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A rate promoted synergistic effect on the solvent extraction system Fe(III)-TTA-SCN
J. inorg, nu¢l. Chem,. 1967. Vol. 29, pp. 2431 to 2440, Pergamon Press Ltd. Printed in Northern Ireland,
A RATE PROMOTED SYNERGISTIC EFFECT ON THE SOLVENT EXTRACTION SYSTEM Fe(III)-TTA-SCN* H. L. FINSTON and Y. INOVZt Department of Chemistry, Brooklyn College of the City University of New York, Brooklyn I0, New York (First received 28 November 1966; in revised form 20 February 1967)
Abstract--The promotion of the extraction rate of iron by TTA-benzene in the presence of ammonium thiocyanate was previously attributed to the rapid extraction of Fe(SCN)3 and the subsequent replacement of SCN- by TTA- in the organic phase. The replacement of the benzene solvent by a benzene-hexone mixture was found to yield both an even greater enhancement of the extraction rate and a definite synergism. It was confirmed that these effects are attributable to the mechanism previously proposed. INTRODUCTION IN A PREVIOUSpaper, tl) the addition of thiocyanate was found to enhance the rate of extraction of Fe(TTA)3 considerably and the reason for this effect was attributed to the replacement of a slow step, the direct formation of F e - T T A complex, by a series of fast reactions, i.e. the formation of F e - S C N complex in the aqueous phase followed by its extraction into the organic phase and the replacement of S C N - in the complex with TTA, in the organic phase. The extraction rate, however, was not as fast as expected from the series of fast reactions and true equilibrium was not reached, nor was a synergistic effect observed, mainly because of the unfavourable competitive reaction of thiocyanate and T T A complex formation and partly because of the reduction of ferric iron by thiocyanate in the course of extraction. These results were attributed to the extremely low distribution coefficient of thiocyanate complex in benzene. Assuming the validity of this argument, a much faster extraction rate could be expected when a better solvent for F e - S C N extraction is used; for example, the addition of oxygenated solvent in benzene. The forward distribution coefficient might then reach the equilibrium value within a practical contact time and synergism based on this mechanism may be expected. The present study is an investigation of this point using methyl iso-butyl ketone (hexone)-benzene mixture as the organic phase. Both a much more pronounced effect of thiocyanate on the rate of F e - T T A extraction and synergism based on this new principle have been observed. EXPERIMENTAL Reagents Methyl iso-butyl ketone was a Baker Analysed reagent. Ammonium thiocyanate solution was standardized titrimetrically by a standard silver nitrate solution. All other chemicals including SgFe tracer were the same as previously described,a~ * This work is partially supported by the U.S. Atomic Energy Commission, contract No. AT(30-1)-3417. ~"Present address: The Research Institute for Iron, Steel and other Metals, Tohoku University, Sendai, Japan. tl~ H. L. FINSTONand Y. INOUE,J. inorg, nucL Chem. 29, 199 (1967). 2431
2432
H . L . FINSTON and Y. INOUE
Apparatus All the apparatus were the same as previously described (x) except for the use of a Beckman DB spectrophotometer for the determination of thiocyanate ion.
Procedure Determination of the per cent thioeyanato ion extracted: Five millilitr¢ of 0.1007 M ammonium thiocyanate solution which had been adjusted to a desired pH was contacted with 5 ml of hexone-benzene mixture having a desired composition for 30 min.
0.8
0.6
a
,q=.._0.4
0.(
0.4
0.8 I. a NH4SCN CONG. { M )
1.6 X 10-4
2.0
FIG. 1.---Calibration curve of ammonium thiocyanate.
tOC
o
LU
)0
BENZENE CONC.(VoI.%) 80 60 40 20 0 i
i
=
i
i
95
m I,..- 9 0 O3 85
ab 4b
do e'o
IEXONE CONC.(VoL%)
JdO
FIG. 2.--Stripping of thiocyanate as a function of hcxone concentration. After separation of both phases, an aliquot of the organic phase was taken and benzene was added to it to make 5 ml. Then, 5 ml of 0.4 w/vyo ferric chloride solution (0.04 N HCI) was added and shaken by hand 200 times. After both phases were separated, the aqueous phase was diluted with the ferric chloride solution to a suitable concentration for the spe~trophotometric measurement. Absorbance was measured with reference to the reagent blank solution at 460 m/z, 30 rain after the stripping of thiocyanate. Concentration of thiocyanate ion which was read from the calibration curve (Fig. 1), was corrected for the incomplete stripping of thiocyanate (Fig. 2) and multiplied by the dilution factor. Per cent thiocyanate extracted was calculated from this value.
A rate promoted synergistic effect on Fe(III)-TTA-SCN
2433
The pH was adjusted with perchloric acid in all the experiments. All the other procedures such as preparation of ferric perehlorate stock solution, extraction and stripping of iron were the same as previously described,c1~except as otherwise noted. RESULTS
Effect of hexone on the extraction rate of iron by TTA from perchloric acid solution The effect of hexone concentration on the extraction rate of iron by 0" 1 M TTA from perchloric acid solution is shown in Fig. 3. It is apparent that the addition of hexone to the organic phase decreases the extraction rate at any p H and the effect is larger, the higher the hexone concentration. l0
•
:
---
80
60
•
lg
w
i
2
3
~
;
CONTACT T I M E (HRS.)
