5826
J . Phys. Chem. 1987, 91, 5826-5832
Interaction of Urea with Weak Acids and Water Prabbat K. Das Gupta and S. P. Moulik* Department of Chemistry, Jadavpur University. Calcutta 700 032, India (Received: October 24, 1986) The results of deactivation of hydrogen ions of strong and weak acids (hydrochloric acid, acetic acid, potassium hydrogen phthalate, and benzoic acid) by urea are presented. Hydrogen ions get bound to the carbonyl oxygen of urea monomer with an overall increase of pK of weak acids. A reasonable scheme for the binding equilibrium together with the dissociation equilibrium of the weak acid is presented. The hydration of urea found from the compressibility measurements is reported; the enthalpy of hydration has been found to be -28.9 kJ/mol. From the thermodynamics of the solubility of benzoic acid in urea solutions, the nature of water structure at low and high concentration of urea has been looked into. The amide breaks the water structure in two steps with organized environment at higher concentrations (>5 mol/dm3) through urea-urea aggregate formation. Partial molar volumes of urea at various temperatures have been also reported in which hydration contributes significantly. Introduction The compound urea has versatile characteristics. It interacts with hydrogen ion;’ in its presence both pH and pK of weak acids increase.2 It is a protein denaturant3 and it breaks water s t r u c t ~ r e . ~It. ~also hinders the association of hydrophobes which is well manifested by its demicellization pr~perty.~.’In aqueous medium amide aggregates and urea dimers have been reported.8 Bull et al. reported’ the effects of urea on the pH of hydrochloric acid, acetic acid, and ammonium hydroxide. Schlfer2 presented the effects urea can have on acetic acid at various degrees of its neutralization; on the basis of urea dimer and monomer he has related the influence of the amide on the pK of the acid. Very recently we have studiedg the kinetics of acid-catalyzed ester hydrolysis in the presence of urea and several substituted ureas and have established that the model of Schafer overestimates the effect of urea. Our reported activities of HCl in the presence of urea agreed closely with those of Bull et al. obtained from pH measurements. We have also shown that urea combines with H+ ion through its carbonyl oxygen and not through the nitrogen of the amide group. The nature and mechanism of deactivation of hydrogen ion by urea is not yet a settled matter and we have undertaken a detailed study with a few typical acids. Furthermore, amide groups which are prevalent in biology often undergo interaction with H+ ion. Urea can serve as a good representative amide for such a study. Herein we report results of the study of the interaction of urea with hydrochloric acid, acetic acid, potassium hydrogen phthalate, and benzoic acid. A simple general scheme has been proposed for the treatment of the data. The solubility of benzoic acid in aqueous urea medium and the thermodynamics of the process have also been studied and the results have been compared with recent reports.1° Since urea is a potent water structure breaker and concentrated solutions of it have been used, the hydration of the amide, its partial molar volume, and the compressibility of its aqueous solution have been also investigated. This information is required for a general understanding of the behavior of the amide, but a comprehensive study is rare in the literature. Materials and Methods Hydrochloric acid, acetic acid, potassium hydrogen phthalate, benzoic acid, and urea were of AnalaR BDH grade. The purity of the latter two compounds was checked by melting point de(1) Bull, H. B.; Breese, G. L.; Ferguson, G. L.; Swenson, C. A. Arch. Biochem. Biophys. 1964, 104, 297. (2) Schafer, 0. F. Ber. 1976, 80, 529. (3) Kauzmann, W. Ado. Protein Chem. 1959, 14. (4) Rupley, J. A. J. Phys. Chem. 1964, 68, 2002. (5) MacDonald, J. C.; Ser Philips, J.; Guerrera, J. J. JPhys. Chem. 1973, 77, 370. (6) Emerson, M. F.; Holtzer, A. J . Phys. Chem. 1967, 71, 3320. (7) Gratzer, W. B.; Beaven, G. H. J . Phys. Chem. 1969, 73, 2270. (8) Stokes, R. H. Aust. J. Chem. 1967, 20, 2087. (9) Das Gupta, P. K.; Bhattacharya, P. K.;Moulik, S. P. Zndian J . Chem. 1984, 23A, 192. (IO) Das, K.; Das, A. K.; Bose, K.; Kundu, K. K. J. Phys. Chem. 1978, 82. 1242.
