Solvent Extraction and Spectrophotometric Determination of Vanadium(V) as a Mixed Ligand Complex with N-Phenylcinnamohydroxamic Acid and Thiocyanate.
MikrochimicaActa [Wien]1983I, 255--262 9 by Springer-Verlag1983
Department of Chemistry, Addis Ababa University Addis Ababa, Ethiopia
Solvent Extraction and Spectrophotometric Determination of Vanadium(V) as a Mixed Ligand Complex with N-Phenylcinnamohydroxamic Acid and Thiocyanate By Mulugeta Assefa and B. S. Chandravanshi
(Received February 24, 1982) N-Arylhydroxamicacids have been used as versatile reagents for the determination of metal ions1-7. Further, these reagents have gained wide popularity due to their very sensitive colour reactions with vanadium(V)4-L Recently, the mixed ligand complexes of vanadium(V) with thiocyanate and few organic reagents have been reported in the literatures, 9. Hence, it was thought worth while to study the mixed ligand complexes of vanadium(V) with thiocyanate and hydroxamic acid. With this view, N-phenylcinnamohydroxamic acid, PCHA, has been employed for the extraction and photometric determination of vanadium(V) in thiocyanate medium. The present investigationdeals with the solvent extraction and spectrophotometric determination of vanadium(V) as mixed ligand complex with PCHA and thiocyanate. PCHA reacts with vanadium(V) to give a pink coloured 1 : 2 (metal : ligand) complex. The vanadium (V)-PCHA complex reacts with thiocyanate in acetic acid medium to form a 1 : 2 : 2 (V : PCHA : SCN) mixed ligand complex accompanied by a hyper and bathochromic shift. This reactoin is capable of forming the basis for the development of a sensitive and selective method for the extraction and photometric determination of vanadium(V). 17
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M. Assefa and B.S. Chandravanshi:
Experimental Apparatus
and Reagents
A Beckman | model 24 UV-Vis Spectrophotometer equipped with 1-cm quartz cells was used for all absorbance measurements. A stock solution of vanadium(V) was prepared by dissolving a known amount of analytical grade ammonium metavanadate in double distilled water and standardized volumetrically ~~ The solutions of foreign ions were prepared according to the procedure of WesttL Standard solution of thiocyanate (0.5 M) was prepared by dissolving analytical grade potassium thiocyanate in distilled water. N-Phenylcinnamohydroxamic acid was prepared by reported method1% A 0.1% (w/v) solution of the reagent in ethanolfree chloroform was used for all extraction work. All other chemicals used were of analytical grade.
Procedure An aliquot of the solution containing 15--150 #g of vanadium(V) was transferred to a 100-ml separatory funnel. Sufficient quantities of distilled water and glacial acetic acid were added to maintain the acidity between 3 and 7 M and volume of the aqueous phase to about 10 ml. T e n ml of a 0.1% P C H A solution in chloroform was added and the mixture was shaken vigorously for a minute. T h e n 5 ml of 0.SM thiocyanate solution, required quantities of distilled water and glacial acetic acid were added to readjust the acidity between 3 and 7M and volume of the aqueous phase to about 25 ml. T h e mixture was shaken vigorously for two minutes and allowed to separate the two phases. T h e chloroform phase was collected in a 25-ml volumetric flask after drying over anhydrous sodium sulphate. T h e aqueous phase was shaken again with 2--3 ml portions of P C H A solution and then with 2--3 ml of chloroform to ensure the complete extraction and to remove any residual colour. T h e washings were combined with the main extract and the volume was made up to the mark with chloroform. T h e absorbance of the green coloured chloroform extract was measured at 590 n m against chloroform as blank. Results and Discussion
Absorption
Spectra
T h e absorption spectra of PCHA, vanadium(V)-PCHA complex and vanadium(V)-PCHA-SCN complex in chloroform have been determined. T h e reagent shows strong absorption m a x i m u m around
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290 n m and has negligible absorption in the visible region. The pink coloured vanadium(V)-PCHA complex has m a x i m u m absorption at 510nm with a molar absorptivity of 43001.mole -1 c m - k W h e n the vanadinm(V)-PCHA complex is extracted from thiocyanate medium, the colour of the complex changed to green and the wavelength of m a x i m u m absorption shifts to 590 n m with a consequent increase in molar absorptivity. Thus the formation of a mixed ligand complex is accompanied with hyper and batho-chromic shifts.
