Solvent extraction of chromium(VI) from base metal ions with diphenyl-2-pyridylmethane as a liquid anion exchanger

Journal of Radioanalytieal Chemistry, Vol. 43 (1978) 199-207

SOLVENT EXTRACTION OF CHROMIUM(VI) FROM BASE METAL IONS WITH

DIPHENYL-2-PYRIDYLMETHANE AS A LIQUID ANION EXCHANGER

M. IQBAL, M. EJAZ*

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

(Receivcd August 8, 1977)

The extraction of chromium(VI) from aqueous hydrochloric, nitric and sulfuric acid solutions by diphenyl-2-pyridylmethane(DPPM) dissolved in chloroform has been studied. Chromium(VI) is quantitatively extracted from hydrochloric acid solutions in the range 0.1-1M. With increasing acid concentration, the extraction of chromium diminishes and in concentrated acid solutions practically all the chromium remains in the aqueous phase. The quantitative back-extraction of chromium from the organic phase is possible with HCI or HNO3at concentrations higher than 5M through the use of reducing agents. The composi- tion of the extracted chromium(VI) species was studied in solution. The complexes (DPPMH)+HCrO~ and (DPPMH)~Cr~O~ are extracted for tracer and macro amounts of chromium(Vl) respectively. The data have been utilized for the separation of chromium(VI) from base metal ions.

Introduction

The alkyl-substituted pyridines are an interesting and versatile class o f ligands,

especially when considered as a homologous series in which the methylpyridines

are the simplest members. In all probabil i ty, the higher homologues 1,2 have com-

plexing abilities similar to those of the lower members of the series with the added

feature of being able to form clean phases in acidic solutions. The subject of con-

tinuing program in this laboratory is the study of extraction equilibria involving

high molecular weight pyridine amines. To date, the studies 1,2 have included the

extraction of different metal ions by 4-(5-nonyl)pyridine (NPy) and 2-hexylpyridine

(HPy) from different aqueous solutions. The present work is concerned with studies

on the extract ion of chromium(VI) from different mineral acid solutions by an-

other high molecular weight pyridine, diphenyl-2-pyridylmethane (DPPM), dissolved

in chloroform. The extract ion of chromium(VI) by NPy and HPy from different

*Author to whom correspondence should be addressed.

J. Radioanal. Chem. 43 (1978) 199

M. IQBAL, M. EJAZ: SOLVENT EXTRACrlON OF CHROMIUM(VI)

acid solutions has been reported. 2-s Data on the extraction of chromium(VI) with DPPM are interesting for comparison of the results with those obtained for long chain alkyl-substituted pyridines, i.e. NPy and HPy, and can be applied for the separation of chromium from iron(Ill) and other metal ions which generally interfere with the determination of this metal by modern analytical techniques. 6-8

Experimental

Reagents, radionuclides and instrumentation

DPPM was obtained from Aldrich Chemical Co. Inorganic chemicals were of reagent grade. High specific activity s 1Cr was obtained by the Szilard-Chalmers effect. 9 The other isotopes used in this study were prepared as previously reported. 1 The equipment used for the radiochemical assay has been described. 1'1~

Determination o f distribution coefficients

The distribution ratio of chromium(V1), D, ratio of the chromium metal concentration in the organic and aqueous phases, was determined radiometrically, tracing with S~Cr solutions of known concentrations. In each D determination, equal volumes of the organic and aqueous phases [the latter containing initially the chromium(VI)] were shaken for 5 min at 23 -+ 2 ~ After equilibration the phases were separated by centrifugation and equal portions were radiometrically assayed as previously reported. 1 The distribution coefficients of other "),-emitter nuclides were determined similar/y,"while the procedures for ix-~and 3-emitters are described elsewhere.1 o �9

Extraction procedure

In a 15 rrd separating funnel was placed 2 ml of a 0.05M chromium(V/) solution (spiked with s 1 Cr) containing a known concentration of the test components (6~ 64Cu, s4'S6Mn, 14~176 indicator amounts, Fe(III), I0 mg/ml). The

solution was shaken once with 2 ml of pyridine solution in chloroform at a ratio of Vo : V a q = 1 : 1. The organic extract was scrubbed twice with equal volumes of 1M HC1. The purity of the separated chromium(VI) was checked by gamma-spectroscopy.

