June, 1956
PROTOLYSIS CONSTANTS OF
SUBSTITUTED PHENYLPHOSPHONIC ACIDS
787
ULTRAVIOLET ABSORPTION STUDY OF PROTOLYSIS CONSTANTS OF SOME PARA-SUBSTITUTED PHENYLPHOSPHONIC ACIDS IN WATER BY WALTERJ. POLESTAK AND HOWARD K. ZIMMERMAN,JR. Department of Chemistry, Agricultural and Mechanical College of Texas, College Station, Texas Received November 19, 1966
Ultraviolet absorption spectra have been observed for aqueous solutions of phenylphdsphonic, p-bromophenylphosphonic, pchlorophenylphosphonic and p-methoxyphenylphosphonlc acids. The wave lengths for the secondary band maxima of the phenyl- and p-chlorophenylphosphonic acids agree closely with values earlier observed in 95% ethanol solutions, although the molar extinction coefficients are depressed by 10.9 and 15.5%, respectively, as a result of the solvent change. The absorption band for p-methoxyphenylphosphonicacid shows a small bathochromic shift with increasing pH. Stoichiometric ionization constants for both the first and second dissociations of all four acids are calculated from the spectral data and compared with values in the literature.
I n . recent years considerable data have been reported concerning both the ultraviolet absorption spectra and the stoichiometric dissociation constants of a variety of substituted arylphosphonic However, in surveying these data, one finds numerous gaps, as well as a few disagreements in the reported results. At the risk of adding to the discords, we undertook to carry out an independent study of the dissociation constants for four of these arylphosphonic acids. In our study of the dissociation constants, the spectrophotometric approach was chosen not only because it provided a method of attack entirely different from that used to obtain the majority of the literature data, but also because it could provide new information on the ultraviolet spectra of these acids in water as a solvent.
Experimental Materials.-The compounds studied, namely, phenylphosphonic acid (m. 162.&163'), p-chlorophenylphosphonic acid (m. 187-188.5"), p-bromophenylphosphonic acid (m. 201-202 ") and p-methoxyphenylphosphonic acid (m. 164164.5') were obtained through the courtesy of Dr. L. D. Freedman. Their preparation has been described elsewhere.6SB Acid solutions were prepared using stock solutions of reagent grade hydrochloric acid; carbonate-free sodium hydroxide stocks were used for solutions in the basic range. All solutions were made up from de-ionized distilled water. Apparatus.-Spectra were recorded using the Beckman DU quartz spectrophotometer with silica cells of 1.000 cm. light path. The calibrated cell correction was 0.1% ?f measured absorbance. The spectrophotometer was calibrated with a Beckman 2260 mercury lamp as radiation source. For measurements, slit widths between 0.5 and 0.7 mm. were set as required. In preliminary work, and for part of the final data, the Warren Spectracord attachment for the spectrophotometer was used.' The recording paper used for this portion of the work had a dimensional stability such that it suffered less than 0.25% change for relative humidities between 20 and 90%. Measurements of pH employed the Beckman model G pH meter. I n solutions of pH 10 or greater, the Beckman type B sealed glass electrode was used. (1) H. H. Jaffe and L. D. Freedman, J . A m . Chem. Soc., 74, lOG9 (1952). (2) P. Lesfauries and P. Rumpf, Compt. rend., 888, 1018 (1949). (3) C. V. Banks and R. J. Davis, Anal. Chim. Acta, 12, 418 (1955). (4) H. H. Jaffe, L. D. Freedman and G . 0. Doak, J . A m . Chem. SOC.,76, 2209 (1953). ( 5 ) G. 0. Doak and L. D. Freedman, ibid., 78, 5658 (1951). (6) R . W. Bost, L. D. Quinn and A. Roe, J . 0 ~ g Chem., . IS, 362 (1953). (7) Use of the Spectracord in this work resulted from the courteey of the Fisher Scientific Company in loaning us the use of the instrument. This assistance is gratefully acknowledged.