0022-3654/87/2091-5826$01.50/0
termination of dried samples. The disagreements between measured and literature values were within f0.6 OC. Further purifications were not essential. The aqueous solution of urea showed neutrality; the conductance of the solution was close to that of water used. The solution did not undergo any chemical reaction with acid which was confirmed by titrating a known strength of acid with and without urea by alkali. Doubly distilled conductivity water of specific conductance 1.5-2.0 pmho/cm and p H 6.7at 310 K was the solvent. If not stated otherwise, all measurements were undertaken at 310 K in a thermostated bath of accuracy f0.02 O C . Methods. pH measurements were taken with a ELICO digital pH meter (Model LI-120, Electronic Corp. India, Hyderabad) using a glass electrode and a calomel electrode. The glass electrode was calibrated in aqueous medium by using NBS buffers of pH 4.02 and 6.84 at 310 K accepted by IUPAC. With the glasscalomel assembly as well as the glass-Ag/AgCl combination, ShaferZhas shown that the liquid junction potential affects the measurements in urea solutions by less than 0.02 pH unit. We have neglected this and the results are uncorrected for this effect. We have also neglected any effect that urea might have on the normal functioning of the glass membrane. The objective was to measure the H+ ion activity in the presence of urea on the consideration that the glass electrode only registers the activity of the free H+ions and not the urea-bound ions. Ultrasound velocities were measured by the method of interference in an ultrasonic interferometer from Mittal Enterprises, New Delhi, India. The instrument generated ultrasound waves by a pulsating quartz crystal. Densities were measured with a calibrated pycnometer of long uniform neck with close graduations. All solutions were prepared fresh prior to the experiments. Different concentrations of the respective acids were prepared in the presence and absence of urea. pH measuements were taken after equilibrating the solutions in the thermostated bath for at least an hour. The same procedure was followed in the measurement of density. Excess benzoic acid was equilibrated in water as well as in aqueous urea solutions of different concentrations at constant temperature for a period of over 30 h and the pH of the solutions were measured. A portion of the solution was pipetted out and titrated against standard N a O H solution with phenolpthalein indicator. The standard deviations of pH, density, solubility of benzoic acid, and the ultrasound velocity were *0.30%, f0.006%, f0.113%, and f0.357%, respectively. Results Effect of Uren on the pH of Acids. The pH of HCl, CH3COOH, and potassium hydrogen phthalate at various concentrations of urea at 310 K are given in Table I. It is seen that, for a particular concentration, pH increases with urea content. The activity of hydrogen ion is reduced by the amide. The reduction follows the order HC1 > CH3COOH > potassium hydrogen 0 1987 American Chemical Society