FIG. 3.--Effect of hexone on the extraction rate of iron by TTA;:'0•1 M, no NH4SCN. - - O - - : pH, 2.0; hexone, 0~o; ¢ : pH, 2.0; hexone, 10~o; --O--: pH, 2.0; hexone, 50~; - - A - - : pH, 0.61; hexone, 0~o; ~ & - - : pH, 0.61; hexone, 10~o.
Effect of hexone on the extraction rate of iron by TTA from ammonium thiocyanate solution The effect of hexone on the extraction rate of iron by T T A was investigated in the presence of 0.1 M ammonium thiocyanate at various pH's and T T A concentrations. Figure 4 shows the effect at 2.00 pH and T T A concentration 0-1 M. The addition of hexone remarkably promotes the extraction rate; that is, the higher the hexone concentration, the faster the extraction rate. For example, when no hexone is present, percentage extracted does not reach constant value within 20 rain, while it does after a shaking period of several minutes when 1 : 1 mixture of hexone and benzene is used. In this case, however, if the extraction curve is examined precisely, it shows a maximum at about 20 rain, 30 rain and 60 rain of shaking time for 50, 30 and 20 per cent hexone concentration, respectively. This phenomenon is more dearly seen in the case of 0.61 pH. As shown in Fig. 5, no maximum appears in the absence ofhexone but in its 20
2434
H.L. FINSTONand Y. INOUE I00
80
60
~ ,o
2O
O0
I0 20 CONTACT TIME (MIN,)
FIG. 4.--Effect of hexone on the extraction rate of iron by TTA in the presence of
ammoniumthiocyanate. pH" 2.0; TTA: 0.1 M; NH4SCN:0.1 M. --V--: hexone, 0%; --O--: hexone, 10~; --A--: hexone, 20%; --A--: hexone, 30%; --e--: hexone,50%.
presence a definite peak appears. When hexone concentration is increased, the position of the peak shifts to shorter contact times corresponding to faster extraction rates. This feature of the extraction curve is common to all pH values and TTA concentrations, as shown in Figs. 6 and 7.
Synergistic effect of ammonium thiocyanate-hexone on the extraction of Fe-TTA The effect of hexone and ammonium thiocyanate on the forward distribution coefficients at 2-0 pH and TTA concentration 0.1 M is summarized in Table 1. All the data were obtained by Contacting both phases for 4 hr. A single asterisk shows the values from backward extractions and a dagger shows the calculated values assuming the additivity 0findependent extraction by TTA and ammonium thiocyanate. In the absence of hexone, the addition of ammonium thiocyanate increases the extraction rate considerably. The distribution coefficient, however, is lower by one third than that obtained in the absence of ammonium thiocyanate as mentioned in the previous paper. However, in the presence ofhexone, the addition of 0.1 M ammonium thiocyanate increases the extraction rate to a much greater extent and also increases the distribution coefficient. For example, the addition of 0.1 M ammonium thiocyanate increases the distribution coefficient by one order and two orders of magnitude, respectively for hexone concentrations of 10 and 50 per cent. It should be noted here that 4 hr of contact time is rather arbitrary and does not mean the optimum contact time for getting the largest synergistic effect.
A rate promoted synergisticeffect on Fe(III)-TTA-SCN
2435
c~ 8C
6C
~
4O
=-.-.-._.__..¢ ,
0
CONTACT TIME {HRS.)
FIG. 5.--Effect of hexone on the extraction rate of iron by TTA in the presence of ammonium thiocyanate. pH: 0"610; TTA: 0.1 M; NH4SCN: 0.1 M. Extraction S t r i p p i n g Hexonec o n c . --~7---v~ 0% ©, --e~ lo% --A---A-20% _~_ _._.~ 309/oo --¢---(~50%
Absorption spectra of iron extracted into TTA-hexone-benzene mixturefrom ammonium thiocyanate solution The absorption spectra of iron extracted into TTA-hexone-benzene mixture from ammonium thiocyanate solution of pH 2.00 and 0.610 were measured in relation to the hexone concentration as shown in Figs. 8 and 9, respectively. The concentration of TTA in the organic phase and that of ammonium thiocyanate in the aqueous phase were 0.1 M. Concentrations of iron in the organic phase were normalized in each case, i.e. 6"96 ppm and 4.70 ppm for Figs. 8 and 9, respectively. Curves 1 in Figs. 8 and 9 are obtained in the absence of hexone and are exactly the same as those obtained by TTA-benzene extraction from perchloric acid solution as reported previously. It is interesting to note here that the spectra corresponding to varying concentrations of hexone from 10 to 50 per cent and to 100 per cent hexone containing no TTA all pass through the same point, an isosbestic point. The spectra in 0.1 M TTA solutionin pure hexone (curve 5 in Fig. 8 and curve 6 in Fig. 9), however, do not pass through the isosbestic point.
