ANALYST, DECEMBER 1990, VOL. 115 1539

Solvent Extraction of Cadmium, Cobalt, Copper, Iron, Manganese, Nickel and Zinc into lsobutyl Methyl Ketone With 2-(2-Benzoxazolyl)malonaldehyde and 2-(2-Benzoxazoly1)cyanoacetaldehyde. Extraction - Atomic Absorption Spectrometric Determination of Trace Amounts of the Metals in Molybdates

kina Karadjova, Sonja Arpadjan," Petko Mandjukov and Todor Deligeorgiev Faculty of Chemistry, University of Sofia, Sofia 1126, Bulgaria

The distribution of 2-(2-benzoxazolyl)malonaldehyde and 2-(2-benzoxazolyl)cyanoacetaldehyde (2-BOCA) and their complexes with Cd, Co, Cu, Fe, Mn, Ni and Zn in the water - isobutyl methyl ketone (IBMK) system was investigated. The disssociation and distribution constants of the ligands studied and also the composition of the extracted species and their conditional extraction constants were evaluated. The extraction system 2-BOCA - IBMK was applied to the determination of Cd, Co, Cu, Fe, Mn, Ni and Zn in high-purity ammonium and sodium molybdate.

Keywords: Ligand; equilibrium constant; composition; trace metals in molybdates

The extraction systems 2-(2-benzoxazolyl)malonaldehyde (2- BOMA) - isobutyl methyl ketone (IBMK) and 2-(2-benzoxa- zoly1)cyanoacetaldehyde (2-BOCA) - IBMK have been re- ported for the separation and pre-concentration of trace elements from various salts, e .g . , lead and bismuth salts.1.2 The advantage of these extraction systems i s their selectivity towards certain metal ions. The organic reagents investigated, 2-BOMA and 2-BOCA, form complexes with Ag1, Cd", Co", Cull, Fell, Fel11, Mn", Nil1 and Zn1l that can be extracted into IBMK. Metal ions with a higher charge such as Au"', As111, Bi"1, MoVl, Sbf11 and WVI do not form extractable complexes. Lead can be extracted into IBMK over a very narrow pH range (pH 7-8).

0 ;;c-c;CHO Y H O H pJ:;c4<:;o \

2-BOMA 2-BOCA

As part of a wider study to assess the utility of 2-BOMA and 2-BOCA as reagents for cxtraction - atomic absorption spec- trometry (AAS), this paper reports the study of the distribu- tion equilibria of the complexes of Cd, Co, Cu, Fe, Ni, Mn and Zn with 2-BOMA and 2-BOCA. Conventional methods were used to calculate the dissociation and distribution constants of the ligands investigated and to estimate the composition of the extracted species and their conditional extraction constants. The application of the 2-BOCA-IBMK system to the de- termination of Cd, Co, Cu, Fe, Mn, Ni and Zn in high-purity molybdates is also described.

Experimental Apparatus

A Carl Zeiss (Jena) Specord 10 spectrophotometer with 10-mm quartz cells, a Perkin-Elmer 1 l00B atomic absorption spectrometer and a Radelkis OP-208 p H meter equipped with glass - calomel electrodes were used.

Reagents

The organic ligands studied, 2-BOMA and 2-BOCA, were synthesised in the Laboratory of Organic Chemistry, Univer-

sity of Sofia. The reagents were purified by recrystallisation from dioxane.3 Stock standard solutions of metals were prepared from Titrisol standards for AAS (Merck). Isobutyl methyl ketone and xylene were analytical-reagent grade solvents for extraction (Merck) and were used as received. All other reagents were of analytical-reagent grade.

