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Solvent extraction of acidic, chloride solutions of iron(III) by phenylphosphonate diesters in various organic solvents
J.inorg.nucLChem.,1971,Vol.33,pp. 3537to 3545. PcrgamonPress. PrintedinGreat Britain
SOLVENT EXTRACTION OF ACIDIC, CHLORIDE SOLUTIONS OF IRON(III) BY PHENYLPHOSPHONATE DIESTERS IN VARIOUS ORGANIC SOLVENTS W. R. M O U N T C A S T L E , Jr., W. H. MARTIN*, P. D. BALLARDi" and D. H. MILES$ Department of Chemistry, Auburn University, Auburn, Alabama 36830 (First received 23 November 1970; in revised form 18 January 1971 ) Abstract-The extractibility of iron(Ill) in acidic, chloride aqueous media by CoHsPOsR2 dissolved in various organic solvents has been studied; where R = n-butyl, s-butyl, n-octyl, and phenyl. The organic solvents were kerosene, cyclohexane, cyclohexane-cyclohexanone, and benzene. For effective complexing to occur the concentration of chloride and hydrogen must exceed I M. Retrograde extraction is observed for hydrochloric acid solutions greater than 6M. At a phase ratio of one, and the optimum {H+}, and {Ci -1} - 4 to 6 M - t h e degree of extractibility measured by the percent of iron(Ill) not found in the aqueous layer was 9 9 + : 9 9 + : 9 1 : 5 for R = n-butyl, s-butyl, n-octyl, and phenyl. For nonpolar solvents, i.e. kerosene and cyclohexane the n-butyl and s-butyl phenylphosphonate complexes of iron(Ill) are insoluble and appear in a third layer that separates between the organic and aqueous phase. The third layer does not appear in benzene but extractibility does not exceed 92.5 per cent. Cyclohexane-cyclohexanone, 5:: 1, is by far the most effective solvent in eliminating third layer and 9 9 + % of iron(II1) known to be in the sample is extracted. INTRODUCTION
THE EXTRACTION of complexes of the lanthanides and actinides with organic esters of various acids of phosphorous have been extensively studied by Blake et al., McKay et al., Navr~Itil, Mason and Peppard, and by Sato[1-5]. In general, micro amounts of the extracted elements were used and the extractibility measure by means of radioactive tracer methods. White and Ross [6] have systematically studied the extractibility of Groups l-VIII, the lanthanides, and actinides in micro and semi-micro amounts with tri-n-octylphosphine oxide (TOPO). Particularly, White and Ross [7] reported "complete extraction of as much as 12 mg of iron(III) can be achieved from 7 M HCI in a single, 10-min equilibration with 0-5 m-mole of TOPO". This amount of iron(Ill) is easily measured by standard photometric and polarographic methods. Accordingly, a study of the extractibility *Senior Project, Auburn University. tSenior Thesis, Birmingham-Southern College, Birmingham, Alabama 35204. Present address: Jefferson State Junior College, Birmingham, Alabama. ~Senior Thesis, Birmingham-Southern College, Birmingham, Alabama 35204. Present address: Mississippi State University, Starkville, Mississippi. 1. C. A. Blake, Jr., A. T. Gresky, J. M. Schmitt and R. G. Mansfield, ORNL-3374 (1963). 2. H. A. C. McKay, T. V. Healy, I. L. Jenkina and A. Naylor, Solvent Extraction Chemistry of Metals MacMillan, London (1965). 3. O. Navr~til, J. inorg, nucl. Chem. 29, 2007 (1967). 4. G.W. Mason, A. F, BoUmeier and D. F. Peppard, J. inorg, nucl. Chem. 29, 1103 (1967). 5. T. Sato,J. inorg, nucl. Chem. 29, 555 (1967). 6. J. C. White and W. J. Ross, NAS-NS 3102, U S A E C (1961 ). 7. Ref.[6]. p. 37. 3537
3538
W.R.
M O U N T C A S T L E , Jr. et al.
of aqueous solutions of iron(III) by di-n-butyl-, di-s-butyl-, di-n-octyl-, and diphenyl-phenylphosphonate was undertaken. The formation of a third layer by the iron(Ill) complex with di-n-butyl-, and di-s-butyl-phenylphosphonate when kerosene was used as the organic phase added to the complexity of this study. The concentration of iron(III), HC1 and CI-, used in this study preclude any meaningful quantiative treatment of the data in terms of formation constants. Consequently, only a qualitative treatment of the results is presented. EXPERIMENTAL
Apparatus Spectrophotometer. Bausch and Lomb Spectronic 505; after careful cross-checking a Bausch and Lomb Spectronic 20 was substituted for routine determinations. pH meter. Leeds and Northrup, Indicator No. 7664; used to adjust pH of solution to be run on the spectrophotometers. Reagents Standard solutions of Fe(llI) were made by a modification of the method of Dodson, et al.[8]. 0.1 mole of electrolytic iron was dissolved in 14 ml of concentrated HCI and diluted to one liter with distilled water. To make certain that no Fe(IIl) was present, USP oxygen was bubbled through the solution for 30min. The Fe(Ill) titer was established by the differential technique using the Zimmerman-Reinhardt method, and an added check, Fe(IIl) was determined polarographically using the method of Lingane [9]. For all solvent studies other than that involving kerosene, Fe(l I I) solutions were prepared by the method of Snell and Snell[10]. Small amounts of nitrates did not interfere at the higher concentrations of Fe(lll) used in these latter studies. Phenylphosphonate diesters. 