Interaction of Urea with Weak Acids and Water
The Journal of Physical Chemistry, Vol. 91, No. 22, I987 5827
TABLE I: pH of HCI, CH3COOH, and Potassium Hydrogen Phthalate in Urea at 310 K ~~
urea concentration, mol/dm3 DH of 0.025 mol/dmg 0.05 mol/dm3 0.075 mol/dm3 0.1 mol/dm3
0
0.5
1.o
2.0
3.0
4.0
5.0
6.0
7.0
8.0
1.61 1.32 1.16 1.03
1.85 1.54 1.34 1.21
2.01 1.70 1.50 1.37
2.27 1.96 1.77 1.63
2.49 2.18 1.98 1.85
2.67 2.36 2.17 2.04
2.87 2.54 2.34 2.21
3.05 2.70 2.49 2.36
3.22 2.86 2.63 2.50
3.38 3.02 2.77 2.64
3.18 3.02 2.94 2.87
3.39 3.21 3.09 3.02
3.55 3.36 3.24 3.15
3.80 3.58 3.45 3.37
4.01 3.78 3.63 3.55
4.18 3.95 3.80 3.71
4.36 4.1 1 3.95 3.87
4.54 4.26 4.10 4.02
4.73 4.42 4.24 4.17
4.91 4.57 4.39 4.31
4.12 4.05 4.01 3.99 3.97
4.23 4.10 4.05 4.02 4.00
4.34 4.16 4.09 4.06 4.04
4.52 4.25 4.18 4.14 4.1 1
4.73 4.36 4.27 4.22 4.19
4.90 4.50 4.36 4.32 4.27
5.09 4.64 4.45 4.42 4.35
5.26 4.77 4.53 4.5 1 4.44
5.43 4.89 4.61 4.59 4.53
5.61 5.02 4.70 4.67 4.64
6.0
7.0
HAC 0.025 mol/dm3 0.05 mol/dm’ 0.075 mol/dm’ 0.1 mol/dm3
KHP 0.01 mol/dm3 0.025 mol/dm’ 0.05 mol/dm3 0.075 mol/dm3 0.1 mol/dm3
TABLE 11: pH, pK, and Solubility (S) of Benzoic Acid in Urea at Different Temperatures
urea concentration, mol/dm3 0 S, (mol/dm’) X 10’
PH PKa S, (mol/dm’) X lo’
PH PKa S, (mol/dm’) x lo3
PH PKa
0.2
0.5
1.o
2.0
3.0
4.0
5.0
62.05 3.50 4.92
73.58 3.66 5.13
88.68 3.71 5.13
105.66 3.76 5.15
123.58 3.83 5.21
66.82 3.50 4.96
84.91 3.68 5.23
101.89 3.74 5.26
121.70 3.80 5.30
140.57 3.87 5.35
72.62 3.61 5.21
97.64 3.73 5.30
116.98 3.83 5.51
138.68 3.92 5.61
155.66 4.00 5.68
92.56 3.71 5.53
100.47 3.98 5.91
122.64 4.06 5.99
146.23 4.14 6.07
166.98 4.22 6.13
Temperature = 303 K 32.59 3.16
33.93 3.18 4.80
36.91 3.24 4.81
35.12 3.12
37.80 3.17 4.77
39.88 3.22 4.79
36.90 2.97
39.88 3.04 4.61
44.05 3.17 4.74
49.40 2.86
52.38 2.98 4.61
56.10 3.16 4.83
41.07 3.30 4.82
51.19 3.42 4.89
Temperature = 308 K 43.90 3.26 4.76
54.46 3.40 4.87
Temperature = 313 K 48.66 3.28 4.85
64.43 3.47 5.10
Temperature = 318 K S, (mol/dm’) X lo3
PH PKa
6i.50 3.35 5.1 1
33r
77.98 3.58 5.40
TABLE 111: Thermodynamic Parameters of Solubility of Benzoic Acid in Urea at 308 K
3 15
3 20
3 25
3 30
3 35
T-’ xlo’
Figure 1. Plots of solubility vs. T’in various aqueous urea media. Numbers against the lines represent urea concentrations in mol/dm3.
