Analyst, April, 1979, Vol. 104, pp. 323-327 323 Sulphochlorophenol N as a Spectrophotometric Reagent for Vanadium(V) M. Zenki Department of Chemistry, Okayama College of Science, 1-1, Ridai-cho, Okayama-shi, 700, Japan A spectrophotometric method for the determination of trace amounts of vanadium (V) with sulphochlorophenol N is described. With this reagent, vanadium forms a blue complex, which is stable in the pH range 3.7-6.0. The coloured complex obeys Beer's law at 627 nm in aqueous solution with a molar absorptivity of 3.12 x lo4 1 mol-l cm-l. Copper and cobalt ions interfere in this method. Keywords Bisazochromotropac acid dye ; s+ectrophotometry ; sulphochloro- Phenol N ; vanadium determination Many bisazochromotropic acid derivatives, such as arsenazo 111, are well known as highly sensitive spectrophotometric reagents for various metal ions1 Alimarin and co-~orkers~,~ reported that some derivatives of 4,5-dihydroxynaphthalene-2,7-disulphonic acid (chromo- tropic acid) were useful reagents for the photometric determination of niobium and other metals (zirconium, hafnium, vanadium, molybdenum, scandium, aluminium, indium, gallium and palladium). Sulphochlorophenol S (3,6-bis [ (5-chloro-2-hydroxy-3-sulphophenyl) azo] - 4,5-dihydroxynaphthalene-2,7-disulphonic acid), which is a symmetrical derivative, has been used for the determination of niobium in steel^,^^^ tantalum metal,5 titanium oxide,6 uranium' and zirconium alloys.2 However, no derivatives that could be used for the deter- mination of vanadium, which is in the same Group as niobium in the Periodic Table, have been reported.Further investigation of bisazo derivatives of chromotropic acid has revealed that the introduction of a nitro group in the @am-position in one of the benzene rings, such as arsenazo-f~-N0,8,~ or carboxynitrazo,lO produces a high sensitivity and selectivity and a wide absorption peak shift. Preliminary studies have shown that an asymmetric bisazo derivative, sulphochlorophenol N (3- [ (5-chloro-2-hydroxy-3-sulphophenyl) azo]4,5-dihydroxy- 6- [ (4-nitrophenyl)azo]naphthalene-2,7-disulphonic acid) (I) reacts with vanadium rather than niobium to form a blue complex under suitable conditions. This paper reports a sensitive and selective spectrophotometric method for the determina- tion of vanadium using sulphochlorophenol N.Attempts to determine a small amount of vanadium in steels have been made. OH OH Experimental Apparatus Absorption spectra were recorded with a Hitachi, Model 124, automatic recording spectro- photometer and other spectrophotometric measurements were made with a Hitachi, Model 139, spectrophotometer, with 1-cm matched cells. A Hitachi-Horiba pH meter, Model F-~ss, was used for the pH measurements. Reagents in all dilutions. All reagents used were of analytical-reagent grade. De-ionised distilled water was used324 ZENKI: SULPHOCHLOROYHENOL N AS A Analyst, VoJ. 104 3- [ (5-Chloro-2-hydroxy-3-sul~hophenyl)azo] -4,5-dihydroxy-6- [ (4-nitrophenyl)azo]naphthalene- 2,7-disulp honic acid (sulphochloro~henol N ) . Diazotise 2-amino-4-chlorophenol-6-sulphonic acid (0-5 "C) and couple with an equimolar amount of chromotropic acid in dilute sodium hydroxide solution.Salt-out the monoazo dye with saturated sodium chloride solution, and then couple with an equimolar amount of 4-nitroaniline diazonium salt, in the presence of calcium hydroxide, to produce the bisazo dye. Allow the reaction mixture to stand at room temperature for 12 h and acidify it by adding concentrated hydrochloric acid. Filter off the precipitate and wash it with hydrochloric acid (1 + 4). Re-precipitate 2-3 times by dissolving in water by the addition of dilute sodium hydroxide solution, followed by hydrochloric acid, until one spot appears on a paper chromato- gram, developed with 2 M ammonia solution saturated with butan-2-01.11 The extraction method12 was employed to remove calcium and sodium from the purified compound com- pletely.Dissolve 0.2 g of sulphochlorophenol N in 1 1 of de-ionised distilled water. This solution is stable for several months. Vanadium( V ) standard solution. Dissolve 0.585 0 g of ammonium metavanadate (NH,VO,) in water in a 500-ml calibrated flask to make a 1 0 - 2 ~ stock solution. Dilute 100-fold with water to make a working solution. This solution contains 5.1 pg ml-l of vanadium. A 0.1 M acetic acid - 0.1 M