STRUCTURAL STUDIESOF CHELATES BY ULTRASONIC WAVES
3325
Structural Studies of Chelates by Ultrasonic Waves
by Som Prakash and Satya Prakash Ultrasonic Laboratories, Department of Chemistry, Allahabad University, Allahabad, India (Received November 4, 1966)
~
~~
~~~
Ultrasonic velocity and adiabatic compressibility lowering at three different temperatures have been studied as a function of the composition for the systems (1) zirconium(1V)catechol-water, (2) cadmium(I1)-catechol-water, and (3) thorium(1V)-catechol-water. The position of the breaks in the velocity-composition and compressibility-composition curves indicates the point of maximum chelate formation. I n systems 1 and 2, 1:1 chelates are formed. I n system 3, however, 1:1 and 1:2 chelates are formed.
Introduction
Experimental Section
The influence of the proportion of the constituents of mixtures of organic liquids on ultrasonic velocity has been studied by several workers like Giacomini,' Richardson, Prasad, B ~ r t o n ,and ~ others. When nonpolar molecules are mixed it is to be expected that the ultrasonic velocity and other properties take values intermediate between those for the two pure liquids. When, however, the molecules of one of the liquids are polar, this simple rule does not hold and the ultrasonic velocity may go through a maximum or minimum. Such types of results are also observed in the solution of electrolytic mixtures. In simple electrolytic mixtures such as NaN03-Sr(NO& and KCl-NaCI, etc., velocity variations are linear.5 However, when the ions or molecules of the two mixed solutions interact to form a complex ion or molecule, the variation of velocity with the composition of the system is characterized by a number of minima. Prakash and co-workers6have obtained such results in mixtures of citric acid and Baf2, Cd+2,Pb+2,and Ag+. With a view to ascertain the variation of ultrasonic velocity and adiabatic compressibility in chelateforming systems, mixtures of catechol (o-dihydroxybenzene) with Zr+4,Cd+2,and Th+4have been studied. Catechol, having two hydroxyl groups in the ortho position, is a good chelating agent. A chelate with Zr+4has been studied pH-metrically by Mehrotra and Kapoor.' Polarographic studies of the Cd+2 chelate have been done by Gorokhovskii and Levin.* Our results on these complexes are in agreement with the results of these workers.
Ultrasonic velocities at the frequency of 5 Mc/sec and at three different temperatures have been measured by the light diffraction method introduced by Debye and L u c a ~ . The ~ source of the ultrasonic waves was a generator comprising an oscillator and a quartz crystal transducer unit. Solutions of zirconium oxychloride, cadmium iodide, thorium nitrate, and catechol were prepared by dissolving accurately weighed pure substances (B.D.H. Analar or E. blerck G.R.) in doubly distilled water. Each time a fresh solution of catechol was used. The solution of thorium nitrate was estimated gravimetrically. Before making up the volume of these solutions, the pH was adjusted and kept at 2.3. The continuous variation method of Job'O was used for preparing the mixtures of different compositions. After the attainment of equilibrium the pH of these mixtures was again brought back to (1) A. Giacomini and B. Pesce, Nuovo Cimento, 6 , 39 (1940).
(2) E.G. Richardson and A. E. Brown, Phil. Mag., 4, 705 (1959). (3) R. Prasad, J . Chem. Phys., 15, 418 (1947). (4) C. J. Burton, J . Acoust. SOC.Am., 20, 186 (1949). (5)T.Satyavati, P. J. Reddy, and S. V. Subrahmanyam, J . Phys. SOC.Japan, 17, 1061 (1962). (6) S. Prakash, F. M. Ichhaporia, and J. D. Pandey, Naturwissenschaften, 5 2 , 1 (1965). (7)R. C.Mehrotra and R. C. Kapoor, 2. A m r g . Allgem. Chem., 293,
94 (1957). (8)V. M. Gorokhovskii and Y . A. Levin, Zh. h'eorgan. Khim., 2 , 343 (1957). (9) P. Debye and F. W. Sears, Proc. Natl. Acad. Sci. U.S.,18, 410 (1932);R. Lucas and P. Biquard, Compt. Rend., 194, 2132 (1932). (10) P. Job, Ann. Chim., 6 , 97 (1936).
Volume 70, Number 10 October 1966
3326
SOMPRAKASH AXD SATYA PRAKASH
0.033M ZmC12 0.033 M Cahchol I
0.2
0.4
I
0.6
04
1.0
Figure 2a. Ultrasonic velocity variation a t 25'.
~ u t v j o c k r t or water nrculdn
Figure 1. Ultrasonic cell for the measurement of velocity at different temperatures.