Effect of Acidity The optimum acidity range for the complete extraction and m a x i m u m colour intensity was found to be 3.0 to 7.0M with respect to acetic acid. Acetic acid was found to be the most suitable medium for the extraction of vanadium(V)-PCHA-SCN complex. Hydrochloric and sulphuric acids are unsuitable for adjusting the acidity of the aqueous phase because the time required for the complete extraction varies from 5 to 10 minutes and is too long for the unstable complex in these media. Nitric acid is also unsuitable because it oxidizes the reagent, whereas both the reagent and the complex extracted from acetic acid medium are stable and the extraction is readily accomplished within two minutes. Extraction of the vanadium complex from acetic acid medium is a distinct advantage over the hydrochloric acid medium which is used for the extraction of vanadium(V) with many reagents and in which the extraction is not quantitative due to the partial reduction of vanadium(V) to vanadium(IV) 13,14
Effect o~ Amount o~ Reagents A 4- and 300-fold molar excess of PCHA and thiocyanate respectively were found to be adequate for the complete extraction of the complex. A further excess of PCHA and thiocyanate has no effect on the extraction and determination of vanadium(V). H o w ever, the order of addition of reagents is found to be critical. There is no colour reaction if thiocyanate is added before the extraction of vanadium(V)-PCHA complex in chloroform. Hence, thiocyanate is added after the extraction of the pink coloured vanadium(V)P C H A complex in chloroform.
Effect of Other Variables A shaking time of two minutes is sufficient for the complete reaction between vanadium(V)-PCHA complex and thiocyanate to 17"
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M. Assefa and B. S. Chandravanshi:
form the mixed ligand complex. The chloroform extracts are stable at least for two days. The absorbance of the chloroform extracts remains unaltered by variation in temperature of the aqueous phase from 200 to 350 C. The absorption of the coloured system and the extraction of vanadium(V) complex are not affected by the change in ionic strength of the aqueous phase from 3.0 to 8.0 moles per litre with potassium chloride, sodium chloride, a m m o n i u m chloride and a m m o n i u m nitrate. The volume of the aqueous phase can vary from 10 to 50 ml without affecting the mixed ligand complex formation and its subsequent extraction. Chloroform was found to be the most suitable solvent amongst the common non-polar organic solvents for the extraction of the ternary complex. Chloroform was also preferred because the reagent is more soluble in it than in the other solvents and with it the quantitative extraction of the complex is readily accomplished.
Beer's Law, Optimum Concentration, Molar Absorptivity and Sensitivity The coloured system obeys Beer's law from 0.8 to 6.4 ppm of vanadium. T h e optimum concentration on the basis of Ringbom's plot la is 1.5 to 5.0 ppm of vanadium. The molar absorptivity of the ternary complex in chloroform is 7500_+50 1.mole -1 cm -~ with a photometric sensitivity 16 of 0.0068 #g of vanadium per cm ~.
Precision and Accuracy The relative standard deviation for ten determinations of vanadium (100 #g) was 0.55%.
Effect of Foreign Ions In order to assess possible analytical applications of the reaction, the effect of diverse ions on the extraction and determination of vanadium(V) was studied by adding a known amount of the ion in question to a solution containing 100 #g of vanadium and proceeding as in recommended procedure. AI(III), As(V), Ba(II), Be(II), Bi(III), Ca(II), Cd(II), Ce(IV), Co(II), Cr(III), Cu(II), Ga(III), Hg(II), In(III), La(III), Mg(II), Mn(II), Ni(II), Pb(II), Sn(IV), Sb(V), Sr(II), Th(IV), TI(I), U(VI), Zn(II), ammonium, acetate, borate, citrate, oxalate, perchlorate, phosphate, sulphate, and tartrate do not interfere with the determination of vanadium even when the weight ratio of each of these ions to vanadium is
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Determination
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200 : 1. Ti(IV), Zr(IV) and Mo(VI) also do not interfere when the weight ratio of each of these ions to vanadium is 50 : 1. Fe(III) and W(VI) interfere with the determination of vanadium. Interference due to Fe(III) is a serious drawback of the method. However, it may be eliminated by the use of a suitable masking agent.
