Analytica Chimica Acta, 218 (1989) 341-344 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

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Short Communication

INDIRECT DETERMINATION OF VANADIUM BY ATOMIC ABSORPTION SPECTROMETRY

DEBASIS CHAKRABORTY and ARABINDA K. DAS*

Department of Chemistry, University of Burdwan, Burdwan-713104 (India)

(Received 20th July 1988)

Summary. An indirect method for the determination of vanadium as vanadate by atomic absorp- tion spectrometry is described. In neutral medium, vanadate forms a stable ion-association com- plex with copper (II) and biguanide, which is extractable into butanol with an efficiency higher than 99%. The copper content in the extract (and hence indirectly VO; ) is determined by aspir- ating it directly into an acetylene flame. The calibration graph is linear up to 3.4 pg ml-’ of VO, . The limit of detection is 16 ng ml-‘. Only chromium interferes.

Numerous methods have been reported for the determination of vanadium by atomic absorption spectrometry (AAS) [ 1,2]. With direct atomization of an aqueous solution the sensitivity is low owing to the formation of stable oxide species in the flame and to various other interferences [2]. Micromolar and nanomolar vanadium levels can be measured by graphite-furnace AAS [ 1 ] and inductively coupled plasma/atomic emission spectrometry [ 3-71, both of which involve costly instrumentation.

The indirect determination of vanadium by AAS has been reported. Two methods [8,9] involve the liquid/liquid extraction molybdovanadophosphoric acid and aspiration of the extract into a dinitrogen oxide/acetylene flame. In another method [lo] vanadium is determined by utilizing the fact that vana- dium (V ) depresses the atomic absorption signal of chromium (VI).

Complexes of biguanide and its derivatives with copper were described long ago [ 11-141. The constitution of the complexes of biguanide with copper (II) has been discussed by Ray and Saha [ 151. The ion-association complexes of biguanides with copper(I1) and various anions (F-, Cl-, Br-, I-, NO,, NO~,CO~-,SO~-,SpO~-,SCN-,SpO~-,CrO~-,H,PO; andSO:-) have been extensively studied [ 16-181.

In this communication, a rapid and sensitive indirect AAS method for the determination of vanadate is proposed, based on the extraction, from neutral solution, of an ion pair of vanadate with a copper (II) biguanide where biguan- ide is HzNC (-NH )NHC ( =NH)NH2 and abbreviated to BigH in this com- munication. The copper in the extract is determined by flame AAS.

0003-2670/89/$03.50 0 1989 Elsevier Science Publishers B.V.

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Experimental Reagents. A stock 4000 pug ml-’ copper(I1) solution was prepared by dis-

solving copper perchlorate crystals obtained by reaction of copper oxide (BDH) with concentrated perchloric acid. The solution was standardized titrimetri- tally [ 191. Working standards were prepared from this solution by serial di- lution with doubly distilled water. A stock 2000 ,ug ml-’ vanadate solution was prepared by dissolving NaVO,*H,O (Riedel de Htien) in doubly distilled water. It was standardized gravimetrically [ 191 and diluted as required. Biguanide, in a very pure state, was prepared from dicyandiamide by a standard method

[201. All other chemicals were of analytical-reagent grade. Apparatus. Absorbance measurements were made with a Shimadzu Model

646 atomic absorption spectrometer with a copper hollow-cathode lamp cur- rent of 7 mA, wavelength 324.7 nm, slit width 0.38 nm, acetylene flow-rate 2.3 1 min-‘, air flow-rate 10 1 min-’ and burner height 4 mm.

The pH values were measured with a Sambros Model 335 digital pH meter. Procedure. To a separating funnel the following solutions were added in the

order given: 1 ml of 80 ,ug ml-’ Cu(II), 0.16 ml of 0.08% (w/v) biguanide solution, 1 ml of sample solution containing not more than 34 pg of VO; (for the blank, 1 ml of doubly distilled water was used), 0.5 ml of methanol, 0.5 ml of pH 7.4 buffer and 5 ml of butanol. The funnel was shaken for 50 s and allowed to stand for 2 min to achieve equilibrium, then the aqueous phase was discarded. The organic phase was washed twice with 2 ml of doubly distilled water and the washings were discarded. The organic phase was diluted with butanol to 10 ml in a volumetric flask and the absorbance was measured by aspirating the organic phase directly into an air/acetylene flame, with the in- strument zeroed with butanol solution aspirated.

Results and discussion Effect of conditions. The rate of extraction of the vanadate complex into

butanol was investigated by shaking vigorously 5 ml of solvent with the aqueous phase for different times. Shaking for 50 s was sufficient for complete extraction.

The pH of the solution strongly affected the formation and extraction of the ion-association complex. The percentage extraction was constant and maxi- mal over the pH range 6.4-7.6. Subsequent work was carried out at pH 7.4 (Na,HPO,/citric acid buffer).

Twelve solvents were investigated as possible extractants, with the following results (percentage extraction): butanol (99.8% ), isobutyl methyl ketone (78.3% ), chloroform (74.4% ), xylene (68.5% ), benzene (58.7% ), amyl methyl ketone (45.0% ) , cyclohexane (43.1% ), isoamyl alcohol (43.1% ), butyl acetate (33.3%), ethyl acetate (33.3%), mesitylene (27.4%) and isoamyl acetate (23.5% ). The recommended solvent is butanol, because it is highly selective and gives the maximum extraction.

The polarity of the aqueous phase was varied by adding 0.5 ml of methanol, ethanol or 1,4-dioxane. Water-methanol gave the highest sensitivity for 1.6 pg

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ml-’ vanadate, the percentage extraction being methanol 99.8%, ethanol 64.1% and dioxane 71.5%. The absorbance of the blank solution (5~10~~) is not affected by the addition of methanol to the aqueous phase.

