June, 1956
CONDUCTANCE AND
775
PHYSICAL P R O P E R T I E S O F TJRE.4 SOLUTION
CONDUCTANCE AND OTHER PHYSICAL PROPERTIES OF UREA SOLUTIONS BY V. K. VENKATESAN AND C. V. SURYANARAYANA Physico-Chemical Laboratory, Annamlai University, South India Received October $4, 1066
A systematic study of electrical conductance of aqueous solutions of urea has been made from 0.001 to 20 N . Viscometric and refractometric studies have been made at higher concentrations. Urea behaves as a weak electrolyte in aqueous solutions. At higher concentrations the variation of viscosity with concentration is best re resented by an equation q/qo = AcZ B where q/vo is relative viscosity, C the concentration and A and B, constants. El%,rolytic behavior of urea is best explained by a zwitterion structure.
+
Introduction Aqueous solutions of urea are known to be electrolytically conducting.lV2 The object of this investigation was to make a systematic study of electrolytic conductance of aqueous solutions of urea ranging from 0.001 to 20 N . Refractometric studies have also been made. At higher concentrations, ranging from 0.1 to 20 N , viscosity measurements have also been made.
Experimental Procedure Purest E. Merck urea was used in these investigations. A platinized vertical cell with a cell constant value of 0.1845 was used for measuring conductance. Resistance of the solutions was measured with a Dawe’s Impedance Bridge utilizing an amplifier detector instead of a telephone. The built-in microphone hummer supplied 1000 cycles per second. To eliminate stray capacitance a mitable part of the circuit was grounded by attaching firmly a copper wire to a cold water pipe. Temperature was controlled at 35 f 0.05’ using Townson and Mercer’s thermostat S427. The same thermostat, being provided with glass panels was used for viscometric measurements with an Ostwald’s viscometer. Hilger’s Abbe Refractometer with Hoppler’s ultra-thermostat was used for making refractometric measurements. Results obtained are recorded below. Conductivity water prepared with Vogel’sa conductivity still was used for making solutions and the specific conductivity of it was subtracted from that of solution everytime.
Discussion Column (3) of Table I shows the variation of equivalent conductance with Concentration of urea solutions. With increase of concentration equivalent conductance falls off. It is clear from the results that in solutions urea behaves as a weak electrolyte, a t all concentrations. TABLE I N 0.0009165 .0018330 ,0045825 ,009 1650 .018330 ,045825 ,09165 ,18330 ,45825 ,9165 1.833 5.000 10.000 15.000 20.000
SDeoific conduotivity 0.00000176 .00000678 .00000500 .00000366 .00000955 .0000076T, .00001090 ,00001594
Eouiv. conhuctance
1,9200 3.6980 1.2350 0.3989 ,5210 . 16710 ,11900 . 08690 .00002360 ,05163 ,00003156 ,03443 ,00005928 ,03232 ,000 12853 ,02589 .00011183 .01118 .00010603 .07068 .00011813 ,059005
Density
Vis-
Refrar-
oos,ity, poises
tive index
0.99580 0 007326 0,99884 ,007357 1 ,00828 ,0075 15 1 .00893 ,007520 1.03394 .008128 1 ,07009 ,009491 1.10423 .011880 1,14129 ,01454
1.3323 1.3324 1.3333 1.3323 1.3324 1.3325 1 ,3330 1.330 1.3345 1,3400 1.3400
1.3535 1 ,37375 1.39325 1.41400
Evidence was presented4J that urea in solution is neither wholly nor in part amphoterically ionic (1) Trubsbaoh, Z. physib. Chem., 1 6 , 709 (1918). (2) T. N. Shivapuri, Unzu. Allahabad Studies, Chem. S e d . , 1 (1950). (3) A. I. Vogel and G . H. Jeffery, J . Chem. Soc., 1201 (1931). (4) J. Walker and J. K. Wood, ibid., 83, 490 (1903). ( 5 ) J. Bell, W. A. Gillespie and D. B. Taylor, Trans. Faraday Soc., 39, 137 (1943).
