KINETICSOF EXTRACTION OF ZIXC DITHIZONATE
Jan., 1962
KINETICDATA FOR
THE
TABLE I1 DECARBOXYLATION O F P I C O L I S I C ACIDAND ht9LONIC ACID I N POLAR SOLVENTS~
-----
Solvent
AH* (kcal./mole)
Picolinio acid AS* (e.u./mole)
THE
MOLTENSTATE AND
.----I_--._
4Fisoo*
(kcal./mole)
127
AH* (kcal./mole)
Malonic acdi-
as*
(e.u./mole)
+
IN
SEVERAL
*
7
~FIFO~
(kcal./mole)
~~it.9 39.8 +13.2 33.8 33.0 4.5 31.0 p-Cresol 35.8 3.4 34.3 - 6.0 29.3 4.5 33.8 29.0 35.9 Phenetole10 - 7.15 30.3 0.5 34.1 28.1 34.4b Nitrobenzene" 34.0 27.8 - 7.9 31.4 2.0 33.1 p-Chlorophentole'o -10.8 32.0 34.0 27.1 32.06 4.6 p-Dimethoxybenzenc - 4.5 28.9 31.gb 4.4 33.9 26.9 Aniline12 The superscript after the name of the solvent refers to the source of the malonic acid data. * These results agree fairly closely with those obtained by Cantwell and Brown using these solvents. See ref. 2.
+ + + -
bring about a lowering of the AH* in the case of picolinic acid, the rate of reaction is faster in the molten state than in solution as shown by the values of AF18,"* for the reaction. This may be ascribed to the fact that more steric hindrance is encountered by the zwitterion in attacking a nucleo-
philic atom of solvent t8han in uniting with a picolinic acid molecule or ion, as a comparison of the AS* values in Ta,ble I1 indicates. Acknowledgment.-The suppart of this research by the National Science Foundat,ion, Wa,shington, D. C., is gratefully acknowledged.
KINETICS OF EXTRACTION OF ZINC DITHIZOYATE BY CARLB. HONAKER AND HENRY FREISER Department of Chemistry, The University of Arizona, Tucson 25, Arizona Received August 10, 1961
The rate of extraction of zinc ion from aqueous solution, using organic solutions of dithizone, has been found t o be first order with respect to zinc, first order with respect t o dithizone, and inverse first order with respect to hydrogen ion. It is possible, from the data obtained in this study, to formulate the rate-controlling step as consisting of the reaction of a dithizonate ion with a hydrated zinc ion in the aqueous phase. The unexpected slowness of the addition of the first ligand is probably due to the fact that the formation of the monodithizonatodiaquo complex involves the breaking of four waterzinc bonds, whereas the addition of the second ligand requires the breaking of only two such bonds. The rate constant for the reaction was found to be on the order of 1 X log1. mole-' min.-l. The temperature dependence of the extraction rate was determined and the energy and entropy of activation have been calculated.
Introduction A survey of the literature shows that little attention has been given to the study of solvent extraction under non-equilibrium conditions. TValkleyl has published a brief study of the extraction of zinc dithizonate aiid Irving, Andrew and Risdon2 have reported work on the separation of copper and mercury by extraction with dithizone under noli-equilibrium conditions. Geigera has reported the effect of pH on the rate of extraction of copper(X1) dithizonate. Under certain conditions dithizone extractions of zinc give rise to a transient, reddish-purple color in the aqueous phase. This observation seemed in sharp contrast to the well-known rapid formation of zinc complexes. This paper presents the results of a detailed study of the kinetics of the reactions involved in the zinc-dithizone extractions. The formation constants of zinc dithizonate also were determined for the first time. Experimental Apparatus.-All extractions were performed by agitating the samples in separatory funnels mounted on a Burrell (1) A. Walklny, Proc. Aust~alia?iChem. Ins!.,9, 29 (1942). (2) H. Irving, G. Andrew and E. J. Risdon, J . Chem. Soc., 541 (1949). (3) R. W. Geiger, Ph.D. thesis, University of Minn., 1951.
