20 to 25% of amorphous material. After a sample was dehydrated at 70' C. for several months until weight loss was constant, x-ray analysis showed that the only crystalline material was tetrasodium pyroThe maximum phosphate (5, 7). weight loss a t 70' C. was exactly equal to five sixths of water. At 70' C. and below, it never exceeded five molecules of water per molecule of hexahvdrate. At this stage, the material was very hygroscopic and became sticky when exposed to room air for a few hours. Hydrolysis of tripolyphosphate anion apparently increases with the amount of water removed. T o remove a fractional amount of water from hexahydrate, the sample was dehydrated at room temper-
' 9O c
SYMBOL 0
I
Experimental
after which the sample container was immersed in a constant temperature bath. T h e vapor pressure was measured from the increase in manometer reading.
recrystallized several times ( 5 ) and a nearly saturated solution of such crystals was covered with a thin layer of benzene and an upper layer of acetone. Slow diffusion of acetone into the aqueous solution over several weeks produced large l / r to 1-inch crystals of high purity. Paper chromatographic analysis (4) and weight loss on fusion showed that the crystals contain a t least 99.3y0 sodium tripolyphosphate hexahydrate. Large crystals were grown because the sodium tripolyphosphate content of the recrystallized crystals is seldom over 99.07,, probably because of slight decomposition of small crystals during drying. Apparatus and Procedure. The apparatus was made of a standard 45 '50 taper joint sealed to form a 5-inch cylinder, in which was placed a 3-inch layer of anhydrous magnesium perchlorate. T h e sodium tripolyphosphate hexahydrate sample was stored in a small glass cell and placed in an open: shallow weighing bottle on the perchlorate bed, T h e other part of the joint was connected with a vacuum-cup type stopcock. T h e assembled unit was evacuated to less than 0.05 mm. of mercury and placed in a constant temperature bath. The unit was then taken out and the weighing bottle was immediately sealed to protect the sample from moist air. Water lost through dehydration was measured by change in weight of sample plus container. For vapor pressure measurements, the hexahydrate samples were stored in a small glass tube connected to a mercury manometer. T h e crystal container was first frozen in a dry ice-acetone mixture and the whole system was evacuated,
V
I I00
TIME
Figure 1.
70
CRUSHED,, CRYSTALS
50
LARGE CRYSTALS
I
I
zoo
FORM OF
SAMPLE
C.
A
ti',.' 0
TEMFRATURE
,
hr.
300
I 400
The rate of water loss depends on surface area exposed VOL. 51, NO. 5
M A Y 1959
717
ature; initial dehydration at elevated temperatures was too high to control. The anhydrous tripolyphosphate in the dehydrated products was not detected by x-ray diffraction analysis. which could detect a n increase in sodium tripolyphosphate hexahydrate in the dehydrated sample (from 70 to 85%) after lost water had been restored by exposure to moist air a t room temperature. This suggested that the products of low-temperature dehydration of sodium tripolyphosphate hexahydrate contain anhydrous tripolyphosphate. Inability to detect such appreciable quantities of anhydrous tripolyphosphate by x-ray analysis indicates that the material is amorphous. Water Vapor Pressure. Water vapor pressure of the hexahydrate at room temperatures was low and apparently reversible (about 0.5 mm.). This indicated that the starting crystals were dry and the apparatus was free from leakage. Above 60 C., the hexahydrate showed a substantial water vapor pressure, which at a given temperature approached a maximum asymptotically with time. O n subsequent cooling, the pressure dropped fast but did not return to the original reading even after 3 days. T h e maximum value for any temperature, however, could be reproduced either starting from a freshly evacuated system or reheating without evacuation. The residual pressure following heating and cooling cycles in water vapor pressure determinations is probably the result of hexahydrate decomposition. This was checked by repeated heating and cooling with removal of the residual water vapor after each cycle. T h e weight loss on the crystals as measured gravimetrically agreed fairly well with calculated results based on chromatographic analyses, assuming that the water lost was from decomposition of tripolyphosphate hexahydrate to equimolar pyro- and orthophosphate. Even though the hexahydrate can be dehydrated to a n amorphous tripolyphosphate, some hydrolysis to pyro- and orthophosphates occurs. An absolute equilibrium vapor pressure for the hexahydrate appears nonexistent; however, below 50" C. hydrolysis is so slight that its effect on water vapor pressure is not easily detectable. At lower temperatures, the meas-
Table I. Temp., OC. b
20-25 36.8 70 70 70 70 a
30
P
a-
i 4
i
1 i/l
I
j
1
1
J
Figure 2. The vapor pressure depends on Na5P3O1o.6Hz0content of the sample ured pressure over the hexahydrate is essentially an equilibrium vapor pressure. The apparent vapor pressure of commercial hexahydrate, which usually contains some tetrasodium pyrophosphate, exhibits a much higher water vapor pressure than the values given above (Figure 2). A sodium tripolyphosphate hexahydrate containing appreciable quantities of tetrasodium pyrophosphate decahydrate will have a vapor pressure higher than that of pure hexahydrate. This explains the tenfold higher hydration pressures reported earlier (8), because the anhydrous sodium tripolyphosphate used in those studies and prepared by heating recrystallized sodium tripolyphosphate hexahydrate normally contains a few per cent of pyrophosphate (chromatographic analysis). Discussion The rate of dehydration of sodium tripolyphosphate hexahydrate in vacuo and analyses of the partially dehydrated hexahydrate indicate that the reaction is carried out in two regions. Unlike most dehydration reactions of hydrated salts ( Z ) , the initial dehydration rate of the hexahydrate is very fast and is temperature-dependent. The dehydrated prod-
Heating, Hr.