and that it may be regarded as a weak base. Studies on the diamagnetic susceptibilities6 of urea pointed to a resonating amphoteric ion changing through an amino-imino structure. Reporting on the dielectric constants of amphoteric ions and polar molecules it was concluded7 that urea is not amphoterically ionic, but rather is a very weak base with a very high permanent dipole moment. A balanced view seems to be8 that urea exists to a large extent as a zwitterion NHz+:C(NH2)0-. Dipole moment studies9 strengthen the above view. Harris and R,obsonl0 also support the same view. I n summaryll i t is concluded that urea exists as a resonance hybrid of two zwitterion forms (1 and 2 ) “2-C: +NHz N&: C-NH~
-6
I
I 0-
I I1 Column V of Table I gives 7 from a concentration of 0.1833 to 20 N of urea. At lower concentmtions of not more than 4 N many workers,12-16 found 7 a t different Concentrations of urea solutions. Also datal7,’*are available up to a concentration of 5 M . Hence in the present investigatioti emphasis is laid on the 7 a t higher concentrations. Viscosity of dilute solutions of urea up to 0.1 M conformed16 to an equation previously given on experimental basis by Jones and Doleig and Joy and WolfendenZ0whereby the viscosities of solutions of strong electrolytes vary linearly with the square root of the concentration a t high dilution. The equation given was q/qo = 1
+ Av‘C + BC
(6) Arehibald Clow, ibid., 88, 381 (1937). (7) W. J. Dunning and W. J. Sohutt, ibid., 34, 479 (1938). ( 8 ) Ernest Bergmann and A. Weizmann, ibid., 34, 783 (1938). (9) W. D. Kumler and Geo. M. Fohlen, J . A m . Chem. Soc., 64, 1041 (1942). (10) J. 0. Harris and A. H. Robson, Nature, 161, 98 (1948). (11) I. D. iMorton and E. Hozzarth. “Chemistrv of Carbon Com-
pounds,” Edited by E. H. R o d d r i o l . IB, Elsevier-Press, New York. N. Y..D. 906. 1952. (12) H . M. Chadwell and B. Agnes, 3. A m . Chem. Soc., 52, 3597 ( 1930).
(13) G. Rudolf, Z. Phyoik. Chem., 43, 257 (1903). Edinburgh, 25, 52 (1904). (14) E. Fawsitt, Proc. Roy. SOC. (15) C. Ranken and W. W. Taylor, ibid., 45 [iil, 397 (190fi). (16) G . Jones and S . K . Talley, J . A m . Chem. Soc., 55, 624 (1933). (17) A. N. Campbell and E. M. Kartzmark, Can. J . Research, 28B, 161 (1950). (18) N. D. LitvinovandA.1. Melnikova,J. A p p l . Chem. (U.S.S.R.), 9, 583 (in English) (1936). (19) C: Jones and M. Dole. J. A m . Chem. Soc.. 51. 2950 119291. .~ (20) W. E. J O Y and J. H. Wolfenden, Proc. ‘Roy. Soc. (London), 8134, 413 (1931). I~
776
V. K. VENKATESAN AND C. V. SURYANARAYANA
Vol. 60
where q/vo is relative viscosity and A and B are parameters to be obtaiiied by plotting the lefthand member of the equation
between 0.003 and 4.10 N showed that the refraction curve is very similar to that of strong electrolytes. Investigationz2 of the apparent molal refractivity of aqueous solutions of urea as a function of qM= A + B,//c concentrations up to sufficiently high concentrad C tions showed the same behavior as that of an elecagainst l / C . A becomes zero for non-electrolytes. trolyte. From concentrations ranging between Jones and Talley16 found that for urea q / q o = 1 0.001 to 20 N we have recorded in the last column of O.O3784C, in dilute solutions up to a concentration Table I the refractive index of urea solutions. At of 0.1 M . low concentrations values obtained by an interferAt higher concentrations, using the Jones and ometer23 are more reliable and a t higher concentraDole equation for urea we have calculated v / q o tions Abbe refractometer is sufficiently reliable. using our results, and tabulated the values