Journal of Radioanalytical Chemistry, Vol. 4 7 (19 78) 3 7-46

EXTRACTION OF IRON(Ill) WITH DIPHENYL-2-PYRIDYLMETHANE DISSOLVED

IN BENZENE FROM AQUEOUS CHLORIDE SOLUTIONS

SUHAIL AHMED, SHAMAS-UD-ZUHA, ABDUL GHAFOOR, M. EJAZ*

Nuclear Chemistry Division, Pakistan Institute of Nuclear Science and Technology, P.O. Nilore, Rawalpindi (Pakistan/

(Received March 17, 1978)

The mechanism of extraction has been investigated by partition, slope analysis and loading-ratio data. The results obtained give a picture of the mechanism of extraction of FeCI~ ions in relation to the hydration and solvation of the .compound extracted. The possible formula of the extracted species is (DPPM)3H30+(H:O)n-FeCI~. In dilute aqueous hydrochloric acid systems the influence of the concentration of a number of salts with cations of different electrical potentials (Ze/r), on iron(Ill) extraction, has been studied. Splitting of the organic phases occurs at high acid and/or high salt concentrations. The phenomenon is explained on the basis of the variability of the hydration number. Investigations have been made to understand the parameters controlling the extraction of the metal and it is shown that the extraction offers a simple, fast and selective separation method of iron from solutions

Introduction

The use of high molecular weight alkyl-substituted pyridines, e.g. 4-(5-nonyl)-

pyridine and 2-hexylpyridine as extractants for the complex chloride acids of some

tervalent metals has been reported in our earlier communicat ionsJ -s The studies

on the use of these pyridine amines for the extract ion of i ron(I l l ) from aqueous

chloride media 2-3 indicate the importance of the type of pyridine, the concentra-

t ion o f chloride and hydrogen ions and the concentration of the metal on the

extraction efficiency. It has also been possible to determine the i r o n ( I l l ) - chloride

enti ty in the extracted organic phases through slope analysis and/or by comparing

the spectra of the organic pyridine solutions with known spectra o f i ron(I l l ) com-

plexes. This information coupled with the results obtained from loading curves

allows conclusions on the stoichiometry of pyridine - i ron(I l l ) - chloride extracted

species. The investigation reported herein was undertaken to acquire extraction data

*Author to whom correspondence should be addressed.

J. Radioartal. Chem. 47(1978) 37

SUHAIL AHMED et al.: EXTRACTION OF IRON(III)x

for iron(II1) using another high molecular weight pyridine, viz. diphenyl-2-pyridyl- methane dissolved in benzene under various experimental conditions and to attempt to characterize the extracted ferric species. The results show that this pyridine is more selective for the extraction of iron(III) than 4-(5-nonyl)pyridine and the data can be employed for the extraction of iron prior to the determination of base metal ions in solution, by modern instrumental analytical techniques such as atomic and molecular absorption spectrometry.

Experimental Reagents

Diphenyl-2-pyridylmethane dissolved in benzene was used as a solvent. The characteristics of this compound are given elsewhere. 4 The stock ferric chloride so-

lution was prepared from reagent grade FeC13," 6H20, obtained from May and Ba-

ker Chemical Company, and diluted to the required concentrations with demineral- ized water (1.1 �9 1 0 6 ohm resistance). A few drops of nitric acid per litre of ferric chloride solution ensured the iron(Ill) state. The iron concentration was generally determined by a volumetric method involving reduction of iron(III) with stannous

chloride and then titrating with a standard solution of potassium dichromate. All hydrochloric acid solutions were prepared as reported in a previous paper) Solu- tions of lithium, sodium, magnesium and aluminium chlorides were pre- pared from A.R. grade salts obtained from B.D.H. Chemicals, Ltd., Poole, England.

Tracers and instrumentation

The course of extraction was followed by a tracer technique in all cases, s 9 Fe was obtained by the reported procedure. 6 The other radioisotopes employed in this study were either obtained from Radiochemical Centre Amersham, or were most often prepared by (n, 7) reaction in the 5 MW research reactor PARR-1 of this Institute or by separation of the daughter nuclide from the parent without a car- der. The equipment used for the radiochemical assay is described elsewhere. 1,7

Extraction procedure

Equal volumes of an aqueous phase containing ~10-SM iron(III) spiked with s 9 Fe, and in some cases a certain amount of neutral chloride salts, and benzene

solutions of DPPM (usually 0.1M) were agitated mechanically for a period of 5 min,

sufficient for equilibration. After centrifugating, the phases were separated and ra- diometrically assayed. The percent extraction and partition values were determined in the usual manner.

