Kinetics and Mechanism of Extraction of Zinc and Nickel with Substituted Diphenylthiocarbazones Joon S. Oh' and Hervy Freiser Depnrtment o j Cliernistiy,, Unirersity of Arizona, Tucson, Ariz. 85721
The kinetics of extraction of zinc and nickel with CHCl3 solutions of the following substituted diphenylthiocarbazones were investigated: di-p-fluoro-, di-pchloro-, di-p-bromo-, di-p-iodo-, di-m-trifluoromethyl-, di-p-methyl-, and di-p-methoxyphenylthiocarbazones. Each reaction was first-order in metal ion and reagent but inverse first-order in hydrogen ion, which signifies that the formation of the 1 to 1 metal-ligand complex in the aqueous phase i s rate-determining. Substituents of both electron-releasing and withdrawing types increased the rate constants of zinc as well as nickel complex formations. The role of inductive and resonance substituent effects on the rate and significance of these results in an understanding of the structures of the nickel and zinc dithizone complexes is discussed. The newly reported di-m-trifluoromethylphenylthiocarbazone should be a useful analytical reagent, since it appears to be sigriificantly more resistant to oxidation than dithizone itself.
IN AN EARLIER PAPER ( I ) it was demonstrated that solventextraction techniques could be conveniently applied to the quantitative determination of the kinetics of relatively fast reactions (>106M-1 sec-'). In the study of the kinetics of extraction of Cd, Zn. 120, and Ni chelates of diphenylthiocarbazone and its di-o-methyl analog, the formation of the 1 to 1 metal ligand complex in the aqueous phase was found t o be the rate-determining step in every case. The secondorder rate constants followed the order observed in the rate of water exchange of the, hydrated metal ion, Cd>Zn>Co>Ni, indicating the importance of the role of the metal-water bond in the mechanism of the formation of the chelates. It was rather surprising t o find that the rate constants of the formation of both the Zn and Ni chelates of the di-o-methylphenylthiocarbazone are faster than those of the parent compound, in view of an earlier finding ( 2 ) of the decrease in chelate stability associated with th s ortho substituent. I t was decided to investigate further the mechanism of metal dithizonate formation by examining dithizone derivatives in which the subihtuents would be in either meta or para position, so as t o permit a distinction among inductive, resonance, and steric effects. EXPERIMENTAL
Apparatus. The apparatus is essentially the same as that previously reported ( I ) . The samples were agitated at the high speed setting of the Eberbach shaker. Cary Model 14 and Beckman D U spectrophotometers were used for absorbance measurement:. Materials. The di-n-fluoro-, di-p-chloro-, di-p-bromo-, di-p-iodo-, di-p-methyl-, di-p-methoxy-, and di-m-trifluoromethylphenylthiocarba.!ones were synthesized by Bamberger's
On leave from Seoul National University, Seoul, Korea.
(1) B. E. McClellan and H. Freiser, ANAL.CHEM.,36, 2262 (1964). (2) K. S. Math, Q. Fernando, and H. Freiser, ANAL.CHEM.,36, 1762 (1964).
Table I. Elemental Analysis Theoretical Diarylthiocarbazone N Halogen Di-p-fluorophenyl(Ci3HioFzN4S) 19.17 12.99 Di-p-chlorophenyl( C ~ ~ H ~ O C ~ ? N ~ S ) 17.23 21.80 Di-p-bromophenyl(C13HioBrzN4S) 13.53 38.59 Di-p-iodophenyl(ClaH1~I~N4S) 11.03 49.95 Di-p-methoxyphenyl(CijH16N40aS) 17.71 .,. Di-m-trifluoromethylphenyl(CIjHioF84s)
14.28
29.06
Found ____ N
Halogen
19.00
12.59
16.78
21.76
13.48 10.86
38.20 48.78
1.696
.,.