Procedure

The pH of the aqueous phase was adjusted by means of acetate (pH 4-7) and borate (pH 8-11) buffer solutions. Sodium perchlorate was added to give a constant ionic strength of 0.1 moll-'. A 0.01 moll-' solution of 2-BOMA or 2-BOCA in IBMK or xylene was used as the organic phase. The volume ratio of the aqueous to organic phase was 1 : 1 (10 ml : 10 ml). The concentration of the ligands in the two phases (after attaining equilibrium) was measured spectro- photometrically, at 590 nm for 2-BOMA and at 580 nm for 2-BOCA, using thcir complexes with Fe"1. The concentration of the elements in the two phases was measured by flame AAS

1 2 3 4 5 6 7 8

PH Fig. 1. Influence of pH on the distribution of A, 2-BOMA; and B, 2-BOCA in the water - IBMK system. [2-BOMA] = [2-BOCA] = 1 X 10-3 moll-1

Table 1. Distribution (KDR) and dissociation (KRH) constants of 2-BOMA and 2-BOCA

Ligand KDR KRH 2-BOMA . . . . . . 61.77 (2.05 k 0.05) x 10-4 2-BOCA . . . . . . 18.66 (4.94 k 0.07) x * To whom corrcspondcncc should be addressed.

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1540 ANALYST, DECEMBER 1990, VOL. 115

Table 2. Conditional extraction constants ( K e x ) and composition [ligand (L) : metal (M)] of the extracted complexes of the metals with 2-BOMA (LIM) and 2-BOCA ( L 2 4 in IBMK according to Job's method.(I) and the equilibrium shift (straight-line regression) method (11)

Methodlligand Log K e x Extracted

Cd Co Cu Fe" Fell1 Mn Ni . . Zn

Metal PH . . . . . . 6 . . * . . . 5 . . . . . . 5 . . . . . . 4 . . . . . . 4 . . . . . . 7 . . . . . . 5 . . . . . . 6

I I L l M

2 2 2 2 3 2 2 2

IIL2C 2 2 2 2 3 2 2 2

IIILIM 2.12 2.11 1.92 1.95 2.86 2.17 2.11 1.85

II/L2c 2.15 2.07 2.05 1.89 3.06 1.95 I .92 1.82

LIM

-1.8 -0.3

1.9 1.8 3.1

-6.5 -0.1 - 1.3

L,C complex -1.5 ML2(1BMK), -0.5 ML,(IBMK),

2.3 ML, 2.1 Mb(IBMK), 3.0 ML3

-5.5 ML2(IBMK), 0.2 ML2(IBMK),T

-1.2 ML2

Table 3. Effect of the solvent on the percentage extraction of metal complexes with 2-BOMA

Extraction, 76

Solvent Cd co c u Fe Mn Ni Zn IBMK . . . . . . . . . . >Y9 >99 >99 >99 >99 >99 >99 Xylene . . . . . . . . . . (1 <1 >99 >99 <1 <1 95 k 2 Xylcne - IBMK (2 + 1) . . . . 95 k 2 >99 >99 >99 96 f 3 >99 >99 Xylcne - acetone (2 + 1 ) . . . . 87 k 2 92 k 3 >99 >99 85 & 2 93 * 2 >99 Xylene - cthanol(2 + 1) . . . . 70 k 2 47 k 3 >99 >99 22 & 5 45 If: 3 >99

m

A 2

1

0

-1

-2

1 2 3 4 5 6 1 2 3 4 5 6 PH

Fig. 2. Influence of pH on the distribution ratio for the metal complexes at various ligand concentrations. ( a ) Fel** - 2-BOMA; (b) Fell1 - 2-BOCA; ( c ) Cu" - 2-BOMA; and (d ) Cull - 2-BOCA. 0, 1 x 10-2; A , 5 x 10-3; and 0, 1 x 10-3 mol 1-1

against appropriate standard solutions. The concentration of metal ions in the aqueous phase was in the range 0.3CL1.50 pg ml-1, depending on the sensitivity of the flame AAS measurements. The pH of the aqueous phase was measured after the extraction.

