Aggregation equilibria in solvent extraction of iron(III) halides

J. inorg, nucl. Chem., 197 I, Vol. 33. pp. 551 to 557.

Pergamon Press.

Printed in G reat Britain

A G G R E G A T I O N EQUILIBRIA IN SOLVENT EXTRACTION OF IRON(Ill) HALIDES O. LEVY, G. MARKOVITS and A. S. KERTES Department of Inorganic Chemistry, The Hebrew University,Jerusalem, Israel (Received 8 April 1970)

Abstraet-Osmometric measurements on benzene solutions containing trilaurylammonium chloride or bromide, TLAHX. and tetrachloro or tetrabromoferrate, 1 LAHFeX~, (X ~ CI and Br) and data on the partition of iron(ll !) chloride between aqueous hydrochloric acid solutions and benzene solutions of trilaurylammonium chloride indicate that in the equilibrium organic phase the predominate metal complexes are TLAHFeX4 and TLAHX.TLAHFeX4 in addition to the various homogeneous oligomers of both TLAHX and TLAHFeX4. Their formation constants are given. The water content of the benzene phase has no significant effect upon the aggregation equilibria. INTRODUCTION THE EXTRACTION of simple m i n e r a l acids by l o n g - c h a i n a l k y l a m i n e s is fairly well u n d e r s t o o d [ l ] a n d is e x p l a i n e d [2] in t e r m s of m a s s - a c t i o n law aggregation e q u i l i b r i a of the a m i n e salts in the o r g a n i c phase, b u t the e x t r a c t i o n of the prot o n a t e d a n i o n i c halo metal c o m p l e x e s of the type H M X 4 or H 2 M X 4 is more complicated[3]. T h e m a i n r e a s o n for this c o m p l e x i t y is the c o e x t r a c t i o n of the supp o r t i n g m i n e r a l acid leading to the f o r m a t i o n of m i x e d c o m p l e x e s , a n d , p o s s i b l y , their aggregates in the o r g a n i c p h a s e [ 1]. D i s t r i b u t i o n data o n i r o n ( I l l ) chloride, o n e of the m o s t e x t e n s i v e l y s t u d i e d s y s t e m s [ 4 - 7 ] , are in g e n e r a l , i n c o n s i s t e n t with s p e c t r a l o b s e r v a t i o n s . T h e a m i n e t o - m e t a l ratio of the o r g a n i c p h a s e varies b e t w e e n 1 a n d 2, d e p e n d i n g o n the conc e n t r a t i o n of the metal salt a n d the s u p p o r t i n g acid in the a q u e o u s phase, a n d that of the a l k y l a m i n e in the o r g a n i c phase. T h e a b s o r p t i o n s p e c t r a [ 7 - 9 ] of the a m i n e e x t r a c t s , h o w e v e r , i n v a r i a b l y c o r r e s p o n d to the FeC14 a n i o n , r e q u i r i n g a value of u n i t y for the a b o v e ratio. S t u d i e s i n v o l v i n g o t h e r t e c h n i q u e s a p p e a r to be n e c e s s a r y for a b e t t e r u n d e r s t a n d i n g of this a n d s i m i l a r metal e x t r a c t i o n s y s t e m s . T h e r e f o r e , as an e x t e n s i o n of o u r p r e v i o u s o s m o m e t r i c m e a s u r e m e n t s on the aggregation e q u i l i b r i a of tri1. Y. Marcus and A. S. Kertes, lon-Exchanee and Solvent Extraction q[lMetal Complexes. WileyInterscience, London 1969. 2. A. S. Kertes and G. Markovits, J. phys. Chem. 72, 4202 ( 1968); and references therein. 3. A. S. Kertes, P. J. Lloyd, K. Kimura. J. F. Byrum and J. W. Irvine, Jr., USAEC Report TID20451 ( 1964); and references therein. 4. G. Duyckaerts, J. Fuger and W. Miiller, Euratom Rep. EUR 426 (1963); W. Miiller, G. Duyckaerts and E. Maino, Euratom Rep. EUR 2245 (1965); A. D. Nelson, J. L. Fasching and R. I . McDonald, J. inorg, nucl. Chem. 27, 439 (1965). 5. J. M. White, P. Kelly and N. C. Li.J. inorg, nucl. Chem. 16,337 ( 1961 ). 6. L Kuca and E. HBgfeldt,Acta chem. Scand. 22, 183 (1968). 7. M. L. Good and S. E. Bryan,J. Am. chem. Soc. 82, 5636 (1960). 8. S. Lindenbaum and G. E. Boyd, J. phys. Chem. 67, 1238 ( 1963). 9. O. Levy and A. S. Kertes, J. ihorg, nucl. Chem. 31,888 (1969). 551

