ANALYST, JANUARY 1984. VOL. 109 35 Study of the Formation of Vanadium(1V) Complexes with Some Triphenylmethane Reagents and Cationic Surfactants Maciej Jarosz and Zygmunt Marczenko Department of Analytical Chemistry, Technical University, ul. Noakowskiego 3, 00-664 Warsaw, Poland The optimum conditions for the formation of ternary complexes of vanadium(1V) with Eriochrome Cyanine R (ECR), Chrome Azurol S (CAS) and Pyrocatechol Violet (PV) in the presence of zephiramine, cetyltrimethylam- monium (CTA) or cetylpyridinium (CP) ions have been determined. The complexes are formed a t pH 3.5-5.5 and suitable molar absorptivities are 5-8 x 1041 mol-1 cm-l. The higher sensitivities of the methods based on ternary systems are due t o the higher R : V(IV) molar ratios in ternary complexes (with cationic surfactants) than in binary complexes.Of the methods studied, that based on the system with ECR and CTA is proposed for the determination of vanadium as it has the highest sensitivity ( E = 7.9 X lo4 I mol-1 cm-1 a t 575 nm). This method has been applied t o the determination of vanadium in steel. Keywords: Vanadium determination; ternary complexes; spectrophotometry; triphen ylmethane reagents; cation ic surfactan ts Spectrophotometric methods for the determination of vanad- ium generally exhibit low sensitivity. 1 The molar absorptivi- ties of the most sensitive methods with triphenylmethane reagents do not exceed 3.7 x 1041 mol-1 cm-1.2 A significant increase in the sensitivity of the methods based on binary complexes of metals with chelating reagents (mainly tri- phenylmethane reagents) can be obtained by introducing a third component, a cationic surfactant, into the system."-" The mechanism of the action of cationic surfactants has been the subject of many publications. The effects observed after introducing cationic surfactants to the binary system, viz., a bathochromic shift of the absorption maximum of the ternary complex relative to that of the binary complex and an increase in molar absorptivity, were attributed to the inter- action of the cationic surfactants with anionic forms of the reagent3.6.7 or to solubilisation of the binary complexes with micelles of the cationic surfactants.83' Savvin et al.9 introduced the term hydrophobic interactions.This paper describes a study of the conditions of the formation of ternary vanadium complexes with Eriochrome Cyanine R (ECR), Chrome Azurol S (CAS), Pyrocatechol CH3 CH3 I Pyrocatechol Violet Eriochrome Cyanine R CH3 CH3 I 1 Chrome Azurol S Violet (PV) and the cationic surfactants cetyltrimethyl- ammonium bromide (CTA), cetylpyridinium chloride (CP) and tetradecyldimethylbenzylammonium chloride (zephiramine) in order to find the best system for the spectrophotometric determination of vanadium.The systems vanadium(1V) - CAS - zephiramine/CTA have been described elsewhere. 10~1 Experimental Apparatus The absorbances were measured using a VSU2-P spectropho- tometer land the absorption spectra were recorded on a Specord ultraviolet - visible spectrophotometer. In all meas- urements 1-cm cells were used.The pH measurements were carried out using an ELPO N-517 pH meter. Reagents Vanadium(V) standard solution, 1 mg ml-1. A 2.295 0-g amount of ammonium vanadate (NH,V03) was dissolved in water and 5 ml of concentrated ammonia solution were added. The solution was acidified with 10ml of concentrated hydrochloric acid and diluted to 1 1 with water in a calibrated flask. Working solutions of vanadium(1V) were prepared by taking suitable aliquots and reducing the vanadium(V) by means of 5% ascorbic acid solution. Eriochrome Cyanine R (Loba), Chrome Azurol S (BDH) and Pyrocatechol Violet (POCh). The reagents were purified as described elsewhere. 