Extraction and spectrophotometric determination of iron(III) with N

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N-Phenylcinnamohydroxamic acid (PCHA) was first introduced as an analytical reagent for the extraction and spectrophotometric determination of vanadium(V) ...

Mikrochimica Acta [Wien] 1984II, 7--15 9 by Springer-Verlag 1984

Department of Chemistry, Addis Ababa University, Addis Ababa, Ethiopia

Extraction and Spectrophotometric Determination of Iron(III) with N-Phenylcinnamohydroxamic Acid By B. S. Chandravanshi and Alemayehu Amsalu

(Received October 10, 1983) N-Phenylcinnamohydroxamic acid (PCHA) was first introduced as an analytical reagent for the extraction and spectrophotometric determination of vanadium(V) 1. The analytical application of PCHA was further extended for the determination of other metal ions 2-8. It has also been reported as a reagent for the extraction and spectrophotometric determination of iron(III) 9. However, the method is neither sensitive nor selective. Hence, the reaction of iron(III) with PCHA has been studied in detail under different experimental conditions in the present investigation to improve the sensitivity and selectivity of the method. The present investigation has led to the development of a new method for the extraction and spectrophotometric determination of iron(III) based on the formation of an orange coloured 1 : 3 (Fe : PCHA) complex quantitatively extractable into benzene from acidic medium. The method has been found to be simple, precise, sensitive, and highly selective in comparison to the widely used thiocyanate 1~ and most of the other methods reported in literature 12-15. It has further been extended for the successive extraction and spectrophotometric determination of iron and vanadium in steels.

Experimental Apparatus and Reagents A Beckman| Model 24 UV-Vis spectrophotometer equipped with 1-cm quartz cells was used for absorbance measurements. A Beckman Chem Mate pH meter was used for the measurements of pH.

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A stock solution of iron(III) was prepared by dissolving 4.0402 g of iron(III) nitrate nonahydrate (BDH, AnalaR) in 100 ml of 1 : 5 (v/v) nitric acid and diluted to 1.0 litre with distilled water in a volumetric flask. This solution was standardized gravimetrically16. A stock solution of vanadium(V) was prepared by dissolving 0.5850 g of ammonium metavanadate (BDH, AnalaR) in distilled water in a 500-ml volumetric flask and standardized volumetricallylL Working solutions of both the metal ions were prepared by appropriate dilutions of the stock solutions with distilled water to give 50/~g m1-1 of the metal respectively. Solutions of other ions were prepared by dissolving known amounts of reagent grade salts in distilled water to give 100 mg ml -z of the ion in question. N-Phenylcinnamohydroxamic acid was prepared by the reported method 18. A 0.15% (w/v) solution of the reagent in ethanol was used for the reaction with iron(III). A 0.1% (w/v) solution of the reagent in ethanolfree chloroform was used for the extraction of vanadium(V). All the other chemicals used were of analytical grade.

Preparation of Steel Solution A weighed quantity (approximately 0.1 g) of the steel sample was transferred into a 400-ml beaker and treated with 10 ml of concentrated nitric acid. The mixture was heated to remove the oxides of nitrogen. To this were added about 10--15 ml of concentrated hydrochloric acid and the solution was evaporated to almost dryness and cooled. To the cold pasty mass about 50 ml of distilled water were added and the solution was boiled. Tungsten was precipitated as tungstic acid. The undissolved silica and tungstic acid were filtered off and washed out several times with hot distilled water. The filtrate and washings were collected in a 500-ml volumetric flask and diluted to volume with distilled water. A suitable aliquot of the sample solution was used for the analysis.

Procedure for Extraction of Iron(HI) An aliquot of the solution containing 1 5 - - 1 5 0 # g of iron(III) was transferred into a 50-ml beaker and 5 ml of 0.15% (w/v) solution of the reagent in ethanol were added to it. T h e solution was diluted to 25 ml with distilled water and the p H was adjusted between 2 and 6 using 1 M hydrochloric acid or 1 M ammonia solution. T h e solution was transferred to a 100-ml separatory funnel and the beaker was washed with 25 ml of benzene. T h e washings were added to the funnel, the mixture was shaken vigorously for t w o minutes, and allowed to separate the t w o phases. T h e organic phase was collected in a 25-mi volumetric flask after drying over anhydrous sodium sulphate and diluted to volume with benzene. T h e absorbance of the coloured benzene extract was measured at 440 n m against the reagent blank.

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For calibration, 0.5, 1.0, 1.5, 2.0, and 2.5 ml of the standard solution (50/~g Fe m1-1) were used through the procedure.