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0. LEVY, G. MARKOVITS and A. S. KERTES

laurylammonium chloride and bromide [2], T L A H X , and the corresponding tetrahaloferrates[10], T L A H F e X 4 , in benzene, we now report o s m o m e t r i c results on the equilibrium in benzene solutions containing two solutes, the simple T L A H X and the metal bearing T L A H F e X 4 , together with results on the extraction of iron(I II) chloride by T L A H C I in benzene.

EXPERIMENTAL The preparation and identification of the solid trilaurylammonium salts has been described elsewhere [9]. The theory and practice of the osmometric measurements has also been described [ 10]. In extraction experiments, equal volumes of the aqueous phase, containing ferric chloride and hydrochloric acid in varying concentrations, and the benzene phase containing TLAHC1, were equilibrated by a thermostat-immersed agitator for 2 hr at 37°C. The metal concentration was determined spectrophotometrically. The activity of hydrochloric acid in the aqueous phase was kept constant, and the total chloride ion content was checked potentiometrically. Calculations were made by means of a CDC-6400 computer using the program described earlier [10]. RESULTS O s m o m e t r i c results. O s m o m e t r i c results on mixtures of equimolar benzene solutions of T L A H X and T L A H F e X 4 in varying ratios (Tables 1 and 2) were interpreted in terms of self-association equilibria[2, 10] of pure T L A H X and T L A H F e X 4 and mixed-adduct formation according to the reaction, ( T L A H X ) z + ( T L A H F e X 4 ) y ~ ( T L A H X ) z ( T L A H FeX4)u

(1)

with an equilibrium constant flzu. T h e treatment of results e m p l o y e d previously[2, 10] for simple binary systems of benzene and one solute, has now b e e n modified, to express the total solute concentrations B, and the osmometrically m e a s u r e d concentrations S, in form of B = a + pflpa p + qflqa q + b + rflrb r + tfltbt+ (z + y)flzyab

(2)

S = a + flpa p + flqa q + b + flrb r + fit bt q- flzyab

(3)

where a and b are the molar concentrations of the m o n o m e r i c species of T L A H X and T L A H F e X 4 respectively, tip and fl~ the formation constants of oligomers in the T L A H X , and fir and fit in the T L A H F e X a systems. Taking the values of the aggregation numbers p, q, r, t, and the values of the respective oligomer formation constants tip, fl~, fl~, fit, found previously [2, 10] in binary s y s t e m s under otherwise identical experimental conditions, as compiled in T a b l e 3, and using a non-linear least square program[10] to search for a set of values for z, y and flz~ that would minimize the error square sum F = Y~ (hobs -- ~calc) 2 10. A. S. Kertes, O. Levy and G. Markovits, J. phys. Chem. 74, 3568 (1970).