12 Aqueous solutions of the reagents were used, of concentrations 1 x l o - 3 ~ ECR or CAS and 2 x 10-3 M PV. Cetyltrimethylammonium bromide (International Enzy- mes), cetylpyridinium chloride (Loba) and zephiramine (ICN).The commercial preparations do not require further purification. The results of elemental analyses (C, H, halogen) agreed with the theoretical values. Aqueous solutions of these reagents were used of concentration 1 X 10-2 M. Procedure for Determining Vanadium(1V) with ECR and CTA Pour a solution containing 2-16yg of vanadium(1V) into a 25-ml beaker. Add 3 ml of the CTA solution and 3 ml of the ECR solution. Adjust the pH to 4.9k0.3 using ammonia solution. Transfer the solution into a 25-ml calibrated flask and dilute to the mark with water. After lOmin, measure the absorbance at 575 nm against a reagent blank. Results Preliminary studies have indicated that vanadium(V) , unlike vanadium(IV), does not form ternary complexes with the reagents considered here.36 ANALYST, JANUARY 1984, VOL.109 Vanadium(1V) - Eriochrome Cyanine R - Cationic Sur- factant Systems Effects of acidity and excess of reagents The effect of acidity on the measured absorbances was examined in the pH range 3-6. The optimum pH ranges for the formation of the ternary complexes were 4.9k0.3, 4.5 kO.1 and 5.2 k 0 . 2 for CTA, CP and zephiramine, respectively. Maximum absorbances were obtained when the excess of ECR with respect to vanadium was higher than 10. In the system with CP the absorbance increased at a higher excess of ECR (Fig. 1). A hypsochromic shift (ca. 10nm) of the absorption maximum in the ternary system was observed as the excess of ECR increased.The optimum molar excesses of the cationic surfactants with respect to vanadium were 75-200,75-125 and 75-150 for CTA, zephiramine and CP, respectively. At a low excess of zephiramine (below the turbidity ranget3) a significant bathochromic shift (from 550 to 620nm) of the absorption maximum of the formed complex with respect to the position of A,,,, of the binary complex V-ECR, and also a hyperchromic effect, were observed (Fig. 2). At an excess of the cationic surfactants above the turbidity range the A,,,,, of the ternary complex exhibited a hypsochromic shift to about 580 nm and the measured absorbances attained maximum values. Similar changes in the absorption spectra were observed when CTA and CP were used. The complexes of vanadium(1V) with ECR and zephir- amine/CTA were stable for at least 30min after the pH had been adjusted to a suitable value.With CP the absorbance decreased after 5 min. Composition of the ternary complexes The molar ratio of ECR to vanadium(1V) in the ternary complexes, determined by the method of isomolar series (Fig. 3), was close to 1 : 1.5 (when zephiramine was used as the third component) or 1 : 2 (with CTA or CP). The complexes formed are weak, as indicated by the lack of sharp maxima on the curves. The molar ratio of vanadium to the cationic surfactant in the ternary complexes leading to the maximum absorbances cannot be determined because a large excess of the cationic surfactant is necessary for the formation of these complexes. Spectrophotometric determination of vanadium (IV) Ternary complexes of vanadium with ECR and cationic surfactants can form a basis for its spectrophotometric determination.Calibration graphs obtained under optimum 0.8 a, 0.6 0 S m -F a 0.4 0.2 [Vl : [ECRI Fig. 1. Effect of the excess of ECR with respect to V(1V) on the systems V - ECR - CTA/CP/zephiramine. cv = 1 X M; C c r A = ccp = c,, h = 1 X 1 0 - 3 ~ . Ternary systems (absorbances measured against byank): 1, with CTA (A = 575 nm); 2 , with CP ( h = 575 nm); and 3, with zephiramine (A = 580 nm) conditions obeyed Beer's law up to a vanadium(1V) concen- tration of 0.7 pg ml-1 when CTA and zephiramine were used and in the range 0.1-0.5 pg ml-1 when CP was used. The molar absorptivities were 7.9 x 1041 mol-1 cm-1 at 575 nm, 7.7 X lo4 1 mol-1 cm-1 at 580 nm and 7.1 x 1041 mol-1 cm-1 at 575 nm for CTA, zephiramine and CP, respectively.The precision of the most sensitive method, based on the system V(1V) - ECR - CTA, is indicated in Table 1. The method is not selective. Numerous metals that readily hydrolyse in slightly acidic media interfere. Hence, before the determination, vanadium should be separated from other metals, e.g., by using highly selective extraction with N-benzoyl-N-phenylhydroxylamine (BPHA).' The determination of vanadium in steel was carried out as follows. A small amount of steel (ca. 20 mg) was dissolved by heating with 10ml of concentrated hydrochloric acid plus