Procedure.-No stock solutions of the arylphosphonic acids could be kept for more than three days because of the development of a non-crystalline flocculation in them, probably as a result of mold growth.* Aqueous solutions of the four acids were prepared to 0.002 M by weighing all constituents. Desired amounts of HC1 or NaOH solutions were added, and the result diluted to 0.001 M . Spectra were recorded for each acid a t approximate pH's of 1.5, 7, and 11 over the range from 220 to 360 mp, usin as comparison standard an aqueous solution of HC1 or NaOd corresponding to the stoichiometric amount added to each test sample. From these spectra as obtained with .the Warren Spectracord, wave lengths giving maximum llght absorption were selected for further point-to-point study a t additional pH values. Triple checks of absorption data obtained by the manual procedure showed a reproducibility of transmittance results within 0.5%.
Results All acids showed a triple set of maxima in the plot of extinction us.. wave length except for p methoxyphenylphosphonic acid. The results at these maxima are shown in Table I. For this latter acid, only the two peaks appeared. TABLE I EXTINCTION OF AQUEOUS P-SUBSTITUTED PHENYLPHOSPHONIC Acms First
Compound CsHrPOiHa pBrCsHdPOiHx pClCsHiPOaHa pHaCOCsHiPOaHz
Xmax
mp 257.3 258.0 257.5
max.
emax
348 316 232
Second max." Xmax,
nip 263.3 263.8 263.5 270
Third max. Xmax
emsx
mlr
trnax
467 360 246 929
269.2 270.7 269.9 276
364 268 212 743
a This is the secondary band maximum according to the definition of Doub and Vandenbelt?
The data of Table I apply to aqueous solutions of the acid compounds under study, with no additional acid or alkali. It is of some interest to notice that, although the wave length of the maximum for the secondary band is not shifted as one passes from alcoholic solution' to an aqueous one, the extinction coefficient for the band is depressed from a value of 524 in alcohol (95% EtOH) to a value of 467 in water in the case of the phenylphosphonic acid, while the extinction coefficient for p-chlorophenylphosphonic acid is depressed from 292 to 246 by the same solvent change. I n view of the fact that the light absorption of each of the first three acids listed in Table I decreases with increasing pH of the solution, it is not unreasonable to suppose that this depression reflects an increasing ( 8 ) L. D . Freedman, private communicatioo. (9) L. Doub and J. M. Vandenbelt, J . Am. Chem. Soc., 69, 2714 (1947).
WALTERJ. POLESTAK AND HOWARD K. ZIMMERMAN,JR.
788
Vol. 60
TABLB I1 COMPARISON OF DISSOCIATION CONSTANTS' Subst. phosphonic
acid
Phenyl"
Ab
1.50
B '
K~ x IOU
1.50
CO
2.51
D
12.3" 11.ob
Ab
0.851
K: x B5
0.851
r07b
CO
0.141
D
3.89" 3.0gb
gBromopheny1" 2.14 . . .e 2.51 1.45 1.48 2.51 p - Chlorophenyl' 2.51 2.19 2.04 1.78 pMethoxyphenyld 0.832 0.955 0.955 0.795 Average deviation of pK values, present work, Potentiometric measurements. b Spectrophotometric measurements. was =t0.06. d Average deviation of p K values, present work, was f0.08. E Jaffe, Freedman and Doak state that this acid waa insufficiently soluble in water to permit determination of this value by their potentiometric method. 1 A, This report; B, Jaffe, Freedman and Doak, ref. 4; C, Lesfauries and Rumpf, ref. 2; D, Banks and Davis, ref. 3.
