Hydrotropic Solvents for Benzoic Acid PHASE EQUILIBRIA WLLIAJI LICHT, J R . , ,4ND L. D. WIENER’ University of Cincinnati, Cincinnati 21, Ohio Phase equilibrium data at 30°, 40”, and 60” C. have been determined for the system watersodium o-xl-lenesulfonate-benzoic acid. The solubility of benzoic acid at 40” C. has also been determined for systems in which sodium o-xylenesulfonate was replaced by sodium benzenesulfonate, sodium p-toluenesulfonate, sodium mxylenesulfonate, sodium p-xylenesulfonate, sodium p bromobenzenesulfonate, sodium rn-benzenedisulfonate, and sodium cinnaniate. The hydrotropic effect of the various solutions on the solubility of benzoic acid has been compared and analyzed. It was concluded that the solubility of benzoic acid in these solvents is due to a “salt effect” rather than to a solubilizing effect due to similarity in structure.
H
YDROTROPIC solutions are aqueous salt solutions capable of dissolving substances normally either insoluble or slightly soluble in water at the same temperature. The hydrotropic substance, as well as the substance dissolved, may be either an organic or an inorganic compound. Xeuberg ( 1 0 ) has presented eytensive lists of both hydrotropic substances and substances which were dissolved. Booth and Everson ( 2 ) have determined the solubility of many substances in some of the best hydrotropic solutions. McKee (9) has presented many industrial applications for this phenomenon. Hydrotropic solution is often described as a “salting-in” effect as opposed to the more commonly known “salting-out” effect, and several theories are based on this line of reasoning. A study of these tm-o effects together, called simply the “salt effect,” has been reviewed by Gross (4). Debye ( 3 ) also treats salting-in and salting-out as opposing effects. Some authors attribute hydrotropic solution to the formation of a compound, soluble in water between the hydrotropic salt and the substance to be dissolved. Others rule out compound formation as hydrotropic solution. This seems logical, since benzoic acid and many other insoluble acids are readily “dissolved” in sodium hydroxide solution owing to the formation of their soluble sodium salts. These neutralization reactions are not considered as examples of hydrotropic solution. Hence, compound formation may be ruled out as an explanation of hydrotropy by definition. According to Bancroft ( 1 ) the solution of hydrotropic salt in water acts as a mixed liquid solvent, the dissolved hydrotropic substance, whether it be solid, liquid, or gas, acting as a second liquid. Kuthy (6) suggests that the action of hydrotropic substances resembles that of emulsifying agents in that the hydrotropic substances act by presenting new physical conditions rather than new chemical complexes. Kruyt and Robinson ( 6 ) believed that the increased solubility was due to the polar properties of the hydrotropic solution. Lindau ( 7 ) explained hydrotropic solution on a phase-rule basis by considering the salt equivialent to a third miscible liquid. 1
Present address, Magnolia Petroleum Company, Dallas, Tex
There are many other explanations of h j drotropy, but curiently there is no one generally accepted theory. There is the possibility that all abnormal solubility cannot be explained by one theory-that is, there may be different types of hydrotropic solution depending on the nature of the nonelectrolyte dissolved. PHASE EQUILIBRIA