phthalate. Binding of H+ ion with the oxygen of the carbonyl group >C=O of urea is envisioned. A working scheme for this phenomenon is presented in the next section. Behavior of Benzoic Acid in Urea Solution. The pH values of saturated solutions of benzoic acid and its solubility both in water and aqueous urea media at different temperatures are presented in Table 11. The results show an increase of both pH and solubility with increasing urea concentration. The thermodynamic parameters for solubility were found. Log (solubility) I (Figure 1). Both convex and concave was not linear with T patterns of variation were observed. Fitting to a polynomial form was not satifactory. Newton’s general interpolation formula” was
urea concn, mol/dm’
AH,kJ/mol
AG,kJ/mol
AS, J/(mol K)
0 0.2 0.5 1.o 2.0 3.0 4.0 5.0 6.0 7.0
10.04 12.19 15.06 15.78 15.11 13.39 22.95 22.24 22.00 18.65
18.52 18.32 18.16 17.89 17.27 16.68 16.00 15.45 14.91 14.46
-27.53 -19.89 -10.06 -6.85 -7.01 -10.68 22.59 22.03 23.01 13.60
tried to find the slopes at each temperature with the help of a Burroughs B 6700 computer. The calculated enthalpies were both negative and positive, showing nonsystematic variation with urea concentration; the results were not convincing. Therefore enthalpies for all the curves were found from the linear portion (Figure 1) in the range 303-3 13 K. Above 3 13 K urea is not a very stable compound,12 and the appreciable curvature toward the 318 K direction was anticipated due to some physicochemical change of the amide in solution. The energetic parameters are presented in Table 111. The solubility values used for the calculation are in mole fraction unit; hence the derived thermodynamic parameters are in the unitary scale. The parameters are not activity derived, so they are not truly standard-state values. (1 1) Numerical Mathematical Analysis; Scarborough, J. B.; Oxford &
IBH Publishing Co.: New Delhi, India, 1966: p 70. (12) Shaw, W. H.R.; Bordeaux, J. J. J . Am. Chem. SOC.1955, 77,4729.
Gupta and Moulik
5828 The Journal of Physical Chemistry, Vol. 91, No. 22, 1987
TABLE I V Density ( d ) and Compressibility (8) of Urea Solutions and Hydration Number (nh), Apparent Molar Volume (&), and Partial Molar Volume (&O) of Urea at Different Temperatures" urea concentration, mol/dm' 2.0 3.0 4.0
temu. K
0
0.2
0.5
1.o
5.0
6.0
7.0
d, g/cm'
303 308 313 318
0.99565 0.99403 0.99221 0.99022
0.99884 0.99717 0.99525 0.99323
1.00352 1.00176 0.99978 0.99769
1.01198 1.00995 1.00783 1.00569
1.02668 1.02513 1.02288 1.02043
1.04238 1.040 1.03759 1.03516
1.05709 1.05439 1.05178 1.04920
1.07222 1.06945 1.06652 1.06377
1.08429 1.08152 1.07853 1.07562
1.10092 1.09795 1.09504 1.09198
6, (cm2/dyn) X 10"
303 308 313 318
4.4647 4.3772 4.3303 4.2871
4.3697 4.3025 4.2647 4.2295
4.2862 4.2329 4.2066 4.1821
4.1768 4.1393 4.1139 4.0911
4.0219 3.9909 3.9872 3.9873
3.8332 3.8128 3.7963 3.790
3.6685 3.6578 3.6511 3.654
3.5152 3.5113 3.5158 3.5216
3.3999 3.4003 3.4047 3.4119
3.2843 3.2878 3.2932 3.2999
mol/mol
303 308 313 318
7.67 6.33 5.33 4.47
5.83 4.67 4.13 3.66
4.32 3.55 3.07 2.63
3.40 2.86 2.62 2.40
2.49 2.21 1.98 1.75
2.25 2.05 1.95 1.83
2.01 1.85 1.76 1.66
1.81 1.69 1.60 1.51
1.60 1.49 1.42 1.36
1.42 1.34 1.28 1.23
&, mL/mol
303 308 313 318
44.37 44.74 45.10 45.41
44.26 44.65 45.23 45.47
44.51 44.87 45.28 45.57
43.92 44.41 44.79 45.03
44.74 44.78 45.08 45.40
44.68 45.01 45.29 45.53
44.89 45.24 45.52 45.76
44.94 45.25 45.55 45.80
45.48 45.75 46.03 46.28
45.22 45.49 45.73 45.97
.,
nh,
Onh
and
@v
values in column 2 refer to true hydration and partial molar volume at the respective temperatures.