sodium acetate solution system was used. The yield is approximately 40%. Bufer solution, pH 5.0. Citric acid solution, 0.1 M. Dissolve 21 g of citric acid in water and dilute to 11. Calibration Using a pipette, introduce aliquots of the standard vanadium(V) solution containing 0, 10.2, 20.4, 30.6 and 40.8 pg of vanadium into 25-ml calibrated flasks.Add 5 ml of buffer solution (pH 5.0), 1 ml of 0.1 M citric acid solution and 5 ml of 0.02% sulphochlorophenol N solution, then dilute to the mark with distilled water. Allow the mixture to stand for 10 min and measure the absorbance at 627 nm, with a reagent blank solution prepared at the same time as the sample solution as reference. Draw a calibration graph; this should be linear and pass through the origin. Absorption Spectra at apparent pH 5.0. Results and Discussion Fig. 1 shows the absorption spectra of sulphochlorophenol N and its vanadium(V) complex The absorption maximum of the complex occurs at 627 nm, and there 600 620 640 660 680 7 10 Wavelength/nm Fig. 1. Absorption spectra: A, vanadium - sulpho- chlorophenol N complex containing 20.4 pg of vanadium(V) measured against reagent blank; and B, reagent blank measured against water.April, 1979 SPECTROPHOTOMETRIC REAGENT FOR VANADIUM(V) 325 was no shift in the wavelength when either the pH was varied from 3.4 to 9.1 or the molar ratio of vanadium to sulphochlorophenol N was varied from 1 : 5 to 5: 1.Sulphochlorophenol N reacted similarly with vanadium(1V) to form a blue complex (absorption maximum 630 nm) under the same conditions. Further investigation was not carried out because the sensitivity was fairly low compared with the absorption of vanadium (V) complexes. Influence of pH from 1.8 to 12.9 (Fig. 2). pH 3.7 and 6.0. Therefore, the pH was adjusted to about 5.0.The influence of the pH on the absorbance at 627nm was examined over a pH range A maximum and constant absorbance was obtained between a, 0.61 O*** 0 2 4 6 I PH Fig. 2. Effect of pH on absorbance measured a t 627 nm. Reagent Concentration The effect of the amount of sulphochlorophenol N on the absorbance was examined by varying the molar ratio of sulphochlorophenol N to vanadium(V), the amount of the latter being kept constant (Fig. 3). The results showed that the absorbance of the complex was constant over a 1.5-fold molar excess of the reagent. The recommended volume of reagent, 5 ml, corresponds to a concentration of about 5.7 x 10-5 M in the final solution. This represents a 1.6-fold excess over the highest point on the calibration graph, i.e., 40 pg of vanadium in 25 ml of solution.Molar ratio of sulphochlorophenol N to vanadium(1V) Fig. 3. Effect of reagent concentra- tion on the formation of the vanadium- (V) complex. Effect of Time complex was examined. The time necessary for complete formation of the sulphochlorophenol N and vanadium Even when a 2-fold molar excess of sulphochlorophenol N was326 ZENKI: SULPHOCHLOROPHENOL N AS A Analyst, Vol. 104 added to vanadium, about 5min were sufficient for complete reaction at 20 "C and the absorbance of the complex did not change for at least 2 d. Therefore, the measurement of the absorbance was carried out a t least 10 min after addition of the reagent. Beer's Law The calibration graph was linear in the range 0-1.6 pg ml-l of vanadium(V), Le., up to at least 40pg of vanadium in 25ml of solution, and its slope corresponded to a molar absorptivity for the complex of 3.12 x lo4 1 mol-l cm-1 at 627 nm. Composition of the Complex Attempts to determine the composition of the complex in aqueous solution were made by the continuous-variation and molar-ratio methods (Fig.3). Both methods revealed that vanadium(V) forms a 1 : 1 complex with sulphochlorophenol N. This is in agreement with Alimarin and Savvin's observation2 that sulphochlorophenol S forms a 1 : 1 complex with niobium. Interferences The effect of several possible interferences on the determination of 20.4pg of vanadium is shown in Table I. No interference was noted with large amounts of acetate, bromide, chloride, citrate, fluoride, iodide, nitrate and sulphate. EDTA and tartrate interfered in the determination.The serious interferences arising from cobalt(I1) and copper(I1) may be caused by complexation with sulphochlorophenol N, which cannot be suppressed by the addition of the usual masking agents. TABLE I EFFECT OF DIVERSE