4.31
28.39
methods (3-5), which involve the coupling of 2 moles of diazonium compound with an alkaline ethanolic CH3NOs solution, followed by reduction of the resulting nitroformazyl compound with ethanolic (NH&S solution to give the thiocarbazide which, in turn, is oxidized to give the corresponding diphenylthiocarbazone. The compounds prepared were purified by precipitation with ethanol from solutions in freshly distilled, NHnOH.HC1-treated ( 2 ) chloroform. The elemental analyses of these compounds are shown in Table I. Inasmuch as the di-p-methyl derivative is known, its spectrum was examined but no elemental analysis was obtained. Since it is difficult to prepare standard solutions of reagents by direct weight, the reagent solutions were prepared and standardized by the method of Landry and Redondo (6). The absorbancy data, including the molar extinction coefficient and the peak ratio, are shown in Table 11. The stability of the solution was checked at regular intervals by spectrophotometric methods using the data of Table 11. The stability of the solutions of halogen-substituted phenylthiocarbazone decreases in the order F > C1 >Br >I, as expected. Di-177-trifluoromethylphenylthiocarbazone,the most stable derivative, showed no evidence of decomposition even after several months. All compounds used as buffer components were of reagent grade purity. NaClO? was added to each solution to adjust the ionic strength to 0.1. The maximum final phosphate and acetate ion concentrations used in the bufTered solutions were 1 X 10-3M and 2 X 10-3M, respectively, to avoid the presence of significant amounts of zinc and nickel complexes of these ions. All buffer solutions were freed from trace metal impurities by extraction with CHCI3 solution of dithizone. The active zinc solution and nickel solution were prepared ( I ) and determined ( I , 7) by the same methods. Partition of Reagent. Values of the ratio of the acid dissociation constant, K,, t o the reagent distribution con(3) E. Bamberger, R. Padova, and E. Ormerod, Ann., 446, 260 (1920). (4) A. I. Busev and L. A. Bazhavova, Russian J . Iiiorg. Cliem., 6, 1416 (1961) (Eng.). (5) D. M. Hubbard and E. W. Scott, J . Am. Chem. SOC.,65, 2390 (1943). (6) A. S. Landry and S. F. Redondo, ANAL.CHEW., 26,732 (1954). (7) A. M. Mitchell and M. G. Mellon, Ind. Eng. Cliem.. Ana/. Ed., 17, 380 (1945). VOL. 39, NO. 3, MARCH 1967
295
Table 11. Absorbancy Data for Dithizones in CHCll
Table V.
10-4
Diarylthiocarbazone DiphenylDi-p-fluorophenylDi-p-chlorophenylDi-p-bromophenylDi-p-iodophenylDi-p-methylphenylDi-p-methoxyphenylDi-m-trifluoromethylphenyla
hmax
Xmin
Elmax
EZmax
6055 605 607 62 1 626 635 61 1 635
445 445 447 458 458 464 449 461
505 505 507 515 522 5 30 508 515
4.0 4.0 6.9 4.4 5.1 5.5 5.1 6.1
2.5 2.5 2.6 2.6 2.5 2.6 3.3 5.0
610
445
510
3.4
1.4
Xlln,,
Metal Nickel
Reaction Orders for Metal Dithizonates Reaction order in Substituted dithizone [HR], [H+l P-F
p-CI p-Br P-1
P-CK p-OCH3 m-CF3 Zinc
P-F
p-Cl p-Br P-1
Reported value ( I ) .
P-CH~ p-OCH3 rn-CF3
Tabie TII. Distribution Characteristics of Dithizones between CHC18 and Water at 25" C. and p = 0.10 Diarylthiocarbazone -logKa/KD'? Diphenyl-
Di-m-trifluoromethylphenylQ
Reported value at ionic strength
=
0.25 ( I ) .