model BB Wrist Action Shaker. Thesc funnels, supplied by Eck and Krebs, were jacketed so that the systems could be maintained a t constant temperature by means of water circulated from a thermostat. Measurements of radioactivity were made with a Nuclear-Chicago Model DS5-5 Well-Type Scintillation Detector connected to a iModel 183B Scaler. For pH measurements a Beckman Model G pH meter was used. This meter was standardized against pH 7 buffer before each set of measurements. Spectrophotometric measurements were made with a Beckman DU spectrophotometer. Materials.-Specially processed, high activity zinc-65 in the form of ZnCln was obtained from Oak Ridge National Laboratories. Reagent grade dithizone was further puri. fied according to the method cited by Welcher.4 The dithizone solutions were standardized spectrophotometrically immediately before use. All other chemicals were reagent grade. Further purification was found to be superfluous since none of the materials gave a positive test with dithizone. Water was purified by passing it through a column of Deeminite L-10 ion-exchange resin supplied by Crystal Research Laboratories, Inc. Procedure.-All extractions were made from aqueous solutions having an ionic strength of one produced by the addition of sufficient perchloric acid or sodium perchlorate. With the exception of measurements made to determine the effect of temperature on extraction rates, all extractions were performed a t 25 & 0.05'. Concentration of zinc ion was 2.0 & 0.2 X 10+ ill. Determination of Equilibrium Constant for the Extraction of Zinc Dithizonate into Carbon Tetrachloride.The pH of the aqueous zinc solution was adjusted to an ap(4) F. J. Welcher, "Organic Analytical Reagents," Vol. 3, D. Van Nodrand Co.. Inc., New York, N. Y . , 1983, p. 466.
128
CARL
B. HOXAKEIZ AND HEXRY FREISER
Vol. 66
proximate value; 15-ml. portions were shaken with equal volumes of carbon tetrachloride solutions of dithizone for a period of 12 hr. The concentration of the dithizone solution was on the order of 1 X 10-8 M. At the end of 12 hr. shaking a 5-ml. aliquot of the aqueous phase was removed and counted. A sample of the original zinc solution, diluted 100-fold to give a reasonable count, rate, was counted. Since the count rate is directly proportional to the total concentration of zinc in the sample, the distribution ratio of zinc could be calculated. Sufficient counting time was allowed on each sample to amass a total count of a t least 10,000, so that the statistical counting error would be less than 1%. The pH of the aqueous phase was measured to the nearest 0.01 p H unit. Extractions were made from unbuffered solutions, with the exception of those in the pH range of 4.5-6.0. These were buffered with 0.1 M acetate. Partition of Dithizone .-Equal volumes of dithizone in either chloroform or carbon tetrachloride and aqueous Clark and Lubs buffer solution (ionic strength one) were shaken for 15 minutes. The funnels were allowed to stand for 15 minutes to ensure complete separation of the layers. The organic phase was drained into a spectrophotometric cell (after discarding the firbt few milliliters) and the absorptivity was mcusured. From a plot of ( A , orig - A, final)/A, final os. 1/[H+] values were found for KJKD,. The pH of the aqueous phase was measured. Kinetics Studies.-A 15-ml. portion of dithizone solution was placed in a jacketed separatory funnel. The pH of the aqueous zinc solution was adjusted and a 15-ml. portion of this solution was pipetted into the funnel in such a way as to produce virtually no mixing and hence no extraction before the shaker was started. The two solutions previously had been brought to the temperature selected for the study. The sample was shaken for a definite time interval on the Burrell shaker, then allowed to stand for five minutes in order to ensure complete separation of the two phases. Preliminary experiments had shown that the rate of extraction during these separation periods was negligible. An aliquot to be counted was removed from the aqueous phase. The aliquots were of 50 A, 25 X or 5 ml., depending upon the amount required to give a reasonable count rate. In the case of the micro samples, enough water was added to the aliquot to give a 5-ml. volume of solution to be counted, thus standardizing the geometry and self-absorption of the count sample. In cases where micro volumes of the aqueous phase were removed, the experiment was continued with further samples being taken a t suitable intervals until the volume of the aqueous phase had been changed enough to introduce a significant error. The activity of the aliquot was compared with the original activity of the aqueous phase.