500g of di-n-butyl phenylphosphonate, D n B P P , di-s-butyl phenylphosphonate, DsBPP, di-n-octyl phenylphosphonate, D n O P P , and di-phenyl phenylphosphonate, DPPP, were obtained from the Victor Division of Stauffer Chemical Company. Subsequently, additional quantities of D n B P P , D s B P P , and D n O P P were prepared by the method of Blake, et a/.[1]. No measurable differences were noted in the complexing abilities of the material from the two sources, and refractive indices were within ___0.002 of each other and the accepted literature values. Solvents. All solvents were best available commercial g r a d e - "spectro" grade wherever possible. Reagents. All other reagents were ACS "reagent" grade. Procedure Spectrophotometric determination of iron. The procedure was used as outlined by Snell and Snell [10]. The concentration of iron(Ill) in each run were determined from a "working curve" prepared by standard methods. Standard extraction technique. Pipette the amount of 0.1M Fe(liI) solution desired into a separatory funnel. Add hydrochloric acid, sulfuric acid and/or sodium chloride as required, dilute to a final volume of 20 ml with distilled water. Equilibrate for 20-min (5 min is sufficient) with 20 ml of 5% (w/v) solution of the phenylphosphonate diester in the indicated organic solvent. After settling, separate phases. Backwash the organic phases with measured volumes of 3M sulfuric acid. Take aliquots of all systems and determine iron(Ill) as indicated. RESULTS
When 11.2 mg of iron(III) contained in 20 ml of 6M HCI is equilibrated with 20 ml of a 5% (w/v) solution of D n B P P or D s B P P in kerosene an immiscible third layer forms between the aqueous and organic phases. Backwashing the 8. R. W. Dodson, G. J. Forney and E. H. Swift, J. Am. chem. Soc. 58, 2573 (1936). 9. J. J. Lingane,J.Am. chem. Soc. 65, 2448 (1946). 10. F. D. Snell and C. T. Snell, Colorimetric Methods of Analysis 3rd Edn. Vol. II, pp. 307-313. Van Nostrand, New York (1949).
Extraction of iron(IIl)
3539
kerosene layer with 3M sulfuric acid and subsequent spectrophotometric determination of iron(liD revealed no detectable amount of iron. The ternary phase formed by the extraction procedure contained only about 15 per cent of the total iron known to be in the original sample. Yet, the aqueous phase was entirely colorless and immediate spectrophotometric analysis for iron(III) revealed only background amounts or less than 1.3 per cent of total original iron. However, on standing at room temperature, the separated aqueous phase turned into the characteristic color of an iron(Ill) hydrochloric acid solution. On standing for 27 days, the remaining 85 per cent of undetected iron(III) could be measured by the spectrophotometric method used in this study. This effect is illustrated by Fig. 1 and summarized in Table 1. The release of uncomplexes iron(III) can be speededup by refluxing the aqueous layer at atmospheric pressure, Table 1. Table l. The decomposition of the water soluble complexes of iron(l I 1). phenylphosphonate diesters on standing
Time (days) 0 1
2 3 4 5 6 7 8 I0 12 14 15 18 20 25 27 29
DsBPP-5%(w/v) DnBPP-5%(w/v) Iron(Ill) found, mg Room Reflux Room Reflux Temp. Temp. Temp. Temp. 0-2 1.2 2.4 3.2 3.5 3.7
0.1 1.1 2.3
0.1
0.1 2-7
1.8 4.8 3.0
5.9
6.2 3.8
4.9 5.0
7.4
4.9
9.3
7.1
8.4 9.7 9.8
6.6 8.0 9.3
9.3 9.3
9.3
8.4 8-6 9.4 9.7 9.7
9.9
9-9
Total i r o n = l l . 2 0 m g , in 20ml; { H + } = 6 . 3 M ; {CI-} = 6.3M Organic phase s o l v e n t - k e r o s e n e phase r a t i o - 1 Atmospheric pressure.
The amount of iron(Ill) complexed in the aqueous and in the ternary layers is dependent on the chloride and hydrogen ion concentration. The effect of hydrogen ion concentration is shown by Fig. 2, summarized in Table 2. The combined effect of chloride and hydrogen ion concentration is shown by Fig. 3, summarized in Tabi~ 3. As indicated, these effects are manifested by D n B P P and DsBPP but apparently D n O P P does not form a ternary phase under the conditions studies. However, the complexing ability of the D n O P P is much less at the same
3540
W.R. MOUNTCASTLE, Jr. I
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-
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L E G E N D ~ I.ImO.Fe)l(l
~
0-(3" 1511.6~Fe(Fe~l li),
/
'i
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4
8
12 16 20 24 TIME STANDING(DAYS)
28
32
36
40
Fig. 1. Effect of standing on amount of iron(Ill) detected in aqueous layer. Di-n-butyl phenylphosphonate-5%(w/v) in kerosene 6-3M hydrochloric acid, phase ratio-1 (20 ml: 20 ml) atmospheric pressure. !
I
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-
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Fe(=)COMPLEX
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5
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CONEENTRATIONOF H" (M)
I
7
I
8
I
9
I0
Fig. 2. Effect of varying the hydrogen ion concentration on iron(III) complexing by din-butyl phenylphosphonate. 11.2 mg iron(Ill) in 20 ml [Cl-q = 6-3M; Organic phase solvent-kerosene phase ratio- 1.