Table IV. Kinetic Data for Extraction of Nickel with Di-p methoxyphenylthiocarbazone [Reagent] X lo4 [H+l X 10' Slopea k ' X lo5 1.12 1.12 1.12 1.12 2.21 2.21 2.21 2.21 2.66 2.66 2.66 2.66 1.91 1.91
k'av.
= =
Q
b
0.0200 0.0138 0.0047 0.0038 0.0314 0.0265 0.0122 0,0070 0,0450 0.0305 0.0141 0,0090 0.0342 0,0050
0.53 0.79 2.24 3.55 0.53 0.79 2.24 3.55 0.53 0.79 2.24 3.55 0.53 3.55
2.2 2.3 2.2 2.7 1.8 2.2 2.8 2.7 2.1 2.1 2.7 2.1
2.2 2.0
2.3 X 0.31 x 10-5
Slope of plot of log [Nil 1-0 os. time (min.). [Nil t k'
=
2.303 X slope X [HDz
1.0 1.1 1 .o 1.1 1.2 1.2 1.2
d[M2+]
- k[M2+] [&-I
for which the constant, k , can be calculated from the value of k' as shown in Table IV by the relation [see equation 8 in (111 k = k ' X -KDR (2) K, Values of k thus obtained are listed in Table VI. DISCUSSION
The rate constants of the formation of the 1 to 1 metal complexes are all more rapid than that for the complex with the parent compound with zinc as well as with nickel. The higher rates observed can help distinguish between the mechanism proposed by McClellan and Freiser and that used by Margerum (9), Wilkins ( I O ) and others. It was suggested by the former ( I ) that first the hydrated metal ion (specifically
Table VI. Rate Constants for Formation of 1 to 1 Metal Chelates with Dithizones at 25" C. and p Rate constant, M-' sec-1 Metal ion Parent P- F p-c1 Ni+2 1.1 x 103 1 . 3 x 103a 2 . 7 x 103 3 . 3 x 103 Zn+2 6.9 X lo6 6 . 2 x 1050 3 . 7 x 107 4 . 6 x 107
Ni+2 Zn+2 a
Reported values at 25" C. and p
296
0
O-CH3 5 . 2 x 1035 7.5 x 1065
ANALYTICAL CHEMISTRY
=
0.25 ( I ) .
(1)
dt
(8) C. B. Honaker and H. Freiser, J . Pliys. Cliem., 66, 127 (1962). (9) R. K. Steinhaus and D. W. Margerum, J . Am. Clzem. SOC.,88, 441 (1966). (10) R. G. Wilkins and M. Eigen, Adcnn. Chem. Ser.., No. 49, 55 (1965).
[H+l - OCH31,.
PI 1 . 3 X lo6 6 . 4 X lo8
-1.1 -1.0 -1.1 -1.1 -1.0 -1.0 -1.0 -0.9 -1.0 -1.0 - 1 .o -1.0 -1.0 -1.0
stant, KDR, were determined as previously described and are listed in Table 111. Determination of Kinetics of Reaction. The procedure followed is the same as that previously described ( I ) . A typical set of kinetic data obtained in this study is shown in Table IV. Reaction orders obtained in the manner previously described (8) are listed in Table V. These findings signify that the rate-determining step is the formation of the 1 t o 1 metal-dithizone complex
10.17 10. 185 9.72 10.46 11.01 11.36 11.89 11.26 10.36
Di-p-fluorophenylDi-p-chlorophenylDi-p-bromophenylDi-p-iodophenylDi-p-methylphenylDi-p-methoxyphenyl-
1. o 1.0 1.0 1.1 1.0 1.0 1.0
=
0.1 p-Br 2.0 x 104 1.9 X 108
Rate constant, A4-lsec-l p-CHa
POCHa
PCF3
2.4 x 104 6 . 9 X lo8
7.0 x 104 2 . 7 X 108
2.7 x 104 1 . 7 X 108
crI
I
I
I
c
2 (5,
0 -
60
30
uFigure 1. Variation of formation rate constants of nickel and zinc dithizonates with substituent electronegativity zinc ion) and the ligand anion form a 1 to 1 complex having the same coordination number in a rapid reversible step. The initially formed complex then slowly loses water in the first-order rate-determining step. Although the change in coordination number involved is satisfactory for zinc, this may not be as easily ap Aicable t o the other metal ions studied. An alternate proposal not involving change in coordination number is formally similar-namely, the first rapid reversible step is that of simple ion pair formation of the hydrated metal ion and the ligand anion from which water is lost in a firstorder rate-determining step. In both mechanisms, rate constant k is a composite ,quantity, the product of the formation constant of a rapidly formed initial complex and the firstorder rate constant of the loss of water from this complex or ion pair. Inasmuch a:; the rate constants varied so widely in the series of substitutsd dithizones, it would appear more likely that the initial complex involves metal-ligand bonding of some sort. Simple ion-pair formation would not be expected to depend so strongly on the relatively minor variation in the large anion of the position of a methyl group, for example, yet the rate constants for the zinc complex of the 0- and p methyl derivatives varied by almost a hundredfold, and even those for nickel by fivefAd. The effect of the eectronegativity of the substituent as reflected by its Hammett c value on the rate of the formation of the metal complexe: (Figure 1) is similar to that observed by Joy and Orchin (ZZ) in a study of the stability of a series of substituted platinum-styrene complexes. They reasoned from a molecular orbital point of view that an increase in (11) J. R.Joy and M. Oichin, J. Am. Chem. SOC.,81, 305 (1959).
35
40
45
50
55
60
Figure 2. Nickel and zinc dithizonate formation rate constants stability arose from the strengthening of the sigma bond between metal and ligand by the action of electron-releasing substituents. The sigma bond weakening that arose from electron-withdrawing substituents was amply compensated by their beneficial effect on the extent of overlap of pi bonding integrals. Hence both types of substituents would increase stability. It might be possible to apply analogous reasoning to the case of the rate constants of the nickel complex formation, since back pi bonding would be feasible from a d8 ion. It would seem highly unlikely, however, that these arguments could apply t o zinc ion with its dl0 configuration. Yet, as may be seen in Figure 2, the effect of substituents on the rate of formation of both nickel and zinc complexes is similar. No alternative explanation can be advanced at this time to the increase in rate observed with ligands having substituents of either positive or negative values. The dashed line in Figure 2 is drawn with a slope of unity through the datum point for dithizone itself and indicates the consequences of the hypothesis that the effect of substituents on the rates of the chelate formation of both metal ions is identical. The points on or close to this line are those of dithizones whose substituents (CFs, Br, and I) are incapable of transmitting electronic effects through conjugation. Most of the points corresponding to those of dithizones having substituents capable of electron release via conjugation fall above the line. This points t o the greater influence of such substituents on the formation rate of the zinc complexes. This would tend io confirm the conclusion of Math er al. (2), based on stability and spectral studies, that a phenyl ring of dithizone is coplanar with the chelate ring in the zinc chelate but not in the nickel chelate. Substituents that can transmit electronic effects via conjugation would therefore be expected VOL. 39, NO. 3 , MARCH 1967
297
t o have greater effects in the zinc chelate. From this point of view, it is not too surprising t o see the p-CH3 point somewhat further above the line than the p-F and p-C1 points, whose electron-release contributions by conjugation are offset by electron-withdrawing effects by induction. The great differences in the behavior of the pair of methylsubstituted dithizones with nickel and zinc is noteworthy.
