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Spectrophotometric study of reactions of scandium, ytrium and lanthanum ions with some triphenylmethane dyes in the presence of cationic surfactants
Analytica Chimica Acta, 159 (1984) 309-317 Elsevier Science Publishers B.V., Amsterdam -Printed
in The Netherlands
SPECTROPHOTOMETRIC STUDY OF REACTIONS OF SCANDIUM, YTTRIUM AND LANTHANUM IONS WITH SOME TRIPHENYLMETHANE DYES IN THE PRESENCE OF CATIONIC SURFACTANTS
M. JAROSZ and 2. MARCZENKO*
Department
of Analy tical Chemistry, Technical University, 00-664 Warsaw (Poland)
(Received 23rd September 1983)
SUMMARY Optimum conditions for the formation of ternary complexes of scandium, yttrium and lanthanum ions with chrome azurol S, eriochrome cyanine R and pyrocatechol violet in the presence of cetyltimethylammonium, cetylpyridinium and tetradecyldimethylbenzylammonium (zephhamine) ions are described. The spectrophotometric determination of scandium with chrome azurol S and zephiramine exhibits the greatest sensitivity (E = 1.50 x lo* 1 mol-’ cm-i at 610 nm). In the spectrophotometric determination of scandium with eriochrome cyanine R and cetylpyridinium ion (E = 9.2 X 10’ at 600 nm), the interference caused by yttrium is the least. In the best method for yttrium (with pyrocatechol violet and zephiramine), the molar absorptivity is 3.3 X 10’ at 660 nm. Lanthanum does not form ternary complexes of analytical interest in these systems. Some aspects of the formation of ternary complexes with cationic surfactants are discussed.
Ternary systems containing metals that hydrolyze in weakly acidic or neutral medium, chelating triphenylmethane reagents and cationic surfactar&, form the basis of very sensitive spectrophotometric methods [l-3]. Methods involving binary systems (without the surfactant) provide considerably smaller sensitivities. Table 1 shows a comparison of molar absorptivities for the spectrophotometric determinations of scandium, yttrium and lanthanum based on binary complexes with chrome azurol S (CAS), eriochrome cyanine R (ECR) and pyrocatechol violet (PV), as well as those based on ternary complexes with these chromophoric reagents and cetyltrimethylammonium (CTA), cetylpyridinium (CP) or tetradecyldimethylbenzylammonium (zephiramine; zeph) ions, which are cationic surfactants. The data are taken from the literature or were obtained in this work. The work described here was designed to examine the conditions for the formation of ternary complexes of SC, Y and La with the above-mentioned reagents in order to establish optimum systems for spectrophotometric purposes, and also to try to explain some aspects of the formation mechanism of such ternary complexes. Previous papers have dealt with similar ternary complexes of aluminium [ 131 and vanadium [ 141.
310 TABLE 1 Some data on spectrophotometric systems Binary system
E (10’ 1 mol-’ cm+)
M:R*
SFECR S?PV Y-ECR
5.2 1.7 2.9
Y-PV La-ECR La-PV
2.6 2.4 0.9
methods for SC, Y and La based on binary and ternary Ref. Ternary system
;104 1 mol-’ cm-l)
1:2 1:l 1:2
4 5 4
SdAS-CTA Sc-CAS-zeph
NDb 1:2 1:l
6 4 7
S&AS-CP Sc-ECR-CTA SFECR-CP SFPV-CTA Y-CAS-CTA Y-PV-zeph
*R is the triphenylmethane
M:Ra
Ref.
13.2 13.7 15.0
1:2 NDb 1:2
8
14.2 15.0 9.2
1:2 1:2 1:2
6.9 3.2 3.3
1:2 1:2 1:2
9
This work 10 11 This work 12 8 This work
dye. bND means no data.