1510
A
2.3 by adding a few drops of alkali. The cell used for the measurement of ultrasonic velocity at different temperatures is shown in Figure 1. The desired temperature of the mixtures was maintained by immersing the flasks in the thermostated bath for about 1 hr. During the exposure the same water from the thermostated bath was circulated in the outer jacket of the cell. Densities of the mixtures were measured by a specific gravity bottle. Adiabatic compressibility was then calculated using the equation
Temp. 2d'C
I 0.I
04
016
de
110
Figure 2b. Ultrasonic velocity variation at 20".
p=- 1 V2P where V is the ultrasonic velocity and p the density of the medium.
Temp. (22"C
Results and Discussion From all the curves we find that the variation of ultrasonic velocity is not linear (Figures 2a-c). As the metal ions are gradually added in the solution of catechol, the curve decreases gradually, reaches a minimum value at a certain point, and then increases again. When Zr+* and Cd+2 ions are involved, the minima occur at the [A!l]/([MJ [catechol]) ratio of 0.5, the corresponding molar ratio being 1 : l . This indicates the formation of maximum chelate a t this point. Had there been no interaction between the respective metal ions and catechol, the variations would have been linear. I n the case of a Th+ecatechol system, however, two minima at the ratios corresponding to the molar ratios 1 : l and 1 : 2 are observed. Thus the variation of ultrasonic velocity gives us informa-
+
The Journal of PhysicaE Chemistry
I
0.2
a.4
0.6
0.8
i!o
Figure 2c. Ultrasonic velocity variation at 22'.
tion not only regarding the chelate formation, but the positions of minima also indicate the compofiition a t which the maximum chelate formation is taking place.
STRUCTURAL STUDIESOF CHELATES BY ULTRASONIC WAVES
However, since ultrasonic velocity in aqueous solutions cannot be determined additively unless a number of corrections are made, it is better to express the results in terms of adiabatic compressibility. Let us now consider the physical and mathematical concept of adiabatic compressibility. Adiabatic compressibility is defined by the equation
p
=
3327
0 . 1 M ZrOClz 0.!6. I M Catechol
Temp. 25 'c
-
50
x
x
3 1.6
8 .f1.2
3
-L(qS
-2l 0.6
3
v hP
v)
When an electrolyte is dissolved in water the compressibility of the solution is lowered. The lowering is attributed to the electrostatic field of the ions on the surrounding water molecules. This electrostatic pressure increases the total internal pressure and the solution becomes harder to compress. Let 0 0 and p be the compressibility of water and solution at a particular temperature T . The potential of an ion in solution may be written from the Debye-Huckel theory of electrolyte as $/i
=
2
p,2 0.4
I
I
0.2
I
I
0.4
0.6
0.8
I
1.0
Figure 3a. Compressibility lowerings a t 25".
Zi€k D
where Zi is the valency of an ion, E is the electronic charge, D is the dielectric constant of the medium, and
nr2ZniZi2 '" k = ( 4 DKT
)
(2)
where K is the Boltzmann constant, T is the temperature, and ni is the number of ions per milliliter of the ith kind. From the well-known laws of electrostatics the force per unit area between two charged particles a t a potential difference of $/ is given by force per unit area
w2
= -
8n
" I
I
0:2
0:4
0:s
fli8
i:O
Figure 3b. Compressibility lowerings a t 35".
(3)
where D is the dielect'ric constant of the medium. Hence force per unit area, i.e., pressure P , is given by
Now since this electrostatic pressure is responsible for the lowering in the compressibility, the compressibility lowering (compressibility of water minus the compressibility of the solution) may be regarded as equal to this pressure. po - /3 = d@= K'Zi2L'nrZi2 (5) For uni-univalent electrolytes Zi = 1 and this equation reduces to ds = K'Bni
(6)
Figure 3c.
Compressibility lowerings at 45".