Extraction of Fe by hexone-benzene mixture from ammonium thiocyanate solution Figure 10 shows the extraction of iron by hexone-benzene mixture from 0.1 M ammonium thiocyanate solution as a function of hexone concentration. It is obvious
2436
,oo
H . L . FINSTON and Y. INotm
;
-
;
V
~
8O
,-, 6 0
~_. ,,x,,m #
f
2o
£
i
.~
CONTACT TiME (HRS.)
6.--Effect of pH on the extraction rate of iron. TTA: 0.1 M; NI-I4SCN: 0.1 M. - - A - - : pH, 0.61; hexonc, 10~o; - - & - - : pH, 0.61; hcxone, 0~o; - - © - - : pH, I-0; hexone, 10~o; ----0~: pH, l.2; hexone,0~'o; ~XT--: pH,2.0; hoxone, 10yo; - - T - - : pH, 2.0; hcxone, 0~o. FIG.
100~-bAe
~
±
-'
•
A
~6
"0
I CONTACT
2
5 TIME
4
5
(HRS.)
FIG. 7.--Effect of TTA concentration on the extraction rate of iron. NH4SCN: 0.1 M; Hexone: 10~. --@---: pH,0.61; TTA, 0.05 M; - - © - - : pH, 0-61; TTA, 0.1M; - - , - - : pH,2.0; TTA. 0.01 M; - - A - - : pH, 2.0; TTA, 0-1 M.
A rate promoted synergistic effect on Fe(II1)-TTA-SCN
2437
TABLE 1.--Sx't,mRClSrIC eFFECT oF THIOCYANATEON TTA EX-rRACrXOr4OF Fe(III) TTA cone. (M)
Hexone conc. (Vol. ~o)
NH4SCN conc. (M)
~o Extracted
0.1 0.1" 0.1 0.1" 0.0 0"lt 0.1 0.1 0"0 0"lt 0"1 0"1 0"0 0"It
0 0 0 0 0 0 10 10 10 10 50 50 50 50
0-0 0.0 0.1 0.1 0.1 0.1 0.0 0.1 0"1 0-1 0"0 0"1 0"1 0"1
98-80 99.96 96.35 99.91 <0.02 98.80 99.46 99.93 0'02 99"46 95"19 99"96 13"4 95"84
Distribution coefficient 82.2 4- 1.8 (2.9 -/- 1.4) x 26.4 4- 3.7 (1.1 4- 0.3) x <1.6 x 10-4 82.2 185 4- 20 (1.7 4- 0.5) x (2"0 q- 0"4) × 185 19-8 4- 2"9 (2"5 4- 0"5) x 0"155 ± 0.00 23"04
10a 10a
10a 10-4
10s
* Backward distribution coefficients. t Calculated values assuming addivity of extraction by TTA and NH4SCN.
1.6
] o
==i 420
460 500 540 WAVE LENGTH (rn/~.)
580
FIG. 8.--Absorption spectra. pH: 2.0; Fe: 6.96ppm; NH4SCN: 0.I M. 1: hexone, 0~o, TTA, 0.1M; 2: hexone, 10~o, TTA, 0.1 M; 3: hexone, 50~, TTA, 0.1 M; 4: hexone, 100yo, TrA, 0.0 M; 5: hexone, 100yo, TTA, 0.1 M.
f r o m Fig. 10 t h a t the increase in b o t h the h e x o n e c o n c e n t r a t i o n a n d p H increases the e x t r a c t i o n o f iron.
Extraction of ammoniurn thiocyanate T h e e x t r a c t i o n o f a m m o n i u m t h i o c y a n a t e was investigated a t a n a m m o n i u m t h i o c y a n a t e c o n c e n t r a t i o n o f 0-1007 M as s h o w n in Figs. 11 a n d 12. T h e e x t r a c t i o n o f a m m o n i u m t h i o c y a n a t e increases with increasing h e x o n e c o n c e n t r a t i o n a n d decreasing p H . This p H d e p e n d e n c e seems to reflect the fact t h a t the e x t r a c t e d species is n o t a m m o n i u m t h i o c y a n a t e b u t t h i o c y a n i c acid.
2438
H. L ~ . . . . . . . . . .
A
v lNotrE
L4 I-
1.0
o
o.2F
]
4~o
4zo
sSo
WAVE
s~o
sSb-
LENGTH (mF)
FtG. 9.--Absorption spectra. pH: 0.61; Fe: 4-70ppm; NH4SCN: 0.1 M. 1: hexone, 0~,TTA, 0.1 M; 2: hexone, 10%,TTA, 0.1 M; 3: hexone, 20~, TTA, 0.1 M; 4: hexone, 50~o, TTA, 0-1 M; 5: hexone, 100K, TTA, 0.0 M; 6: hexone, 100~o, TTA, 0.1 M.
BENZENE CONC. {Vol.%)
I0000 8o
SO 40
2,0
F BOF
I 6ol
x~o2
40
j
20r
O0 " ZO 8'0 HEXONE CONC. (Vol.%)
I/O
FIo. 10.~Extraction of Fe-SCN as a function of hexone concentration. ---A--: pH, 0.61; --C)--: pH, 2-00.
