1 Kinetics and Mechanism of Extraction of Iron(ll1) with ,6- Is0propyltropolone B. E. McClellan' and Oscar Menis* Analytical Chemistry Division, National Bureau of Standards, Washington, D. C. 20234
HONAKER AND FREISER (I) and McClellan and Freiser (2) pointed out that, in favorable cases, solvent extraction can be used to study the rates of quite rapid chelation reactions. Many workers (3-5) have studied the extraction characteristics of metal ions with P-isopropyltropolone (HIPT). Dyrssen (3) indicated that the extraction of Fe(II1) with HIPT occurs very slowly from acid solution, and postulated that some step in the formation of the extractable complex, Fe(IPT)3, is slow. The purpose of this study was to apply the extraction technique for studying the kinetics of reaction to the Fe(II1)HIPT system. It was hoped that a n analysis of the kinetic data would lead to the elucidation of the mechanism and ratedetermining step in the extraction process. Iyer and Menis (6) found VOs+ extraction to reach equilibrium relatively rapidly (less than 1 hour from 0.01M HIPT) compared to the slow (1 to 4 days) equilibrium for Fe3+reported by Dyrssen. Both of these metal ions extract quantitatively from strong acid solutions (1M). Hopefully, a detailed study of the kinetics of extraction of the system would lead to a method for the separation of Fe(II1) and VOz+ based on the large difference in the extraction rates. EXPERIMENTAL
Apparatus. All extractions were performed in 60-ml separatory funnels having polytetrafluoroethylene stopcocks and stoppers. The samples were agitated on a mechanical shaker. Radioactivity measurements were made with a modular counting system connected to a 2-in. x 2-in. NaI (TI) well-type scintillation detector. Measurement of pH was done with an expanded scale pH meter. Reagents. The radioactive isotopes jgFe and 4%' were obtained from International Chemical and Nuclear Company. The P-isopropyltropolone (HIPT) was obtained from Columbia Organic Chemicals Co., Columbia, S. C. It was recrystallized from 95 ethanol and dried under vacuum prior to use. Triple distilled water was used throughout the study. All other reagents were of Reagent Grade quality. Procedure. All extractions were performed using equal volumes (10 ml) of aqueous and organic phases at room tem-
Present address, Department of Chemistry, Murray State Uni-
versity, Murray, K y . 42071. 2 To whom inquiries should be addressed.
(1) C. B. Honaker and Henry Freiser, J. Pkys. Chem., 66, 127 (1 962). ( 2 ) B. E. McClellan and Henry Freiser, ANAL. CHEM.,36, 2262 (1964). ( 3 ) David Dyrssen, Tm/is. Roy. Zrist. Techuol., Stockliolm, No. 188, 1962. (4) R . P. Singh and Yat Dutt, ZdiuiiJ. Chem., 4,214 (1966). ( 5 ) Kuan Pan and T. M. Hseu, J . Cliiiiese Chem. Soc., 2, 23 (1955). (6) C. S . P. Iyer and Oscar Menis, National Bureau of Standards, Washington, D. C., unpublished data, 1970. 436
ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