where K' is a constant equal to Zi2r4/2D2KT. Thus from eq 6 we see that the compressibility lowering caused b y an electrolyte is a measure of the total
-
Volume 70, Number 10 October 1966
3328
SOMPRAKASH AND SATYA PRAKASH
number of free ions as has been regarded by some author^.^^^ If in eq 6 nl is replaced by CiN/1000 where Ci is the ionic concentration in moles per liter and R / N is written for K (Boltzmann constant), we get
'Temp. 20 C'
where p is the ionic strength equal to 1 / 2 X i Z , 2 . So, for any electrolyte, compressibility lowering is dependent upon the ionic strength of the solution. In the above derivation it has been assumed that the lowering in the compressibility of any solution is due to the electrostatic field of the ions. This is true onIy in the case of the solutions of strong electrolytes. I t is known that when a nonelectrolyte is dissolved in water the compressibility of the solution is also decreased and this decrease may be due to the molecules of the substance alone, for there are no ions in this solution. Thus for solutions containing free ions as well as a nonionized substance, the above equation will not hold strictly. An allowance will have to be made for the effect of the un-ionized part or molecules of the substance upon the compressibility of the solution. However, the electrolytes involved in the present work are mostly ionized and thus the results obtained could be explained on the basis of eq 7. From the compressibility lowering-composition curves (Figures 3 4 ) , we find that the values of compressibility lowering (d,) are minima at the stoichiometric ratios. These minima, as is clear from eq 7, are due to the minimum ionic strength at the stoichiometric ratios, for it is here that the maximum number of ions has combined to form the chelate. The compressibility lowering variation is thus similar to the velocity variation. Breaks in the curves are, however, more sharp (in Zr+4and Cd+2systems) in the compressibility lowering curves than the velocity-composition curves, for compressibility is the property which is more closely related with the behavior of the ions than the velocity. Temperature Effect. The systems in this paper have been studied a t three different temperatures with a difference of 10" (Figures 3a-c, 4a-c, 5a-e). In all cases the ultrasonic velocity-composition curves show minima at the points corresponding to the stoichiometric ratios. At higher temperatures the lowering in ultrasonic velocity is less a t all compositions than at lower temperature. This temperature effect is probably due to the breaking up of the chelate a t higher temperature and thus a t any composition the number of ions or other species increases resulting in less velocity lowering. The Journal of Physical Chemistry
Figure 4a.
/"D
0.1
M cddol
Compressibility lowerings a t 20".
P
I
I
0.2
Figure 4b.
I
Q.6
0.8
PO
I
0.2
Figure 4c.
0.4
Compressibility lowerings a t 30".
k
0!6
of8
Compressibility lowerings at 40'.
Compressibility lowering-composition curves a t all temperatures and concentrations except in Figure 5c show minima a t the points corresponding to the stoichi-
STRUCTURAL STUDIES OF CHELATES BY ULTRASONIC WAVES
3329
I
2-41
Temp. 4 2 'c
P 0.1 M cufechol 0.1 M Th[N+), O.O5M Cofechol 0.05M Th(N@)g 0.033 M Caferho1
i
0.053 M Th (NO,),
I
1
012
0!4
0!6
Oh
b'0
Figure 5a. Compressibility lowerings at 22'.
n
I
0.2
I
L
0.6
0.8
,
1.0
Figure 5c. Compressibility lowerings at 42".
small that practically no minimum could be detected. This is only in the case of Th+4chelates, indicating that these chelates break up more readily with the increase in temperature (small value of the stability constant). This temperature effect is probably caused by the breaking up of the chelate at higher temperatures into simpler ions or other species resulting in more compressibility lowering values. I n order to ascertain the nature of the charge on these chelates, electrophoretic studies have also been done. I t has been found that the 1 : 1 chelates of Zr+4 and Cd+2 and 1: 2 chelate of Th+4 are neutral molecules while the 1:1 chelate of Th+4 is positively charged. From these studies the structure of the chelates formed in Zr+4-, Cd+2-, and Th+4-catechol systems can be given as
I
Figure 5b.
I
0.4
Compressibility lowerings at 32"
ometric ratios. At higher temperature, the compressibility lowering values are more at all compositions, than a t lower temperatures. I n Figure 5c (42"), compressibility lowering values go on increasing with the addition of metal ions and no minimum is observed at the lower concentration (5c, lower curve). However, a t the points of maximum complex formation, a change in the gradient of the curves is observed. It is interesting to note here that in the Th+"catechol system the compressibility lowering-composition curves are not similar to the velocity-composition curves. It appears that, although a t lower concentrations and higher temperature (Figure 5c, lower curve) some chelate is formed in the system, its concentration is so
CaH40z-ZrO I (1:l) CCH402-T h +' IIIa (1: 1)
CsHrOz-Cd I1 (1:1) (CsHJ&)z-Th IIIb (1:2)
Structures I and I1 are in agreement with the results of Alehrotra and Kapoor' and Gorokhovskii and Levin,* respectively. Thus, we see that the nonlinear curves obtained from the study of the variation of the ultrasonic velocity and compressibility not only indicate the formation of the chelates but also the composition and their structure. Acknowledgment. S. P. takes this opportunity to thank the Council of Scientific and Industrial Research, India, for providing financial assistance.
Volume 70, Number 10 October 1966