(4)

Aggregation equilibria of iron(l I 1) halides Table 1. O b s e r v e d and calculated values for r/in the T L A H F e C I 4 + T L A H C I s y s t e m at 37°C B

0.01 0.01 0.01 0.01 0.01 0.01 0.01 0-01 0-01 0.02 0.02 0.02 0.02 0-02 0-02 0.0278 0.0278 0.0278 0.0278 0.0278 0.0278 0.0278 0.06

CTLAHCI

0.01 0.008 0.007 0.006 0.005 0.004 0.002 0.001 0.014 0.012 0.008 0.006 0.004 0.0278 0.0250 0.0222 0.0195 0.0165 0.0083 0-0028 0-03

('TLAHFeCI i

0.002 0.003 0.004 0.005 0.006 0.008 0.009 0.01 0.006 0008 0-012 0.014 0.016 0-02 0.0028 0.0055 0.0083 0.0111 0.0195 0.0250 0.03

nol)~

~calc

1.13 1.16 1.25 1-40 1.58 1.57 1-51 1-47 1.43 1-69 1.94 2.22 2-19 2-07 1.90 1.26 1.33 1.51 1.76 2-16 2-70 2.40 3.03

1.13 1.16 1-25 1.40 1.64 1.63 1.54 1.49 1.43 1.75 1.99 2.26 2-23 2.19 1.96 1.28 1.33 1.52 1.83 2.20 2.73 2.44 3.03

Table 2. O b s e r v e d and calculated values for fi in the T L A H F e B r 4 + T L A H B r s y s t e m at 37°C B

CTLAHBr

0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.02 0.02 0.02 0.02 0.02 0-02 0.03 0-03 0.03

0"007 0.006 0-005 0.004 0.002 0.001 0.012 0.01 0.008 0-006 0-004 0-002 0-018 0.015 0-0t2

CTLAHFeBr4

0'003 0.004 0'005 0.006 0-008 0-009 0.01 0-008 0.01 0.012 0.014 0'016 0-018 0-02 0.012 0.015 0.018

/'lobs

/lc ale

1.38 1-46 1.54 1-66 1.82 1-86 1.82 1.66 1-89 1-98 2-17 2.38 2"36 2.60 1.83 2.05 2-30

1.94 1.46 1.54 1-66 1.82 1.85 1-82 1-67 1-89 1-98 2-17 2.38 2.36 2-59 1-83 2.05 2.29

553

554

O. L E V Y , G. M A R K O V I T S and A. S. K E R T E S

in terms of h~= B/S, we found that the value of z = y = 1 fits the experimental results, as shown in Tables 1 and 2, with log/311 = 2.82±0.01 for the TLAHC1. TLAHFeC14, and logfln = 3"165±0.001 for the T L A H B r . T L A H F e B r 4 mixed dimers. Distribution results. Partition of macro amounts (0.05-0.1M) of iron(Ill) chloride between an aqueous solution of constant chloride concentration (2.4M) and constant hydrochloric acid activity (2-2M), and a benzene solution of T L A H C I (0.05--0-1M) has been measured. In addition to the equilibria leading to the formation[2, 10] of (TLAHCI)2, (TLAHFeCI4)2, (TLAHFeC14)8 and T L A H C 1 - T L A H F e C I 4 , the equilibrium /310 T L A H C I + FeCI3 ~ T L A H F e C I 4

(5)

Table 3. Aggregation numbers and constants used in solving Equations (2) and (3)

TLAHCF TLAHFeCIfl ° TLAHBr4 TM T L A H F e B r 4 ~°

p

/3p

q

2

15"5

0

2

17"4

3

flq

r

/3r

t

/3,

2

1.01 X 102

8

2-37 X 10j4

2

1.63 × 10z

8

2.08 × 10j~

2"63 × 102

has been invoked to express the experimentally determined distribution ratio O = [ F e ( l I 1 ) ] / [ F e ( I I I ) ] = uv/3~o{1 +2uV/3r/31o+8flt(uvfl~o)r+ v/3n}/[FeC13]

(6)

and the material balance equation for the calculation of the concentration of the monomeric T L A H C I , CTLAHC1

~---

V+2V2/3v+UVfllo{1-k-2UV/3r/31o+8/3t (UV/3,o)7+2v/3H}

(7)

where u = [FeCla] and v = [ T L A H C I ] , and/3p,/3r,/3t and/311 a r e the osmometrically determined oligomerization constants [2, 10] (Table 3). For the aqueous phase, the equilibrium [FeCla] is given by the relationship,