perature (21 i 1 "C). I n the experiments to determine the reaction order in HIPT, the initial Fe(II1) was 1.0 X 10-3M, the [H+] was l M , and the ionic strength was unity. The iron solutions contained enough activity as 59Fe to give an initial count rate of at least 10,000 counts/minute for a 5-ml aliquot. The iron stock solution was prepared by dissolving pure iron metal in HC104, and the [H+] was varied using HCI04. The ionic strength ( p ) was usually unity, but in experiments in which p was varied NaC104 was used. The HIPT dissolved to 2.0 X 10-2M. The samin CHC13varied from 3.0 X ples were agitated for time intervals ranging from 2 minutes to 2 hours. Following agitation, the solutions were allowed to stand until complete phase separation had occurred (about 5 minutes). The phases were then physically separated and 5-ml aliquots were taken for gamma counting in disposable counting vials. The log of the ratio of the initial aqueous phase count rate, C,, to the aqueous phase count rate, C,,, at time, tl, (log C,jC,,) is equal to log [FeIii1t= o/[FeiriIl. This was plotted cs. time and the slopes were measured. A similar procedure was used to determine the effect of [H+] on the reaction rate. The Fe(II1) was again 1.O X The [HIPT] in CHCI3 was held constant at 1.OX 10-2M. I n the [H+] range of 0.1 to lM, the p was constant at 1, while in the range of 2 to 5M[H+], p was 5 . and the [HIPT] The [H+] was 1 , the Fe(II1) was 1.0 X was 1.0 X in the study of the effect of ionic strength on the extraction rate. The p was varied from 1 to 5 with NaC104, and the shaking times varied from 2 to 20 minutes. RESULTS AND DISCUSSION
The reaction order with respect to metal ion was determined us. time. These plots from plots of log [Fe"']t=o/[Fe"']t yielded straight lines, indicative of a first-order reaction with respect to metal ion in all cases. An example of such a plot is shown in Figure 1. The deviation from zero intercept reflects the statistical error of counting of the ratio of activities in two phases. The change in the values of the slopes of these lines at various reagent concentrations was used to obtain the reaction order with respect to reagent. The kinetic data for theextraction of iron with HIPT are shown in Table I. As shown in Figure 2, a slope of 1.1 was obtained indicating a first-order reaction i n HIPT. A similar study of the effect of [H+] o n the rate of reaction showed the rate to be independent of [H+], at constant ionic strength, between 0.1 and 5M Hf. Since the pK, for the reaction KO
HIPT
H+
+ IPT-
is 7.5 at p = 1, a t high H+ only the HIPT will exist. Therefore, n o dependence o n H+ in the high acid range would be expected. However, at lower [H+]one would expect a negative first-order dependence on [H+] as was observed for the zinc-dithizone system by McClellan and Freiser (2). In SUP-
.4
.3
Figure 1. Plot of log [Fellllt,o/[Fellllt us. time [HIPT] = 3.0 x 10-3, [H+] = 1.0, [FeXI1]= 1.0
x 10-3
Slope
=
1
I
1
I
I
I
I
I
I
I
I
I
I
--
r +a
1
I
.2-
AA m
J
0.0020
,I
-
-
/*
/' I
I
I
I
1
0
2
I
I
I
I
I
I
12 14 Tlminl
16
I
I
Table I. Kinetic Data for Extraction of Iron with p-Isopropyltropolone [HIPTI, W I Slope" ktb
x 7.0 x 1.0 x 3.0
10-3
2.0
10-3 10-2 10-2
5.0 x 10-3
a
b
x
1.o 1.0 1.0 1 .o
1 .o
[FelI1]t 0 Slope of a plot of log ____ [FeIIIIt k' = 2.303
x
0.0020 0,0038 0.0061 0,0063 0.0163
1.54 1.75 2.01 1.45 1.88 5' = 1.73 u = 0.23
time'
[H+I slope X _ _ [HIPT]o'
Figure 3. strengths
4
6
8
IO
20
22
24
Plot of log [Fellll,,o/FelI1l, us. time a t various ionic
[H+] = 1.0, [FelI1] = 1.0 X a. p = 1, slope b. p = 2, slope c . p = 3, slope d. p = 4, slope e . p = 5 , slope
port of this, one experiment a t a p H of 4 and [HIPT] of 5.0 X showed very rapid extraction. The extraction was complete in about 10 minutes. However one can also postulate the existence under those conditions of a mixed hydroxy FeOH(1PT)Z complex. Also, an experiment to determine the effect of H+ on the distribution ratio showed much slower extraction a t higher [H+]. The extraction was essentially