200 J. Radioanal. Chem. 43 (1978}

M. IQBAL, M. EJAZ: SOLVENT EXTRACTION OF CHROMIUM(VI)

Results and discussion

The dependence of the extraction process of trace (<10-6M) and macroamounts

(0.05M) of chromium(VI) on mineral acid concentration is shown in Fig. 1. A 0.1M

solution of DPPM in chloroform was used as extractant. As is seen from Fig. 1, the

increase of the hydrogen ion concentration improves the extraction, which reaches a

maximum value for 0 .5 -2M acids and then decreases. The increase in extract ion with

increasing hydrogen ion concentration is due to the increase of the ratio of the

extractable species (oxymetal acids) in the aqueous phase and also because the ratio of

the combined acid species in the organic phase increases by forming mixed acid

_m

Fig. 1. Variation of the partition coefficient of chromium(VI) with mineral acid concentration in the equilibrium aqueous phase. Curves for trace Cr(VI): (1) - HC1, (2) - HNO3, (3) - H2SO,; curves for 0.05M Cr(VI): (4) - HC1, (5) - HNO3, ( 6 ) - H~SO4

J. Radioana l. Chem. 43 (1978) 201

M. IQBAL, M. EJAZ: SOLVENT EXTRACTION OF CHROMIUM(V])

-2 - ] tg [ a n i o n ] , M

9 Q em

]

Fig. 2. Dependence of the partition coefficient of chromium(Vl) on the concentration of neutral chloride, nitrate and sulfate ions from IM HCI. Curves: (1) - chloride, (2) - nitrate, (3) - sulfate

complexes since the solute is a hydrogen bond acceptor. However, water, which

is also a strong electron donor and acceptor, would be replaced by the solute by

forming a symmetrical and stable H30 § with a proton dissociated from a strong

acid. On the other hand, it also has the opposite effect because it increases the

extraction of mineral acids with DPPM and then decreases the free DPPM con-

centration. The distribution ratio of oxychromium acids results from the com-

bined effect of these two factors in addition to solute-solvent and solute-water

interactions. The extraction in these three systems decreases in the order:

chloride > nitrate > sulfate

Fig. 1 indicates that the net extraction in sulfate and nitrate systems is not effec-

tive; this may be due to the very large hydrophilic tendency of the hydrogen sul-

fate and nitrate ions. A comparison of the extraction data obtained in these three

systems with those of NPy 3-s reveals that this reagent extracts chromium(VI) less

efficiently than NPy, and this difference can be attributed to an inductive effect,

i.e. the strong electron withdrawing nature of the phenyl groups.

The effect of the addition of sodium chloride, nitrate and sulfate on the ex-

traction of chromium(VI) was investigated from IM hydrochloric acid using a

0.1M solution of the reagent. The results presented in Fig. 2 show that the addi-

202 .I. Radioanal. Chem. 43 (1978)


~g [DPPM], M o

t

0

Fig. 3. Variation of the partition coefficient of chromium(VI) with concentration of DPPM in chloroform from 1M HC1

tion of sodium chloride does not exert any depressing effect but rather increases

the extraction efficiency. Thus extraction is expected to improve in the presence of unextractable metal chlorides. The increase in extraction could be either due to the absence of acid competition or due to a salting-out effect. The formation and extraction of some chloride species also could be accounted for since KATS

et al. 11 and TUCK et alJ 2 have considered the formation and extraction of

chloride complexes in order to explain the high extractability of chromium(VI) fiom hydrochloric acid by certain extractants. Sulfate ions have little effect on the extraction, while nitrate ions decrease the extraction similarly to the NPy and HPy systems; 2,4 this may be due to the competition of nitrate and oxymetal ions

for association with the base cations.