EXPERIMENTAL
Reagents and apparatus For the scandium standard solution (1 mg SC ml-‘), 0.1530 g of scandium oxide was dissolved in 10 ml of hot 2 M hydrochloric acid and the solution was diluted with water to exactly 100 ml. For the yttrium standard solution (1 mg Y ml-‘), 0.1270 g of yttrium oxide was dissolved in 5 ml of hot hydrochloric acid (1 + 1) and the solution was diluted with water to exactly 100 ml. For the lanthanum standard solution (1 mg La ml-‘), 0.1170 g of lanthanum oxide was dissolved in 5 ml of hot hydrochloric acid (1 + 1) and the solution was diluted with water to exactly 100 ml. The oxides used were first dehydrated and freed from CO2 by ignition. Chrome azurol S (BDH), eriochrome cyanine R (Loba) and pyrocatechol violet (POCh) were purified as described earlier [15]. Aqueous solutions (5 X lo4 M) of the reagents were used. Cetyltrimethylammonium bromide (International Enzymes), cetylpyridinium chloride (Loba) and zephiramine (ICN) were used as aqueous solutions (5 X lo3 M). The absorbances were measured with a VSUB-P spectrophotometer and the absorption spectra were recorded on a Specord UV-VIS spectrophotometer. The pH was measured with an ELPO N-517 pH-meter. Geneml procedure A solution containing an appropriate amount of scandium, yttrium or lanthanum was placed in a 25-ml beaker. The solutions of triphenylmethane dye and cationic surfactant were added and the pH was adjusted to the
311
optimum value for the system examined. The solution was transferred to a 25ml volumetric flask and diluted with water to volume. After 5 min, the absorbance of the solution was measured at the appropriate wavelength against a reagent blank. In the examination of the ternary complexes with chelating triphenylmethane reagents and cationic surfactants, the metal ion concentrations were 7.5 X 10” M (SC) and 1.5 X lo* M (Y and La). Recommended procedure for scandium with ECR and CP A solution containing not more than 10 pg of scandium is placed in a 25-ml beaker and 5 ml of the ECR solution and 5 ml of the CP solution are added. The pH is adjusted to 5.5 f 0.1, and the solution is transferred to a 25-ml volumetric flask and diluted with water to volume. After 5 min, the absorbance of the solution is measured at 600 nm against a reagent blank. RESULTS
The ternary systems of yttrium with ECR and a cationic surfactant cannot be regarded as a basis for spectrophotometric methods. When CTA or zephiramine is used, the complexes form only to a small extent at about pH 5.5. In the Y-ECR-CP system, no ternary complex is formed in the pH range 4-9. It was shown that lanthanum does not form ternary complexes with CAS or ECR and a cationic surfactant in the pH range 2-11. Introduction of a cationic surfactant into the La-PV system at about pH 9 causes a shift of the peak maximum of the absorption spectra from 600 to 670 nm, which proves the formation of ternary complexes; but the absorbances measured are so small that none of these ternary systems can be regarded as the basis of a sensitive spectrophotometric method for lanthanum. Ternary systems of scandium and yttrium with chrome azurol S and a ca tionic surfac tan t The optimum pH ranges for the formation of ternary complexes of scandium and yttrium with CAS and a cationic surfactant do not differ substantially. Scandium complexes form at pH 5.2 + 0.4 (zeph), 5.5 f 0.2 (CTA) and 5.5 * 0.1 (CP); yttrium complexes form at 5.1 + 0.1 (CTA) and 5.2 f 0.1 (CP, zeph). The molar excesses of CAS with respect to metal that guarantee the greatest absorbances are relatively narrow: for scandium, the optimal molar excesses are 2-3 (zeph), 4-6 (CP) and 4-8 (CPA) ; and for yttrium, the optimal molar excesses are 3-4 (CP), 7-8 (CTA) and 9-10 (zeph) (all at a loo-fold molar excess of the surfactant with respect to the metal). Any increase in the excess of chromophoric reagent above the optimum leads to the occurrence of hypsochromic effects and lower absorbances. Cationic surfactants ensure the greatest absorbances if they are used in excesses above the turbidity ranges [13]. The optimum molar excesses of
312
cationic surfactants with respect to metal are: for scandium 50-100 (CP), loo-125 (zeph) and 100-150 (CTA), and for yttrium 25-30 (CTA, CP) and 100-120 (zeph). The influence of increasing amounts of CP on the absorption spectra in the ternary scandium-CAS-CP system is shown in Fig. 1. The absorption maximum of the ternary complexes of scandium and yttrium with CAS and the cationic surfactants examined is situated at 610 nm. The complexes with CTA and zeph are stable with time; the absorbances reach their maximum 5 min after mixing of the reagents and establishing the pH, and then remain unchanged for 30 min. If CP is used, the absorbances reach their maximum in 5 min and then decrease. The mole ratios CAS:Sc and CAS:Y in the ternary complexes, evaluated by Job’s method of isomolar series are 1.5:1 for the Sc-CAS-CTA and YCAS-zeph systems, slightly below 2:l for the systems of both these metals with CP, and 2:l for the Sc-CAS-zeph and Y-CAS-CTA systems. The mole ratio of scandium to the cationic surfactant was not evaluated because the complexes with maximum absorbance are formed at a large excess of the cationic surfactant and undoubtedly micelles rather than simple ions participate in the reaction. Curves showing the dependence of absorbance on scandium concentration based on the systems with CAS and a cationic surfactant satisfy Beer’s law up to a concentration of 0.4 ,ug SC ml -l. Molar absorptivities (at 610 nm) are: 1.50 X lo5 (zeph), 1.42 X lo5 (CP) and 1.22 X lo5 1mol-1 cm-’ (CTA). As far as yttrium is concerned, the spectrophotometric methods based on the CAS ternary systems are considerably less sensitive. The linear calibration ranges and molar absorptivities are: 0.5-1.6 c(g Y ml-’ and 8.3 X lo3 1 mol-’ cm-’ at 610 nm (zeph), and 0.5-1.6 fig Y ml-’ and 8.0 X lo3 1 mol-’ cm-’ at 610 nm (CTA). The standard curve for the Y-CAS-CP system does not satisfy Beer’s law.