Procedure [or Extraction o[ Vanadium(V) An aliquot of the solution containing 20--200 #g of vanadium(V) was transferred to a 100-ml separatory funnel. Sufficient quantities of distilled water and concentrated hydrochloric acid were added to maintain the acidity between 4 and 6 M and volume of the aqueous phase to about 25 ml. A 10-ml aliquot of 0.1% (w/v) solution of the reagent in chloroform was added and the mixture was shaken vigorously for two minutes and then proceeded as for the extraction of iron(III). The absorbance of the coloured chloroform extract was measured at 540 nm against chloroform as blank. For calibration, 1.0, 1.5, 2.0, 2.5, and 3.0 ml of the standard solution (50 #g V m1-1) were used through the procedure.

Results and Discussion

Solvent [or extraction and absorption spectra. Several organic solvents such as benzene, toluene, o-xylene, chlorobenzene, o-dichlorobenzene, chloroform, carbon tetrachloride, diethylether, ethylacetate, and amyl-alcohol were found to extract the iron(III)-PCHA complex from the aqueous phase. Benzene was found to be the most suitable solvent because the quantitative extraction of the iron(III)-PCHA complex is readily accomplished in it. It was also preferred due to the higher sensitivity of the colour reaction in it than in other solvents. The absorption spectra of the iron(III)PCHA complex in the visible region were found to be similar in all nonpolar organic solvents with an absorption maximum invariably at 440 nm. However, variations in the absorbance values were noticed. The absorption spectra of the reagent has also been measured in the visible region. The benzene solution of the reagent showed an almost negligible absorption in the region 400--500 nm. However, a reagent blank is necessary for the precise measurements of absorbance of the complex at 440 nm. Effect o[ pH. The optimum pH range of the aqueous phase for the complete extraction of iron(III) was found to be 1.5--7.0. At a lower pH the absorbance decreases while at a higher pH the separation of the two phases becomes difficult as well as the absorbance decreases due to the precipitation of iron(III) as hydroxide. Effect o[ amount o[ reagent. A 1 : 6 molar ratio of iron to reagent was found to be adequate for the complete extraction of

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iron(III). A large excess of the reagent up to 50-fold molar excess had no adverse effect on the extraction of iron(III) from the aqueous phase. As the reagent is insoluble in water, it was dissolved in ethanol and used for the reaction with iron(III). It has been found that the extraction of the iron(III)-PCHA complex is unaffected by the amount of ethanol in the aqueous phase up to 30% (v/v). However, increasing amounts of ethanol more than 30% (v/v) in the aqueous phase decreases the percent extraction of iron(III) due to the increasing solubility of the complex in aqueous ethanol. Effect o[ other variables. The volume of the aqueous phase can be varied from 10 to 100ml with respect to a fixed volume of 25 ml of the organic phase without any variation in the absorbance values or extraction efficiency. Variation in the temperature of the aqueous phase between 20 o and 40 o C did not produce any change in the absorbance value of the benzene extract. The ionic strength of the aqueous phase can also be varied between 0.1 and 1.0 mole per litre with potassium chloride, ammonium chloride, and ammonium nitrate. The iron(III)-PCHA complex is completely extracted into benzene within two minutes. The benzene extract of the complex is stable for at least 3 days at 25 o + 2 ~ C.

Beer's Law, Optimum Concentration Range, Sensitivity and Molar Absorptivity The coloured system obeys Beer's law in the concentration range 0.6--7.0 ppm of iron. The optimum concentration range for the determination of iron as evaluated from Ringbom's plot 19 was found to be 1.5--6.0 ppm of iron. The photometric sensitivity 2~ and molar absorptivity were found to be 0.007 #g of iron per cm ~ and 8000 1mo1-1.cm -1 respectively at 440 nm.

Precision The precision of the method was evaluated from the analyses of ten different samples each containing 50 #g of iron. The relative standard deviation was found to be +_0.6%. These results indicate that the method is highly precise and gives reproducible results.

Effect o[ Foreign Ions The influence of foreign ions on the extraction and determination of iron(III) with PCHA have been studied by adding a known quantity of a desired ion to a solution containing 50 #g of iron(III).