There have been many instances of hydrotropic salts and nonelectrolytes, dissolved by aqueous solutions of these salts, reported in the literature. However, complete phase equilibrium data for any three component system, water-hydrotropic saltnonelectrolyte, have never been published. Complete phase equilibrium data include solubility data a t various temperatures of the salt, the nonelectrolyte and compounds or double salts formed between them, and/or any hydrates in equilibrium vith the aqueous solution of these compounds. Phase equilibrium data were first determined for the system, water-sodium o-xylenesulfonate-benzoic acid, a t several temperatures. Sodium xylenesulfonate Tvas chosen as the hydrotropic salt because it is frequently encountered in the literature as an example of an effective as well as relatively inexpensive hydrotrope. The 4-sulfonate of o-xylene was arbitrarily selected. Benzoic acid was chosen as the nonelectrolyte because it offers an easy means of analysis in that it may be titrated with sodium hydroxide solution using phenolphthalein as indicator. Sodium xylenesulfonate, a salt of a strong acid and strong base, is essentially neutral and should offer no interference. This was found t o be true. Benzoic acid is only slightly soluble in water, but, as reported in the literature, its solubility is increased appreciably in sodium xylenesulfonate solutions. The solubilizing effect of several other similar salts on benzoic acid was also investigated with the hope of correlating increased or decreased hydrotropic effect with the structure and molecular weight of the salt. EXPERIMENTAL
Phase equilibrium data were obtained for the system waterbenzoic acid-sodium o-xylenesulfonate at 30°, 40°, and 60” C., by determining the solubility of benzoic acid in aqueous solutions of various concentrations of sodium o-xylenesulfonate, as well as the solubility of sodium o-xylenesulfonate in aqueous solutions of various concentrations of benzoic acid, and the composition of the solid phase in equilibrium with these solutions. -4dditional data were obtained a t 40” C. for the solubility of benzoic acid in aqueous solutions of various concentrations of the following salts: sodium benzenesulfonate, sodium p-toluenesulfonate, sodium m-xylenesulfonate, sodium p-cymenesulfonate, sodium p-bromobenzenesulfonate, sodium p-phenolsulfonate, sodium m-benzenedisulfonate, and sodium cinnamate. C.P. benzoic acid was used and the hydrotropic salts were the best grades obtainable from Eastman Kodak Company with the exception of the sodium benzenesulfonate and sodium p-cymenesulfonate which were obtained from Kyandotte Chemicals Corporation. 1538
.
August 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
The determinations for the svstem involving sodium o-xvlenesulfonate were made in the following manner: Saturated solutions of benzoic acid in aqueous sodium o-xylenesulfonate solutions and of sodium o-xylenesulfonate in aqueous benzoic acid solutions were made by having benzoic acid and sodium o-xylenesulfonate in excess, respectively. The ingredients, roughly weighed out and enough to make up about 100 grams of solution, were placed in a stoppered 125-ml. Erlenmeyer flask. This mixture was heated to a temperature above that of the determination, but not to boiling, in order to dissolve all or nearly all of the solid material. The solution was then cooled and placed in a constant temperature bath and allowed to come to equilibrium. The solution was stirred from tjme to time, and 24 t o 48 hours were allowed to ensure equilibrium. If excess solid material did not separate out on cooling, additional material was added, and the above procedure repeated. After equilibrium was considered to have been reached, 5 portions of supernatant saturated solution of 3 to 5 ml. each were drawn off by means of a pipet, the flask remaining in the constant temperature bath. After removal of these samples, an additional amount of sodium o-xylenesulfonate (or benzoic acid) was added to the remaining mixture to produce the desired incremental increase in concentration, and the above procedure was repeated. I n the determination of the solubility of benzoic acid in aqueous sodium o-xylenesulfonate solution, the initial concentration of the salt used was 10 to 15% and its concentration was increased in increments of 5 to 10% until the solution became saturated with both substances. In the determinations of the solubility of sodium o-xylenesulfonate in benzoic acid solutions, the initial concentration of benzoic acid was 50/, or less and the increments of increase were smaller than above. The solubility of pure sodium o-xylenesulfonate in water was determined a t 30°, 40°,and 60" C. The solubility of benzoic acid in water a t 30" and 40" C. was obtained from the literature (11). ANALYSIS OF SOLUTIONS