Fig. 1. Effect of the excess of CP with respect to scandium on absorption spectra in the Sc-CAS-CP system. ca = ‘7.5 X lo4 M, CC&? = 3.8 x 10-I M, pH 5.5 f 0.1. Molar excess of CP: (1) without CP; (2) 3:l; (3) 1O:l; (4) 25:l; (5) 5O:l; (6) 15O:l. (A turbidity appears between the molar ratio 1O:l and 25:l.)
313
Ternary systems of scandium with eriochrome cyanine R and a cationic surfac tan t The optimum pH ranges for the formation of ternary complexes of scandium with ECR and a surfactant are: 5.5 f 0.1 (CP), 5.5 f 0.2 (CTA) and 5.5 f 0.2 (zeph). Maximum and reproducible absorbances are ensured by making the molar excess of ECR with respect to scandium 8-10 with zephiramine, lo-15 with CTA, and 15-20 with CP. As in the case of the CAS systems, increases in the amounts of the chromophoric reagent lead to the hypsochromic shift of the absorption maximum. Absorbances are maximal for the following excesses of cationic surfactants with respect to scandium: 40-100 (CTA), 50-150 (CP), and 50-200 (zeph). At these concentrations of the surfactant, there is no turbidity. The changes in the absorption spectra of the complexes caused by increasing amounts of CP are shown in Fig. 2. The stability with time of the ternary complexes of scandium with ECR and a surfactant is similar to that found for the CAS complexes. The mole ratios ECR:Sc in ternary complexes evaluated by Job’s method are 2:l in the presence of CTA and a little over 2:l when CP or zephiramine is used. The spectrophotometric methods of measuring scandium based on these ternary systems with ECR exhibit lower sensitivity than those based on CAS. The absorption maximum of the complexes is situated at 600 mn. Molar absorptivities are: 9.8 X lo4 (zeph), 9.3 X lo4 (CTA) and 9.2 X lo4 1 mol-’ cm-’ (CP). Standard curves satisfy Beer’s law up to the concentration of 0.4 /.Ig SC ml-‘. Ternary systems of scandium and yttrium with pyrocatechol violet and a cationic surfactant The optimum pH ranges for the formation of the ternary complexes of scandium and yttrium with PV and the surfactant lie at higher pH than in
Fig. 2. Effect of the excess of CP with respect to scandium on absorption spectra in the SC-ECR-CP system. csc = 7.5 X lo* M, CECR = 1.2 x lOa M, pH 5.5 2 0.1. Molar excess of CP:(l) without CP;(2) 3:1;(3) 10:1;(4) 50:1;(5) 2OO:l. Fig. 3. Effect of the excess of CP with respect to scandium on absorption spectra in the SC-PV-CP system. csc = 1.5 x 10” M, cpv = 4.5 x 10” M, pH 8.5 * 0.2. Molar excess of CP: (1) without CP;(2) 5:1;(3) 10:1;(4) 25:1;(5) 100:1;(6) 150:l.