a few drops of concentrated nitric acid. The solution obtained was evaporated almost to dryness and then diluted with 5h1 hydrochloric acid to 20ml and shaken with two successive portions of 0.1% BPHA solution in chloroform.The combined extracts were evaporated to dryness and the residue was mineralised with a mixture of concentrated sulphuric and nitric acids. After dilution of the solution, vanadium was reduced to vanadium(1V) using ascorbic acid and determined by means of ECR and CTA [see Procedure for Determining Vanadium( IV) with ECR and CTA]. The content of vanadium in the steel sample was (2.70 2 0.15) x lo-*% (seven determinations). The recovery of vanadium was 91% (the amount of vanadium added was 5 pg). Vanadium(1V) - Chrome Azurol S - Cationic Surfactant Systems Conditions of complex formation The optimum pH ranges for the formation of the ternary complexes of vanadium(1V) with CAS and the cationic surfactants were 4.2 k 0.2, 4.3 k 0.3 and 4.5 k 0.2 for CP, CTA and zephiramine, respectively.Maximum absorbances in the presence of CTA were obtained when the molar excess of CAS with respect to vanadium was higher than 10. With CP the absorbance increased continuously with increasing excess of CAS. When zephiramine was used a maximum was observed at a molar excess of CAS of 8-10. An increasing excess of CAS with respect to vanadium resulted in a hypsochromic shift of h,,,, of the ternary complex (cu. 5nm). 0.8 0.6 (r, c m a 0.4 2 0.2 5 500 560 580 600 Wavelengthhm 700 Fig. 2. Effect of the excess of zephiramine with respect to V(1V) on. absorption spectra in the system V - ECR - zeph. cv = 1 x 10-5 M ; cECR = 1.2 x 10-4 M . Molar excess of zephiramine: 1, without zephi- ramine; 2, 1 : 1 ; 3 , 1 : 3; 4, 1 : 50; 5 , 1 : 100; and 6, 1 : 200ANALYST, JANUARY 1984, VOL.109 37 0.6 cu 0.4 C m +2 z n a 0.2 I 1 I 1 1 : l 1 : 2 I I I I Molar ratio V : ECR Fig. 3. Determination of the molar ratio of V(IV) to ECR in the ternary complexes with CTA/CP!zephiramine by means of the method of continuous variations. cv + cECR = 2.5 x M; cnA = ccp = C,,ph = 1 x 1 0 - 3 ~ . 1, With CTA (h = 575 nm); 2, with CP ( h = 575 nm); and 3, with zephiramine (A = 580 nm) The optimum molar excesses of the cationic surfactants were 75-125 for CTA, 150-200 for CP and 100-175 for zephiramine. When CTA or zephiramine was used the absorbance increased up to a maximum value with increasing excess of the surfactant (above the turbidity range). An increase in the excess of CP up to 150-fold resulted in a linear decrease in absorbance.The absorption spectra of the V(1V) - CAS - cationic surfact- ant ternary complexes with various excesses of the cationic surfactants showed similar courses to those of the complexes with ECR. The V(1V) - CAS - CTA/zephiramine ternary complexes were stable for at least 30min. When CP was used the maximum absorbance decreased after 5 min. Composition of the ternary complexes The molar ratios of CAS to vanadium in the ternary complexes, determined by the method of isomolar series (Fig. 4), were approximately 1 : 1 , 2 : 1 and more than 2 : 1 for CTA, zephiramine and CP, respectively. Spectrophotometric determination of vanadium(IV) The molar absorptivities for the methods of vanadium determination based on the ternary complexes involving CAS and cationic surfactants were 7.1 x 104 1 mol-1 cm-1 at 595 nm for zephiramine, 6.0 x 1041 mol-1 cm-1 at 600 nm for CTA and 5.4 x 104 1 mol-* cm-1 at 600 nm for CP.The calibration graphs were straight lines for vanadium concentrations up to 0.7 pg ml-1 (zephiramine), up to 0.8 pg ml-1 (CTA) and in the range0.15-0.7 pgml-l (CP). Vanadium(1V) - Pyrocatechol Violet - Cationic Surfactant Systems Conditions of complex formation Ternary complexes of vanadium( IV) with PV and cationic surfactants were formed in the pH ranges 3.6 k 0.2,4.0 k 0.3 and 4.3 k 0.3 for CP, zephiramine and CTA, respectively. Table 1. Precision data for the determination of vanadium(1V) Vanadiumlpg Standard Confidence 7.6 x 10-2 1.97 zk 0.07 2.0 1.97 8.0 8.01 4.9 x 10-2 8.01 f.0.05 16.0 16.20 1.08 x 10-I 16.20 k 0.10 Added Found deviation*/pg limits? * For seven determinations. t Probability level = 