18
[HIPT], =
=
1.0 x 10-2
0.0058
= 0.0086 =
0.0105
= 0.0182 =
0.0256
complete in 30 minutes a t p H = 2, while a t least 6 hours were required to attain equilibrium from 1M HCIOl using 1.0 X lo-* HIPT in CHC13. It was observed that the reaction rate was effected by ionic strength as changed by the addition of NaCIOa. Figure 3 shows plots of log [Felll]t=o/[Felll]t us. time at various ionic strengths ranging from 1 t o 5 . Figure 4 shows a plot of log (slope) us. log (NaClO,). A slope of 1.1 was obtained indicating a possible first-order dependence on NaClO,. Although the rate was dependent on the NaClO,, the equilibrium distribution ratio was found to be independent of the NaC104 concentration in the range of 0.1 to 4M acidity. Since perchlorate forms ion pair complexes in iron chelate systems, the observed rate effect may be due t o this process. However, this phase of study was not pursued further. ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
437
*04
*
These reactions are analogous to those for the zinc-dithizone system which was studied by Honaker and Freiser ( I ) and by McClellan and Freiser (2). These workers reported the addition of the first ligand to be the rate-determining step in the extraction of zinc with dithizone. Similarly, in the iron-IPT system, the reaction order in IPT was unity indicating that the addition of the first ligand is the rate-determining step. The rate expression is me"'] dt
- k[Fe'"][IPT-]
(7)
Substitution from Equations 1 and 2 above yields the expression
$
s
u)
4Fe" dt
.003
or
- -d[Fe"']
.002
dt
1'
- k'
[Fe"'][HIPT],
WI
K, [Fe"'][HIPT], = k - x K D ~ [H+l
(8) (9)
Hence, dividing the value of k' by K , / K Dyields ~ the rate constant, k, for the reaction. The value for K , at p = 1 is 3.16 X lo-* and the value for K Dis~2.83 X l o 2at p = 1 (7). The value for k is then 1.5 X 1 O l o M-l minute-'. This value is somewhat higher than the k values reported for the metaldithizonates by McClellan and Freiser (2). The addition of ethanol to the aqueous phase was found to greatly increase the rate of extraction for Fe(II1). This may reflect the change of Kn, in mixed solvent media, whereas as much as 2 to 6 hours may be required to reach equilibrium in the case of the extraction of Fe(II1) from strongly acid (1M) solutions with 0.01M HIPT, equilibrium is reached in 30 minutes in the presence of ethanol under the same conditions. On the other hand, although no detailed kinetic study was made on the VOz+ system in the absence of ethanol, its extraction was found to be complete in 5 minutes from a 1M H+ with 0.1M HIPT in CHCla. Therefore, the separation of V 0 2 + from Fe(JI1) should be possible, based on the wide difference in the extraction rates with HIPT dissolved in CHC13 and free from alcohol.
t1 1
.001
2
I
4
6
8
IO
Na(C104)
Figure 4. Plot of log (slope) us. log [NaClOd [H+] = 1, [FeIrl] = 1.0 X Slope = 1.1
lo+,
[HIPTI, = 1.0 X
The steps involved in the reaction for the extraction of iron with HIPT are as follows : K + HIPT(o,
(1)
+H+ + IPT-
(2)
HIPT HIPT
DI
KO
+ IPT- +Fe(IPT)2+ (at low [H+]) (3a) KI Fe8++ HIPT Fe(IPT)1+ + H+ (at high [H+]) (3b) Fe(IPT)?+ + IPTFe(IPT)*+ (4) Fe(IPT)2+ + IPT- a Fe(IPT)3 (5) Fe(IPT)3 % Fe(IPT)3 (6) Fea+
KI
(0)
438
0
ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
RECEIVED for review June 5, 1970. Accepted November 9, 1970. (7) Oscar Menis, B. E. McClellan, and D. S. Bright, ANAL.CHEM.,
43,431 (1971).