But activity coefficients of benzoic acid at different temperatures are considered unity and the energetic parameters are taken to be standard-state values. A discussion on this is presented in the next section. Behaviors of Aqueous Urea Solution. In Table IV are presented the densities, compressibilities, apparent molar volumes, and hydration numbers of urea at various concentrations. The apparent molar volumes were calculated from the relation
where 4,,, C, M,, d, and do are the apparent molar volume, molar concentration, molecular weight of urea, density of solution, and density of solvent, respectively. The standard partial molar volume 4,,O = V 2 O was found from graphical extrapolation of I& against C according to the equation 4 v = 4 v o + svc (2) where S,, is the slope. Good linearity was observed. The $vo values are presented in Table IV. The ultrasound velocities were used in the following relation to find the adiabatic compressibilities of water and urea solution 1
(3)
where (3, is the compressibility, Vis the volume, P is the pressure, u is the sound velocity, and p is the density of solution. The compressibility values were substituted in eq 4 to compute the hydration number nhI3
L A
1
5
x,
'C
15
x lo2
Figure 2. Excess compressibilities of urea solutions plotted against mole fractions of urea: curves 1-4 a t 303, 308, 313, and 318 K.
tempted to explain it. Recently, we have shown from kinetic study' that the >C=O center of the amide attaches to the H+ ion and the decrease in the activity is an exponential function of urea concentration that closely corresponds to the findings of Bull et al. Schafer considered the monomer-dimer equilibrium of urea and binding of the H C ion to both the species. For a strong acid the following three equilibria may then be written (5)
(4) where n, and nu are the number of moles of water and urea in a solution, respectively, and (3, and (3, are the respective compressibilities of the solvent and the solution. The hydration numbers are presented in Table IV. The excess compressibilities (3, = (@&d - (3,,bsd)14 are plotted against mole fraction of urea in Figure 2. Discussion Urea deactivates Hf ion. From pH metric studies Bull et al.' and Schafer2 have demonstrated the phenomenon and have at(1 3) Modern Electrochemisrry; Bockris, J. OM.; Reddy, A. K. N. Plenum: New York, 1977; Vol. I, p 127. (14) For a noninteracting mixture, the compressibility is given by the additive rule, paid = where 6, and Gi are the compressibility and volume fraction of the ith species. For a water-urea binary mixture, the solute = is assumed noncompressible, so that
xipi@,,
U,
+ Hf
U,H+
(6)
Ud
+ H+ '4udH+
(7)
Kd
where U,, ud, U,Hf, and UdH+ represent urea monomer, urea dimer, hydrogen-ion-bound monomer, and hydrogen-ion-bound and K", are the dimerization dimer, respectively, and Kd, constant, monomer-Hf ion association constant, and dimer-H+ ion association constant, respectively. From the conventional algebraic forms of the equilibrium constants of equilibria 6 and 7, we get
c,
aU,H+
+ aUdH+ + c a u , a H + + K h d a H +
(8)
At equilibrium, in concentration notations CU,H+
+ C U d H + = c,+
-
c,+
(9)
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 5829
Interaction of Urea with Weak Acids and Water
005r
12.0 r
J
I
/'
0 02'-
I
I
I
10
20
I
3.0
I
I
I
LO
50
60
U mt
Figure 3. Test of eq 11 for the HC1-urea system at 310 K. Concentrations of HCI are given against the curves. Broken lines represent