0.95. Maximum absorbances of the ternary complexes were obtained at molar excesses of PV of 10-20 (zephiramine), 15-20 (CP) and 20-25 (CTA). When zephiramine was used a constant absorbance was obtained at molar excess of PV higher than 10. When CP was used at least a 15-fold excess of PV was necessary. In the presence of CTA the optimum PV excess was 20-25. The absorption maximum did not shift with increasing excess of PV. The optimum molar excesses with respect to vanadium of the cationic surfactants for the formation of ternary complexes involving PV were to 80-200 (CP), 200-275 (zephiramine) and 100-175 (CTA).As the excess of CP or zephiramine (above the turbidity range) increased the absorbance increased up to a constant value. With an increasing excess of CTA the absorbance initially decreased and attained a constant value only at a 100-fold molar excess. Ternary complexes of vanadium(1V) with PV and cationic surfactants were not formed below the turbidity range. The absorbance of ternary complexes of vanadium(1V) with PV and CTA/zephiramine did not change for 30min. When CP was used the absorbance decreased after 5 min. Composition of the ternary complexes The PV:V(IV) molar ratio in the ternary complexes was determined by the method of isomolar series (Fig. 5). It was slightly higher than 2 when CTA was used and slightly lower than 3 when zephiramine or CP was used. The number of PV molecules per vanadium atom in the complexes was, on average, higher than in the systems with ECR and CAS.Spectrophotometric determinution of vanadium (IV) The sensitivity of the spectrophotometric determination of vanadium using PV and cationic surfactants depends strongly on the nature of the surfactant. For zephiramine E = 7.5 X lo4 at 660nm and for CP E = 5.0 x 1041 mol-'cm-1 at 660 nm. The determination ranges are up to 0.5 pg ml-1 of vanadium for zephiramine and 0.15-0.6 pg ml-1 of vanadium for CP. The system V(1V) - PV - CTA cannot be used for the spectro- photometry of vanadium because the calibration graph is not straight for a wide range of vanadium concentrations. Discussion and Conclusions Of the nine systems of vanadium(1V) with selected triphenyl- methane reagents and cationic surfactants examined as a basis for the spectrophotometric determination of vanadium(IV), that with ECR and CTA, which exhibit the highest sensitivity (molar absorptivity 7.9 x l o 4 1 mol-1 cm-I), can be recom- mended.The methods based on the V(1V)-ECR- zephiramine and V( IV) - PV - zephiramine systems have similar sensitivities. In the presence of cationic surfactants, spectrophotometric methods for determining vanadium with the chromophoric reagents ECR, CAS and PV are far p o r e sensitive than those with binary systems. Compared with molar absorptivities of 1.8-3.7 x l o 4 for binary systems,2 values of 7.1-7.9 X lo4 can be attained for ternary systems. An advantageous increase in Ah ( L a x .complex - L a x . reagent) is also observed.38 ANALYST. JANUARY 1984. VOL. 109 Molar ratio V : CAS Fig. 4. Determination of the molar ratio of V(1V) to CAS in the ternary complexes with CTAICPlzephiramine by means of the method of continuous variations. cv + cCAS = 2.5 x lops M ; CCTA = ccp = C,,ph = 1.5 x l o - 3 ~ . 1, With CTA (h = 600nm); 2, with CP (h = 600 nm); and 3, with zephiramine (h = 595 nm) The increased sensitivity results from the fact that in the binary systems the V(1V) : R molar ratio is 2 : 1-1 : 1,2 whereas in ternary systems the ratios, as shown in this work, are in the range 1 : 1-1 : 3. Undoubtedly cationic surfactants (and espe- cially their micellar forms) permit higher R : V(1V) molar ratios in ternary complexes.This effect is observed with high concentrations of cationic surfactants, above the turbidity range.13 The shifts of the absorption maxima that occurred with ternary complexes with ECR or CAS following an increase in the concentration of added cationic surfactants or chromo- phoric reagents are closely connected with the character of the interactions between the triphenylmethane reagent and the cationic surfactant. The large bathochromic shift of I.,,,, below the turbidity range is a result of electrostatic