A rate promoted synergistic effect on Fe(III)-TTA--SCN
2439
BENZENE C0 NC.( Vol.;/o)/
60
!
-I/
i
40
12O
° ~ ~'o ~'o
'BO
HEXONE CONC. (Vol.%)
,;o°
FIO. 11.--Extraction of ammonium thiocyanate as a function of hexone concentration. NH4SCN: 0.I M., --(3--: pH, 2.00; --O--: pH, 0.61.
pH
FIG. 12.--Extraction of ammonium thiocy-
anate as a function of pH. NH4SCN: 0.1 M. --&--: hexone, 07o;--A--: hexone, 1070; --O---: hexone, 20 70; -- O--: hexone, 50 70.
DISCUSSION Table 1 clearly shows the synergistic effect which had been predicted in the previous paper. The forward distribution coefficients of iron extracted into TTA-hexone-benzene mixture from 0,1 M ammonium thiocyanate solution fall between the values obtained from forward and backward extraction of Fe-TTA-benzene system, but, it is higher than the backward distribution coefficient obtained in the system, F e - S C N TTA-benzene. The reason for this observation is understood by means of the extraction curve and will be given later. T h e promotion effect of hexone on the extraction rate of F e - T T A from ammonium thiocyanate solution and its dependence on hexone concentration a r e both well explained by the fact that the distribution coefficient of F e - S C N increases with increasing hexone concentration as shown in Fig. 10. Thereremains some doubt whether addition of hexone accelerates the extraction even in the absence of ammonium thiocyanate. As can be seen from Fig. 3, the addition of hexone to TTA-benzene system does not increase the extraction rate but decreases it. To understand this phenomena, absorption spectra of TTA and F e - T T A in hexone-benzene mixture were measured. The absorption spectra of F e - T T A in pure benzene and hexone-benzene mixture were the same but those of TTA were not the same; a small increase in molar extinction coefficient with increasing hexone concentration was observed. It is suspected from these results that the decrease of the extraction rate by the addition of hexone perhaps results from the presence of some interaction between T T A and hexone. In any case, the presence of thiocyanate is essential for the acceleration of extraction.
2440
H.L. FINSTONand Y. INOUE
Another important feature of the extraction curves which is observed in the presence of both hexone and ammonium thiocyanate is the appearance of maxima. This is understood by considering that iron is over-extracted beyond the equilibrium point because of the very fast rate of Fe-SCN extraction in contrast with the slow rate for attaining Fe-TTA extraction equilibrium. The gradual decrease in distribution coefficient after the maximum may reflect the recovery from over-extraction. The equilibrium, however, cannot be reached within 7 hr of contact time. In the absence of hexone, however, the promoting effect of thiocyanate on the rate of extraction is not so great as to cause the over-extraction and therefore maxima are not observed. This accounts for the higher forward distribution coefficient of Fe-TTA into hexone-benzene mixture from 0. I M ammonium thiocyanate solution than the backward distribution coefficient obtained in Fe-SCN-TTA-benzene system. The backward extraction curves at a pH of 0.610 presented in Fig. 5 show that addition of hexone also promotes the stripping rate. This can be explained by the fact that the formation of F e - S C N is favoured in the organic phase due to the higher concentration of thiocyanic acid as shown in Fig. 11. The anomalous fact that the extraction curve and stripping curve corresponding to 10 per cent hexone concentration first cross and then approach each other cannot be explained at the current stage of experiments. Also not explained is the fact that in a higher hexone concentration, extraction and stripping curves do not appear to converge. When hexone concentration is increased, the absorption spectra of iron extracted by TTA from ammonium thiocyanate solution tend toward the spectrum of pure Fe-SCN complex; on the other hand, they tend toward the spectrum of pure Fe-TTA complex with decreasing hexone concentration. This difference in absorption spectra with different hexone concentrations and the presence of an isosbestic point in a series of curves clearly show that the extracted species is not a mixed ligand complex, but is the mixture of two species, Fe-TTA and Fe-SCN. The spectra, 5 in Figs. 8 and 6 in Fig. 9, which do not pass the isosbestic point, may indicate the formation of some mixed ligand complex in this medium although definite conclusions are not warranted from these spectra only. If the above argument concerning the absorption spectra is true, the composition of extracted species can be calculated by the method usually adopted for the simultaneous determination of more than one component. The proportion of Fe-SCN in the extracted iron species is higher at pH 0.610 than at pH 2.00. For example, when 1 : 1 hexone-benzene mixture is used as an extraction solvent, about 50 per cent of the extracted species are present as thiocyanate complex at pH 0.610 and only a few per cent is present at pH 2.00. At least qualitatively, this dependence on pH is attributable to the pH dependence of the distribution of thiocyanate ion between the two phases. As can be seen above, a synergistic effect by thiocyanate on the extraction of Fe-TTA was actually observed using a hexone-benzene mixture as the organic solvent as had been previously predictedJ 1~ Furthermore, all the results including both the extraction curves and the absorption spectra are well understood in terms of the mechanism "Case 2", given in the preceding paper; formation of Fe-SCN complex followed by the extraction of Fe-SCN into organic phase, and then, the replacement of SCN- in the complex with TTA in organic phase. This entirely new principle for synergism in solvent extraction will be extended t o the other TTA systems and will be reported in the near future.