Fig. 3 gives the distribution ratio of tracer amounts of chromium(VI) as a function of free DPPM concentration in the organic phase when the aqueous phase was 1M HC1. The slope below 0.1M concentration is approximately unity, which indicates the extraction of (DPPMH)§ A non-linear region occurs above the

concentration of 0.1M. The non-linear dependence of lg D on CDppM and the lack

of a slope of unity are experimental evidence for the coexistence of several species in equilibrium in the organic phase, at least one of them being aggregated. It has been noted previously 2'4 that the chloride salts of NPy and HI~ are strongly self-

associated through dipole-dipole interaction of ion-pairs.

The variation in the concentration of chromium(VI) in the organic phase as a

function of initial aqueous chromium concentration gave the results shown in

J. Radioanal. Chem. 43 (1978) 203


A

Fig. 4. Extraction of chromium(VI) by 0.1M DPPM in chloroform as a function of the metal concentration from 1M (e) and 4M (o) HC1

Fig. 4. In 1M acid system, the chromium(VI) to pyridine mole ratio approaches the limiting value of unity, implying that the chromium(VI) complex extracted into the organic phase contains chromium(VI) in the mole ratio of 1 : 1. The

fact that one molecule of the compound is associated with each chromium(VI) atom is also supported by the dependence of the distribution coefficient on the solvent concentration. The partition coefficient decreases when the concentration

of chromium(VI) in the aqueous phase increases above 25 mg/ml. Similar results were obtained in the case of NPy 4 and HPy, 2 and have been explained as due to the decrease in the concentration of H* ions, because of the condensation poly- merization of large amounts of chromium(VI) in the aqueous phase, so that the

stability of the amine cation is considerably decreased. However, in 4M HCt no

decrease in the uptake of chromium(VI) by the reagent was observed even when

the chromium(VI) concentration in the aqueous phase exceeds 100 mg/ml. The maximum uptake of chromium(VI) at 4M was about 7.5 mg/ml, indicating the extraction of a mixture of compounds possibly as (DPPMH)2Cr207 and (DPPMH)+HCr2 027-.

Fig. 5 shows the isotherms of chromium distribution as a function of citrate, oxalate, ascorbate, thiosulfate and thiocyanate concentrations in the aqueous phase from 1M HC1 by 0.1M DPPM/chloroform. There is a regular decrease in the extraction of chromium with increasing concentration of these ions in the aqueous

phase, except for the acetate system, where the distribution coefficient remains

fairly constant up to 0.25M concentration and then decreases. These results are very similar to those for the hexylpyridine system. 2 The data indicate that a 1M

204 J. Radioanal. Chem. 43 {1978)


Fig. 5. Extraction of chromium(VI) (0.05M) with 0.1M DPPM/chloroform in the presence of various anions. Curves: (1) - acetate, (2) - citrate, (3) - oxalate, (4) - ascorbate, (5) - thiosulfate, (6) - thiocyanate

concentration of these ions can be successfully used for the back-extraction of

chromium. A dilute solution of perchloric acid, was also found useful for back-

extraction o f the metal, presumably due to the low aqueous hydration o f the

large perchlorate ions.

The selectivity of the extraction separation o f chromium(VI) with 0.1M DPPM in chloroform was studied from 1M HC1. The partition behaviour of a number Of

metal ions including the base metals was studied by the method described. The

data presented in Table 1 show that chromium(VI) can be separated from a num-

ber of elements including Fe(III), Cr(III), La(III), Co(II), Ni(II), Mn(II) and Ba(II).