ob- We have therefore taken into consideration refractained in column 3 of Table 11. As expected there tive index values of urea solutions ranging from is a wide divergence between observed and calcu- 0.1833 t o 20 N . A plot of refractive index against lated values. Variation of relative viscosity with concentration (not given here) gives a straight line concentration of urea solutions is best represented beyond 5 N solutions. From 0.1833 to 0.9165 N by a n extension of the Jones and Dole equation in- the curve risc s continuously, short of linearity, and volving the square of concentration. between 0.9165 and 5 N there is a sharp break in the curve intensifying at 1.833 N . I n this connection T/qo = A c e B it may be noted that the density values in column 3 where for urea A = 0.009971 and B = 1.02860. of Table I when plotted against the respective conObserved and calculated values of q/qo based on the centrations give a similar curve (not given) as for above equation are recorded in column 4 of Table 11. refractive index with a break at the same concentration. A plot of viscosity against concentration TABLE I1 of urea solutions (not given) also shows a break at 7 / 7 0 aalcd. identically the same concentration. Curve 2 in AccordAccording to ing to graph I also shows an inflection at concentrations Jones theexConcn.. and tended round about 0.9165 and 1.833 N . Further from 7hQ obsd. Dole eq. form E. graph I1 (not given) i t is clear that from about moles/l. (1) (11) (111) 1.833 N upwards t o 10 N the fall in A with increas0,09165 1,0090 1.003468 1.02868 +0.005532 -0.01968 ,22913 1.01407 1.00867 1.02912 +O .005400 -0,01505 ing 7 is less and becomes lesser from 10 N upwards. ,45825 1.0350 1.01735 1.0307 +0,01765 +O. 0043 ,91650 1.03700 1.03468 1.036975 +O ,00232 +0.000025 These observations point t o the conclusion that a t 2,5000 1,1200 1.09461 1.09096 4-0.02549 +0.02904 about 1.833 N in aqueous solutions of urea some 5,000 1.3080 1.1892 1.2778 +0.0302 +O. 1182 7.5000 1,5540 1.2839 1.5896 -0,0356 f O . 2701 notable intermolecular rearrangement is occurring 10.0000 2.0257 1.3734 -0,0217 $0.6306 2 004 manifesting itself in arresting the rate of growth of Graph I1 (not given) shows the variation of physical properties of the solutions. Both the solequivalent conductivity (A) with q a t higher con- ute and the solvent being polar, dipole-dipole incentrations above 0.1833 N . It is interesting that teractions must be profoundly inflqencing the oriA falls off rapidly with increasing viscosity. The entation of the solvent and solute molecules at difdipole moment of urea is high and is about 8.6 ferent concentrations, perhaps with a noticeable Debye units according t o Bergmann and Weiz- transition occurring a t a concentration of about mann.* A rapid fall of A in urea solutions of high 1.833 N . Gurneyz4discusses the various possibiliviscosity must therefore be ascribed t o d i p o l d i - ties of orientation of molecules in solution taking pole interactions of the zwitterions. This factor into account different forces existing between them. should contribute significantly to all abnormal be(22) Biagio Peace, Atti V. Congr. nart. chim. pura applicata, Rome, 1935,Pt. I, 441 (1936). havior of highly concentrated urea solutions (23) A. E. Brodskii, J. M. Sherahever and N. S. Filippova, Acta Investigations21of refraction of urea solutions. Phyeicochem. U.S.S.R.,8 , 685 (1935).
+
+
(21) J. M. Shershever and A. E. Brodskii. 2. phyeik Chem., BS4, 145 (1936).
(24) R. W. Gurney, “Ionic Processes in Solution,” McGraw-Hill I3ook Co.. New York. N. Y., 1953.