The portions of saturated solution were transferred to tared weighing bottles with ground-glass covers. The evaporation of water was considered to be negligible during transfer, and the weight of saturated solution was obtained by difference. Benzoic acid was determined by titration with standard sodium hydroxide solution using phenolphthalein as indicator. Three of the five samples taken from each solution were titrated directly in the weighing bottles. The remaining two portions of saturated solution were placed in a drying oven at a temperature of 110' to 120' C. At this temperature the water was driven off by evaporation, and benzoic acid was removed by sublimation. The samples were left in the oven until a constant weight was obtained on two successive weighings. Since pure chemicals were used, the dried residue was assumed to be only sodium o-xylenesulfonate, and the amount of water in the solution was obtained by difference. The results are expressed as weight per cent of each component. ANALYSIS OF WET RESIDUES
The composition of solid sodium o-xylenesulfonate in equilibrium with saturated solution was determined by the netresidue method. This consists in analyzing the moist solids in equilibrium with the solution a t two different concentrations. On a plot of these data on a triangular diagram, the two lines joining the points representing the corresponding concentrations of the solution and the moist solids and extended past the latter points, intersect a t a point representing the solids in equilibrium with solution. The moist solids were recovered and sampled as follows: As much supernatant saturated solution was removed as possible by means of a pipet, the flask remaining in the constant temperature bath. I n order to obtain uniform samples, the stoppered flask containing the moist solids was heated until all went into solution. Samples were then poured off into weighing bottles and analyzed in the same way as the solutions. There is no known hydrate of benzoic acid, hence it was as-
1539
TABLE I. SOLUBILITY DATAFOR
THE SYSTEM BENZOIC ACID (BzOH )-SODIUM0-XYLENESULFONATE ( N~XS)-WATER
BBOH, Wt. % 0.41 1.10 1.65 3.09 4.34 5.12 0.00 1.07 2.36 2.99 3.66 0.55 1.38 2.41 3.30 4.73 5.59 6.97 9.32 9.54
0.00
2.69 5.60 8.35 1.18 2.81 4.83 8.98 10.34 13.28 19.20 20.39 0.00 5.07 9.25 11.17 11.68 13.38 15.64
NaXS,
Hz0,
Wt. %
Wt. %
0.00
99.59 84.34 79.01 69.71 62.77 59.82 71.25 68.98 66.02 64.57 63.08 99.45 85.20 77.57 72.64 65.91 61.93 56.49 47.37 46.66 59.91 55.46 51.50 48.28 98.82 84.76 76.43 63.85 59.51 51.57 36.20 33.61 53.47 47.53 43.44 41.28 40.96 39.42 37.48
14.56 19.34 27.20 32.89 35.06 28.75 29.95 31.62 32.44 33.26 0.00 13.52 20.02 24.06 29.36 32.48 36.54 43.31 43.80 40.09 41.85 42.90 43.37 0.00 12.43 18.74 27.17 30.15 35.15 44.60 46.00 46.53 47.40 47.31 47.31 47.36 47.20 46.88
Grams BzOH/ Gram NaXS (Corrected) 0 0.052 0.068 0.103 0.124 0.139
Moles NaXS/ 1000 Grams Hg0 0.000 0.829 1.176 1.874 2.517 2.815
...
... 0
0.067 0,099 0.121 0.149 0.162 0.183 0,209 0.212
...
... 0.000 0,762 1.239 1.591 2.140 2.519 3.107 4.391 4.508
...
...
... 0
...
...
2.38 4.09 7.61 8.76 10.12 16.27 17.27
0.000 0.703 1.1777 2,044 2.434 3.274 5.917 6.574
...
... ...
... ...
... ...
... ...
...
... ... ...
Analysis of Wet Residue in Equilibrium with CorreEponding Saturated Solutions (5) 9.88 54.33 35.89 ... ... (v) 12.07 52.90 35.03 ... ... (2) 14.22 51.80 33.98 ... ...
