ADDITION COMPOUNDS OF PHOSPHOROUS ACID WITH CERTAIN ORGANIC COMPOUNDS1 H. L. REDFIELD
AND
G . B. KING
Laboratory of Ueneral Chemistry, The State College of Washington, Pullman, Washington Received June 18, 1966
In a previous communication (5) it has been shown that orthophosphoric acid forms addition compounds with several types of organic compounds; furthermore, Kendall’s (2) rule of acidity held in all systems examined. It was the purpose of the present investigation to make a comparison of phosphorous and phosphoric acids with regard to their ability to form addition compounds with organic substances and to test further the validity of Kendall’s rule of acidity. In the study of these addition compounds, two methods have been employed. The most direct method is the determination of freezing-point curves in two-component systems. This method has certain advantages in that the compound is isolate4 and furthermore the composition may be determined from the phase diagram. The second method of study is described by Knox and Richards (6). This method is based on the fact that strong acids added to solutions of weak acids decrease the solubility of the latter because of common-ion effect. In certain cases, however, decrease in solubility takes place to a certain point only, after which an increase in solubility occurs. This increase in solubility is attributed to compound formation. EXPERIMENTAL
Phosphorous acid was obtained from Eimer and Amend and purified by recrystallization. The recrystallized acid melted at 74.4OC. (corrected), which is higher than previously reported. Most of the organic compounds used in this investigation were prepared by the Eastman Kodak Co., and in only a few cases was any special purification necessary. The method of experimentation in determining freeaing points has previously been given (5). 1 The material of this paper was presented by Herbert L. Redfield in partial fulfillment of the requirements for the degree of Master of Science in Chemistry at The State College of Washington.
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e
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H. L. REDFIELD AKD G . B. KING
The solubilities of several organic acids and phenol were determined in solutions of progressively greater concentrations of phosphorous acid. About 20 cc. of the solution of phosphorous acid was placed in a large test tube, together with an excess of the solute. The tube and contents were heated to 50" to 60°C. for a short time to near saturation. The tube was then placed in a thermostat at 25"C., and the contents stirred several times daily for a period of ten days to two weeks. This was found to be a sufficient length of time for attainment of equilibrium. Portions of the samples were then pipetted off and analyzed. METHODS O F ANALYSIS
Although several methods of analysis have been employed by other investigators, the method by which the solvent acid is determined gravimetrically and the solute acid by difference from total acidity, proved satisfactory. It was found that phosphorous acid could be oxidized quantitatively with concentrated nitric acid to which a small amount of hydrochloric acid was added, after which the phosphorus was determined as magnesium pyrophosphate. Total acidity was determined by titration with standard sodium hydroxide, using phenolphthalein as indicator. The end point is reasonably sharp. For analysis of the phenol-phosphorous acid mixture, the phenol was precipitated as tribromophenol and weighed as such. The procedure was as follows: A suitable aliquot containing from 0.08 to 0.2 g. of phenol was diluted to 25 cc. and bromine water added slowly with rapid stirring. The phosphorous acid was oxidized slowly with the excess bromine. Any tribromophenol bromide formed is changed to tribromophenol by the phosphorous acid. The former bromine compound is yellow, while the latter is white. The precipitate was transferred to a Gooch crucible, washed, and dried over phosphorus pentoxide to constant weight. This method gave results which were uniformly 2.6 per cent low; however, by carefully standardizing the procedure and applying the above correction, very satisfactory results were obtained. The phosphorous acid was determined gravimetrically. The oxalic acid was determined directly by titration with potassium permanganate after calcium oxalate had been precipitated in a buffered acetic acid solution. EXPERIMENTAL RESULTS
The results or the freezing-point determinations are given in table 1 and of the solubility determinations in table 2. The compounds used were considered representative for studies of binary systems to test further the rule of acidity. Curves are shown in figure 1. No evidence of compound formation was found in any of the
MOLE PER CENT
Hapoi
1
TABLE 1 Freezing-point data FBEEZINC POINT
dOCE PER CENT
HiPo,
I
FREEZINQ POINT
“C.
73.2 68.3 64.0 60.0 55.2 51 . O 47.4 42.7 38.7 33.4 28.4 23.2 17.7 13.5 13.5 13.2 11.o 10.0 11.7 13.6 16.4
Trichloroacetic acid 100 92.4 85.0 80.2 75.7 70.0 66.4 61.5 56.3 53.7 50.8 48.6 43 3 38.3 33.3 26.9 22.0 18.0 14.2 8.9 0 .o Pyrui 100 85.3
81 .o
75.8 70.2 65.6 60.4
Hap08
Pyruvic acid-Cont’d
Acetic acid 100 92 85.9 79.8 73.7 68.5 63.7 58.4 53.8 47.4 42.8 36.9 31.2 27.4 26.0 25.6 24.8 20.0 14.3 7.8 0.0
MOLE PER CENT
73 6 69 7 67 8 66 4 65 2 64 1 63 3 61 9 60 7 60 3 59 7 58 9 57 3 55 2 53 6 50 9 49 0 50 0 51 0 52 7 57 2 acid 72 8 63 8 607 57 6 54 4 51 7 48 2
OC.