weight volume concentration, see Table 4. The D P P P is an ineffective complexing agent for iron(Ill) under the conditions of this study, see Table 4. Also, it is noted that very little Fe(lll) is extracted by the cyclohexane/cyclohexanone 5 :: 1 (v/v) solvent when no phenylphosphonate diester is present. Obviously, the system D n B P P or D s B P P 5% (w/v) in kerosene is unsuitable for the separation of iron(III) in hydrochloric acid under the conditions used for this study. Cyclohexane, as expected, is equally ineffective as the organic solvent. Benzene, as the organic solvent, leads to the elimination of the ternary phase but
Extraction of iron(Ill) (
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3541 I
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LEGEND ..---o - - - ° '
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POINTS DESIGNATED BY OPEN-FIGURES REPRESENT TOTAL AMOUNT OF COMPLEX FORMED; THOS[ DESIGNATED BY SOLID FIGURES REPt~SENT AMOUNT OF CONPI.EX DISSOt.VED tN AQU(OU$ LA~I[R "0---0-O"-O-
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MOLARITY OF CHLORIDE ION
Fig. 3. Effect of varying the chloride ion concentration on iron(Ill) complexing by Din-butyl phenylphosphonate in various hydrogen ion solutions. 11,2 mg of iron(Ill) in 20 ml organic phase solvent-kerosene; phase r a t i o - 1. Table 2. Effect of concentration of hydrogen ion on complexing of iron(l 11) by the phenylphosphonate diesters {H +}
M 6"3 6'0 5"3 4"8 3'5 2"1 1"0 0"4 0.0
DsBPP-5%(w/v) DnBPP-5%(w/v) Iron(I 11) f o u n d - mg Complexed, Complexed, aqueous Total aqueous Total layer complexed layer complexed 6.7 6-4 5"3 5-0 4-6 4"2 2'0 1'5 0.0
9"2 9.6 8.9 8.0 6"5 6.6 6'2 5"0 0.7
7.0 6.5 5.6 5'1 4"8 4'5 3.1 2.0 0.0
10,0 9.5 9.1 8.7 7.5 6-6 6.2 5.4 1.1
Total iron = 11"20 mg in 20 ml; {C1-} = 6.3M Organic phase solventkerosene phase r a t i o - I.
the complexing abilities of DnBPP and DsBPP is reduced, see Table 4. LeBlanc [11] had found 5:: 1 mixture of cyclohexane/cyclohexanone to be a suitable solvent for the ternary phase that separated in the iron(Ill); 6M HCI; D s B P P 5% (w/v), kerosene system. This mixed solvent was used to effect a number of separations at various concentrations of iron(Ill) and hydrochloric acid. These results are summarized in Table 5. Additionally, Table 5 includes several other 11. J.T. LeBlanc, J. AlabamaAcad. Sci. 36, 175 (1965).
7,0 4-7 3-8
0"5
6.7 2-7 1'1
0'5
6,6 5.3 4.1 2.5 2"1 2.2
6-6 8.6 9"5 10"1 10"2 8-2
6.7 4.7 4.8 4.6 4.3 4.4 4-9 0-7
9"3 9.2 9.8 10.4 10.6 10,6 10.7 10,4 0.3
6.7 2.6 1.0 0.4
7.0 5.4 2.5
Iron-I 1i - F o u n d - M g 6"3 M -- H + 0.9 M -- H + Complexed Total Complexed Total aqueous cornaqueous coinlayer plexed layer plexed
Total iron = 11.20 mg in 20 ml; Organic solvent - K e r o s e n e ; P h a s e r a t i o - 1.
6-3 5-4 4"5 3"6 2-7 1"8 0-9 0-~
{C1-1} (M)
D s B PP 0-9 M -- H + 4"5 M -- H + C o m p l e x e d Total Complexed Total aqueous cornaqueous cornlayer plexed layer plexed
6.3 5.4 4.9 4.0 3"6 3.1
6-4 8-0 9.5 9.9 9,7 10.0
10.0 9.0 10"2 10-6 10-9 10.7 10-5
6"9 4.4 4.2 3-9 3.3 3.0 1"1
DnBPP 4,5 M -- H + 6-3 M - - H + Complexed Total Complexed Total aqueous cornaqueous tomlayer plexed layer plexed
T a b l e 3. Effect of the c o n c e n t r a t i o n o f h y d r o g e n a n d chloride ion o n t h e complexing o f i r o n ( I I I ) by di-s-butyl- a n d d i - n - b u t y l - p h e n y l p h o s p h o n a t e
tin
C) 2~
I'o
Extraction of iron(Ill)
3543
organic solvents for DnBPP at various concentrations of iron(llI) and hydrochloric acid. It can be seen that the cyclohexane/cyclohexanone 5:: 1 (v/v) mixture is by far the most effective solvent system for the separation of iron(Ill) from hydrochloric acid solutions by solvent extraction. DnBPP appears to be slightly better as a complexing agent for iron(Ill) than D s B P P under the conditions of this study. However, Blake[12] reports D s B P P to have good hydrolytic stability and excellent ability to separate uranium from thorium. Table 4. Comparison of the extracting ability of various phenylphosphonate diesters in various organic solvents for iron(III) solutions in hydrochloric acid Hydrochloric acid (M)
(20 ml)
extracted
4-0-6.0
11.2-28.0
99+
4.0- 6"0
11.2-28.0
99+
4.0-6.0
11.2-28.0
< 1-17
Cyclohexane
4'0-6"0
11.2-22'4
96-91
Benzene
6.0-6.0
I 1.2-22.4
92'5
Xylene
6.0
11.2-22.4
5-0
Agent 5% (w/v)
Organic solvent
Di-n-butyl phenylphosphonate Di-s-butyl phenylphosphonate None
Cyclohexane/ cyclohexanone 5: 1 Cyclohexane/ cyclohexanone 5:1 Cyclohexane/ cyclohexanone 5:1
Di-n-octyl phenylphosphonate Di-n-butyl phenylphosphonate Di-phenyl phenylphosphonate Phase ratio- 1.