sumed that the solid phase in equilibrium with saturated solution of benzoic acid was anhydrous benzoic acid. ANALYSIS O F OTHER SYSTEMS
The same type analysis was applicable for all other hydrotropic salts used. Only solutions saturated with respect to benzoic acid were analyzed. Hence it was not necessary to analyze any solid residues. The following salts were used in these determinations: sodium benzenesulfonate, sodium p-toluenesulfonate, sodium m-xylenesulfonate, sodium p-cymenesulfonate, sodium p-bromobenzenesulfonate, sodium benzenesulfonate, and sodium cinnamate. Solubility data for the system benzoic acid-sodium p-bromobenzenesulfonate-water were determined in the manner described. However, after completion of this work, it was found that the original sodium p-bromobenzenesulfonate was not pure: it contained an acid impurity as indicated by the fact that it was necessary t o add a considerable volume of approximately 0.1 N sodium hydroxide solution t o an aqueous solution of the salt to produce a pink color with phenolphthalein. Rather than discard the data, it was decided to determine if the acidity of t h e impure salt was constant throughout the lot of it. The volume of standard sodium hydroxide solution required to neutralize a gram of several samples of the salt was found t o be constant (2.42 ml. of 0.1331 N base), and the benzoic acid determinations were corrected accordingly. Thus, the hydrotropic effect was not determined for pure sodium p-bromobenzenesulfonate, but for the salt with free acid present. Calculated as p-bromobenzenesulfonic acid, the salt contained 7.63% free acid by weight. This value would be lower if the acid present was of lower molecular weight.
1540
INDUSTRIAL AND ENGINEERING CHEMISTRY
Voi. 42, No. 8
sodium o-xylenesulfonate in saturated solutions increases with increased benzoic acid concentration. However, the concavity of the curves representing the solutions is reversed a t the two temperatures. At 60" C. there is a maximum in the curve representing the concentration of sodium o-xylenesulfonate in saturated solutions. However, by expressing the concentrations of sodium o-xylene-sulfonate and benzoic acid in moles per 1000 grams of water, the solubility of sodium o-xylencsulfonate increases with increasing concentration of benzoic acid. 0. The rate of increase of solubilitv of sodium o-xylenesulfonate in benzoic acid solutions with increasing temperature decreases with increasing temperature. This is also true for aqueous solutions. 6. The addition of water to an aqueous mixture is represented graphically on a trianW't. % NaXS gular diagram by a line joining the points repFigure 1. Solubility Curves for the System Benzoic Acid-Sodium resenting the composition of the solution and o-Xylenesulfonate- Water the water vertex. The more water added, the closer the point, representing the resulting composition, approaches the water vertex. AddiIn the determination of the solubility of benzoic acid in aqueous tion of water to the concentrated solution of benzoic acid in aqueous sodium o-xylenesulfonate solution produces composition in sodium p-cymenesulfonate solutions, the analysis for the salt the two-phase, benzoic acid-water, region. Thus, benzoic acid proved to be somewhat uncertain. The time required for is precipitated by dilution with water. evaporating a sample to dryness and to constant weight was much 7 . Since there are only two portions to the curve bounding longer than that for the other salts. As much as a month was the one-phase regions, there is no compound formation indicatcd between benzoic acid and sodium o-xylenesulfonate. required for samples high in benzoic acid, This indicates that 8 At 60" C. the solid phase in equilibrium with saturate solubenzoic acid is more difficult to remove by sublimation from tion (with respect to sodium o-xylenesulfonate) as determined by these solutions than from the others. There was also a gradual the wet-residue method was found to be anhydrous sodium darkening of the salt during this time, an indication that deconio-xylenesulfonate. position was taking place. This was confirmed by the fact that Results of the determinations of the solubility of benzoic