A RATE PROMOTED SYNERGISTIC EFFECT ON THE SOLVENT EXTRACTION SYSTEM Fe(III)-TTA-SCN* H. L. FINSTON and Y. INOVZt Department of Chemistry, Brooklyn College of the City University of New York, Brooklyn I0, New York (First received 28 November 1966; in revised form 20 February 1967)
Abstract--The promotion of the extraction rate of iron by TTA-benzene in the presence of ammonium thiocyanate was previously attributed to the rapid extraction of Fe(SCN)3 and the subsequent replacement of SCN- by TTA- in the organic phase. The replacement of the benzene solvent by a benzene-hexone mixture was found to yield both an even greater enhancement of the extraction rate and a definite synergism. It was confirmed that these effects are attributable to the mechanism previously proposed. INTRODUCTION IN A PREVIOUSpaper, tl) the addition of thiocyanate was found to enhance the rate of extraction of Fe(TTA)3 considerably and the reason for this effect was attributed to the replacement of a slow step, the direct formation of F e - T T A complex, by a series of fast reactions, i.e. the formation of F e - S C N complex in the aqueous phase followed by its extraction into the organic phase and the replacement of S C N - in the complex with TTA, in the organic phase. The extraction rate, however, was not as fast as expected from the series of fast reactions and true equilibrium was not reached, nor was a synergistic effect observed, mainly because of the unfavourable competitive reaction of thiocyanate and T T A complex formation and partly because of the reduction of ferric iron by thiocyanate in the course of extraction. These results were attributed to the extremely low distribution coefficient of thiocyanate complex in benzene. Assuming the validity of this argument, a much faster extraction rate could be expected when a better solvent for F e - S C N extraction is used; for example, the addition of oxygenated solvent in benzene. The forward distribution coefficient might then reach the equilibrium value within a practical contact time and synergism based on this mechanism may be expected. The present study is an investigation of this point using methyl iso-butyl ketone (hexone)-benzene mixture as the organic phase. Both a much more pronounced effect of thiocyanate on the rate of F e - T T A extraction and synergism based on this new principle have been observed. EXPERIMENTAL Reagents Methyl iso-butyl ketone was a Baker Analysed reagent. Ammonium thiocyanate solution was standardized titrimetrically by a standard silver nitrate solution. All other chemicals including SgFe tracer were the same as previously described,a~ * This work is partially supported by the U.S. Atomic Energy Commission, contract No. AT(30-1)-3417. ~"Present address: The Research Institute for Iron, Steel and other Metals, Tohoku University, Sendai, Japan. tl~ H. L. FINSTONand Y. INOUE,J. inorg, nucL Chem. 29, 199 (1967). 2431
2432
H . L . FINSTON and Y. INOUE
Apparatus All the apparatus were the same as previously described (x) except for the use of a Beckman DB spectrophotometer for the determination of thiocyanate ion.
Procedure Determination of the per cent thioeyanato ion extracted: Five millilitr¢ of 0.1007 M ammonium thiocyanate solution which had been adjusted to a desired pH was contacted with 5 ml of hexone-benzene mixture having a desired composition for 30 min.
0.8
0.6
a
,q=.._0.4
0.(
0.4
0.8 I. a NH4SCN CONG. { M )
1.6 X 10-4
2.0
FIG. 1.---Calibration curve of ammonium thiocyanate.
tOC
o
LU
)0
BENZENE CONC.(VoI.%) 80 60 40 20 0 i
i
=
i
i
95
m I,..- 9 0 O3 85
ab 4b
do e'o
IEXONE CONC.(VoL%)
JdO
FIG. 2.--Stripping of thiocyanate as a function of hcxone concentration. After separation of both phases, an aliquot of the organic phase was taken and benzene was added to it to make 5 ml. Then, 5 ml of 0.4 w/vyo ferric chloride solution (0.04 N HCI) was added and shaken by hand 200 times. After both phases were separated, the aqueous phase was diluted with the ferric chloride solution to a suitable concentration for the spe~trophotometric measurement. Absorbance was measured with reference to the reagent blank solution at 460 m/z, 30 rain after the stripping of thiocyanate. Concentration of thiocyanate ion which was read from the calibration curve (Fig. 1), was corrected for the incomplete stripping of thiocyanate (Fig. 2) and multiplied by the dilution factor. Per cent thiocyanate extracted was calculated from this value.