Several ion-association systems [ 22,231, e.g., Fe (phen)i+ , Fe (dipy )z’ , Co(phen)i+, Ni(phen)g+, Mn(phen)E+, Cu(phen)g+, Cd(phen)z+, Zn(phen)i+, Cu (neocuproine ) + , Cu (thiourea) +, Cu(BigH)z+ , Ni(BigH)i+, Co(BigH)i+, Cr(BigH)3+, were studied for the extraction of vanadium, and Cu (BigH )g’ gave the greatest absorbance. [ Cu (BigH)g+ ] (Cloy )z gave greater recoveries than the SO:-, NO;, Cl- or acetate salts.

Increasing the concentration of biguanide in the aqueous phase increased the extraction of the vanadate up to 0.08% biguanide; with higher biguanide concentrations the extraction slowly decreased. The overall extraction of the sample remained constant when the amount of 0.08% adduct solution added was 0.1-0.2 ml, so a volume of 0.16 ml was adopted. The absorbance of the blank solution was not affected by the addition of biguanide to the aqueous phase.

Analyticalperformance. The determination of 16 pg of VO, was not affected ( ? 2% ) by the presence of SO;- (125-fold by weight); NO;, NO, (g&fold); Cr,O:- (85-fold); Cl-, BrO; , IO;, B,O;- (75-fold); SeOi- , PO:-, AsO!-, SiOz-, WOZ-, citrate (65-fold); C,Of-, CH,COO-, Zn2+ (56-fold); F-, Ba2+, Cr3+ (in the presence of citrate), Br- (50-fold); SO;-, EDTA (40-fold); I-, A13+ Fe3+, S2- (30-fold); Hg2+, Cd2+ (25-fold); tartrate, Ni2+, MOO:-, Ag+, Mn2’ (20-fold); Co2+, SCN-, Th4+, Pb2+, Be2+ (14-fold) or Ti4+, Zr4+ (8- fold). Only Cr3+ interferes when present above a 2.5-fold excess, but its tol- erance limit can be enhanced (to 50-fold) by addition of citrate.

In the pH range 6.4-7.6, VO; and VOi- may be present. To find the com- position of the complex formed under the experimental conditions, the mole ratio method was applied. To a series of solutions with a fixed biguanide con- centration, fixed VO, concentration and increasing Cu (II) concentration, the mole ratio was determined to be 2. Hence the species extracted is presumably [Cu(BigH)g+] (VOF)~.

The extracted complex is stable for up to 2.5 h, then its absorbance decreases slowly owing to decomposition of the complex. After 24 h, the absorbance re- duces to one third of its original value.

The calibration graph is linear up to 3.4 pg ml-l of VO; ; the 3a limit of detection [24] is 16 ng ml-‘. The sensitivity is 21 ng ml-‘. The relative stan- dard deviation for ten determinations of 1.6 pg ml-’ of VO, is 1.1%.

One of the authors (D.C.) is grateful to U.G.C., New Delhi, for sponsoring a fellowship.

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REFERENCES

1 2 3 4 5 6 7 8 9

10 11 12 13 14 15 16 17 18 19

20 21 22

B. Welz, Atomic Absorption Spectrometry, VCH, Weinheim, 2nd edn., 1988. I. Kojima, T. Uchida, M. Nanbu and C. Iida, Anal. Chim. Acta, 93 (1977) 69. H. Uchida, T. Uchida and C. Iida, Anal. Chim. Acta, 116 (1980) 433. V. Rett and I. Hlavacek, Hutn. Listy, 34 (1979) 428. I. Tanak, S. Tahara, T. Ohtsuki, K. Sato and R. Matsumoto, Bunseki Kagaku, 28 (1979) 37. D.C. Bankston, S.E. Humphris and G. Thompson, Anal. Chem., 51 (1979) 1218. K.D. Hassell, D.A. Rose and J. Warren, ICP Inf. Newsl., 4 (1979) 343. R. Jakubiee and D.F. Boltz, Anal. Lett., 1 (1968) 347. H.N. Johnson, G.F. Kirkbright and T.S. West, Analyst, 97 (1972) 696. M. Toshihisa and T. Tsugio, Bunseki Kagaku, 22 (1973) 1067. R. Rathke, Chem. Ber., 12 (1879) 779. F. Emich, Monatsh. Chem., 4 (1883) 395. F. Smolka and K. Friedrich, Monatsh. Chem., 17 (1896) 648. R. Andreasch, Monatsh. Chem., 48 (1927) 145. P. Ray and H. Saha, J. Indian Chem. Sot., 14 (1937) 670. P. Ray and P.N. Bagchi, J. Indian Chem. Sot., 16 (1939) 617. P. R&y and H. Ray, J. Indian Chem. Sot., 21 (1944) 163. P. Ray and A.S. Bhaduri, J. Indian Chem. Sot., 22 (1945) 197. J. Bassett, R.C. Denney, G.H. Jeffery and J. Mendham, Vogel’s Textbook of Quantitative Inorganic Analysis, 4th edn., ELBS/Longman, New York, 1978, pp. 321,489,752. A. Ostrogovich, Bull. Sot. Stiinte Bucuresti, 19 (1910) 641. A. Ostrogovich, Chem. Zentralbl., 11 (1910) 1890. A.K. De, S.M. Khopkar and R.A. Chalmers, Solvent Extraction of Metals, Van Nostrand, London, 1970, p. 225.

23 E.G. Rochow, Inorganic Synthesis, Vol. VI, McGraw-Hill, New York, 1960, p. 65. 24 G.L. Long and J.D. Winefordner, Anal. Chem., 55 (1983) 712A.