acid after constant weight was reached, the residue was acid t o in aqueous solutions of sodium benzenesulfonate, sodium p phenolphthalein, although the original salt was neutral. Altoluenesulfonate, sodium m-xylenesulfonate, sodium p-bromothough these data are not so accurate as those obtained in most benzenesulfonate, sodium p-cymenesulfonate, sodium m-benof the other determinations, they give a good indication of the zenedisulfonate, and sodium cinnamate a t 40" are given in Table hydrotropic effect of sodium p-cymenesulfonate toward benzoic 11. There was a slight decrease in the solubility of benzoic acid. acid in the sodium m-benzenedisulfonate and sodium cinnamate RESULTS AND CONCLUSIONS solutions as compared to its solubility in water a t 40" C. Ilencc, extensive data were not obtained for these salts and these runs The phase equilibrium data for the system benzoic acid-sodium are not plotted. o-xylenesulfonate-water a t 30°, 40°, and 60" C., as obtained from The results of the determinations involving the other salts are the analytical procedure, are expressed as weight per cent of each shown in comparison with the solubility of benzoic acid in aqueous of the components. The experimental results are given in sodium o-xylenesulfonate a t 40' in Figure 2. Since the accuracy Table I and are shown graphically in Figure 1. of the analysis for sodium p-bromobenzenesulfonate and sodium Figure 1 shows a plot of weight 70benzoic acid against weight p-cymenesulfonate was doubtful, a plot of these data is not shol+n. yosodium o-xylenesulfonate on rectangular coordinates. ExperiOf the six salts showing a hydrotropic effect tou-ard benzoic mental points are shown on this plot but have been omitted on acid, sodium p-cymenesulfonate was the most effective. The all subsequent derived plots. others, in order of decreasing effectiveness, are: sodium o-xyleneOn inspection of these plots, the following observations were made :
1. There is a pronounced increase in hydrotropic effect with increase in temperature. This seems to correspond with the relative increase in solubility of benzoic acid in water with temperature. A plot (not shown) of the ratio of the solubility of benzoic acid in sodium o-xylenesulfonate solution (weight % benzoic acid in solution) to the solubility of benzoic acid in water (weight % ' benzoic acid in solution) a t the same temperature against concentration of sodium o-xylenesulfonate (weight % salt in solution) bears out the statement, in that the three curves practically coincide. 2. The maximum Concentration of benzoic acid in saturated solutions a t 60" C. is 20.39%. I n this solution there is about l l / a times as much sodium o-xylenesulfonate present as there is water. 3. The maximum concentration of benzoic acid in saturated solutions a t 40" C. is 9.54%. In this solution there is almost as much sodium o-xylenesulfonate present as there is water. 4. At 30' and 40" C. the concentration of
Figure 2.
Solubility of Benzoic Acid in Hydrotropic Salt Solutions at 40" C.
August 1950
:E
i?
INDUSTRIAL AND ENGINEERING CHEMISTRY
0.1
0 I
3
2
I
4
6
Moles NaXS/1000 Grams HzO
Figure 3.
Ratio of Benzoic Acid to Sodium o-Xylenesulfonate
Corrected for benzoic acid in water present us. moles sodium o-xylenesulfonate per 1000 grams water
sulfonate, sodium m-xylenesulfonate, sodium p-bromobenzenesulfonate, sodium ptoluenesulfonate, and sodium benzenesulfonate. I n the absence of free acid it is entirely possible that sodium p-bromobenzenesulfonate would be more effective than sodium m-xylenesulfonate, Thus, for the homologous series of derivatives of sodium benzenesulfonate, the hydrotropic effect toward benzoic acid increases with increased number and increased size of alkyl groups on the benzene ring-that is, the effect increases with increasing molecular weight and increasing size of the molecule. The presence of a bromine atom on the ring yields a molecule of approximately the size that would result if a methyl group were present. However, the molecular weight is larger and the hydrotropic effect toward benzoic acid is greater. The presence of a
DATAFOR TABLE 11. SOLUBILITY
THE SYSTEMBENZOIC ACID (BZOH)-HYDROTROPIC SALT-WATER AT 40 O C.