Results and Discussion Distribution of 2-BOMA and 2-BOCA

The distribution of 2-BOMA and 2-BOCA between IBMK and 0.1 mol 1-1 sodium perchlorate solution, over the pH range 1-10, is illustrated in Fig. 1. Assuming no polymeric species are formed, the ligand distribution ratio, DR, is given by

[RHIO - KDR - [RH] + [R-] - 1 + KRH/[H+]

D -

where K13R is the distribution constant of 2-BOMA or 2-BOCA, KRH is the acid dissociation constant and RH is undissociated 2-BOMA or 2-BOCA. Species in the organic phase are denoted by the subscript "0".

2

1

a o 0,

-I

-1

-2

I I 1 1 1

-7 -6 -5 -4 -3 Log CR

Fig. 3. Equilibrium shift study: logarithm of the distribution coefficient versus logarithm of the ligand (2-BOCA) concentration. 1, 1 x 10-4 moll-1 Cu2+; 2 , l x 10-3 moll-1 Co2+; 3 , 1 x 10-3 mol 1-1 Fe3+; and 4, 1 X 10-4 moll-' Mn2+

Table 4. Percentage extraction of the metals from 10 g 1-l sodium or ammonium molybdate solution using the extraction systems 2-BOCA - IBMK and 2-BOMA - IBMK

Extraction, %

Cd Co Cu FelI FelI1 Mn Zn Ni

Metal 2-BOCA - IBMK . . . . . . . . >99 . . . . . . . . 95 f 2 . . . . . . . . >99 . . . . . . . . 94 k 3 * . . . * . . . <I . . . . . . . . 95 f 2 . . . . . . . . >99 . . . . . . . . 94 k 3

2-BOMA - IBMK 80 * 2 90 k 3 >99

92 k 4 < 1

78 k 4 80 k 2 89 k 2

A computer program was used to calculate K D R and K H R according to equation (1).3 Results are presented in Table 1.

Distribution of the Metal Complexes With 2-BOMA and

A pH range of 4-8 was used for the quantitative extraction of Cd, Co, Cu, Fe, Ni, Mn and Zn into IBMK as complexes with

2-BOCA

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ANALYST, DECEMBER 1990, VOL. 115 1541

Table 5 . Accuracy and precision of the procedure for the extraction - AAS determination of Cd, Co, Cu, Fe, Mn. Ni and Zn in ammonium molybdate (five parallel determinations j

Analyte

Parameter Cd c o c u Fe Mn Ni Zn Addedyg ml-I . . . . . . 2.00 4.00 2.00 4.00 2.00 4.00 1 .oo Found/pgml-l . . . . . . 1.95k0.15 3.9220.12 2 .1k0 .1 3 .9+0.1 2 .05f0 .05 3 .9k0 .1 1.12k0.07 Recovery, YO . . . . . . . . 97.5 98 105 97.5 102.5 97.5 112 RSD, ”% . . . . . . . . 7.7 3.1 4.8 2.6 2.4 2.6 6.2

* RSD = relative standard deviation.

2-BOMA or 2-BOCA. Quantitative extraction can be achieved after shaking for 2 min. Variation of the concentra- tion of the analytes at pH 4.6 and at a constant reagent concentration of 0.01 mol I-’ 2-BOMA or 2-BOCA in IBMK had no effect on the distribution ratio, which suggests that only mononuclear species were extracted. In this instance the metal distribution ratio, D , is given by

where M represents the metal, H R the extracting agent, X another ligand present in the aqueous phase and the subscripts “0” and “w” denote the organic and aqueous phase, respect- ively; charges have been omitted for simplicity.

Assuming that only the species MR,(HR), is extracted and that the metal is present in the aqueous phase predominantly as the cation Mn+, the following equation can be obtained:

log D = log K,, + npH + (m + n)log[HR], . . (3) where K,, is the extraction constant of MR,(HR),.

Fig. 2 shows values of log D for CdII, Cu” and FeIII, obtained for three different ligand concentrations, plotted against pH. The same results were obtained for all the metal ions investigated. As all the curves tend to the same maximum value of log D , it is unlikely that any adduct species are extracted, hence rn = 0.