interac- tions between a positive charge localised on the nitrogen atom of the cationic surfactant and anionic forms of the triphenyl- methane reagent. These interactions are stronger when the localisation of the positive charge on the nitrogen atom is more compact.It depends on the size of the inductive effect from alkyl groups bonded with the nitrogen atom and also on steric hindrances (screening of the nitrogen atom by large substituents). The hypsochromic shift of the absorption maximum of the ternary complexes above the turbidity range (Fig.2) is caused by hydrophobic interactions9 of micellar forms of the cationic surfactants. Micelles of cationic surfactants are formed more readily when their molecules contain larger hydrophobic substituents. Hydrophobic interactions are hindered by rigid fragments of the molecules, e.g., the pyridine ring in CP. These interactions lead to an increased energy state of the electrons in conjugated bonds and increased reactivity of chromophoric reagents, which results in the formation of complexes with higher R : V(1V) molar ratios than below the turbidity range.In ternary complexes with PV, hydrophobic interactions play an important role. These complexes are formed only above the turbidity range, and the positions of their absorp- tion maxima do not change with increasing concentration of cationic surfactants. Their formation depends on the dissoci- ation of protons of the phenol groups of PV, which is made easier by micellar forms of cationic surfactants. The number of molecules of chromophoric reagents in these complexes is greater than in complexes with ECR or CAS. 1 : l 1.2 1 . 3 Molar ratio V: PV Fig. 5. Determination of the molar ratio of V(IV) to PV in the ternary complexes with CTA/CP/zephiramine by means of the method of continuous variations.cv + cpv = 2.5 x 10-5 M (CP, zeph) or 5 X 10-5 M (CTA). cmA = 2 x l o - 3 ~ ; ccp = 1.5 x 10-3 M; C,,ph = 2.5 X 10-3 M. 1, With CTA (h = 660 nm); 2, with CP (h = 660nm); and 3, with zephiramine (A = 660 nm) In the optimum pH ranges for the formation of the ternary complexes, vanadium(1V) can occur in two forms, VO(OH)+ or VO(OH)2.14 In solutions containing ECR, CAS or PV and cationic surfactants at these acidities doubly and triply ionised forms of these reagents15316 are in equilibrium. It has been found previously13 that the ternary complexes of aluminium with ECR, CAS and PV are formed when aluminium occurs as Al(OH)2+ or A1(OH)3 and the triply ionised form of the reagent (ECR, CAS, PV) predominates. Ternary complexes of vanadium(1V) and aluminium(II1) with the examined reagents are formed in the pH ranges in which these metals occur as univalent hydroxy complexes or hydroxides and the chromophoric reagent in doubly or triply ionised forms.1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. References Marczenko, Z., “Spectrophotometric Determination of Ele- ments,” Ellis Horwood, Chichester, 1976. Janssen, A., and Umland, F., Fresenius 2. Anal. Chem., 1971, 254,286. Tikhonov, V. N., Zh. Anal. Khim., 1977, 32, 1435. Savvin, S. B., Crit. Rev. Anal. Chem., 1979, 8, 5 5 . Marczenko, Z.,Crit. Rev. Anal. Chem., 1981, 11, 195. Chernova, R. K., Zh. Anal. Khim., 1977,32, 1477. Savvin, S. B., Chernova, R. K., Belousova, V. V., Sukhova, L. K., and Shtykov, S . N., Zh. Anal. Khim., 1978,33, 1473. Martynov, A . P., Novak, V. P., and Reznik, B. E., Ukr. Khim. Zh., 1978,44, 203. Savvin, S. B., Marov, I. N., Chernova, R. K., Shtykov, S. N., and Sokolov, A. B., Zh. Anal. Khim., 1981, 36, 850. Horiuchi, Y., and Nishida, H., Bunseki Kagaku, 1969,18,850. Gordeeva, I . N., and Mescherakova, D. N., Vest. Leningr. Tekhn. Univ., 1981, 4, 95. Langmyhr, F. J., and Klausen, K. S . , Anal. Chim. Acta, 1963, 29, 149. Marczenko, Z., and Jarosz, M., Analyst, 1982, 107, 1431. Nazarenko, V. A., Antonovich, V. P., and Nevskaa, E. M., “Metal Ions Hydrolysis in Dilute Solutions” (in Russian), Atomizdat, Moscow, 1979. BureSova, I., Kubaii, V., and Sommer, L., Collect. Czech. Chem. Commun., 1981,46, 1090. Skarydova, V., and cermakova, L . , Collect. Czech. Chem. Commun., 1982,47, 1310. Paper A31104 Received April Ilth, 1983 Accepted August 25th, 1983