...
1.0 425.0 2.0 64.8 347.8 480.0
~~0
Molar pyro/Ortho
Lost,
in
Moles
Product
...
0.395 1.550 1.135 1.830 2.208 4.816
...
1.75 2.20 2.40 1.72 1.18 0.76
Chrornatographic" Analysis, % of Total P Tripoly Pyro Ortho 99.5 0.5 trace 97.3 87.1 94.2 74.6 62.3 11.0
No long-chain or ring phosphates found in dehydrated product.
71 8
i
L?/
Hydrolysis of Tripolyphosphate Increases with Amount of Water Removed
De-
hydration
40
INDUSTRIAL AND ENGINEERING CHEMISTRY
2.1 10.6 4.8 19.6 26.4 53.7
0.6 2.4 1.0 5.7 11.3 35.4
Starting material.
uct contains a significant amount of anhydrous tripolyphosphate. After one to t\vo molecules of water have been lost from the hexahydrate, the dehydration rate slows and is not strongly affected by temperature. T h e dehydrated product in the slow dehydration range consists primarily of hydrolytic-degradation products. The activation energy estimated from the initial reaction rate is about 14.0 kcal. per mole. Although the mechanism appears complicated, a good correlation could be made from the heat transfer rate and initial rate of water removal. This rate-controlling step is applicable for several dehydration reactions (2). T h e heat transfer rate \vas estimated on the assumption that the heat absorbed from the dehydration reaction equals the heat received from radiation and convection from the surroundings. hfter the crystals are well covered with a layer of dehydrated product, the rate of dehydration must be controlled by the rate of release of water from the hydrate a t the interface between the hydrate and its dehydrated product and/or by the rate of diffusion of water through the dehvdrated product. The latter probably is controlling, as it is not appreciably affected by temperature changes. Water molecules diffusing through the dehydrated phase probably cause the amorphous tripolyphosphate and pvrophosphate to hydrolyze; both the anhydrous, amorphous tripolyphosphate content and the pyro- to orthophosphate ratio decrease with further dehydration. This suggests that the composition of the dehydrated product will depend on the bvater vapor pressure in contact, and may cause the variations in composition reported (5: 7). Acknowledgment The authors are indebted to E. J. Griffith for suggesting the method for preparing pure sodium tripolyphosphate hexahydrate. literature Cited (1) Bonneman-Bemia, P., -4nn. chim. 16, 395 (1941). (2) Garner, W. E., "Chemistry of the Solid State," pp. 213-31, Academic Press, New York, 1955. ( 3 ) Griffith, E. J., Anal. Chem. 29, 198 (1957'). (4) Karl-Kroupa, E., Ibid.,28,1091 (1956). (5) Quimby, 0. T., J . Phys. Chem. 5 8 , 603 (1954). ( 6 ) Raistrick, B., Sci. J . Roy. Coll. Sci. 19, 9 (1949). (7) Thilo. E., Seeman, H , 2. anorg. Chem. 267, 65 (1951). (8) Zettlemoyer, A. E., Schneider, C. H., J . A m . Chem. Soc. 78, 3870 (1956). (9) Zettlemoyer, A. E., Schneider, C. H., .4nderson, H. V., Fuchs, R. J., J . Phys. Chem. 61, 991 (1957). RECEIVEDfor review March 24, 1958 ACCEPTEDFebruary 24, 1959
Circle Nos. 64-1, 64-2, 64-3, 64-4, 64-5, 64-6 on Readers' Service Card
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