CONCLUSIONS AND INTERPRETATION OF RESULTS
It is apparent that a number of {iron(III)}~{phenylphosphonate diester}~ {solvent}z complexes are involved in this study. The complex(s) formed in the system involving kerosene as the organic solvent is very soluble in hydrochloric acid solutions greater than 1M in concentration. This water soluble complex is very stable, more stable than Fe(CNS) 2+ but decomposes on standing in hydrochloric acid medium-possibly due to acid hydrolysis of the diester. At higher concentrations of hydrochloric acid, for instance when (Fe(III)} = 0.01M and {HCi} = 6.3M, the FeCl4- complex competes effectively with the water soluble complex of the diester. Marcus[13] reports essentially 100% formation of FeCl4- at this concentration of hydrochloric acid. 12. Ref.[2]. p. 178. 13. V. Marcus,J. inorg, nucl. Chem. 12,287 (1960).
5. E x t r a c t i o n
* Includes
99
and uncomplexed.
28.0 mg F e ( l i l ) - 2 0 ml *" 10 15 75 phase-complexed
N o n e Prod.
all i r o n ( I I I ) f o u n d i n a q u e o u s
Cyclohexane- cyclohexanone
I
J
Trace
Cyclohexane- cyclohexanone
22.4 mg F e ( l l l ) -- 20 ml * Trace None Prod. 99+
< I
71" Trace
22.4 mg F e ( l l l ) - 20 ml I 21 78
11"2 mg F e ( l l l ) - 2 0 ml
I 13
33-6 mg F e ( l l l ) - 20 nfl 3 72 45 21 N o n e Prod. 79 ~
Trace
28-0 mg F e ( l l l ) - 20 ml ~ Trace N o n e Prod. 99+ ~
None None 99+ 43
15 Trace 8
32 84 N o n e Prod. N o n e Prod.
22-4 mg F e ( l l l ) - - 20 ml 25 75 None ' Trace N o n e Prod. 99+ 45 None Prod. 55
.
68* 16 Trace 57
75* 8* < I 8
Organic A q u e o u s Phase Phase
I 1.2 mg F e ( l l l ) -- 20 ml
5.0 M - HCI Third Phase
I 1.2 mg F e ( l l l ) - 20 ml * 70* 30 None * Trace N o n e Prod. 99+
None 99+
35 48
98
None 99 45
None None 99+ 35
Aqueous Phase
D s B P P - 5% (w/v) Kerosene Cyclohexane-cyclohexanone 24 None Prod.
55 None Prod.
None Prod.
48 None Prod. None Prod.
23 42 None Prod. None Prod.
Organic Phase
~
76* Trace
10 52
2
52 1 55
77* 58 Trace 65
4.0 M - HCI Third Phase
Cyclohexane- cyclohexanone
DnBPP-2.5%(w/v) Cyclohexane- cyclohexanone
Cyclohexane-cyclohexanone Benzene
Cyclohexane-cyclohexanone
Cyclohexane Cyclohexane-cyclohexanone Benzene
Kerosene Cyclohexane Cyclohexane-cyclohexanone Benzene
Aqueous Phase
None Prod.
29 None Prod.
None Prod.
59 None Prod.
None Prod.
85 N o n e Prod. None Prod..
27 92 None None Prod.
6-0 M - HCI Third Phase
and di-s-butyl phenylphosphonate-solvent,
acid effects.
Per cent iron(Ill) found in each phase Phase r a t i o - I
hydrochloric
efficiency of di-n-butyl phenylphosphonate
DnBPP-5%(w/v)
Solvent System
Table
99+
None 99+
99+
40 87
99+
None 99+ 92
None None 99 92
Organic Phase
Z
o
Extraction ofiron(III)
3545
Taube[14, 15] has studied the effects of polarizability of organic diluents in extraction. Similarly, the ease of polarization of the organic solvents by chlorides can be used to explain the formation of the ternary phase. Such a series would be as follows: Benzene > Cyclohexane > Kerosene 1. The ease of polarization of benzene by the {phenylphosphonate diester}n {iron(III)}k{hydrochloric acid}t complex permits ready solvalysis of the complex by benzene but does not completely compete for the complex soluble in the aqueous phase. 2. Cyclohexane with a moderately high resistance to polarization permits the formation of a ternary phase which can contain a high concentration of iron(III). The formation of a third phase containing {HCl}p{phosphonate complexer}q{solvent}r has been reported by a number of investigatorsWhite [6] and Foa[16] as examples. 3. The very non-polar, unpolarized kerosene is not competitive for the ternary phase. Thus a large amount of {iron(III)}x{phenylphosphonate diester}u {solvent}z is found in the aqueous phase. 4. The introduction of cyclohexane could lead to two effects: (a) The formation of a solvent system of intermediate polarizability (between benzene and cyclohexane) that does not permit the formation of the ternary phase and does compete effectively for the water soluble complex. or
(b) The formation of another complex- {iron(llI)}a{phenylphosphonate diester}b{Cyclohexanone}c, e t c . - that is very soluble in cyclohexane. At this stage the exact composition of the complexes in the above proposed mechanisms is a matter of conjecture. There is no evidence that iron(II) is complexed by the phenylphosphonate diesters used in this study. Acknowledgement-The principal author, W. R. Mountcastle, Jr. acknowledges a Faculty Development Research Grant from Auburn University, additional support for the senior thesis at BirminghamSouthern College came from N SF-U RP Grants, Numbers G E-6416 and GY-250. 14. M. Taube, J. inorg, nucl. Chem. 15, 171 (1960). 15. M. Taube, J. inorg, nucl. Chem. 12, 174 (1959). 16. E. Foa, N. Rosintal and V. Marcus, J. inorg, nucl. Chem. 23, 109 ( 1961 ).