A rate promoted synergistic effect on Fe(III)-TTA-SCN
2433
The pH was adjusted with perchloric acid in all the experiments. All the other procedures such as preparation of ferric perehlorate stock solution, extraction and stripping of iron were the same as previously described,c1~except as otherwise noted. RESULTS
Effect of hexone on the extraction rate of iron by TTA from perchloric acid solution The effect of hexone concentration on the extraction rate of iron by 0" 1 M TTA from perchloric acid solution is shown in Fig. 3. It is apparent that the addition of hexone to the organic phase decreases the extraction rate at any p H and the effect is larger, the higher the hexone concentration. l0
•
:
---
80
60
•
lg
w
i
2
3
~
;
CONTACT T I M E (HRS.)
FIG. 3.--Effect of hexone on the extraction rate of iron by TTA;:'0•1 M, no NH4SCN. - - O - - : pH, 2.0; hexone, 0~o; ¢ : pH, 2.0; hexone, 10~o; --O--: pH, 2.0; hexone, 50~; - - A - - : pH, 0.61; hexone, 0~o; ~ & - - : pH, 0.61; hexone, 10~o.
Effect of hexone on the extraction rate of iron by TTA from ammonium thiocyanate solution The effect of hexone on the extraction rate of iron by T T A was investigated in the presence of 0.1 M ammonium thiocyanate at various pH's and T T A concentrations. Figure 4 shows the effect at 2.00 pH and T T A concentration 0-1 M. The addition of hexone remarkably promotes the extraction rate; that is, the higher the hexone concentration, the faster the extraction rate. For example, when no hexone is present, percentage extracted does not reach constant value within 20 rain, while it does after a shaking period of several minutes when 1 : 1 mixture of hexone and benzene is used. In this case, however, if the extraction curve is examined precisely, it shows a maximum at about 20 rain, 30 rain and 60 rain of shaking time for 50, 30 and 20 per cent hexone concentration, respectively. This phenomenon is more dearly seen in the case of 0.61 pH. As shown in Fig. 5, no maximum appears in the absence ofhexone but in its 20
2434
H.L. FINSTONand Y. INOUE I00
80
60
~ ,o
2O
O0
I0 20 CONTACT TIME (MIN,)
FIG. 4.--Effect of hexone on the extraction rate of iron by TTA in the presence of
ammoniumthiocyanate. pH" 2.0; TTA: 0.1 M; NH4SCN:0.1 M. --V--: hexone, 0%; --O--: hexone, 10~; --A--: hexone, 20%; --A--: hexone, 30%; --e--: hexone,50%.
presence a definite peak appears. When hexone concentration is increased, the position of the peak shifts to shorter contact times corresponding to faster extraction rates. This feature of the extraction curve is common to all pH values and TTA concentrations, as shown in Figs. 6 and 7.
Synergistic effect of ammonium thiocyanate-hexone on the extraction of Fe-TTA The effect of hexone and ammonium thiocyanate on the forward distribution coefficients at 2-0 pH and TTA concentration 0.1 M is summarized in Table 1. All the data were obtained by Contacting both phases for 4 hr. A single asterisk shows the values from backward extractions and a dagger shows the calculated values assuming the additivity 0findependent extraction by TTA and ammonium thiocyanate. In the absence of hexone, the addition of ammonium thiocyanate increases the extraction rate considerably. The distribution coefficient, however, is lower by one third than that obtained in the absence of ammonium thiocyanate as mentioned in the previous paper. However, in the presence ofhexone, the addition of 0.1 M ammonium thiocyanate increases the extraction rate to a much greater extent and also increases the distribution coefficient. For example, the addition of 0.1 M ammonium thiocyanate increases the distribution coefficient by one order and two orders of magnitude, respectively for hexone concentrations of 10 and 50 per cent. It should be noted here that 4 hr of contact time is rather arbitrary and does not mean the optimum contact time for getting the largest synergistic effect.
A rate promoted synergisticeffect on Fe(III)-TTA-SCN
2435
c~ 8C
6C
~
4O
=-.-.-._.__..¢ ,
0
CONTACT TIME {HRS.)
FIG. 5.--Effect of hexone on the extraction rate of iron by TTA in the presence of ammonium thiocyanate. pH: 0"610; TTA: 0.1 M; NH4SCN: 0.1 M. Extraction S t r i p p i n g Hexonec o n c . --~7---v~ 0% ©, --e~ lo% --A---A-20% _~_ _._.~ 309/oo --¢---(~50%
Absorption spectra of iron extracted into TTA-hexone-benzene mixturefrom ammonium thiocyanate solution The absorption spectra of iron extracted into TTA-hexone-benzene mixture from ammonium thiocyanate solution of pH 2.00 and 0.610 were measured in relation to the hexone concentration as shown in Figs. 8 and 9, respectively. The concentration of TTA in the organic phase and that of ammonium thiocyanate in the aqueous phase were 0.1 M. Concentrations of iron in the organic phase were normalized in each case, i.e. 6"96 ppm and 4.70 ppm for Figs. 8 and 9, respectively. Curves 1 in Figs. 8 and 9 are obtained in the absence of hexone and are exactly the same as those obtained by TTA-benzene extraction from perchloric acid solution as reported previously. It is interesting to note here that the spectra corresponding to varying concentrations of hexone from 10 to 50 per cent and to 100 per cent hexone containing no TTA all pass through the same point, an isosbestic point. The spectra in 0.1 M TTA solutionin pure hexone (curve 5 in Fig. 8 and curve 6 in Fig. 9), however, do not pass through the isosbestic point.