Extraction and Spectrophotometric Determination of Iron(III)

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The pH of the solution was adjusted to about 2 and iron(III) was extracted and determined by following the procedure described earlier. Vanadium(V), titanium(IV), zirconium(IV), molybdenum(VI) and tungsten(VI) react with the reagent and interfere in the determination of iron(III) by the recommended procedure. However, the interferences due to vanadium(V), titanium(IV), and zirconium(IV) were eliminated by extraction of the metal ions with 0.1% solution of the reagent in chloroform from 4 M, 8 M, and 2 M hydrochloric acid solutions respectively prior to the extraction of iron(III). Iron(III) remains in the aqueous phase under these conditions and it was extracted and determined with PCHA after raising the pH of the aqueous phase to 2.0. The interferences due to molybdenum(VI) and tungsten(VI) were also eliminated by using an excess reagent to precipitate these metal ions completely. These precipitates are highly soluble in benzene giving colourless extracts and did not interfere in the determination of iron(III). The tolerance limits of foreign ions are as follows: 2000 ppm: Na +, K +, Li § Ca 2+, Sr 2+, Ba 2+, Mg 2+, acetate, chloride, nitrate, sulphate, NH4 +, C104-; 1000 ppm: Zn 2+, Cd 2+, Hg 2§ A1a+, Cr a+, UO~ 2+, citrate, tartrate, thiocyanate; 800 ppm: Mn 2+, Ni 2+, Co 2+, Cu 2+, Sn 2+, Pb 2+, As 5+, Sb 5+, Bi a+, borate; 200 ppm azide; 100 ppm: Ti 4+, Zr 4+, VO2 + (prior extraction is necessary); i00 ppm: Ce4+; 80ppm: MoO42-, WO42- (excess reagent is necessary); 80ppm: PO4 a-. These results clearly indicate that almost all common ions which are normally associated with iron in ores, alloys, steels, and complex materials do not interfere in the determination of iron(III) with PCHA. Thus, the method is highly selective and can be applied for the determination of iron in any type of samples.

Composition and Stability Constant o/the Complex The composition of the iron(III)-PCHA complex was determined by mole ratio 21 and continuous variations 22 methods. In the later method a series of solutions was prepared in which the mole fractions of iron and PCHA were continuously varied between 0 and 1 at constant total molarity. The absorbance of the solutions of different composition was measured according to the procedure described earlier and plotted against the mole fraction of the ligand, PCHA. The abscissa of the maximum absorbance gave the number of ligands in the complex. The stability constant was also calculated from the same curve by drawing tangents to initial and final parts of the curve, and using the co-ordinates of certain points on t h e tangents and the curve.

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The results obtained by both the mole ratio and continuous variations methods indicate the formation of 1 : 3 (Fe : PCHA) complex. The overall stability constant was found to be 6.15 • 1011 by the continuous variations method.

Application of the Method Vanadium(V) reacts with PCHA to form a stable violet coloured complex quantitatively extractable into chloroform from strongly acidic solutions 1. Iron(III) remains in the aqueous phase under these conditions. Thus it is possible to separate both metal ions from the mixture by solvent extraction at different pH and determine the metals in the organic phase by spectrophotometry. Hence, on the basis of these studies the proposed method has been extended for the successive extraction and spectrophotometric determination of iron(III) and vanadium(V). Vanadium(V) was first extracted from 4--6 M hydrochloric acid solutions containing both the metal ions with 0.1% (w/v) solution of PCHA in chloroform and determined by the procedure described earlier. Iron(III) remained in the aqueous phase under these conditions and it was extracted after raising the pH of the solution to 2.0 and determined by the procedure described earlier. The results Table I. Separation and Determination of Iron and Vanadium Iron taken mg

Vanadium taken mg

Iron found mg

Vanadium f o u n d mg

0.10 0.50 1.00 5.00 10.00

0.50 0.40 0.30 0.20 0.10

0.098 0.500 0.990 5.020 10.010

0.502 0.393 0.302 0.201 0.099

of the analysis of artificial mixtures are given in Table I which shows that the two metals can be separated and determined accurately from mixtures composed of 1 : 5 to 100 : 1 (Fe : V).

Determination of iron and Vanadium in Steels To test the reliability of the newly developed method, the method has been applied for the separation and determination of iron and vanadium in British Chemical Standard, BCS, Steel No. 64 a and 241/1. The iron and vanadium were separated and determined by the procedure described earlier. The results of the analysis are given

Extraction and Spectrophotometric Determination of Iron(III)

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Table II. Determination of Iron and Vanadium in Steel Steel sample B. C. S. No.