SOLVENT EXTRACTION OF ACIDIC, CHLORIDE SOLUTIONS OF IRON(III) BY PHENYLPHOSPHONATE DIESTERS IN VARIOUS ORGANIC SOLVENTS W. R. M O U N T C A S T L E , Jr., W. H. MARTIN*, P. D. BALLARDi" and D. H. MILES$ Department of Chemistry, Auburn University, Auburn, Alabama 36830 (First received 23 November 1970; in revised form 18 January 1971 ) Abstract-The extractibility of iron(Ill) in acidic, chloride aqueous media by CoHsPOsR2 dissolved in various organic solvents has been studied; where R = n-butyl, s-butyl, n-octyl, and phenyl. The organic solvents were kerosene, cyclohexane, cyclohexane-cyclohexanone, and benzene. For effective complexing to occur the concentration of chloride and hydrogen must exceed I M. Retrograde extraction is observed for hydrochloric acid solutions greater than 6M. At a phase ratio of one, and the optimum {H+}, and {Ci -1} - 4 to 6 M - t h e degree of extractibility measured by the percent of iron(Ill) not found in the aqueous layer was 9 9 + : 9 9 + : 9 1 : 5 for R = n-butyl, s-butyl, n-octyl, and phenyl. For nonpolar solvents, i.e. kerosene and cyclohexane the n-butyl and s-butyl phenylphosphonate complexes of iron(Ill) are insoluble and appear in a third layer that separates between the organic and aqueous phase. The third layer does not appear in benzene but extractibility does not exceed 92.5 per cent. Cyclohexane-cyclohexanone, 5:: 1, is by far the most effective solvent in eliminating third layer and 9 9 + % of iron(II1) known to be in the sample is extracted. INTRODUCTION
THE EXTRACTION of complexes of the lanthanides and actinides with organic esters of various acids of phosphorous have been extensively studied by Blake et al., McKay et al., Navr~Itil, Mason and Peppard, and by Sato[1-5]. In general, micro amounts of the extracted elements were used and the extractibility measure by means of radioactive tracer methods. White and Ross [6] have systematically studied the extractibility of Groups l-VIII, the lanthanides, and actinides in micro and semi-micro amounts with tri-n-octylphosphine oxide (TOPO). Particularly, White and Ross [7] reported "complete extraction of as much as 12 mg of iron(III) can be achieved from 7 M HCI in a single, 10-min equilibration with 0-5 m-mole of TOPO". This amount of iron(Ill) is easily measured by standard photometric and polarographic methods. Accordingly, a study of the extractibility *Senior Project, Auburn University. tSenior Thesis, Birmingham-Southern College, Birmingham, Alabama 35204. Present address: Jefferson State Junior College, Birmingham, Alabama. ~Senior Thesis, Birmingham-Southern College, Birmingham, Alabama 35204. Present address: Mississippi State University, Starkville, Mississippi. 1. C. A. Blake, Jr., A. T. Gresky, J. M. Schmitt and R. G. Mansfield, ORNL-3374 (1963). 2. H. A. C. McKay, T. V. Healy, I. L. Jenkina and A. Naylor, Solvent Extraction Chemistry of Metals MacMillan, London (1965). 3. O. Navr~til, J. inorg, nucl. Chem. 29, 2007 (1967). 4. G.W. Mason, A. F, BoUmeier and D. F. Peppard, J. inorg, nucl. Chem. 29, 1103 (1967). 5. T. Sato,J. inorg, nucl. Chem. 29, 555 (1967). 6. J. C. White and W. J. Ross, NAS-NS 3102, U S A E C (1961 ). 7. Ref.[6]. p. 37. 3537
3538
W.R.
M O U N T C A S T L E , Jr. et al.
of aqueous solutions of iron(III) by di-n-butyl-, di-s-butyl-, di-n-octyl-, and diphenyl-phenylphosphonate was undertaken. The formation of a third layer by the iron(Ill) complex with di-n-butyl-, and di-s-butyl-phenylphosphonate when kerosene was used as the organic phase added to the complexity of this study. The concentration of iron(III), HC1 and CI-, used in this study preclude any meaningful quantiative treatment of the data in terms of formation constants. Consequently, only a qualitative treatment of the results is presented. EXPERIMENTAL
Apparatus Spectrophotometer. Bausch and Lomb Spectronic 505; after careful cross-checking a Bausch and Lomb Spectronic 20 was substituted for routine determinations. pH meter. Leeds and Northrup, Indicator No. 7664; used to adjust pH of solution to be run on the spectrophotometers. Reagents Standard solutions of Fe(llI) were made by a modification of the method of Dodson, et al.[8]. 0.1 mole of electrolytic iron was dissolved in 14 ml of concentrated HCI and diluted to one liter with distilled water. To make certain that no Fe(IIl) was present, USP oxygen was bubbled through the solution for 30min. The Fe(Ill) titer was established by the differential technique using the Zimmerman-Reinhardt method, and an added check, Fe(IIl) was determined polarographically using the method of Lingane [9]. For all solvent studies other than that involving kerosene, Fe(l I I) solutions were prepared by the method of Snell and Snell[10]. Small amounts of nitrates did not interfere at the higher concentrations of Fe(lll) used in these latter studies. Phenylphosphonate diesters. 500g of di-n-butyl phenylphosphonate, D n B P P , di-s-butyl phenylphosphonate, DsBPP, di-n-octyl phenylphosphonate, D n O P P , and di-phenyl phenylphosphonate, DPPP, were obtained from the Victor Division of Stauffer Chemical Company. Subsequently, additional quantities of D n B P P , D s B P P , and D n O P P were prepared by the method of Blake, et a/.[1]. No measurable differences were noted in the complexing abilities of the material from the two sources, and refractive indices were within ___0.002 of each other and the accepted literature values. Solvents. All solvents were best available commercial g r a d e - "spectro" grade wherever possible. Reagents. All other reagents were ACS "reagent" grade. Procedure Spectrophotometric determination of iron. The procedure was used as outlined by Snell and Snell [10]. The concentration of iron(Ill) in each run were determined from a "working curve" prepared by standard methods. Standard extraction technique. Pipette the amount of 0.1M Fe(liI) solution desired into a separatory funnel. Add hydrochloric acid, sulfuric acid and/or sodium chloride as required, dilute to a final volume of 20 ml with distilled water. Equilibrate for 20-min (5 min is sufficient) with 20 ml of 5% (w/v) solution of the phenylphosphonate diester in the indicated organic solvent. After settling, separate phases. Backwash the organic phases with measured volumes of 3M sulfuric acid. Take aliquots of all systems and determine iron(Ill) as indicated. RESULTS
When 11.2 mg of iron(III) contained in 20 ml of 6M HCI is equilibrated with 20 ml of a 5% (w/v) solution of D n B P P or D s B P P in kerosene an immiscible third layer forms between the aqueous and organic phases. Backwashing the 8. R. W. Dodson, G. J. Forney and E. H. Swift, J. Am. chem. Soc. 58, 2573 (1936). 9. J. J. Lingane,J.Am. chem. Soc. 65, 2448 (1946). 10. F. D. Snell and C. T. Snell, Colorimetric Methods of Analysis 3rd Edn. Vol. II, pp. 307-313. Van Nostrand, New York (1949).