Composition
of the Complex
T h e composition of the complex was determined by different methods. T h e continuous variations 17 and mole ratio is methods were employed to determine the ratio of PCHA to vanadium and curve fitting method 19 was employed to determine the ratio of thiocyanate to vanadium. T h e results obtained indicate the formation of 1 : 2 : 2 (V : PCHA : SCN) ternary complex. T h e probable parition equilibrium of the vanadium(V)-PCHA complex with and without the thiocyanate between water and chloroform may be expressed as follow 9,2~ 21: Vanadium(V) reacts with PCHA to form 1 : 2 ( V : P C H A ) complex extractable into chloroform VO2 %q + 2H Acorg ~ - VO (Ac) 2OHorg + H +~q and the equilibrium constant can be expressed as ill, 2 = [VO (Ac)2OH]org [H+]aq [VO2+]aq [H AcJ2org
T h e vanadium(V)-PCHA complex reacts with thiocyanate to give 1 : 2 : 2 (V : PCHA : SCN) ternary complex VO(Ac) 2OHo~g + 2SCN -~q + H %q~- VO(Ac) (SCN) 2" HAco~g + OH-~q, which is defined by the equilibrium constant ill, 2, 2 --
[VO (Aa) (SCN) 2" H ac]org [OH-]aq [VO(Ac)2OH]org [SCN-]2aq [H+]aq
Thus the overall extraction mechanism may be expressed as VO2 %q + 2SCN-~q + 2HAcorg ~ VO(Ac) (SCN)2 9HAcorg + OH-~q and the extraction constant is given by
Kex-
[VO(Ac)(SCN)2- HAc]org [OH-]aq [VO2+]~q [SCN_]~aq [HAc]2org
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M. Assefa and B. S. Chandravanshi:
Then the distributionratio is given by D1, 2, 2 = =
[VO(Ac)~OH]org q- [ V O ( A c ) ( S C N ) 2
.
HAc]org
[VO2+]aq ill,2 [HAc]%~g([H+]-l~q+fix,~,2 [SCN-]2~q[OH ]aq).
The subscript aq, org, and HAc denote aqueous phase, organic phase and PCHA. Thus the distribution ratio of vanadium is a function of the concentration of reagent (PCHA) in the organic phase and that of thiocyanate in the aqueous phase. The following structures are proposed for the mixed ligand complex. HAc : 0
0
SCN\/O=C- SCN\v "O=CScN/V\0-N S C N / . \ 0 - N HAc Acknowledgement The authors are thankful to the chairman, Department of Chemistry, Addis Ababa University, Addis Ababa, Ethiopia, for providing facilites. Summary Solvent Extraction and Spectrophotometric Determination of Vanadium(V) as a Mixed Ligand Complex with N-Phenylcinnamohydroxamic Acid and Thiocyanate N-Phenylcinnohydro• acid (PCHA) reacts with vanadium(V) in thiocyanate medium, giving water insoluble mixed ligand complex, extractable into chloroform. On the basis of sensitive colour reaction, a new method for the extraction and photometric determination of vanadium(V) has been developed. The mixed ligand complex can be quantitatively extracted into chloroform from 3.0--7.0M acetic acid media. The green coloured ternary complex has a broad absorption band around 590 nm. The photometric sensitivityof the colour reaction is 0.0068 #g of vanadium per cmL The effect of foreign ions and several experimental parameters have been studied. The composition of the complex has been found to be 1 : 2 : 2 (vanadium : PCHA : SCN).
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Zusammenfassung Extraktion gernischter
und speletrophotometrische Bestimmung von Vanadin(V) als Komplex mit N-Phenylzimthydroxamsdure und Rhodanid
N-Phenylzimthydroxams/inre reagiert mit Vanadin(V) in Gegenwart yon Rhodanid unter Bildung eines mit Chloroform extrahierbaren, gemischten Komplexes. Auf der Grundlage dieser Farbreaktion wurde eine neue Methode der Extraktion nnd photometrischen Bestimmung von Vanadin(V) entwickelt. Aus 3,0--7,0 M Essigs~iure 1/itgtsich dieser Komplex quantitativ mit Chloroform extrahieren. Der griine tern~ire Komplex hat eine breite Absorptionsbande bei 590 nm. Die Empfindlichkeit der Farbreaktion betr/igt 0,0068#g V per cm2. Die Wirkung von Fremdionen und einzelner experimenteller Parameter wurde nntersucht. Die Zusammensetzung des Komplexes betdigt 1 : 2 : 2 (V : PCHA : SCN).
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1'/ p. Job, Ann. chim. 9, 113 (1928). 18 j. H. Yoe and A. L. Jones, Ind. Eng. Chem., Analyt. Ed. 16, 11 (1944). 19 L. G. Sillen, Acta Chem. Scand. 10, 185 (1956). 20 I. Kojima and Y. Miwa, Analyt. Chim. Acta 83, 329 (1976). sl R.S. Kharsan, K.S. Patel, and R.K. Mishra, Mikrochim. Acta [Wien] 1979 I, 353.
Correspondence and reprints: Dr. B.S. Chandravanshi, Department of Chemistry, Addis Ababa University, P . O . Box 1176, Addis Ababa, Ethiopia.