constant was 1.13 X while the stepwise con, values of 5.6 X 10’ and 1.4 X stants, k1 and k ~had lo’, respectively. Kinetics of Extraction of Zinc Dithizonate.A preliminary series oE experiments was performed to determine the effect on the rate of extraction of varying the shaker speed. It was found that the rate of extraction increased quite rapidly as the shaker speed was increased, up to a maximum value beyoid which the increase in agitation had no significant effect on the rate of extraction. In all subsequent determinations a shaking speed within this “plateau” region was used. On this plateau the rate of extraction was chemically controlled, whereas a t slower rates of agitation the transfer processes tended t o become rate controlling. The “plateau” region using carbon tetrachloride involved more vigorous agitation than when using chloroform. This was to be expected in the light of the greater viscosity of carbon tetrachloride. I n the first runs carbon tetrachloride was used as the organic solvent. Since the extraction was quite rapid, it seemed advisable to shift to chloroform as the extracting medium. Irving6 had predicted that the rate of extraction of the dithizonates should be much slower with chloroform than with carbon tetrachloride. This choice seemed particularly wise in view of the need to determine rates toward the beginning of the extraction before the magnitude of the reverse reaction became great enough to introduce a serious error inlo the computations. Extractions were made using various concentrations of dithizone in the organic phase and varying values of pH in the aqueous phase. As a working hypothesis, the reaction was assumed to be first order with respect to concentration o€ zinc ion iii the aqueous phase. This assumption was validated when plots of log ( [%n]orig/ [%n]t) VS. time were made and the poiiits were found to lie on sensibly straight lines. A tabulation of [H+], [HDzIoand slope is made in Table I. An inverse first order of the reaction with respect to [ a + ] Results was found from a plot of log slope us. log [H+], Partition of Diehizone.-In order to calculate keeping [HDzIo constant. In similar fashion the both the concentration of the dithizonate ion in the order with respect to [ H D z ] at ~ constant pH was aqueous phase and the formation constants €or found to be one. zinc dithizonate, it was necessary to determine the The four rate determinations made with carbon value of the ratio K,/KD, for dithizone. It can tetrachloride can be compared with those using be shown that chloroform. It is seen from the data in Table I that these extractions are more rapid than the chloroform extractions by a factor of 58. Extractions were made a t various temperatures. At least three samples of different pH values were studied at each temperature. Results are given Thus from the slope of plots of D, us. l/[H+], in Table 11. values of the quotient K,/Knr for dithizoiie were Discussion calculated to be 2.0 * 0.2 x 10-9 and 1.3 f 0.2 The steps involved in the extraction of zinc with x for carbon tetrachloride and chloroform, respectively. These values were determined a t dithizone can be delineated as WDz(0) HDz(aq) (1) 25’ and with an aqueous phase whose ionic strength was 1.0. At Go this ratio for the partitioning of HDz(aq) r‘H + Dz(2) dithizone between chloroform and the aqueous Zn+2 DeZnDz+ (3) phase had a value of 2.5 f 0.4 X ZnDz + Dz- 7-f ZnDzz and ( 4) Formation Constants of Zinc DithizonsPte.-From ZnDzdaq) r’ZnDzdO) ( 5) a plot of D,vs. ~ [ D z ]stepwise , formation coiistants were calculated using the graphical method of (6) D. Dyrsven and L. G. Sillen, Acta Chem. Scand., 7 , 663 (lW3). (6) IT. Irving and R. J. F. Williams, J. Ckenc. Am., 1841 (1949). Dyrsseii and SiIIen.5 The over-all formation
+
+
+
KINETICS O F EXTRACTION
Jan., 1.962 'rABIiE
TABLE I1
I k'
Run no.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26O 2,7' 28" 29O
129
DITHIXONATE
02' Z I N C
[HDzl; x 10
[=+I x 104
Slopen (rnin. -1)
1.10 1.31 1.34 1.34 1.51 1.51 1.51 1.51 1.51 1.51 2.42 3.50 3.91 6.93 7.92 7.92 7.92 7.92 7.92 7.92 8.70 8.70 10.3 10.3 10.4 0.084 .44
1.26 1.20, 2.14 3.17 0.89 1.59 2.12 0.31 0.58 0.85 1.26 0.81 1.26 0.81 19.9 7.95 6.32 1.26 0.80 0.40 0.63 1.41 I .26 1.26 0.81 0.71 1.45 3.18 7.42
1.10 0.12
b 1. q h e - 1
kf
Run
-1
no.
Temp., O K .
HDz x 104
0.32
30 31 32 33 34 35 36 37 38 39 40 41
279 279 279 288 288 288 293 293 293 308 308 308
8.7 8.7 8.7 7.9 7.9 7.9 10.1 10.1 10.1 9.8 9.8 9.8
min.