56.2 51.8 51.3 48.2 43.9 39.1 33.4 30.1 24.8 20.4 12.7
,
45.7 42.8 43.2 40.6 38.1 34.4 31.3 29.3 24. ( f 0 . 5 ) 19. ( 1 1 . 0 ) 7. ( f 2 . 0 )
,
Phenol 73.6 70.0 68.0 66.7 65.0 64.1 63.0 62.7 62.0 61.1 60.3 57.5 56.7 56.2 55.4 54.2 53.8 51.1 48.9 45.9 (36.2) 37.6 40.6
100 93.0 87.8 83.5 78.1 72.8 68.4 66.8 63.5 59.8 55.6 44.6 40.7 36.9 33.4 30.4 26.9 22.4 17.9 13.5 7.9 0.0 Acetc 100 94.0 82.8 78.0 72.2 67.4 61.7 56.9 51.9 47.2 42.8 41.2 37.8 34.6 31.7 28.6
73.1 70.0 63.8 60.1 55.3 51 .O 45.0 40.3 35.2 30.2 26.3 24.9 19.3 17.5 16.1 16.3 921
(I
FREEZINQ POINT
Acetophenone-Cont’d 4c. 24.7 16.7 20.5 17.1 16.5 17.7 11.2 18.3 5.7 19.0 0.0 19.7 Pip onal 100 73.0 91.5 68.4 87.2 66.2 81.4 63.2 77.2 60.0 73.1 56.6 69.4 53.2 64.5 49.2 58.1 42.2 54.1 38.6 53.0 37.3 51.8 36.2 46.2 32.8 40.8 32.9 37.0 30.7 34.3 31.1 29.0 32.0 23.9 32.6 18.4 33.2 13.9 33.8 8.8 34.2 0.0 35.1 c o .arin 100 74.5 69.4 90.4 63.6 82.4 59.0 77.1 53.3 47.5 44.5 39.3 58.8 40.4 54.8 43.7 50.8 48.8 49.8 50.5 45.9 54.6 41.6 57.8 37.7 60.1 32.8 62.1 30.1 63.1 25 .O 64.2 20.1 65.3 14.0 66.4 8.1 67.3 0.0 68.7
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H. L. REDFIELD AND G. B. KING
TABLE 2 Solubility data: normalities of solvent and solute when varying concentrations of solvent are saturated with the solute SAMPLE
OXALIC ACID
-
XO.
1 2 3 4 5 6 7 8 9 10 11
CzHiOi
HaPO:
N
N
2.407 1.71 1.51 1.38 1.21 1.13 1.oo 0.77 0.71 0.81 0.84
CITRIC ACID
N
N
8.48 12.78 15.11 18.38 20.21 23.39 30.77 36.66 42.02 44.73
I
BUCOINIO ACID
-
HaPOi
12.61 11.42 9.13 8.07 6.15 5.12 3.70 2.66 2.22
N
6.18 11.64 15.21 21.02 25,61 30.77 37.38 41.42
1.347 0.90 0.69 0.71 0.75 1.34 1.51
PEXlNOL
-
Hip01
CsHaOH
N
N 0.896 0.808 0.881 0.918 1.060 1.214
9.79 16.61 18.60 25.55 34.20 35.97
HIPOZ N
9.59 16.02 17.99 22.95 26,65
-
I .tJ
I
m .
NOOLE PERCENT
YPG
I 7s
I
FIQ.1. Freezing points of binary systems of phosphorous acid and certain organic compounds. Curve I, phenol and phosphorous acid; subtract 90 from temperature scale. Curve 11, piperonal and phosphorous acid; subtract 100 from temperature scale. Curve 111, trichloroacetic acid and phosphorous acid; subtract 60 from temperature scale. Curve IV, coumarin and phosphorous acid; subtract 55 from temperature scale. Curve V, pyruvic acid and phosphorous acid; subtract 30 from temperature scale. Curve VI, acetophenone and phosphorous acid; subtract 25 from temperature scale. Curve VII, acetic acid and phosphorous acid. above systems. Although some difficulty was encountered in determining freezing points in some of the systems, particularly in the region of the
ADDITION COMPOUNDS OF PHOBPHOROUS ACID
923
eutectic, still they are sufficiently accurate to leave little doubt as to the ndn-exktence of a compound.