38 J. Radioanal. Chem. 47 (1978}

SUHAIL AHMED et al. : EXTRACTION OF IRON(lID

. 100 ~ _" ..~ .7. * ?

Q

6O

SO

~0

30 - -

2O

10

I I - i 2 3 . s s 7 8 9 , o ~

[HCLI,

Fig. 1. Percent extraction of ixon(III) as a function of hydrochloric acid concentration in the aqueous phase by 0.1M solution of the pyridine in benzene. �9 - NPy, + - DPPM, o - HPy

Results and discussion

Fig. 1 shows that the extraction of iron(III) (<10-SM) with 0.1M DPPM/ben-

zene starts from about 3M hydrochloric acid and becomes almost quantitative

from 6M acid onwards. A rise in extraction is observed in dilute acid solutions

followed by a plateau at high acidities. The former is probably due to salting-out

from the aqueous phase by chloride ions, whereas the latter effect may be attributed to acid competition for the available DPPM, which has a tendency to level off the extraction of iron(III). For the sake of comparison, we have also plotted in Fig. 1

the extraction of iron(HI) by O.IM solutions of 4-(5-nonyl)pyridine (NPy) and

2-hexylpyridine (HPy) under similar conditions. The curves indicate that the ex-

traction is dependent on the hydrogen ion concentration. If hydrogen ions do not take part in the extraction process, then there is no valid reason for the extraction to start at different molarities of the acids for these pyridines. DPPM is pronounced- ly less basic than NPy and HPy and consequently a greater acidity is required

for extraction on weakening of the basic character of the extractant. However, .~n

the extraction of different tervalent metal complex acids by the same reagent, the

position of the ascending portion of the extraction curve is determined not only

J. Radioanal. Chem. 47 (1978) 39

SUHAIL AHMED et al.: EXTRACTION OF IRON(Ill)

i - 70 f ? o c 60

w 40 f ~ , t t ~

,oI- ...::.'/ IO

" I I 0 t 2 3 4 5

[ N o C t ] , M

Fig. 2. Effect of the concentration of sodium chloride on the percent extraction of iron(III) by 0.1M DPPM/benzene from hydrochloric acid of different acidities. + - 0.1M HCI, o - 0.5M HCI, �9 - 1.0M HC1

(or even, perhaps, not so much) by the hydrogen ion concentration but also by

factors favouring the formation of a complex anion, such as the stability constant and concentration of ligand. For example the chloro-complexes of gold are extracted

at lower acid concentrations than iron complexes, primarily because the chloro-

complexes of gold of the type AuCI~ are considerably more stable. If the forma-

tion of the protonated ions (BH +) plays a decisive part, then strong acids should.

under otherwise identical conditions, be extracted better, which is not altogether so.

The effect of neutral salts on complex formation and salting-out of extractable iron(III) species was investigated at different concentrations of hydrochloric acid.

The salting-out agents investigated were the chloride salts of lithium, sodium,

magnesium and aluminium. The results presented in Figs 2 - 4 show that the presence of these cations, having different electrical potentials, serves to exert a salting-out effect, resulting from their high aqueous hydration and common ion

effect. Such an interpretation is in agreement with the conclusions drawn from a systematic study of the influence of several salting-out agents on iron(Ill) extrac-

tion by 4-(5-nonyl)pyridine. 2 At constant chloride concentration, a high charge and

small radius of the non-extractable mineral salt favours iron extraction. In all these experiments it was observed that a loss of activity occurs when the concentration

of the supporting acid or that of the salting-out agent increases. This may be due

to third phase formation. Since at high concentrations of the salts in the aqueous

solutions, there is practically no free water, the whole of the water is used up in

the hydration of ions; the phenomenon can be explained on the basis of different

40 J. Rodioanal. Chem. 47 (1978)

SUHAIL AHMED et al.: EXTRACTION OF IRON(I l l )

L - 6 0 -

5 0 -

~ o -

30

20

10

I

/ -

t I p . . 2 3

[,gctz] ,M

Fig. 3. Effect of the concentrat ion of magnesium chloride on percent extract ion of i ron( l l I )

by 0.1M DPPM/benzene from hydrochloric acid of different acidities. + - 0.1M HCI,

o - 0.5M HCI, �9 - 1.0M HCI

i

so

c . - . . . . s ~ " , 0 ./ . - ~ i #

60 / ,,-,

5O

.30

2O

lO

v 1 l 3

[A|Cl3] ~ M

Fig. 4. Effect o f the concentrat ion of aluminium chloride on the percent extract ion of i ron(l l I )

by 0.1M DPPM/benzene from hydrochloric acid of different acidities. + - 0.1M HCI,

o - 0.5M HC1, �9 - 1.0M HCI

J. Radioanal. Chem. 47 (1978) 41

SUHAIL AHMED et al.: EXTRACTION OF IRON(III)