Extraction of Fe by hexone-benzene mixture from ammonium thiocyanate solution Figure 10 shows the extraction of iron by hexone-benzene mixture from 0.1 M ammonium thiocyanate solution as a function of hexone concentration. It is obvious
2436
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H . L . FINSTON and Y. INotm
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CONTACT TiME (HRS.)
6.--Effect of pH on the extraction rate of iron. TTA: 0.1 M; NI-I4SCN: 0.1 M. - - A - - : pH, 0.61; hexonc, 10~o; - - & - - : pH, 0.61; hcxone, 0~o; - - © - - : pH, I-0; hexone, 10~o; ----0~: pH, l.2; hexone,0~'o; ~XT--: pH,2.0; hoxone, 10yo; - - T - - : pH, 2.0; hcxone, 0~o. FIG.
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(HRS.)
FIG. 7.--Effect of TTA concentration on the extraction rate of iron. NH4SCN: 0.1 M; Hexone: 10~. --@---: pH,0.61; TTA, 0.05 M; - - © - - : pH, 0-61; TTA, 0.1M; - - , - - : pH,2.0; TTA. 0.01 M; - - A - - : pH, 2.0; TTA, 0-1 M.
A rate promoted synergistic effect on Fe(II1)-TTA-SCN
2437
TABLE 1.--Sx't,mRClSrIC eFFECT oF THIOCYANATEON TTA EX-rRACrXOr4OF Fe(III) TTA cone. (M)
Hexone conc. (Vol. ~o)
NH4SCN conc. (M)
~o Extracted
0.1 0.1" 0.1 0.1" 0.0 0"lt 0.1 0.1 0"0 0"lt 0"1 0"1 0"0 0"It
0 0 0 0 0 0 10 10 10 10 50 50 50 50
0-0 0.0 0.1 0.1 0.1 0.1 0.0 0.1 0"1 0-1 0"0 0"1 0"1 0"1
98-80 99.96 96.35 99.91 <0.02 98.80 99.46 99.93 0'02 99"46 95"19 99"96 13"4 95"84
Distribution coefficient 82.2 4- 1.8 (2.9 -/- 1.4) x 26.4 4- 3.7 (1.1 4- 0.3) x <1.6 x 10-4 82.2 185 4- 20 (1.7 4- 0.5) x (2"0 q- 0"4) × 185 19-8 4- 2"9 (2"5 4- 0"5) x 0"155 ± 0.00 23"04
10a 10a
10a 10-4
10s
* Backward distribution coefficients. t Calculated values assuming addivity of extraction by TTA and NH4SCN.
1.6
] o
==i 420
460 500 540 WAVE LENGTH (rn/~.)
580
FIG. 8.--Absorption spectra. pH: 2.0; Fe: 6.96ppm; NH4SCN: 0.I M. 1: hexone, 0~o, TTA, 0.1M; 2: hexone, 10~o, TTA, 0.1 M; 3: hexone, 50~, TTA, 0.1 M; 4: hexone, 100yo, TrA, 0.0 M; 5: hexone, 100yo, TTA, 0.1 M.
f r o m Fig. 10 t h a t the increase in b o t h the h e x o n e c o n c e n t r a t i o n a n d p H increases the e x t r a c t i o n o f iron.
Extraction of ammoniurn thiocyanate T h e e x t r a c t i o n o f a m m o n i u m t h i o c y a n a t e was investigated a t a n a m m o n i u m t h i o c y a n a t e c o n c e n t r a t i o n o f 0-1007 M as s h o w n in Figs. 11 a n d 12. T h e e x t r a c t i o n o f a m m o n i u m t h i o c y a n a t e increases with increasing h e x o n e c o n c e n t r a t i o n a n d decreasing p H . This p H d e p e n d e n c e seems to reflect the fact t h a t the e x t r a c t e d species is n o t a m m o n i u m t h i o c y a n a t e b u t t h i o c y a n i c acid.
2438
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FtG. 9.--Absorption spectra. pH: 0.61; Fe: 4-70ppm; NH4SCN: 0.1 M. 1: hexone, 0~,TTA, 0.1 M; 2: hexone, 10%,TTA, 0.1 M; 3: hexone, 20~, TTA, 0.1 M; 4: hexone, 50~o, TTA, 0-1 M; 5: hexone, 100K, TTA, 0.0 M; 6: hexone, 100~o, TTA, 0.1 M.
BENZENE CONC. {Vol.%)
I0000 8o
SO 40
2,0
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x~o2
40
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O0 " ZO 8'0 HEXONE CONC. (Vol.%)
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FIo. 10.~Extraction of Fe-SCN as a function of hexone concentration. ---A--: pH, 0.61; --C)--: pH, 2-00.