activity-derived results. where the terms CHl+and CH+denote the total hydrogen ion concentration and its equilibrium concentration, respectively, or
-'U,H+ + - = - - 'UdH' -
H ':
aH+
YU,H+
YH1+
yH+
YU,jH+
(9a)
For HCl of constant ionic strength in urea solutions, the activity coefficients, yH,+, yH+,yumH+, and yudH+,of the species are assumed to be equal. Equation 9a changes into
+ aUdH+ = aH,+- aH+
aUmH+
(9b)
Combining eq 8 and 9b'and eliminating audby eq 5, we have
with the replacement of aumby nu,, (the activity of total mo3 mol/dm3 that allows it to trap H+ ion favorably. The efficient water structure breaking by urea at concentrations >4 mol/dm3 has been shown.4 It is seen from Figure 3 that the tangents of the initial and final curves meet at Cum,= 3.5 mol/dm3. We had proposed association of the H+ ion at the >C=O center of urea; the inertness of thiourea toward the H+ ion had supported this proposition. If it exists, urea dimer should have a structure /NH2
C=O\
----H ZN\
NH2-----O=C
/ H2N
and further association of H+ ion to such a species is not expected because of the lack of a potential center. To our understanding, even though the urea dimer is formed, it does not bind the H+ ion; only the monomer accepts it. Equation 10 is then modified to aH+ = aH,+/( + a U m , c )
(12)
In Figure 4 plots of aH+vs. aH,+are shown. The results are in (15) Hydration of neutral urea should affect its activity. Therefore it is strictly not correct to equate activity with concentration. But the effect is considered small enough to set them equivalent. (16) Wyman, J. (Jr). J. Am. Chem. SOC.1933, 55, 4116.
(17) has a slight dependency on the acid Concentration. Extrapolation to zero acidity gives the true value which in the present situation at 37 O C is 1.47.
Gupta and Moulik
5830 The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 TABLE V 310 K
and K , Values at Different Urea Concentrations at
urea concn, mol/dm’
E
KU
0.5 1.o 2.0 3.0 4.0 5.0 6.0 7.0 8.0
1.424 1.543 1.996 2.570 3.264 4.246 5.376 6.844 8.771
1.356 1.404 1.686 2.047 2.464 3.057 3.718 4.556 5.635
757
”
O
h
e
good agreement with the equation. values obtained from the slopes are given in Table V. If dimerization of urea is ignored, becomes K,, which is also listed in the table. Both and K, have been found to be a function of urea concentration and the dependency changes above 3 mol/dm3, again justifying the special effect at higher concentrations of the amide. For both acetic acid and potassium hydrogen phthalate, the pH at concentrations equal to that of the hydrochloric acid in the presence of urea were measured. values at the respective urea concentrations found with the hydrochloric acid were used to calculate the dissociation constants and hence the pK,. The following equations were used for calculations, taking acetic acid (HAC) as the representative acid.
e
& H+ + Ac+ H+ 5 U,H+
HAC U,
(13)
= aH+
+ aU,H+
(14)
+ aUmfl)
(15)
By eq 6, eq 1 4 becomes aAc‘
= aH+(1
By eq 15 the dissociation constant of acetic acid, K,, is aH+uAc- aH+*(1 Ka=--
+ au,,JT)
(16)
HAC
QHAC
The equilibrium concentration of acetic acid, CHAc, is related with the total concentration, C(HAc),, by the relation
C H A=~ C(HA~), - CAC. With
aHAc
CHAc
and aAc- = CAc-
HAC = ~(HAC),- a ~ c = -
~(HAC),
w
I
0
0.025
- UH+ ( 1 + au,,fl)
and eq 1 6 reads
h
n
I
I
I
0.05
0.075
0.1
0 0
A c e t i c A c i d ( m o l dni31
Figure 5. pK, vs. acetic acid concentration plots in aqueous urea environments at 310 K. Numbers against the curves represent urea concentrations in mol/dm3.