0

- - - 1

Fig. 5. Influence o f the concentrat ion o f DPPM/dissolved in benzene on the extract ion o f iron(IIl)

f rom 7M HCI

hydrates resulting in the formation of two phases, yielding two organic layers which separate out. Two organic phases are also formed in the extraction of iron

with diisopropyl ether s which differ in the number of molecules of water per iron atom but are otherwise identical; LAWERENCE et al.s have shown that it is the

water which is the decisive factor in the formation of two ether phases. They re- garded the system as containing insufficient water for a homogeneous solution to

be formed. Thus our studies indicate that water takes part in the extraction process.

However, it is hardly possible to draw any conclusions on the extent of solvation of the halometal acid hydrate in the organic solution. It is unlikely that there would be a single value for the number of DPPM molecules which are associated with the extracted species through water molecules which are polarized in the field of the simple oxonium ion.

In examining the different possibilities as the nature of the extracted species, we may first assume that HFeCI4 is present in the organic phase as an ion-pair. Even though the acid is quite strong, it is not expected to be present as dissociated ions in benzene. One can formulate the extraction equilibria as follows:

42

H § 4- FeCI~ a-i) 4- xC1- + yDPPM o 4- zH20

(H § yDPPM �9 z H 2 0 . . . FeCl~)o (1)

,1". Radioanal. Chem. 47 (1978J

SUHAIL AHMED et al.: EXTRACTION OF IRON(III)

(where subscript o denotes the organic phase, while no subscript indicates the aque- ous phase).

(H +" yDPPM �9 zH2 0 . . . FeC17~)o K = " (2)

(H § (FeCI! 3-i)) (C1-) x (DPPM)Yo (H2 O) z

[H +" yDPPM �9 zH2 0 . �9 �9 FeC14 ]o 3'o (3)

[H+] [FeCl!3-i)] [C1-]x [DPPMlYo (H20)z 7H + ")'FeCI (.3-i) ~CI- "YYPPM 1

At constant acid concentration, (H20)Z[H § 7 r r [C1-] x 7 x is constant and the ac- c1- tivity of Fe(III) is also nearly constant because of the trace concentrations. The ratio

7o/7~VPM may also be assumed to be constant. Thus Eq. (3) can be written as:

[H +" yDPPM "zH20 . . . FeC14]o K = (4)

[FeCl~ -i ] [DPPM]Yo

Since

D = [H § yDPPM �9 zH2 0 . . . FeCI~, ]o

[FeC1}3-i)]

D K = (5)

[DPPM] y

D = K [DPPM] y

lg D = log K + y log [DPPM] (6)

Thus the plot o f lg D vs. log [DPPM] should give the number of DPPM mole- cules associated in the extraction process. A ptot of log D vs. log [DPPM] from 0.001-0.5M concentration with benzene as diluent is shown in Fig. 5. At low reagent concentrations, the slope of the plot is ~1 and it decreases as the concen- tration of pyridine increases, indicating that polymerization of the hydrochloride salts of the pyridine occurs as the concentration exceeds 0.1M. The loading curve from 7M HC1 for 0.1M DPPM/benzene shown in Fig. 6 indicates that DPPM/Fe(III)

Z Radioanal. Chem. 47 (1978) 43

SUHAIL AHMED et al.: EXTRACTION OF IRON(III)

k

u

,-// 1

00.~0 ~ 0.1

|

I I 1 6 1 lo

C~ ~

Fig. 6. Equilibrium concentration of iron(III) in organic (0.1M DPPM/benzene) and aqueous phases (7M HCI)

is ~3. Since the extraction of strong acids, e.g. HC1, by the basic extractant in-

volves competition between the basic extractant, the anion and water for the pro-

ton, which in aqueous solutions exists as the hydronium ion [H30+(H20)n], in extraction from moderate hydrochloric acid solutions, where the activity of water is decreased (as compared to dilute acid solutions) and in the presence of basic

DPPM, ~ competes with water for the solvation of the hydronium ion, the primary hydration cell of the hydronium ion is probably replaced by a solvent cage and a possible structure for the extracted species of iron(III) can be formul- ated as:

DPPM, DPPM

H H \ / 0 I

H | i t

DPPM

FeC14

where the organic solvent adds to the oxonium ion by hydrogen bonding. How- ever, this scheme is to a considerable extent arbitrary. In systems with relatively dilute acid solutions, where the extraction is poor, DPPM molecules perhaps do

not displace water from the first layer and some or all of the sites in the first

layer are occupied by water and DPPM becomes attached in the second hydration layer of the hydrated simple oxonium, where each of the water molecules periodi- cally becomes the oxonium ion. The loading data also indicates the presence of

44 J. Radioanal. Chem. 47 (1978}

SUHAIL AHMED et al.: EXTRACTION OF IRON(Ill)

9o

i a

80

70

60

50

40

3O

20

lC

t

\

I I I [ i - 0.2 0.4 0.6 0.8 1.0

[Anion] ~ M

Fig. 7. Effect of various anions on the percent extraction of iron(llI) by 0.IM DPPM/benzene. a - acetate, �9 - nitrate, �9 - citrate, o - oxalate, + - ascorbate

highly charged iron chloride species, whose extraction is very unlikely because more charged anions are more strongly hydrated and in addition the neutralization

of a high charge requires two or three voluminous molecules of the pyridine salt,

the accomodation of which around the complex metal halide anion may be hin-

dered by steric factors, and the presence of intra and extrasphere hydration is

perhaps the determining factor for the extraction of the complex anion.

The effect of various masking agents on the extraction of iron was investigated

from 7M HCI solutions by 0.1M DPPM/benzene. The results presented in Fig. 7

show that acetate, nitrate, citrate and oxalate ions have almost no effect on the extraction, which remains nearly constant at very low concentrations of these ions

in the original aqueous solutions. However, there is a slight decrease at relatively

high concentrations of citrate and oxalate ions. Ascorbic acid reduces extraction drastically and it is expected that this acid at a concentration of > I M can be effec- tive to hold back the metal in solution.

The selectivity of the extraction of iron(III) under optimal conditions was in-

vestigated by checking the behaviour of several other elements including the basic

metal ions. The distribution data presented in Table 1 indicate that the extraction of iron is very selective. Several other metal ions which form fairly strong anionic

J. Radioanal. Chem. 47 (1978) 45

SUHAIL AHMED et al.: EXTRACTION OF IRON(III)

Table 1 Distribution coefficients of different metal ions with respect to Fe(IIl) from 7.0M HC1

by 0.1M DPPM/benzene

Distribution Metal ions Concentration, M coefficients

u(vI) Cr(VI) Mo(VI) Mo(V) Hf(IV)

Zr(IV) As(III) La(III) Nd(III) Y(III) Pm(III) TI(II) Co(II) Sr(II) Hg(II) Cd(II) Zn(II) Cu(II) Cs(1)

C.F.: Carrier-free

10-3

C.F. 10-s lO-S 1077 lO-S 10-6

10-~ lO-S lO-S 10-7

10-7 10-7 lO-S 10-7

10 -~ 10-s 10-6

lO-S

0.4 0.001

0.035 0.046 0.002 0.05 0.0058 0.001 0.001 0.001 0.0006 0.0015

0.005 0.006 0.3 0.09 0.001 0.001

0.00008

chloride complexes are very weakly extracted and the selective separation of iron

can be utilized for the measurement of trace elements in different matrices, via

prior removal of ferric ions, in a number of modern analytical techniques.

References

t.M. IQBAL, M. EJAZ, S. A. CHAUDHRY, R. AHMED, Sepal Sci., 11 (1976) 255. 2.D. MOHAMMAD, T. MAHMOOD, MUDASSAR A. QURESHI, M. EJAZ, J. Radioanal. Chem.,

42 (1978) 45. 3. MUDDASSAR.A. QURESHI, M. FARID, A. AZIZ, M. EJAZ, Talanta (in press). 4. M. IQBAL, M. EJAZ, SHAMIM A. CHAUDHRY, ZAMIRUDDIN, J. Radioanal. Chem., 42

(1978) 00. 5. M. IQBAL, M. EJAZ, Radiochim. Acta, 22 (1975) 37. 6.M. IQBAL, M. EJAZ, Radiochim. Aeta, 23 (1976) 53. 7.M. EJAZ, Separ. Sci., 10 (1975) 425. 8.A.H. LAWERENCE, D. E. CAMPBELL, S. E. WIBERLY, H. M. CLARK, J. Phys. Chem., 60

(1956) 901.

46 J. Radioanal. Chem. 47 (1978)