A rate promoted synergistic effect on Fe(III)-TTA--SCN
2439
BENZENE C0 NC.( Vol.;/o)/
60
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i
40
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'BO
HEXONE CONC. (Vol.%)
,;o°
FIO. 11.--Extraction of ammonium thiocyanate as a function of hexone concentration. NH4SCN: 0.I M., --(3--: pH, 2.00; --O--: pH, 0.61.
pH
FIG. 12.--Extraction of ammonium thiocy-
anate as a function of pH. NH4SCN: 0.1 M. --&--: hexone, 07o;--A--: hexone, 1070; --O---: hexone, 20 70; -- O--: hexone, 50 70.
DISCUSSION Table 1 clearly shows the synergistic effect which had been predicted in the previous paper. The forward distribution coefficients of iron extracted into TTA-hexone-benzene mixture from 0,1 M ammonium thiocyanate solution fall between the values obtained from forward and backward extraction of Fe-TTA-benzene system, but, it is higher than the backward distribution coefficient obtained in the system, F e - S C N TTA-benzene. The reason for this observation is understood by means of the extraction curve and will be given later. T h e promotion effect of hexone on the extraction rate of F e - T T A from ammonium thiocyanate solution and its dependence on hexone concentration a r e both well explained by the fact that the distribution coefficient of F e - S C N increases with increasing hexone concentration as shown in Fig. 10. Thereremains some doubt whether addition of hexone accelerates the extraction even in the absence of ammonium thiocyanate. As can be seen from Fig. 3, the addition of hexone to TTA-benzene system does not increase the extraction rate but decreases it. To understand this phenomena, absorption spectra of TTA and F e - T T A in hexone-benzene mixture were measured. The absorption spectra of F e - T T A in pure benzene and hexone-benzene mixture were the same but those of TTA were not the same; a small increase in molar extinction coefficient with increasing hexone concentration was observed. It is suspected from these results that the decrease of the extraction rate by the addition of hexone perhaps results from the presence of some interaction between T T A and hexone. In any case, the presence of thiocyanate is essential for the acceleration of extraction.
2440
H.L. FINSTONand Y. INOUE
Another important feature of the extraction curves which is observed in the presence of both hexone and ammonium thiocyanate is the appearance of maxima. This is understood by considering that iron is over-extracted beyond the equilibrium point because of the very fast rate of Fe-SCN extraction in contrast with the slow rate for attaining Fe-TTA extraction equilibrium. The gradual decrease in distribution coefficient after the maximum may reflect the recovery from over-extraction. The equilibrium, however, cannot be reached within 7 hr of contact time. In the absence of hexone, however, the promoting effect of thiocyanate on the rate of extraction is not so great as to cause the over-extraction and therefore maxima are not observed. This accounts for the higher forward distribution coefficient of Fe-TTA into hexone-benzene mixture from 0. I M ammonium thiocyanate solution than the backward distribution coefficient obtained in Fe-SCN-TTA-benzene system. The backward extraction curves at a pH of 0.610 presented in Fig. 5 show that addition of hexone also promotes the stripping rate. This can be explained by the fact that the formation of F e - S C N is favoured in the organic phase due to the higher concentration of thiocyanic acid as shown in Fig. 11. The anomalous fact that the extraction curve and stripping curve corresponding to 10 per cent hexone concentration first cross and then approach each other cannot be explained at the current stage of experiments. Also not explained is the fact that in a higher hexone concentration, extraction and stripping curves do not appear to converge. When hexone concentration is increased, the absorption spectra of iron extracted by TTA from ammonium thiocyanate solution tend toward the spectrum of pure Fe-SCN complex; on the other hand, they tend toward the spectrum of pure Fe-TTA complex with decreasing hexone concentration. This difference in absorption spectra with different hexone concentrations and the presence of an isosbestic point in a series of curves clearly show that the extracted species is not a mixed ligand complex, but is the mixture of two species, Fe-TTA and Fe-SCN. The spectra, 5 in Figs. 8 and 6 in Fig. 9, which do not pass the isosbestic point, may indicate the formation of some mixed ligand complex in this medium although definite conclusions are not warranted from these spectra only. If the above argument concerning the absorption spectra is true, the composition of extracted species can be calculated by the method usually adopted for the simultaneous determination of more than one component. The proportion of Fe-SCN in the extracted iron species is higher at pH 0.610 than at pH 2.00. For example, when 1 : 1 hexone-benzene mixture is used as an extraction solvent, about 50 per cent of the extracted species are present as thiocyanate complex at pH 0.610 and only a few per cent is present at pH 2.00. At least qualitatively, this dependence on pH is attributable to the pH dependence of the distribution of thiocyanate ion between the two phases. As can be seen above, a synergistic effect by thiocyanate on the extraction of Fe-TTA was actually observed using a hexone-benzene mixture as the organic solvent as had been previously predictedJ 1~ Furthermore, all the results including both the extraction curves and the absorption spectra are well understood in terms of the mechanism "Case 2", given in the preceding paper; formation of Fe-SCN complex followed by the extraction of Fe-SCN into organic phase, and then, the replacement of SCN- in the complex with TTA in organic phase. This entirely new principle for synergism in solvent extraction will be extended t o the other TTA systems and will be reported in the near future.