In almost all the commonly used solvating reagents 13 and liquid anion exchangers,

iron(III) is coextracted at this acidity. Although 2-hexylpyridine 2 also extracts

chromium(VI) at this acidity very selectively, the present reagent (DPPM) extracts

chromium(VI) more efficiently in hydrochloric acid media, in the presence o f

J. Radioanal. Chem. 43 (1978J 205.


Table 1 Separation factors of different metal ions relative to chromium(VI)

in 0.1M DPPM/chloroform - IN HC1 extraction system

Metal

Nb(V) Zr(IV) HffIV) Th(IV) Fe(llI) Cr(IIl) Y(IlI) ka(IlI) Cc(Ill) Pm(llI) Tb(IlI) Ba(II) Sr(II) Ca(If) Mn(II) Ni(II) Co(II) Mg(II) Cu(II) Cs Na

Concentration, mole/1

C . F . *

10-7 i0-~ C.F. 10-~ C.F. C.F. C.F. lO-S lO-S lO-S lO-S 10-9 10-~ lO-S 10 -7 10-5 lO-S

10-s lO-S 10-6

Separation factors > DCr(VI)/D M

1M HC1

106 106 106 106 l0 s 105 106 106 106 106 106 106 106 106 106 106 106 105 10 s 106 106

*C.F. = carrier-free

neutral chloride ions and does not have any odor. Furthermore, in handling high

levels of radiation, the DPPM/chtoroform system is particularly useful for remote

control operations since the organic phase is the lower phase and extractions of

an aqueous solution may be repeated using the same separatory funnel.

The data were checked by separation of S XCr from a synthetic mixture of base

metal ions (indicator amounts) and mg amounts of Fe(III) (10 mg/ml) in a single

extraction, followed by two scrub stages. The separated fraction, as shown by the

7-spectra (Fig. 6) and decay studies, was 99.9% pure.

206 J. RadioanaL Chem. 43 (1978)


S~Cr

4OBa 6~ 56Mn 64Cu

56Mn 51Cr ~ A

I0 ~ ~

c 105

m g (J

10 2

10 0.4 0.8 1.2 1.6 2.0 Energg~ MeV

Fig. 6. Separation of ehromiumfVI) from base metals. Curve A - -r-spectrum of the mixture; curve B - -t-spectrum of the separated S lCrfVI) fraction

References

1. M. IQBAL, M. EJAZ, S. A. CHAUDHRI, R. AHMED, Separ. Sci., 11 (1976)255, and references cited therin.

2. M. IQBAL, M. EJAZ, J. Radioanal. Chem., (communicated). 3. M. IQBAL, M. EJAZ, Anal. Chim. Acta, 74 (1975) 125. 4. M. IQBAL, M. EJAZ, Talanta, 22 (1975) 143. 5. M~ IQBAL, M. EJAZ, Anal. Chem., 47 (1975) 936. 6. A. G. FOGG, S. SOBYMANLOO, D. BURNS, Talanta, 22 (1975) 541. 7. B. SUBRAHMANYAM, M. C. ESHWAR, Bull Chem. Soc. Japan, 49 (1976) 347. 8. N. P. KRIVENKOVA, L I. PAVLENKO, B. YA. SPIVAKOV, I. A. POPOVA, T. S. PLOTNI-

KOVA, V. M. SHKINEV, L P. KARLAMOV, YU. A. ZOLOTOV, Zh. Anal. Khim., 31 (1976) 514.

9. M. IQBAL, M. EJAZ, Radiochim. Acta, 22 (1975) 49. 10. M. EJAZ, Separ. Sci., 10 (1975) 425. 11. S. A. KATS, W. M. MCNABB, J. F. HEZEL, Anal. Chim. Acta, 27 (1962) 405. 12. D. G. TUCK, R. M. WALTERS, J. Chem. Soc., (1963) 1111. 13. Y. MARCUS, A. S. K. KERTES, Ion Exchange and Solvent Extraction of Metal Complexes,

Wiley, New York, 1969, p. 950, 951,954, 955.

J. RadioanaL Chem. 43 (1978) 207

Documents

Solvent extraction of chromium(VI) from base metal ions with diphenyl-2-pyridylmethane as a liquid anion exchanger