Cr~c,J ( 1 +/3_,C-ci_ +/3_2Cc~_ )

(8)

/3--1 = [FeCI2 +] [Cl-] [FeCla] - ' = 126

(9)

13_2 = [ F e C F +] [CI-] 2 [FeCIz] -1 = 25-5

(lO)

[FeCla] =

where [ 11 ]

and

Using a non-linear least squares program, the value of •10 to fit the experimental results. I 1. Y. Marcus, Coordn. chem. Rev. 2,257 (1967),

=

4.03 _ 0-49 was found

Aggregation equilibria of iron(l I !) halides

555

24

lromide 22

\ \ \

\

c,,o,iae

LoL

\

j c,I

L

I

I

I

I

0

0.2

04

06

08

~0

I

I

I

I

I

r

i0

08

06

04

02

0

Fraction

TLAHFeX

Fraction

TLAHX

4

Fig. l. Average aggregation number of mixed dimers, T L A H X - T L A H F e X 4 , as a function of the initial composition of solution at a total of 0.02M solute level. Dashed line. calculated ~ values neglecting mixed dimer formation.

///

HBr/ /

J

~ b~

/ / ,,;/

~

I0

f HC L

i0 ~

i~ 4 --

J

I 10-9

I

I

I

]d 6

10 7

10-6

CTL A " aHX

Fig. 2. Experimental (O for CTLAHX)and corrected (© for a T L A H X ) distribution curves for HC1 and l-lBr (Fig. 4 in Ref. [12], see text).

556

O. L E V Y , G . M A R K O V I T S

a n d A . S. K E R T E S

DISCUSSION

The osmometric results indicate a tendency of the simple T L A H X and the metal bearing TLAHFeX4 to form mixed-dimers rather than homogeneous selfassociates. Figure 1 shows plots of the measured mean aggregation numbers of the mixed oligomer as a function of the TLAHX/TLAHFeX4 molar ratio, and plots representing the calculated n values [2, 10], at the same composition of the solutes, on the basis of additivity, for the formation of homogeneous oligomers. This tendency toward mixed-dimer formation, rather than self-association, is not difficult to understand in view of the dipole-dipole interaction [1] responsible for the ion quadrupole formation. This implies that only the monomeric T L A H X and TLAHFeX4 are capable of forming the mixed dimer, reducing thus the extent of self-association. The best fit of the distribution results equally suggests that it is the monomeric TLAHCI which extracts FeCI3 from an aqueous solution, although the TLAHFeCI4 formed will then undergo both self-association and mixed-dimer formation. This is true, at least, for the solute concentrations covered in this study. The presence of water plays no significant role in the extent and degree of association of alkylammonium salts in benzene and other nonpolar, low dielectric constant diluents. We have, in fact, successfully applied the association constants determined osmometrically in dry benzene to interpret partition results in wet benzene. That the success of this substitution is not fortuitous is demonstrated by the plots shown in Fig. 2 and the log Kt values given in Table 4. Thus, we have Table 4. Apparent and thermodynamic extraction constants of HC1 and HBr into T L A in benzene based on distribution d a t a [ 1 2 ] (Kapp) and corrected for the activities[2] of the organic phase species ( K t )

log K I

~/TLA'IL¥

8 ' 7 0 x 10 z

0-435

TLA-HC1 1-76 x 10 -5

4-69

3 ' 8 8 x 10 -2

0.580

1'80 x 10 -6

4.52

4"29

1'57 x 10 -z 5.40 x 10 -3 1.47 x 10 -3

0.764 0.904 0.972

6 ' 8 0 x 10 -7 3-12 x 10 -7 1-00 x 10 -7

4.36 4"24 4-17

4-25 4"19 4-15

3.10 x 10 -4

0-994

2-02 x 10 -s

4"18

4'18

7 . 0 0 x 10 -5

0.999

3"15 x 10 -9

4'35

4"34

TLA-HBr 4-55 x 10 -7 3"80 x 10 -7

5.93 5"70

3.90 x 10 -J 1"92 x 10 -1

CTLA.aHx

log Kap p.