In the calculations, C(HAc), is used for For no urea dimerization
u(HA~),
+
(6)
The dissociation equilibrium (13) is affected by (6). Both H+ and U,H+ contribute to the formation of acetate ion. Thus, if we assume ?Ac- = yH+ = YU,H+, aAc-
n
a ~ + ~ ( 1a&)
K, = ~(HAC),
(18)
- Q H + (+~ ~ u K J
Because of the assumption that activity = concentration, the derived K, values must not be considered thermodynamic. The pK, values for acetic acid and potassium hydrogen phthalate given in Table VI are seen to depend on both the concentrations of the acid and urea. In Figure 5, extrapolations to zero acid concentration at all urea concentrations for acetic acid are presented. Similar plots for potassium hydrogen phthalate are shown in Figure 6 . The variations of the extrapolated pK, values with urea concentration for both the acids are presented in Figure I . As is seen from Figures 5 and 6, the variation of pKa of potassium hydrogen phthalate is distinctly different from that of acetic acid. The turning zone of pKa is between 3 and 4 mol/dm3 of urea. The pK, of benzoic acid has variations comparable to that of acetic and CUvalues have acid (Table 11). As mentioned above CUmt been used for au,, and aUin the calculation. The product C U , , c = CuK,,; the K, calculated from eq 1 7 and 1 8 are therefore identical and independent of the state of aggregation of urea. The energetic profiles of transfer of benzoic acid from water to urea medium are presented in Figure 8. Two-step structure alteration has been revealed from the entropy values. The water structure breakdown after 3 mol/dm3 urea is significant. At further higher concentrations organized environment through urea hydration and urea-urea aggregate formation is supported from almost unchanged AS, and its lowering after 5 mol/dm3. From
TABLE VI: pK, of CHpCOOH and Potassium Hydrogen Phthalate in Urea at 310 K urea concentration, mol/dm3
DK. of
0
0.5
1.o
2.0
3.0
4.0
5.0
6.0
7.0
8.0
4.74 4.73 4.73 4.73
4.93 4.88 4.82 4.80
5.09 5.03 4.96 4.91
5.33 5.20 5.13 5.08
5.54 5.38 5.27 5.23
5.70 5.54 5.42 5.36
5.88 5.69 5.54 5.5 1
6.09 5.84 5.69 5.65
6.32 6.00 5.83 5.80
6.53 6.15 5.97 5.94
6.23 6.50 6.72 6.85 6.94
6.23 6.37 6.57 6.69 6.77
6.29 6.33 6.50 6.6 1 6.70
6.40 6.25 6.42 6.51 6.58
6.60 6.26 6.38 6.46 6.52
6.76 6.36 6.38 6.48 6.50
6.96 6.46 6.38 6.50 6.49
7.15 6.56 6.39 6.52 6.5 1
7.34 6.65 6.39 6.53 6.54
7.55 6.77 6.43 6.55 6.61
HAC 0.025 mol/dm3 0.05 mol/dm3 0.075 mol/dm’ 0.1 mol/dm3
KHP 0.01 mol/dm3 0.025 mol/dm3 0.05 mol/dm’ 0.075 mol/dm’ 0.1 mol/dm3
Interaction of Urea with Weak Acids and Water
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 5831
8 0
I
-12 4 I
70 76 7 4 7 2 KHP
0 Y
Q
70 6 8
6 6
6 4
a
a -6 0
1/05
-
6 0
I
i
0-
6 2
0 025
0050
0075
01
1
1
I
20
40
60
U t ( m o l dm-j)
K H phtholote(mo1d r f 3 )
Figure 6. pK, vs. potassium hydrogen phthalate concentration in aqueous urea environments at 310 K. Numbers against the curves represent urea concentrations in mol/dm’.
I
Figure 8. Energetic profiles of the transfer of benzoic acid from water to urea medium at different total urea concentrations at 308 K.
L
0 6 r
303 K
-
6
0
E
\
0
E
-
4
L
C
2
0
2
4
6
8
Ut l m o i dm-’)
Figure 9. Dependence of the hydration numbers of urea on its total concentration at various temperatures.