.24
.43 .12 .46 .OB4 .33 .24 .46 .19 .38 .12 .24 .51 ,29 .33 -30 .23 .ll * 24 .34 .63 .35 .26 1.65 * 45 .26 0.046 .20 ,088 .32 .17 .22 .59 .I6 .68 * 12 I99 .18 1.10 ,24 0.64 .29 1 .03 I .03 .29 .49 2.66 14.4 0.74 15.5 1.98 19.3 1.16 .44 20.5 0.53 -44 b k' = a Slope of a plot of log [Zn],,i,,/[Zn]t) us. time. (2.303 X Slope X [H-"])/[HDZI. c Carbon tetrachloride
used as the extracting medium.
The first and last steps can be disregarded as rate controlling, since neither would give rise to a pH dependence. Further, Geigers has reported the results of a study of the rate of attainment of partition equilibrium for dithizoiie between carbon tetrachloride and water. Equjlibrium is essentially reached in less than 15 min. This is considerably less time than is needed to attain equilibrium in the zinc dithizonate system. Step 2 can be dismissed as rate controlling on two counts. First, if step 2 were rate controlling, the first-order dependence with respect t o zinc would not be explained. Secondly, extractions of other metals (such as Hg+2 and Agf) with dithizone proceed much more rapidly than that of zinc. This hardly could be the case if the ionization of dithizone were the rate controlling step. Also the rate of partition of HDz is independent of pH. If step four were rate controlling, a second-order dependence upon the concentration of dithizonate should be observed. This leaves only step 3 which is first order with respect to both zinc ion and dithizonate ion concentration. The expression representing the observed rate can be written as From a consideration of steps one and two, it is apparent that the concentration of dithizonate ion in the aqueous phase is directly proportional to the conceiitration of dithizone in the organic phase and inversely proportional to the concentration of
x
1. indle-1 min. -1
Slope (rnin.-I)
H' 104
0.38 .62 .84 .IO .28 .47 .11 .23 .41 .31 .85 1.44
0.391 0.195 0.048 1.20
0.514 0.289 5.76 3.47 1.82 6.77 2.34 1.00
0.020 .023 .01t .035
.042 ,040
.14 .18 .17 .50 I47 .34
TABLH I11 R 106
IID.
ltrrn
x
no.
Slope X 103 (rnin. -1)
x
106
5.02 0,502 2.51 5.02 2.24 1.26 1.26 2.2 15.9 13.2 lob a Aqueous phase made 0.3 24 in
1
0.81 0.81 1.88 2.18 2.18 2.18 2.18 8.98 2.28 2.24
2 3 4 5 6 7" 8 Sh
x
kf 10%
5.8 2.2 8.8 9.3 7.3 3.1 3 18 354 385
0.5 15.5 2.87 1.76 3.08 4.24 4.24 31.3 22.2 28.4
KCl. b Carbon tetrachloride used as the extracting medium. All other extractions were into chloroform.
TABLE IV k
Temp., OX.
k', 1. mole-' min.-1
0,019
279 288 293 298 308
.069 .12 .25 .87
KXa /10'0 KD~
x lo-*! 1. mole-1 min. - 1 0.76 1.23 1.48 1.92 3.11
0.25 0.56 0.81 1.3 2.8
hydrogen ion in the organic phase and inversely proportional to the concentration of hydrogen ion in the aqueous phase. Therefore, the above rate expression is equivalent to
- d( Zndt
+2)
__I
= k[Zn+2][Dz-] = kK, KD,
x
[Zn +2] __
[K]!