Composition of the Extracted Species

The composition of the extracted complexes was studied by the equilibrium shift method based on equation (3 )5 ,6 and by Job’s method.5.7 Fig. 3 shows plots of the logarithm of the distribution coefficient (log 0) versus the logarithm of the ligand concentration( log cR) for CuII and CO” at pH 5 , for FeIII at pH 4 and for Mn” at pH 7. The parameters of the straight-line regression equations, calculated by the least- squares method, together with the results given by Job’s method and also the calculated conditional extraction con- stants are given in Table 2. These results predict a ratio of the metal ion to the ligand (2-BOMA or 2-BOCA) of 1 : 2 for a metal in the +2 oxidation state, with the liberation of two protons during complexation. Only FeIII forms a 1 : 3 complex, liberating three protons.

Considering the extraction of unsaturated chelate com- plexes, the nature of the organic solvent should strongly influence the degree of extraction. Replacing IBMK with a less polar and more inert solvent such as xylene or a mixture of xylene with acetone or ethanol substantially decreases the percentage extraction for Cd”, Co”, Mn” and NiII (Table 3).

These elements form neutral, unsaturated coordinate com- plexes. The solvation number for IBMK obtained using the equilibrium shift methods was found to be between 1 and 2.

Analysis of Molybdates

The percentage extraction of Cd, Co, Cu, Fe, Mn, Ni and Zn with 2-BOCA or 2-BOMA in IBMK in the presence of 10 g 1 - 1

of Na2Mo04 or (NH4)6M07024 in the aqueous phase is illustrated in Table 4. The recovery of Cd, Mn and Zn is lower with 2-BOMA in comparison with 2-BOCA. The extraction system 2-BOCA - IBMK permits the extraction separation, pre-concentration and flame AAS determination of trace amounts of these metals in Na2Mo04 or (NH4)6M07024. There is no difference in the extraction behaviour of the metals investigated for either the sodium or the ammonium salt, i.e., the cationic part of the molybdate saIt does not influence the extraction.

A typical analysis is carried out as follows. The sample (4 g) is transferred into a tall test-tube, and 40 ml of re-distilled water and 50 mg of solid ascorbic acid are added. Then, 5.0 ml of a 0.01 moll-’ 2-BOCA solution in IBMK are added and the extraction is performed for 5 min. Cadmium, Co, Cu, Fe, Mn, Ni and Zn are determined in the organic phase by flame AAS. A blank and standard additions samples are taken through the whole pre-concen tration procedure. The accuracy and pre- cision of the procedure are demonstrated in Table 5 . The method permits the determination of 5 X 10-6% Cd, Cu, Mn and Zn, and 1 x 10-s% Co, Fe and Ni.

References 1.

2.

3.

4. 5. 6.

7.

Karadjova, I . , Arpadjan, S. , and Deligeorgiev, T., Fresenius 2. Anal. Chem., 1989,333, 9. karadjova, I . , Arpadjan, S., and Deligeorgiev. T., Anal. Chim. Acta, in the press. Chandramohan, M., and Seshadri, S. , Indian J. Chem., 1972. 10, 573. Liem, D. H., Acta Chem. Scand., 1971, 25, 1521. Stari, I . , “Ekstraktsia Chelatov,” Mir, Moscow, 1966. Hartley, F. R., Burgess, C., and Aleoek, R. M., “Solution Equilibria,” Ellis Honvood, Chichester, 1980, p. 133. Zolotov. Yu. A., “Ekstraktsia Ynutrikompleksnikh Socdine- nii,” Nauka, Moscow, 1968, p. 313 (English translation: “Extraction of Chelate Compounds,” Ann Arbor-Humphrey Science Publishers. London, 1970). Anil, K. D., Khopkar, S. M., and Chalmers, R. A., “Solvent Extraction of Metals.” Wiley, London, 1970, p. 14.

8.

Paper 01026556 Received June 13th, 1990

Accepted August 16th, 1990

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