CTLA-HX

4.36

9.34 x 10 -2

0.250

2.65 x 10 -7

5"55

4.97

4.40 x 1"54 x 6.30 x 2-20 x 3-95 x 7-50 x

0.380 0.593 0-718

1.80 x 8"60 x 4-50 x 1.95 x 3'80 x 7'40 x

5.39 5-25 5-14 5-05 5-01 5.00

4.98 5.03 5.00

10 -2 10 -z 10 -~ 10 -3 10 -4 10 -5

10 -7 10 -8 10 -8 10 -8 10 -9 10 -u

1"05 x 10 -5 5.50 x 10 - r

1"12 x 10 -J° 8-00 x 10 -11

4-97 4.83

1.00 x 10 - r

1-00 x 10 -12

5-00

Agregation equilibria of iron(I I 1) halides

557

Table 5. Thermodynamic extraction constants, log K~, based on distribution data[12] corrected for nonideality [2] System

CCI~

TLA-HCI

3.34--+0.03 3-41" 4.22-+0.02 4-04"

TLA-HBr

Cyclohexane

Benzene 4'28-+0"06

2.56+(/.{)2

5.00-.-(I.02

*Corrected i.r. data of Strehlow[ 13]. Table 6. Dielectric constants at 30°C Benzene (dry) Benzene (water sat. ) Benzene +0.02M TI,AHBr (dry) Benzene + 0.02M TI,AH Br (water sat.) Benzene + 0.01M T1,AH FeCI4 (dry) Benzene + 0.01M TI~AHFeCI~ (water sat.)

2.265 2-267 2.388 2.374 2 -4(16 2-294

r e c a l c u l a t e d the c o n s t a n t s K~tpp = [ T L A H X ] [~I-LA] ~, ( b r a c k e t s a n d p a r e n t h e s e s i n d i c a t e m o l a r c o n c e n t r a t i o n a n d a c t i v i t y , r e s p e c t i v e l y ) for the e x t r a c t i o n o f h y d r o c h l o r i c a n d h y d r o b r o m i c a c i d s b y t r i l a u r y l a m i n e o b t a i n e d b y Miiller a n d D i a m o n d [ i 2] using o u r a c t i v i t y coefficients for the o r g a n i c p h a s e s p e c i e s o b t a i n e d in d r y b e n z e n e [2]. C a l c u l a t i o n s for t h e s a m e e x t r a c t i o n s y s t e m s [12] b u t in o t h e r s o l v e n t s g a v e the t h e r m o d y n a m i c e x t r a c t i o n c o n s t a n t s , K~, s h o w n in T a b l e 5. In spite o f the r e l a t i v e l y high w a t e r c o n t e n t o f the o r g a n i c p h a s e [ 12] c o n t a i n ing a l k y l a m m o n i u m s a l t s , with a w a t e r - t o - s a l t ratio o f 0.5-1-1 (after c o r r e c t i o n f o r s o l v e n t w a t e r ) , it is not n e c e s s a r i l y s u r p r i s i n g that the a c t i v i t y coefficients o f the salts are p r a c t i c a l l y u n a f f e c t e d b y the p r e s e n c e o f w a t e r . T h e d i e l e c t r i c c o n s t a n t s o f s e v e r a l b e n z e n e s o l u t i o n s [ 1 4 ] s h o w n in T a b l e 6 m a y at least p a r t i a l l y e x p l a i n the p h e n o m e n o n . 12. W. Miiller and R. M. Diamond, J. phys. Chem. 70, 3469 I 1966). The authors are indebted to Dr. W. Miiller for making available the numerical data on their partition experiments. 13. C. D. Strehlow, M. S. Thesis, Department of Chemistry, Massachuettes Institute of Technology, Cambridge Mass. (1964). 14. O. Levy, PhD. Thesis, Hebrew University. Jerusalem (1969),