[B+I
This is the rate equation for step 3, which must bc rate controlling. It is of interest to compare the above results vith those obtained by Geiger in the extraction of copper(11) dithizonate. He found that in the PH range of 0.0-2.0 the rate of extraction of copper(I1) dithizonate was virtually independent of the acidity. Further, he reported that an increase in concentration of dithizone in the organic phase did increase the rate of extraction somewhat, although he found no quantitative relationship. On the basis of these observations, a mechanism was postulated by Geiger which involved the reaction of molecular dithieone with copper(l1) ion a t the interface. A probable cause of this variance is Geiger's use of a slower shaker speed (100 shakes/ min. compared with 300 shakes/min. in the present study). This could account for the diffusioncontrolled mechanism. On the basis of K,/KD, values, it is to be expected
CARLB. HOXAKER A N D HENRY FREISER
130
that the extraction should proceed some fifteen times more rapidly when carbon tetrachloride is substituted for chloroform as the extracting medium. The actual change in the rate constant (k') a t 25' is from 0.30 to 17.4, a fifty-eight fold change. Further work needs to be done on the role of the solvent in extraction kinetics. The suggestion that the addition of the first ligand is rate controlliiig was proposed by Irving and Williams in 1949,6on the basis of the differing rates of extraction mhen chloroform and carbon tetrachloride were used as the organic media. In their article on the extraction of zinc dithizonate, Irving and eo-workers' presented a variety of kinetic data. They were interested primarily in showing the relative effects of pH, dithizone concentration, and aqueous phase composition on the rate of attainment of equilibrium in the extraction. Therefore, only a few measurements were made under each set of conditions and no quantitative conclusions mere presented concerning the kinetics. On analysis of the data presented by Irving, et al., it is found that the orders with respect to hydrogen ion, dithizoiie and zinc ion concentrations are the same as those determined in the present study. These conditions and rate constants are used in Table 111. The rate of extraction into carbon tetrachloride appears, at first, to be anomalously fast. However, since these extractions were made from acetate buffered solutions, whereas the other extractions were made from phthalate buffered solutions, no strict comparison can be made. Further, since different methods of agitating the samples were used in the two studies, no comparison of rate constants can be made except to note that, in both the work of Irving and the present study, the results are internally consistent. From the data in Table I1 a plot of log IC' US. 1/T was made and the best straight line was fitted to the points using the method of least squares. From this line and from the values of K,/KU~ for dithizone a t different temperatures, the rate constants of the reaction Zn+s
+ Da-
ZnDai
a t different temperatures were calculated. These are given in Table ITT. These values are in accord with the well-known rapid rate of formation and lability of zinc complexes. Bjerrum8 has reported the virtually instantaneous formation of zinc dithi(7) H. Irring, C F. Bell and R. J. P. Williams, J . Ciiem Soc 356 [l952). (8) J. Bjerrum and K. G. Poulson, hTat7,re, 169, 463 (1952).
Vol. 66
zonate in methanol solution a t -75". Taft and co-~vorkers~ have shown that the reaction of zinc ion with TA must involve a rate constant of a t least 1 X lo6 l./mole/min. From the data in Table IV the activation energy and the entropy of activation have been calculated. These values are 7.9 kcal./mole and 2.4 e.u./mole. Although the attachment of the first dithizone is rate controlling, other considerations seem to call for a rapid first step followed by a slower second step. The doubly charged, uncomplexed zinc ion would be expected to react more rapidly than the nionodithizonate species. Further, since the first stepwise formation constant is greater than the second, the concept of a rapid first step would appear to be more tenable. The zinc ion, in water solution, exists as the octahedrally coordinated aquo complex.lo The present authors propose the following mechanism for the chelation. One of the solvent molecules is lost from the aquo complex. This is followed by the attachment of the first dithizonate ion through a sulfur-metal linkage, the loss of a second solvent molecule, and then the linkage of the other coordinating group of the first dithizoiie. The ligand then exerts a bond-weakening effect on the two water molecules which are trans to it; these are lost and the monodithizonatodiaquo complex rearranges to a tetrahedral configuration. The last two water molecules are lost in stepwise fashion, with the second dithizonate attaching to the zinc. It is probable that the monodentate attachment of the first dithizonate ligand t o the metal ion is the rate-controlling step. The formation constant of zinc dithizonate, 1.13 X 10l6,is far less than that reported by Geiger and Sandellll for the copper(I1) dithizonate, 1 X 1023. This conforms to the normal order of chelate stabilities. Further work in dithizonate formation constant determination is underway and the results should reveal much of interest with regard to the role of sulfur in chelation. Acknowledgment.-Financial assistance from the Atomic Energy Commission is gratefully acknowledged. Appreciation is expressed to Tennessee Wesleyan College for granting C. B. H. a leave of absence. (9) R. W. Taft, Jr., and E. H. Cook, J. Am. Chem. Suc., 81, 46 (1959). (10) L. E. Orgel, "An Introduction to Transition-mrtal Chemwtry Ligand-Field Theory," John Wiley and Sons, New York, N. Y.,1960, P. 83. (11) R. W. Geiger and E. B. Sandell. Anal. Chim. Acta, 8, 197 (19%).