Figure 7. Plots of extrapolated pK, vs. total urea concentration for potassium hydrogen phthalate and acetic acid at 310 K.
from the temperature effect is comparable to the results on other nonelectrolytes. l9 The present reported standard partial molar volume, V20 (0.044 37 dm3/mol) at 303 K is close to that reported by Philip et a1.,200.044 25 dm3/mol, at 298 K. The nominal increase by
the thermodynamics of solubility of benzoic acid, Kundu et al.1° observed mild water structure alteration by urea. The excess compressibilities of urea solutions have a similar pattern of variation with concentration; the hump at -2 mol/dm3 smooths out with temperature. An initial structuredness followed by breakdown of the water structure is anticipated. The degree of structuredness is expected to decrease with increased temperature which corroborates the observation. The entropy of transfer data discussed above do not reveal such an initial structuredness in the presence of urea. The derived hydration from compressibility is dependent on urea concentration. The nonlinear concentration-dependent hydration numbers have been smoothly extrapolated to evaluate the zero concentration value (Figure 9, Table IV). This is higher than the value of 2.5 at 25 OC obtained from spectroscopic studies.’* The enthalpy of hydration evaluated
(18) Bonner, 0. D.; Woolsey, G. B. J . Phys. Chem. 1968, 72, 899. The spectroscopic method registers the loss of monomeric water through hydration. Due to the structure-breaking effect the concentration of monomeric water is enhanced in the presence of urea. The hydration is therefore underestimated. Since in addition to hydration water structure is broken, the measured compressibilities in the present study are more than what should have been due to hydration alone. The values calculated by eq 4 are thus somewhat lower than actual. A quantitative account for the discrepancy between our results and that of Bonner and Woolsey is difficult. The sound velocity results at 6 m urea of Beauregard et al. (Beauregard, D. B.; Barrett, R. E. J . Chem. Phys. 1968, 49, 5241) at 313 K when processed according to eq 4 has given a value of nh that fits into our results and is shown in Figure 9 with a closed square symbol. (19) Shiio, H.; Ogawa, T.; Yoshihashi, H. J . Am. Chem. SOC.1955, 77, 4980. (20) Philip, P. R.; Perron, G.; Desnoyers, J. E. Can. J . Chem. 1974, 52, 1709.
U t ( m o l dm-’)
5832 The Journal of Physical Chemistry, Vol. 91, No. 22, 1987
0.27% for a rise of 5 OC is by way of dehydration and expansion. All the reported values are lower than the molar volume of pure urea, 0.045 45 dm3/mol, which means a contraction in volume with urea addition. This is mainly affected by the solute hydration. Acknowledgment. Thanks are due to the University Grants
Additions and Corrections Commission, Government of India, for awarding a Teacher Fellowship to P.K.D.G. Registry No. H', 12408-02-5; urea, 57-13-6; hydrochloric acid, 7647-01-0; acetic acid, 64-19-7; potassium hydrogen phthalate, 877-24-7; benzoic acid, 65-85-0; water, 7732-18-5.
ADDITIONS AND CORRECTIONS 1986, Volume 90
D. Wayne Goodman* and Charles H. F. Peden: CO Oxidation over Rh and Ru: A Comparative Study. Page 4841. On the last paragraph of the page it is erroneously stated that, "The surface oxygen coverage, subsequent to the thermal desorption of CO, is essentially zero even at O2pressures sufficient to attenuate the overall activity." Thus, the following statement, beginning "Surface oxygen is believed ...",is irrelevant. Close inspection of Figures 3a and 4a (on pages 4841 and 4842) reveals that Auger electron spectroscopic (AES) data were not obtained under conditions in which CO oxidation activity on the Rh(ll1) single-crystal catalyst was negative order in O2pressure. In more recent work,' we have found that a sharp increase in surface oxygen, as detected by AES subsequent to reaction at high O2/CO ratios (higher ratios than used in this previous AES study), directly correlates with the change from positive to negative O2 partial pressure dependence. (1) Peden, C. H. F.; Goodman, D. W.; Blair, D. S.; Berlowitz, P. J.; Fisher, G. B.; Oh, S. H. J. Phys. Chem., accepted for publication.