ionizing influence which the water solvent exercises on the acid. If from the spectral data, one plots the absorbance of a 0.001 M solution of acid against pH, a stepped curve is obtained having two regions of extreme sensitivity to p H changes. Such curves were plotted and treated by the method of Stenstrom and Goldsmith,'O as modified by Irving, Rossotti and Harris," making use of a differential plot of the data to locate the pH a t which the ionized and un-ionized forms of the acidic species are of equal concentration. At such a point, the p H is numerically identical to the pK of the acid. The two stoichiometric dissociation constants for the individual acids, as evaluated in this manner, are listed in Table 11,where they are also compared with values reported by other investigators. As is to be seen from this compariqon, our values for the dissociation constants are in good general agreement with those reported by both Jaffe, Freedman and Doak14 and by Lesfauries and Rumpf,2 both of whose reports were based on potentiometric studies. Since our data derive directly from the spectral absorption characteristics of the test substance in solution, which in turn depend predominantly on stoichiometric concentrations, our constants are to be considered as '(apparent dissociation constants, " rather than thermodynamic ones. The results from the potentiometric method cited above were deduced on the same basis, so that the extent of agreement, particularly with the results of Jaffe, et al., leads us to believe that the other constants (for other arylphosphonic and arylphosphinic acids) reported by them are to be preferred over others appearing in the literature. Our findings are in complete disagreement with those of Banks and Inasmuch as these investigators worked in ionic strengths corresponding to salt concentrations of the order of 1.0 M , we believe there is probably a high degree of uncorrected salt effect in the potentiometric results reported by them. Moreover, the molar extinction coefficients used by them in their spectrophotometric method were estimated on the basis of their potentiometric results. Consequently, we feel that their results are not reliable. Certainly the weight of the evidence supports this conclusion. We experienced our greatest difficulty in the determination of the constants for the p-methoxy(IO) W. Stenstrom and N. Goldsmith, THISJOURNAL, 29, 1477 (1925). ( 1 1 ) €1. Irving. H. (1955).
S. Rossotti and G. Harris,
Annlrpf. 80, 83
phenylphosphonic acid, and our results for this case are considerably less precise than are those for the other three cases. The difficulty arose from the fact that the differences in spectral transmission amounted to only about 8% in the pH range from zero to ten. As a result, the increment of absorption with change in p H (used in the differential plot) was not large anywhere in the range, and maximum precision was not possible. In addition the degree of resolution in the spectrum was poor. Optimum resolution appears to occur for these acids in the range of transmittance between 40 and SO%, whereas for the concentrations we had chosen for this substance, the transmittance was of the order of 20%. Solutions even more dilute than ours would probably lead to better values. A further difficulty experienced in employing the spectrum of p-methoxyphenylphosphonic acid for dissociation constant calculations arises from the fact that the wave length of maximum light absorption undergoes a small bathochromic shift with increasing pH. This effect, together with the general increase in absorption as pH increases (contrary to experience with the other three acids) we believe to be attributable to the changing influence of the non-bonding valence electrons of the oxygen atom in the methoxy group on resonance within the phenyl ring. I n acid solutions, the presence of a relatively high proportion of hydrogen ions in the solution makes probable a moderate degree of coordination of such ions with these nonbonding oxygen electrons, immobilizing them. With fewer hydrogen ions present, as in an alkaline solution, such non-bonding electrons are free to participate in a "quinoid" type of resonance with the ring, thereby lengthening the resonance path. This postulate raises the interesting question of whether or not this acid (as well as others of its type) exists to any extent in the form of "zwitterions." We have been able neither to find any mention of such a phenomenon in the literature nor to carry out experimental work to answer the question a t the present time. Insofar as the interpretation to be placed on the experimentally determined dissociation constants is concerned, the values we have obtained in agreement with the results of Jaffe, Freedman and Doak4 lead us to concur in their conclusions. I n particular, it appears from the fact that the p-bromoand p-chloro- derivatives possess larger ionization const,ants (in the order which would be predicted from t,heir well-known inductive influence-actu-
VISCOSITIES OF
June, 1956
THE
ally a perturbation of the resonance pattern in the phenyl group) than the unsubstituted phenylphosphonic acid, while the constant for the electronyielding p-methoxy- derivative is depressed, that there is a very definite communication of electronic charges across the molecule to the site of ionization. Such a communication can only arise from
BINARY GAS MIXTURES
789
the polarizability of the central phosphorus atom in the acid functional group, operating in concert with the resonance pattern within the phenyl ring. Acknowledment.-We wish gratefully to acknowledge the valuable consultations and many helpful suggestions contributed to us during this study by Dr. A. F. Isbell.
VISCOSITIES O F THE BINARY GAS MIXTURES, METHANE-CARBON DIOXIDE AND ETHYLENE-AR.GON' BY W. MORRISON JACKSON Goodyear A t m i c Corporation, Laboratory Division,Portsmouth, Ohio Received November 36,1066
Experimental viscosities at 25" are reported for the binary gas mixtures, methane-carbon dioxide and ethylene-argon over the composition range from 0 to 100 mole yoof each component. The measured viscosities are compared with calculated values and their application to quantitative analysis of these gas pairs is discussed.