I
I1
I t is apparent from Figure 2 that the position of the methyl group has a more profound effect on the formation of the zinc than on the nickel complex. Since, as indicated by molecular models, the phenyl ring in the Ni complex (11) lies out of the plane of the chelate ring, the presence of a methyl substituent in either ortho o r para position would have about the same effect. In contrast, the presence of an ortho substituent on the phenyl ring in the zinc chelate (I) would severely limit the free rotation of this ring. The steric effect shows up as strongly in the rate of formation of the zinc chelates as in their stabilities. At this time it is impossible t o evaluate separately the factors of the rate constant, k , which is a composite of a rapidly formed intermediate complex and a first-order decomposition constant. The behavior of the p-OCH3 derivative should resemble that of the p-CH3 compound. Its failure t o d o so cannot be readily understood. Further work involving rate studies with other metal ions is under way. RECEIVED for review August 26, 1966. Accepted December 15, 1966. The authors are grateful t o the Atomic Energy Commission and the National Science Foundation for financial support in this work.
Effect of pH on Ion and Precipitate Flotation Systems Alan J. Rubinl and J. Donald Johnson Department of Encironmental Sciences and Engineering, Schoo! of Public Health, Unicersity of North Carolina at Chapel Hill, N . C. The effects of pH on the ion flotation and precipitate flotation of several metal ion-collector systems were examined. The anionic collector, sodium lauryl sulfate, was used to remove both soluble and insoluble copper(l1) and iron(ll1) species. Stearylamine, a cationic collector, removed dissolved copper and copper hydroxide, while soluble iron was not removed and ferric hydroxide was only partially removed by this collector. A weak acid collector was found to be less efficient than a strong acid collector for removing iron by precipitate flotation. A method for predicting precipitate flotation as a function of p H is described.
IONFLOTATION and precipitate flotation are foam separation processes used to remove surface inactive substances from aqueous dispersions. This is accomplished by adding a surface active agent, a collector, that will react to form a surface active product with the component to be removed. With these processes the separation is obtained at the interface of the bulk and foam phases without assistance from the extended phase, having the advantage of producing a dry foam of small volume, and thus allowing the use of compact equipment. Both ion and precipitate flotation are sensitive to bubble size and, therefore, an alcohol frothing agent is added along with the collector. The ion flotation technique was introduced by Sebba in 1959 ( I ) , and the precipitate flotation technique by Baarson I Present address, College of Engineering, University of Cincinnati, Cincinnati, Ohio 45221
__ ( I ) F. Sebba, A ’ a ~ e 184, , 1062 (1959).
298
ANALYTICAL CHEMISTRY
and Ray in 1963 (2). Although precipitate flotation has not been extensively investigated (3), Sebba has published several papers on ion flotation (4-7). A detailed study in which the effects of several variables o n the two processes are compared has recently been described by Rubin et 01. (8). With Sebba’s process an insoluble metal ion-collector product is floated, whereas the ion flotation process described in this paper involves the partition of a soluble ion-pair product. Ion and precipitate flotation differ from one another in that with the latter process the component to be removed is precipitated before the addition of collector. In principle this may be accomplished by adding any substance that forms an insoluble compound, however, this work is restricted to hydroxide precipitates. Since surfactant need only react with ions on the surface of the condensed phase to produce a hydrophobic surface, only small amounts of collector are required. Thus, while ion flotation requires stoichiometric o r greater concentrations of collector, precipitate flotation is effective in some systems in which the metal ion concentra-
__________ ( 2 ) R. E. Baarson and C. L. Ray, “Precipitate Flotation-A New
Metal Extraction and Concentration Technique,” Amer. Inst. of Mining, Metallurgical and Petroleum Engineers Symposium, Dallas, Texas, 1963. ( 3 ) J. A. Lusher and F. Sebba, J. Appl. Cllem., 16, 129 (1966). (4) Ihid.,15, 577 (1965). (5) N. W. Rice and F. Sebba, Ihid., p. 105. (6) F. Sebba. “Ion Flotation,” Elsevier, New York, 1962. ( 7 ) F. Sebba, Narzrre, 188, 736 (1960). (8) A. J. Rubin, J. D. Johnson, and J. C. Lamb, Ind. Errg. Ckem. Process Design Decelop., 5, 368 (1966).