Hydrotropic Salt Sodium benzenesulfonate
Sodium p-toluenesulfonate
Sodium m-xylenesulfonate
Sodium p-cymenesulfonate
Sodium p-bromosulfonate
Sodium m-benzenedisulfonate Sodium oinnamate
BzOH Salt Wt. .Id Wt. %
€120
Wt. %
Grams BaOH/ Gram Salt (Corrected)
Moles Salt/ 1OOOGrams Hz0
0.92 1.62 2.12 2.59 3.11
21.06 32.11 36.61 39.92 42.20
78.02 66.27 61.27 57.49 54.69
0,023 0.039 0,049 0,057 0.067
1.498 2.689 3.314 3.854 4.283
1.47 2.23 3.63 4.67 5.32
20.32 25.78 31.81 36.90 39.40
78.21 71.99 64.56 58.43 55.28
0.051 0,071 0.103 0.118 0.127
1.338 1.845 2.537 3.252 3.671
1.75 3.13 4.20 6.16
19.18 26.63 31.49 39.06
79.07 70.24 64.31 64.78
0,068 0.103 0.122 0.150
1.165 1.821 2.352 3.42B
1.05 3.14 4.20 4.91 7.54 11.54
8.22 14.77 26.27 28.74 40.64 53.10
90.73 83.09 70.63 66.35 51.82 35.36
...
..
...
...
1.18 2.68 4.28 4.79
15.52 24.95 32.50 34.99
83.40 72.37 63.22 60.22
... ... ...
... ...
0.24 0.34
36.41 39.44
63.35 60.22
0.28
9.05
90.61
... ... ... ...
I
.
1541
second sulfonate group on the ring eliminates the salting-in effect toward benzoic acid and causes a slight degree of salting-out to occur. Bancroft (1) has stated that hydrotropic solvents may be compared to mixed liquid solvents in their action. The action of mixed solvents is based on similarity in structure betweei one of the liquids and the substance dissolved. The greater the percentage of this liquid, the more the substance is dissolved. In the ideal case the solubility of the third component in the mixed solvent increases linearly with the weight fraction of solubilizing liquid in the mixed solvent. Thus, a plot of solubility of the third component against composition of mixed solvent will yield a straight line. This line of reasoning was applied to the system waterbenzoic acid-sodium o-xylenesulfonate. The solubility data were first recalculated as solubility of benzoic acid in grams per 1000 grams of sodium o-xylenesulfonate solution and weight % ' sodium o-xylenesulfonate in the sodium o-xylenesulfonate solution. These were plotted as ordinate and abscissa, remectivelv. It was believed that a straight line, which could be extended t o the sodium o-xylenesulfonate axis, might result and give a fictitious solubility of benzoic acid in pure sodium o-xylenesulfonate. However, an exponential type of curve was obtained. It was impossible to extrapolate to the pure salt axis. Following the reasoning that there might be a fictitious solubility of benzoic acid in sodium o-xylenesulfonate, it is logical to assume that the solubility of benzoic acid in aqeuous sodium o-xylenesulfonate solutions is due to one of the three following reasons: similarity in structure of benzoic acid and sodium o-xylenesulfonate, alone; a salt effect in addition to similarity in structure; or a salt effect alone. If similarity in structure alone is the cause of the solubilizing action, the ratio of grams of benzoic acid, exclusive of the amount dissolved in the water, to grams of sodium o-xylenesulfonate in the solution should be constant a t all concentrations of sodium o-xylenesulfonate; this is considered to be an ideal case of a mixed liquid solvent. If there is a salt effect in addition t o the solubilizing effect due to similarity in structure, this ratio would no longer be constant, but wTould be expected to increase with increased salt concentration. However, an indication of the solubilizing effect due to similarity in structure can be determined by extrapolating the curve to zero salt concentration, where the salt effect is zero. If the solubility of benzoic acid in sodium o-xylenesulfonate solution is due only to a salt effect, the extrapolation of the curve should intersect the water axis a t a ratio value of zero. I
-
03
r
I
d
I
I
I
I
.
,..
I . .
0
I
I
I
I
I
e
3
4
Figure 4.
...
...
... ...
...
...
Corrected for benzoic acid in water present at 40' C. salt per 1000 grams water 1 = Sodium o-xylenesulfonate 2 = Sodium rn-xylenesulfonate 3 = Sodium p-toluenesulfonate 4 = Sodium benzenesulfonate
...
5
Moles Salt/1000 Grams Hz0
...
...
I
Ratio of Benzoic Acid to Hydrotropic Salt us.
moles
1542
INDUSTRIAL AND ENGINEERING CHEMISTRY
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