Introduction Two inert binary gas mixtures for which no previous experimental viscosity data have been reported are methane-carbon dioxide and ethyleneargon. Calculations of the theoretical viscosities showed that for methane-carbon dioxide the change in viscosity with composition should deviate in a positive manner from the additivity of the individual viscosities, while for ethylene-argon the change in viscosity with composition should show a negative deviation. To verify this theoretical difference in behavior, the viscosities of these gas pairs were measured over the entire composition range using the capillary tube method, and the results were compared with the calculated values. Experimental Apparatus .-An automatic-reading capillary tube viscosimeter, previously described by Junkins,Z was used for the determination of the viscosities of the gas pairs, methanecarbon dioxide and ethylene-argon. A straight platinum Capillary with an internal diameter of 0.03 and 100 cm. in length was used in all of the measurements. Materials .-The gases used to prepare the binary gas mixtures were obtained from commercial cylinders of methane, carbon dioxide, ethylene and argon. The impurities present in each gas as determined by mass spectrometer analysis are as follows. Methane: contained 0.1% oxygen, 0.1% propane and 0.7% ethylene; carbon dioxide: based upon a trace of argon observed, a maximum of 0.1% oxygen and 0.4% nitrogen could have been present; ethylene: contained less than 0.17% propane and/or propene and a trace of air. Small amounts of acetylene would not have been detected; argon: contained 0.06% nitrogen and 0.01% oxygen, carbon dioxide and water. The air used was medicinal quality "breathing air" which was dried by passing through a Dry Ice slush trap and drying towers of anhydrous calcium sulfate and anhydrous magnesium perchlorate. Procedure.-Methane-carbon dioxide mixtures of the approximate composition desired were prepared by filling evacuated cylinders with one of the pure gases through a manifold to a predetermined pressure, and then adding the second pure gas at a higher pressure until the desired total pressure was obtained. To ensure thorough mixing, each cylinder was alternately heated and cooled several times and (1) Based on work performed for the U. 8.Atomic Energy Commission by Union Carbide Nuclear Company, Union Carbide and Carbon Corporation, Oak Ridge, Tenneasee. (2) J. H. Junkins, Rev. Sci. Znstr., 28, 467 (1955).
then allowed t o stand about five days before sampling for analysis. The composition of the methane-carbon dioxide mixtures was determined by Orsat analysis. The chemical methods of analysis investigated were found to be uneatisfactory for ethylene-argon mixtures. Conse uently, ethylene-argon mixtures were prepared in much %e same manner as the methane-carbon dioxide mixtures except that they were mixed with extreme care using a specially designed manifold so as not to exceed a maximum error in composition of &O.l%. For the measurement of the flow time of a gas, whose viscosity was to be determined, the gas was admitted into the capillary forechamber until the pressure was greater than a reference pressure, PI. As the gas flowed through the capillary into a container at a constant pressure, PO, the forechamber pressure dropped until it was lower than a second reference pressure, Pz. The time required for the pressure to drop from P I to PZ was recorded by a timer which was automatically turned on at P I and off at Pa. Measurements were repeated as necessary to establish the precision. After evacuation of the manifold and capillary, the reference gas was run in the same manner without changing the temperature or pressures PI, PZand PO. The temperature of the bath surrounding the forechamber and capillary and the pressures were noted for each measurement. Relative flow times under different pressure conditions were obtained by changing the reference premures PI, Pz and Pa.
Results Measurements of the viscosity of nitrogen at 65", made under different pressure conditions of PI, Pz and Po to establish the precision of the viscosimeter and to determine the effect of varying Reynolds numbers, are shown in Table I. TABLEI
VISCOSITY OF NITROQEN AT 65" Mean pressure, Pm, om.
Mean Reynolds no. Rem
16.64 18.87 25.29 31.91 37.71
16.5 15.3 40.3 58.3 85.7
Exptl. viscosity of nitro5en. cp0-e
195.3 195.4 195.4 195.5 195.4
Experimental viscosities of methane-carbon dioxide and ethylene-argon mixtures, for the composition range from 0 to 100 mole % of each component, are compared to calculated values in Tables I1 and 111.
