INDUSTRIAL AND ENGINEERING CHEMISTRY
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Figure 3 is a plot of this ratio against moles of sodium o-xylenesulfonate per 1000 grams of water for the three temperatures. The curves a t 30°, 40°, and 60” C. extrapolate to zero. I t is therefore concluded that the solubility of benzoic acid in sodium o-xylenesulfonate solution is due solely to a salt effect. The same type curves are shown for solutions involving sodium benzenesulfonate, sodium p-toluenesulfonate, and sodium m-xylenesulfonate in Figure 4. These curves are also extrapolated to zero, indicating that the solvent action of these salt solutions toward benzoic acid is due only to a salt effect.
Vol. 42, No. 8
(3) Debye, P. B., 2.physik. Chem., 130, 56 (1927). (4) Gross, P. +Ma,Chem. Revs., 13,91 (1933).
(5) Kruyt, H. R., and Robinson, C., Proc. Acad. Sci. Amsterdnni, 29, 1244 (1926). (6) Kuthy, A. yon, Biochtm. Z., 237, 380 (1931). (7) Lindau, G., NatlLrwissenschn~ten,20, 396 (1932). (8) Linderstrom-Land, K., Compt. rend. traz. lab. Carlsheig, 15, 1 (1924). (9) McKee, R. H., ISD. ENG.CHmr., 38,382 (1946). (10) Xeuberg, C., Biochem. Z., 76, 107 (1916). (11) Seidell, A., “Solubilities of Organic Co~npounds,”Vol, 11, p. 500, Sew York, D. Van Sostrand Co., 1941.
BIBLIOGRAPHY
(1) Bancroft, R. D., Science, 82, 388 (1935). (2) Booth, H. S., and Everson, H. E., IND. ENG.CHEM.,40, 1491 (1948).
RECEIVED June 16, 1949. This paper is abstracted from a dissertation submitted b y L. D. Wiener in partial fulfillment of the requirements for the Ph.D. degree.
Reversion of Molecularly
Dehydrated Sodium Phosphates JEROME GREEN National Aluminate Corporation, Chicago 38, I l l .
The rate of reversion to orthophosphate of seven molecularly dehydrated sodium phosphates was measured at 150” and 190” F. near pH 5, 7, and 9. Concentrations of 5 and 50 p.p.m. of dehydrated phosphate, expressed as equivalent PO1, were employed. The effects of calcium and magnesium were investigated for a limited number of conditions. The stability of all the dehydrated phosphates studied decreased with decreasing pH. Sodium tripolyphosphate and sodium pyrophosphate behaved similarly. Sodium trimetaphosphate was exceptionally stable under all test conditions. The usual effect of calcium was to increase the rate of reversion. The magnitude of this effect increased with pH. In the presence of magnesium the rate of reversion was usually unaltered or decreased. The results obtained clearly point to the necessity of obtaining reversion data under conditions closely approximating those prevailing in the intended application.
T
HE molecularly dehydrated sodium phosphates, which
include those more commonly known as pyro-, meta-, or polyphosphates, are of considerable importance in water treatment because of the unusual properties which many of them possess. A11 of these materials have a tendency to react with water to form, ultimately, orthophosphate. This process is usually referred to as hydration, hydrolysis, or reversion. If it proceeds to the orthophosphate state it is undesirable in a t least two ways: (1) It reduces the amount of dehydrated phosphate available to serve its useful function; and (2) the introduction of orthophosphate ions into the water may result in the formation of relatively insoluble calcium or magnesium compounds when these latter ions are present. (In boiler feed water treatment it is desirable that the rate of reversion in the boiler proper be rapid.) Two important applications of certain of the molecularly dehydrated phosphates to water treatment are the stabilization of water supersaturated with respect to calcium carbonate and the inhibition of corrosion, From the standpoint of information desired for these applications, for which this work was undertaken, almost all investigations t o date on the rate of reversion of molecularly dehydrated phosphates are deficient in one or more respects. I n particular, no studies have been made under the conditions of dehydrated phosphate concentration used for the purposes mentioned. Unfortunately, the analytical methods available for complex mixtures of dehydrated phosphates are not applicable to these low concentrations. Because of this limitation, in this work the rate of formation of orthophosphate has
been taken as a measure of the stability of the dehydrated phosphate. Although support for this criterion of stability is offered by Watzel ( I S ) , he, and more recently Bell ( d ) , among other investigators, have shown that in the course of reversion of metaand polyphosphates to orthophosphate, intermediate products may be formed. (The pyrophosphates presumably revert directly to orthophosphate.) These intermediates may be either more or less effective than the original dehydrated phosphate in performing its useful function. I t is obvious, however, that where intermediate products are involved, the formation of orthophosphate does not correspond to the disappearance of a n equivalent amount of the original dehydrated phosphate. This consideration should be kept clearly in mind, for the words “reversion” and “stability” in this paper are used in the limited sense defined by measurements of the fraction of total phosphorus converted to orthophosphate. APPARATUS
The experimental apparatus consisted of a N i t e r flask having a mercury-sealed stirrer, a reflux condenser, and provisions for the withdrawal of samples and the addition of acid or alkali for adjustment of pH. A Leeds & Korthrup Standard 1199-22 high temperature glass electrode and a Leeds & Northrup Standard 1199-23 high temperature reference electrode were inserted in the smaller necks of the flask and connected to a Beckman Model G pH meter. The flask wag immersed in a liquid b a t h
f N D U S TRIAL A N D E N G I N E E R I N G C H E M I S T R Y
August 1950
1543
"Sodium metaphosphate," a glassy material, was prepared by heating C.P. monosodium orthophoos hate for 1 hour a t 300" C., 1 hour a t 500" C., 1 hour a t 600 2.5 hours at 900' C., and then cooling rapidly. Sodium trimetaphosphate was prepared by heating commercial disodium dihydrogen pyrophosphate for 2 hours a t 300' C. and for 5 hours a t 575 O C. It gave an x-ray diffraction pattern corresponding to that of NaPOJ of Partridge, Hicks, and Smith (6). Nalco No. 519, sodium metaphosphate, and sodium tetraphosphate gave noncrystalline x-ray diffraction patterns. Other chemicals employed were of a C.P. grade. Calcium and magnesium, when used, were added as the chloride and sulfate, respectively. Sodium hydroxide and hydrochloric acid were used for adjustment of pH.