FIQ.2. Solubility curves of binary system;. Curve I, oxalic acid in phosphorous acid. Curve 11, oxalic acid in phosphoric acid. Curve 111, succinic acid in phosphorous acid. Curve IV, succinic acid in phosphoric acid. Curve VI phenol in phosphorous acid. Curve VI, phenol in phosphoric acid. Curves 11, IV, and V I are taken from data of Kepfer and Walton (4).
I
I
I
8 le NORMALITY
I
a OF
SZ
40
SOLVENT
FIQ.3. Solubility curves of binary systems. Curve I, citric acid in phosphorous acid. Curve 11, citric acid in phosphoric acid (taken from data of Kepfer and Walton (4)). Curves for the various systems are shown in figures 2 and 3. For the purpose of comparison, the curves for phosphoric acid taken from data of
924
H. L. REDFIELD AND G. B. KING
Kepfer and Walton (4) are shown. In all cases there appears to be definite evidence of the formation of addition compounds. DISCUSSION
From the results of this investigation it appears that phosphorous acid forms addition compounds only in solution. Also Kendall’s rule of acidity is borne out well in the solubility study, but in this case is not successful in predicting compound formation in freezing-point studies of phosphorous acid with a second component. On the basis of Kendall’s rule, phosphorous acid would be expect’ed to form addition compounds, as it is of the same approximate strength as phosphoric acid, which yielded several compounds in a similar study of freezing-point curves. As a matter of fact, since phosphorous acid is slightly stronger, the tendency for compound formation should be slightly greater with certain organic compounds. However, Kendall’s rule at best is a qualitative one and the results obtained here indicate that factors other than acidic strength, such as structure and nature of compounds involved, must play a not inconsiderate rBle in compound formation. Too, it is questionable that acid dissociation constants which have been determined in water solution should be carried over to a solvent other than water. However, Kendall’s rule is well borne out in the solubility study, in which curves very similar t o the ones formed in a similar solubilit’ystudy with phosphoric acid are found. For purposes of comparison the curves obtained in the study with phosphoric acid are included in figures 2 and 3. The primary dissociation constants of the acids concerned in this study follow : Acid
Oxalic acid (8).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Citric acid (IO). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Succinic acid (9). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Phenol (3). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Phosphorous acid (7). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Phosphoric acid (1). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
K
3 . 8 X 10-2 8.0 X 10-4 6 . 7 X 10-6 1.08 X 10-10 1 . 6 to 6 . 2 X IO-* 1.1 X 10-2
Compounds were indicated in every case. Assuming sharpness of break as indicative of relative tendency toward compound formation, phenol and succinic acid show the greatest tendency to form addition compounds with phosphorous acid, while citric and oxalic acids show a lesser tendency. This is to be expected in view of Kendall’s rule, since the difference in acidic strengths between phosphorous acid and oxalic and citric acids is less than the difference between phosphorous acid and succinic acid and phenol. When a comparison of phosphorous and phosphoric acids is made, generally speaking, phosphorous acid shows a slightly greater tendency to
ADDITION COMPOUND8 OF PHOSPHOROUS ACID
925
form addition compounds than phosphoric acid. Again this is in agreement with Kendall's rule. It is t o be noted that although phenol exhibits an unusually strong tendency toward compound formation in solution, no evidence whatever is obtained in a freezing-point study. This directly bears out the statement that phosphorous acid appears to form these addition compounds only in solution. BUMMARY
1. The freezing-point diagrams of several binary systems in which phosphorous acid acts as one component have been determined. No compound formation was indicated. 2. The solubilities of phenol, oxalic acid, succinic acid, and citric acid have been determined in solutions of phosphorous acid of varying concentration. 3. Kendall's rule of acidity holds well in solution, but in the case of phosphorous acid fails in freezing-point equilibria. 4. A comparison of solubility curves of phosphorous and phosphoric acids has been made. 5 . The melting point of phosphorous acid was recorded as 74.4"C. (corrected). REFERENCES (1) ABBOTT AND BRAY:J. Am. Chem SOC.31, 729 (1909). (2) KENDALL, J.: J. Am. Chem. SOC.43, 1545 (1921). (3) KENDALL, J.: J. Am. Chem. SOC.39, 7 (1917). (4) KEPFERAND WALTON: J. Phys. Chem. 34, 543 (1930). (5) KINQAND WALTON: J. Phys. Chem. 36, 1745 (1931). (6) KNOXAND RICHARDS: J. Chem. SOC. 115, 508 (1919). (7) KOLTHOFF: Rec. trav. chim. 46, 350 (19%'). (8)LANDOLT-B~RNSTEIN: Physikalisch-chemidle Tsbellen, Vol. 11, p. 1133. J.
Springer, Berlin (1923). (9) OSTWALD: Z.physik. Chem. 3, 282 (1889). (10) WALKER: J. Chem. SOC.61, 696 (1892).