Extraction of iron(IIl)
3539
kerosene layer with 3M sulfuric acid and subsequent spectrophotometric determination of iron(liD revealed no detectable amount of iron. The ternary phase formed by the extraction procedure contained only about 15 per cent of the total iron known to be in the original sample. Yet, the aqueous phase was entirely colorless and immediate spectrophotometric analysis for iron(III) revealed only background amounts or less than 1.3 per cent of total original iron. However, on standing at room temperature, the separated aqueous phase turned into the characteristic color of an iron(Ill) hydrochloric acid solution. On standing for 27 days, the remaining 85 per cent of undetected iron(III) could be measured by the spectrophotometric method used in this study. This effect is illustrated by Fig. 1 and summarized in Table 1. The release of uncomplexes iron(III) can be speededup by refluxing the aqueous layer at atmospheric pressure, Table 1. Table l. The decomposition of the water soluble complexes of iron(l I 1). phenylphosphonate diesters on standing
Time (days) 0 1
2 3 4 5 6 7 8 I0 12 14 15 18 20 25 27 29
DsBPP-5%(w/v) DnBPP-5%(w/v) Iron(Ill) found, mg Room Reflux Room Reflux Temp. Temp. Temp. Temp. 0-2 1.2 2.4 3.2 3.5 3.7
0.1 1.1 2.3
0.1
0.1 2-7
1.8 4.8 3.0
5.9
6.2 3.8
4.9 5.0
7.4
4.9
9.3
7.1
8.4 9.7 9.8
6.6 8.0 9.3
9.3 9.3
9.3
8.4 8-6 9.4 9.7 9.7
9.9
9-9
Total i r o n = l l . 2 0 m g , in 20ml; { H + } = 6 . 3 M ; {CI-} = 6.3M Organic phase s o l v e n t - k e r o s e n e phase r a t i o - 1 Atmospheric pressure.
The amount of iron(Ill) complexed in the aqueous and in the ternary layers is dependent on the chloride and hydrogen ion concentration. The effect of hydrogen ion concentration is shown by Fig. 2, summarized in Table 2. The combined effect of chloride and hydrogen ion concentration is shown by Fig. 3, summarized in Tabi~ 3. As indicated, these effects are manifested by D n B P P and DsBPP but apparently D n O P P does not form a ternary phase under the conditions studies. However, the complexing ability of the D n O P P is much less at the same
3540
W.R. MOUNTCASTLE, Jr. I
I
I
I
I
et al.
I
l
I
t
_1 u')
~
-
z
~ e¢/
L E G E N D ~ I.ImO.Fe)l(l
~
0-(3" 1511.6~Fe(Fe~l li),
/
'i
#_
4
8
12 16 20 24 TIME STANDING(DAYS)
28
32
36
40
Fig. 1. Effect of standing on amount of iron(Ill) detected in aqueous layer. Di-n-butyl phenylphosphonate-5%(w/v) in kerosene 6-3M hydrochloric acid, phase ratio-1 (20 ml: 20 ml) atmospheric pressure. !
I
I
I
I
1
I
i
1
o
EOC ~
-
o
-
-
o
-
Fe(=)COMPLEX
,¢
( (
I
I
I
2
I
3
t
4
I
5
I
6
CONEENTRATIONOF H" (M)
I
7
I
8
I
9
I0
Fig. 2. Effect of varying the hydrogen ion concentration on iron(III) complexing by din-butyl phenylphosphonate. 11.2 mg iron(Ill) in 20 ml [Cl-q = 6-3M; Organic phase solvent-kerosene phase ratio- 1.