8,
PROCEDURE
Figure 1.
Example of Primary Experimental Data
Reversion of tetrasodium pyrophosphate at 190' F. and 50 p.p.m. phosphate as Po4
-4pproximately 2 liters of water were brought to the desired temperature and p H and, in order to provide an approximately constant ionic environment throughout the work, 1700 p.p.m. of sodium chloride were added. A small volume of a freshly prepared solution of the dehydrated phosphate t o be tested was then introduced into the flask to produce a concentration equivalent t o 5 p.p.m. or 50 p.p.m. of PO,. The p H was quickly readjusted, if necessary, by the addition of dilute hydrochloric acid or sodium hydroxide and maintained within 0.1 t o 0.3 p H unit for the duration of the experiment by the occasional addition of a few drops of very dilute acid or alkali. At intervals of 1, 3, 6, and 22.5 hours a sample of the water was withdrawn, cooled, and analyzed immediately for orthophosphate by the colorimetric method referred to below. The initial orthophosphate content of the water was determined by analyzing a sample of the dehydrated phosphate solution after diluting it t o a n appropriate concentration. The p H measuring system was standardized with buffer solutions prepared from materials obtained from the National Bureau of Standards. Although p H values are certified only to 140" F., it is believed (11)that no significant uncertainty was introduced by the extrapolation of the data t o obtain p H values of the buffer solutions a t 150" and 190"F.
whose temperature was maintained within 1" F. of the desired value. MATERIALS
The dehydrated phosphates used in the investigation are listed in Table I. "Sodium tetraphosphate" was manufactured by the Rumford Chemical Works. (The sample used was obtained in 1941 and may not be representative of the material currently manufactured by this company.) The remaining dehydrated phosphates were obtained from the Blockson Chemical Company. They are characterized a s follows: The sodium tripolyphosphate was a commercial material which gave an x-ray diffraction pattern corresponding to that of NasPtOlc I1 of Partridge, Hicks, and Smith (6). Nalco No. 519 is a commercial grade sodium phosphate glass having a mole ratio of Na.0 to P20, of about 1.3.
v)
a I3 0 1
s
SODIUM
TRIPOLYPHOSPHATE I
&DISODIUM
\\\
1
N N
PYROPHOSPHATE
I
a W
tU z P v)
a TABLE
I. DEHYDRATED PHOSPHATES USED IN REVERSIOS EXPERIMENTS
Wt.
w
> W a
..p
o/,
SODIUM
Tetrasodium pyrophosphrte Disodium dihydrogen pyrophosphate Sodium tripolyphogphate Sodium tetraphosphate Nalco No. 619 Sodium metaphosphate Sodium trimetaphosphate
K6~&07
71.5
70
W a i-?
2c TABLE
I
+II
SODIUM
T R I METAPHOS P
rT E
I
5
p7H
Figure 3.
11. EFFECT OF
CONCENTRATIOX O X
22.5 HOURS AT 190" F.
I
Effect of pH on Reversion at 190" F. 50 p.p.m. phosphate as PO1
Dehydrated Phosphate Tetiasodium pyrophosphate Sodium tripolyphosphate Sodium tetraphosphate Nalco S o . 519 Sodium metaphosphate Sodium trimetaphosphate
REVERSION AFTER
PH 9 q0 P.P.M. 5 P.P.M. Reversion, %---93 10 4
pH 5 50 P.P.M. 5 P.P..M.
-98
100 84 69 44
10
87 71
56 36 13
19 24 26 16 3
25 27 33
31 11
August
INDUSTRIAL AND ENGINEERING CHEMISTRY
1950
1545
OF TEMPERATURB ON REVERSION TABLE IV. EFFECT
(50 p.p.m. of dehydrated phosphate) PH 5 pH 7 PH 9 Dehydrated 150' F. 190' F. 150' F. 190" F. 150' F. 190" F. Phosphate , Reversion, yo,after 2 2 , 5 hours-Tetrasodium pyrophosphate 31 98 9 73