IONS ON VANADIUM(V) DETERMINATION Tests were made by adding the interfering ion t o a solution containing 20.4 pg of vanadium(V). Ion added Al(II1) . . Bi(II1) . . Ca(I1) .. Cd(I1) .. Co(I1) .. Cr(V1) .. Cu(I1) .. Cr(II1) . . Fe(II1) . . Mn(I1) .. Mo(V1) . . Ni(I1). . Sb(II1) . . .. .. W V ) Pb(I1) . . Si(1V) .. Sn(I1) . . Ti(1V) .. Zn(I1) .. Te(1V) . . .. .. .. . . .. .. .. .. .. .. .. .. .. .. .. .. . . - . . . .. Amount of ion added/ pg 20 20 100 100 20 1000 20 20 20 100 1000 100 20 100 20 1000 20 20 20 100 Amount of vanadium recovered/ pg 20.2 19.9 20.4 20.4 30.1 19.6 12.6 33.2 21.0 19.4 21.2 21.3 20.6 20.0 20.4 21.0 20.4 20.4 20.2 20.1 Recovery, % 99.0 97.5 100 100 148 96.1 61.2 163 103 104 104 101 100 103 100 100 95.1 98.0 99.0 98.5 Application to the Determination of Vanadium in Steel Two samples of Japanese Standards of Iron and Steel (JSS) were analysed in order to check the validity of the method.When carrying out the determination of vanadium in steels, a large amount of iron(II1) also interferes and a suitable preliminary treatment is therefore required. If the vanadium(V) ion is reduced to vanadium(IV), the extraction method of Specker and Doll13 with 4-methylpentan-2-one should prove satisfactory. Ammonium iron(I1) sulphate is suitable for this purpose as a reducing agent.The preparation of the sample solution is as follows. Transfer a suitable mass (0.05-0.5 g) of the finely divided iron or steel sample into a 100-ml beaker and add 5 ml of water, 5 ml of concentrated hydrochloric acid and 1 ml of concentrated nitric acid. Heat gently untilApril, 1979 SPECTROPHOTOMETRIC REAGENT FOR VANADIUM(V) 327 all of the metal is in solution, then evaporate almost to dryness. Dissolve the residue in 5 ml of concentrated hydrochloric acid and then transfer the solution completely (rinse with 10 ml of water) into a 100-ml separating funnel. Add 10 ml of concentrated hydro- chloric acid, about 3 mg of ammonium iron(I1) sulphate and 30 ml of 4-methylpentan-2-one, shake the funnel vigorously for a few minutes and allow it to stand for 30min.Transfer the aqueous phase into an evaporating dish and evaporate to dryness. Add a small amount of concentrated nitric acid and heat again. Dissolve the residue in distilled water and adjust this solution to about pH 5 with 1 N sodium hydroxide solution. Then dilute to 100 ml with distilled water in a calibrated flask. Transfer 1-5 ml of the solution, with a pipette, into a 25-ml calibrated flask and determine the vanadium as described above. The results obtained are given in Table 11. It can be seen that the experimental results are in good agreement with the certified values. Recovery tests were also carried out by adding 10.2pg of the vanadium standard solution to the steel-sample solution. The mean recovery was 102% with a standard deviation of 0.5%.TABLE I1 DETERMINATION OF VANADIUM(V) IN JSS SAMPLES Average of at least four determinations. Mass of sample/ ------ -, Relative error, Vanadium content, yo Sample g Certified value Found Y O JSS 159-3 . . . . 0.1017 0.31 0.305 - 1.6 0.200 1 0.316 + 1.9 0.5003 0.320 +3.2 JSS 852-1 . . . . 0.0496 0.52 0.528 + 1.5 0.0498 0.534 +2.7 0.062 8 0.495 -4.8 and 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. The author is indebted to Professor Kyoji TGei, Okayama University, for valuable advice discussions and to Mr. K. Kaichida for technical assistance. References Flaschka, H. A., and Barnard, A. J . , Jr., Editors, “Chelates in Analytical Chemistry,” Volume 2, Alimarin, I. P., and Savvin, S. B., Talanta, 1966, 13, 689. Alimarin, I . P., Savvin, S. B., and Okhanova, L. A., Talanta, 1968, 15, 601. Wakamatsu, S., Bunseki Kagaku, 1969, 18, 376. Hashitani, H., and Adachi, T., Bunseki Kagaku, 1975, 24, 303. Ilsemann, K., and Bock, R., 2. Analyt. Chem., 1975, 274, 185. Budesinsky, B., and Savvin, S. B., 2. Analyt. Chem., 1965, 214, 189. PeriSiC-JanjiC, N. U., Muk, A. A,, and CaniC, V. D., Analyt. Chem., 1973, 45, 798. Muk, A. A., and Pravica, M. B., Analyt. Chem., 1974, 46, 1121. Savvin, S. B., Petrova, T. V., and Romanov, P. N., Talanta, 1972, 19, 1437. Budesinsky, B., and Krumlova, L., Analytica Chim. Acta, 1967, 39, 375. Zenki, M., Analytica Chim. Acta, 1977, 93, 323. Specker, H., and Doll, W., 2. Analyt. Chem., 1956, 152, 178. Marcel Dekker, New York, 1969, p. 1. Received July 14th, 1978 Accepted October llth, 1978