weight volume concentration, see Table 4. The D P P P is an ineffective complexing agent for iron(Ill) under the conditions of this study, see Table 4. Also, it is noted that very little Fe(lll) is extracted by the cyclohexane/cyclohexanone 5 :: 1 (v/v) solvent when no phenylphosphonate diester is present. Obviously, the system D n B P P or D s B P P 5% (w/v) in kerosene is unsuitable for the separation of iron(III) in hydrochloric acid under the conditions used for this study. Cyclohexane, as expected, is equally ineffective as the organic solvent. Benzene, as the organic solvent, leads to the elimination of the ternary phase but
Extraction of iron(Ill) (
f
f
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I
3541 I
I
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LEGEND ..---o - - - ° '
2 0",,
~::hO0
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E
POINTS DESIGNATED BY OPEN-FIGURES REPRESENT TOTAL AMOUNT OF COMPLEX FORMED; THOS[ DESIGNATED BY SOLID FIGURES REPt~SENT AMOUNT OF CONPI.EX DISSOt.VED tN AQU(OU$ LA~I[R "0---0-O"-O-
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MOLARITY OF CHLORIDE ION
Fig. 3. Effect of varying the chloride ion concentration on iron(Ill) complexing by Din-butyl phenylphosphonate in various hydrogen ion solutions. 11,2 mg of iron(Ill) in 20 ml organic phase solvent-kerosene; phase r a t i o - 1. Table 2. Effect of concentration of hydrogen ion on complexing of iron(l 11) by the phenylphosphonate diesters {H +}
M 6"3 6'0 5"3 4"8 3'5 2"1 1"0 0"4 0.0
DsBPP-5%(w/v) DnBPP-5%(w/v) Iron(I 11) f o u n d - mg Complexed, Complexed, aqueous Total aqueous Total layer complexed layer complexed 6.7 6-4 5"3 5-0 4-6 4"2 2'0 1'5 0.0
9"2 9.6 8.9 8.0 6"5 6.6 6'2 5"0 0.7
7.0 6.5 5.6 5'1 4"8 4'5 3.1 2.0 0.0
10,0 9.5 9.1 8.7 7.5 6-6 6.2 5.4 1.1
Total iron = 11"20 mg in 20 ml; {C1-} = 6.3M Organic phase solventkerosene phase r a t i o - I.
the complexing abilities of DnBPP and DsBPP is reduced, see Table 4. LeBlanc [11] had found 5:: 1 mixture of cyclohexane/cyclohexanone to be a suitable solvent for the ternary phase that separated in the iron(Ill); 6M HCI; D s B P P 5% (w/v), kerosene system. This mixed solvent was used to effect a number of separations at various concentrations of iron(Ill) and hydrochloric acid. These results are summarized in Table 5. Additionally, Table 5 includes several other 11. J.T. LeBlanc, J. AlabamaAcad. Sci. 36, 175 (1965).
7,0 4-7 3-8
0"5
6.7 2-7 1'1
0'5
6,6 5.3 4.1 2.5 2"1 2.2
6-6 8.6 9"5 10"1 10"2 8-2
6.7 4.7 4.8 4.6 4.3 4.4 4-9 0-7
9"3 9.2 9.8 10.4 10.6 10,6 10.7 10,4 0.3
6.7 2.6 1.0 0.4
7.0 5.4 2.5
Iron-I 1i - F o u n d - M g 6"3 M -- H + 0.9 M -- H + Complexed Total Complexed Total aqueous cornaqueous coinlayer plexed layer plexed
Total iron = 11.20 mg in 20 ml; Organic solvent - K e r o s e n e ; P h a s e r a t i o - 1.
6-3 5-4 4"5 3"6 2-7 1"8 0-9 0-~
{C1-1} (M)
D s B PP 0-9 M -- H + 4"5 M -- H + C o m p l e x e d Total Complexed Total aqueous cornaqueous cornlayer plexed layer plexed
6.3 5.4 4.9 4.0 3"6 3.1
6-4 8-0 9.5 9.9 9,7 10.0
10.0 9.0 10"2 10-6 10-9 10.7 10-5
6"9 4.4 4.2 3-9 3.3 3.0 1"1
DnBPP 4,5 M -- H + 6-3 M - - H + Complexed Total Complexed Total aqueous cornaqueous tomlayer plexed layer plexed
T a b l e 3. Effect of the c o n c e n t r a t i o n o f h y d r o g e n a n d chloride ion o n t h e complexing o f i r o n ( I I I ) by di-s-butyl- a n d d i - n - b u t y l - p h e n y l p h o s p h o n a t e
tin
C) 2~
I'o
Extraction of iron(Ill)
3543
organic solvents for DnBPP at various concentrations of iron(llI) and hydrochloric acid. It can be seen that the cyclohexane/cyclohexanone 5:: 1 (v/v) mixture is by far the most effective solvent system for the separation of iron(Ill) from hydrochloric acid solutions by solvent extraction. DnBPP appears to be slightly better as a complexing agent for iron(Ill) than D s B P P under the conditions of this study. However, Blake[12] reports D s B P P to have good hydrolytic stability and excellent ability to separate uranium from thorium. Table 4. Comparison of the extracting ability of various phenylphosphonate diesters in various organic solvents for iron(III) solutions in hydrochloric acid Hydrochloric acid (M)
(20 ml)
extracted
4-0-6.0
11.2-28.0
99+
4.0- 6"0
11.2-28.0
99+
4.0-6.0
11.2-28.0
< 1-17
Cyclohexane
4'0-6"0
11.2-22'4
96-91
Benzene
6.0-6.0
I 1.2-22.4
92'5
Xylene
6.0
11.2-22.4
5-0
Agent 5% (w/v)
Organic solvent
Di-n-butyl phenylphosphonate Di-s-butyl phenylphosphonate None
Cyclohexane/ cyclohexanone 5: 1 Cyclohexane/ cyclohexanone 5:1 Cyclohexane/ cyclohexanone 5:1
Di-n-octyl phenylphosphonate Di-n-butyl phenylphosphonate Di-phenyl phenylphosphonate Phase ratio- 1.