Iron certifiedvalue %

Iron found" %

Vanadium certified value %

Vanadium found* %

64 a 241/1

83.45 65.87

83.41 65.82

1.57 1.57

1.560 1.556

* Average of 5 determinations. in Table II. The experimental values are in good agreement with the certified values indicating the accuracy of the method. Acknowledgement The authors are grateful to the Chairman, Department of Chemistry, Addis Ababa University, for providing facilities to conduct the research work. Summary

Extraction and Spectrophotometric Determination of Iron(Ill) with N-Phenylcinnamohydroxamic Acid Iron(III) reacts with N-Phenylcinnamobydroxamic acid to form an orange coloured complex quantitatively extractable into benzene from acidic medium. The complex has an absorption maximum around 440 nm with a molar absorptivity of 8000 1.tool -1.cm -1. The effect of foreign ions and several experimental variables on the extraction of iron(III) have been studied. The composition of the complex has been found to be I : 3 (metal : ligand) with an overall stability constant of 6.15 x 1011. On the basis of these studies a simple, precise, sensitive, and highly selective method has been developed for the extraction and spectrophotometric determination of iron(III). The method has been applied for the successive extraction and spectrophotometric determination of vanadium and iron in BCS steels.

Zusammenfassung Extraktion und spektrophotometrische Bestimmung yon Fe(III) mit N-Phenylzimthydroxamsdure Eisen(III) reagiert mit N-Phenylzimthydroxams~iure unter Bildung eines orange gef~irbten Komplexes, der sich aus saurem Milieu mit Benzol extrahieren l~if~t. Er hat sein Absorptionsmaximum bei 440 nm und eine molare

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Absorptivit~it von 8000 l'mo1-1 "cm-1. Die Wirkung yon Fremdionen und verschiedener experimenteller Bedingungen auf die Extraktion des Fe(III) wurde untersucht. Die Zusammensetzung des Komplexes entspricht dem Verh~iltnis Metall : Ligand = 1 : 3; die Stabilit~itskonstante ist 6,15 • 1011. Daraus ergab sich eine einfache, genaue, empfindliche und hochselektive Methode zur Extraktion und Bestimmung von Fe(III). Sie wurde fiir die schrittweise Extraktion und spektrophotometrische Bestimmung yon Vanadin und Eisen in BCS-Stahl verwendet. References x U. Priyadarshini and S. G. Tandon, Analyst 86, 544 (1961). A.K. Majumdar and A.K. Mukherjee, Analyt. Claim. Acta 22, 514 (1960). a K. F. Fouche, H. J. LeRoux, and F. Phillips, J. Inorg. Nucl. Chem. 32, 1949 (1970). 4 N. K. Dutt and T. Seshadri, Indian J. Chem. 6, 741 (1968). 5 N. K. Dutt and T. Seshadri, J. Inorg. Nucl. Chem. 31, 2153 (1969). 6 E. A. Ostroumov and V. A. Kulumbegashvili, Zh. Analit. Khim. 26, 1111 (1971); Chem. Abstr. 75, 94377b (1971). v M. Assefa and B. S. Chandravanshi, Mikrochim. Acta [Wien] 1983 I, 255.

8 M. Assefa and B. S. Chandravanshi, Ann. chim. (Rome), in press. 9 F. G. Zharovskii and R.I. Sukhomlin, Zh. Analit. Khim. 21, 59 (1966); Chem. Abstr. 64, 14950f (1966). 10 R. H. Betts and F. S. Dainton, J. Amer. Chem. Soc. 75, 5721 (1953). 11 E.B. Sandell, Colorimetric Determination of Traces of Metals, 3rd Ed., New York: Interscience. 1959. p. 524. 12 A. Nestoridis, Analyst 95, 51 (1970). 1~ Z. Holzbecher, L. Divis, M. Kral, L. Sucha, and F. Vlacil, Handbook of Organic Reagents in Inorganic Analysis, Sussex: Ellis Horwood. 1976. p. 558. 14 K. S. Patel, K. K. Deb, and R. K. Mishra, Bull. Chem. Soc. Japan 52, 595 (1979). 15 A. I. Vogel, Textbook of Quantitative Inorganic Analysis, 4th Ed., London: Longmans. 1979. p. 741. 16 L. c. 15. p. 466. 17 G. Charlot and D. Bezier, Quantitative Inorganic Analysis, New York: Wiley-Interscience. 1957. p. 623. 18 U. Priyadarshini and S. G. Tandon, J. Chem. Engg. Data 12, 143 (1967). 19 A. Ringbom, Z. analyt. Chem. 115, 332 (1938).

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20 1. c. 11. p. 83. 21 j. H. Yoe and A. L. Jones, Ind. Eng. Chem., Analyt. Ed. 16, 111 (1944). 22 p. Job, Ann. chim. (Paris) 9, 113 (1928).

Correspondence and reprints: Dr. B. S. Chandravanshi, Department of Chemistry, Addis Ababa University, P. O. Box 1176, Addis Ababa, Ethiopia.

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