CONCLUSIONS AND INTERPRETATION OF RESULTS
It is apparent that a number of {iron(III)}~{phenylphosphonate diester}~ {solvent}z complexes are involved in this study. The complex(s) formed in the system involving kerosene as the organic solvent is very soluble in hydrochloric acid solutions greater than 1M in concentration. This water soluble complex is very stable, more stable than Fe(CNS) 2+ but decomposes on standing in hydrochloric acid medium-possibly due to acid hydrolysis of the diester. At higher concentrations of hydrochloric acid, for instance when (Fe(III)} = 0.01M and {HCi} = 6.3M, the FeCl4- complex competes effectively with the water soluble complex of the diester. Marcus[13] reports essentially 100% formation of FeCl4- at this concentration of hydrochloric acid. 12. Ref.[2]. p. 178. 13. V. Marcus,J. inorg, nucl. Chem. 12,287 (1960).
5. E x t r a c t i o n
* Includes
99
and uncomplexed.
28.0 mg F e ( l i l ) - 2 0 ml *" 10 15 75 phase-complexed
N o n e Prod.
all i r o n ( I I I ) f o u n d i n a q u e o u s
Cyclohexane- cyclohexanone
I
J
Trace
Cyclohexane- cyclohexanone
22.4 mg F e ( l l l ) -- 20 ml * Trace None Prod. 99+
< I
71" Trace
22.4 mg F e ( l l l ) - 20 ml I 21 78
11"2 mg F e ( l l l ) - 2 0 ml
I 13
33-6 mg F e ( l l l ) - 20 nfl 3 72 45 21 N o n e Prod. 79 ~
Trace
28-0 mg F e ( l l l ) - 20 ml ~ Trace N o n e Prod. 99+ ~
None None 99+ 43
15 Trace 8
32 84 N o n e Prod. N o n e Prod.
22-4 mg F e ( l l l ) - - 20 ml 25 75 None ' Trace N o n e Prod. 99+ 45 None Prod. 55
.
68* 16 Trace 57
75* 8* < I 8
Organic A q u e o u s Phase Phase
I 1.2 mg F e ( l l l ) -- 20 ml
5.0 M - HCI Third Phase
I 1.2 mg F e ( l l l ) - 20 ml * 70* 30 None * Trace N o n e Prod. 99+
None 99+
35 48
98
None 99 45
None None 99+ 35
Aqueous Phase
D s B P P - 5% (w/v) Kerosene Cyclohexane-cyclohexanone 24 None Prod.
55 None Prod.
None Prod.
48 None Prod. None Prod.
23 42 None Prod. None Prod.
Organic Phase
~
76* Trace
10 52
2
52 1 55
77* 58 Trace 65
4.0 M - HCI Third Phase
Cyclohexane- cyclohexanone
DnBPP-2.5%(w/v) Cyclohexane- cyclohexanone
Cyclohexane-cyclohexanone Benzene
Cyclohexane-cyclohexanone
Cyclohexane Cyclohexane-cyclohexanone Benzene
Kerosene Cyclohexane Cyclohexane-cyclohexanone Benzene
Aqueous Phase
None Prod.
29 None Prod.
None Prod.
59 None Prod.
None Prod.
85 N o n e Prod. None Prod..
27 92 None None Prod.
6-0 M - HCI Third Phase
and di-s-butyl phenylphosphonate-solvent,
acid effects.
Per cent iron(Ill) found in each phase Phase r a t i o - I
hydrochloric
efficiency of di-n-butyl phenylphosphonate
DnBPP-5%(w/v)
Solvent System
Table
99+
None 99+
99+
40 87
99+
None 99+ 92
None None 99 92
Organic Phase
Z
o
Extraction ofiron(III)
3545
Taube[14, 15] has studied the effects of polarizability of organic diluents in extraction. Similarly, the ease of polarization of the organic solvents by chlorides can be used to explain the formation of the ternary phase. Such a series would be as follows: Benzene > Cyclohexane > Kerosene 1. The ease of polarization of benzene by the {phenylphosphonate diester}n {iron(III)}k{hydrochloric acid}t complex permits ready solvalysis of the complex by benzene but does not completely compete for the complex soluble in the aqueous phase. 2. Cyclohexane with a moderately high resistance to polarization permits the formation of a ternary phase which can contain a high concentration of iron(III). The formation of a third phase containing {HCl}p{phosphonate complexer}q{solvent}r has been reported by a number of investigatorsWhite [6] and Foa[16] as examples. 3. The very non-polar, unpolarized kerosene is not competitive for the ternary phase. Thus a large amount of {iron(III)}x{phenylphosphonate diester}u {solvent}z is found in the aqueous phase. 4. The introduction of cyclohexane could lead to two effects: (a) The formation of a solvent system of intermediate polarizability (between benzene and cyclohexane) that does not permit the formation of the ternary phase and does compete effectively for the water soluble complex. or
(b) The formation of another complex- {iron(llI)}a{phenylphosphonate diester}b{Cyclohexanone}c, e t c . - that is very soluble in cyclohexane. At this stage the exact composition of the complexes in the above proposed mechanisms is a matter of conjecture. There is no evidence that iron(II) is complexed by the phenylphosphonate diesters used in this study. Acknowledgement-The principal author, W. R. Mountcastle, Jr. acknowledges a Faculty Development Research Grant from Auburn University, additional support for the senior thesis at BirminghamSouthern College came from N SF-U RP Grants, Numbers G E-6416 and GY-250. 14. M. Taube, J. inorg, nucl. Chem. 15, 171 (1960). 15. M. Taube, J. inorg, nucl. Chem. 12, 174 (1959). 16. E. Foa, N. Rosintal and V. Marcus, J. inorg, nucl. Chem. 23, 109 ( 1961 ).