314
the case of CAS and ECR. The ranges are: for scandium, 8.0 k 0.3 (zeph), 8.5 + 0.2 (CP) and 8.5 + 0.3 (CTA), and for yttrium, 9.0 If:0.1 (CTA, zeph) and 9.0 +_ 0.2 (CP). Maximum absorbances are obtained when the molar excesses of PV with respect to metal are as follows: 3-4 (SC, CTA or CP), 4-5 (SC, zeph), 5-10 (Y, CTA) and 8-10 (Y, CP or zeph). As in the ternary complexes with CAS and ECR, increasing amounts of PV cause hypsochromic shifts of the maximum wavelength. Molar excesses of the surfactant must be greater than the turbidity ranges for the formation of these ternary complexes. The optimum excesses are as follows: for scandium, 40-50 (CTA), 40-125 (CP) and 125-175 (zeph), and for yttrium, 50-75 (CTA), 50-100 (CP) and 75-100 (zeph). With only small excesses of the surfactant, the ternary complexes with PV are not formed at all (in contrast to CAS and ECR). Increasing amounts of the surfactant do not lead to hypsochromic shifts (Fig. 3) such as are characteristic of the complexes with CAS and ECR. The stability with time of these coloured ternary complexes of scandium and yttrium is similar to that found in the CAS systems. Maximum absorbantes appear 5 min after the system reaches the optimum PH. The absorbantes of the solutions of the complexes with CP decrease after 5 min. The mole ratios of PV to metal ion evaluated by Job’s method are slightly over 2:l for the ternary complexes of both scandium and yttrium. The spectrophotometric methods of measuring scandium based on PV and a surfactant exhibit considerably smaller sensitivity than the methods based on CAS and ECR. Standard curves satisfy Beer’s law up to a scandium concentration of 0.7 pg ml-’ (CTA, CP) or in the range 0.2-0.7 pg ml-’ (if zephiramine is the third component), Molar absorptivities are as follows: 5.8 X lo4 at 670 nm (CI’A), 4.7 X lo4 at 690 nm (zeph) and 4.5 X lo4 1 mol-’ cm-’ at 670 nm (CP). Pyrocatechol violet in the presence of a cationic surfactant produces yttrium complexes that show stronger absorbances than the analogous ternary complexes of this metal with CAS. These complexes absorb at 660 nm. If zephiramine is used, the standard curve satisfies Beer’s law up to an yttrium concentration of 1.2 I-(g ml-‘. The range of measurement for the methods based on CTA and CP is 0.1-1.0 pg Y ml-‘. Molar absorptivities are as follows: 3.3 X lo4 (zeph), 3.0 X lo4 (CP) and 2.4 X lo4 (CTA) 1 mol-’ cm-‘. Determination of scandium with ECR and CP in the presence of yttrium and lanthanum As has been said above, yttrium forms ternary complexes with CAS and a cationic surfactant. With ECR and CP, yttrium does not form ternary complexes. Therefore the system with ECR and CP provides the most selective method for determining scandium. The procedure recommended is given under Experimental. The influence of yttrium and lanthanum on this spectrophotometric determination of scandium was examined. The results are presented in
315
Table 2. Above certain limits, both metals cause an increase in the results obtained for microgram quantities of scandium. In these cases, a preliminary separation of scandium by extraction or ion-exchange methods becomes necessary [ 16,171. DISCUSSION
The work described above shows that the most sensitive spectrophotometric methods of determination are as follows: for scandium in the system of Sc-CAS-zeph (E = 1.50 X lo5 1mol-’ cm-’ at 610 nm, pH 5.2 + 0.4) and for yttrium in the system of Y-PV-zeph (E = 3.3 X lo4 1 mol-’ cm-’ at 660 nm, pH 9.0 -+ 0.1). The determination of scandium with ECR and CP is of analytical interest, because the influence of yttrium is the least in this system. Lanthanum does not form ternary complexes with CAS, ECR or PV in the pH range 2-11 in the presence of CTA, CP or zephiramine. This is due to the high basicity of this metal; its reactive forms, hydrolyzed ions, appear only at pH >9. The sensitivity of these spectrophotometric methods based on the ternary systems with CAS, ECR or PV depends mainly on the triphenylmethane reagent. In a weakly acidic or neutral medium (pH 3--8), ECR makes it possible to achieve very high sensitivities for determinations of easily hydrolyzed metals such as aluminium [ 131 and vanadium(IV) [ 141; this reagent reacts with highly hydrolyzed metal forms, mainly VO(OH)2 and A1(OH)S [18]. Methods based on CAS show the highest molar absorptivities for scandium, a metal of stronger basicity. Weakly hydrolyzed ions SC(OH)~* [ 18 ] take part in the formation of the ternary complexes of scandium with CAS. Pyrocatechol violet reacts with more hydrolyzed metal ions such as Y(OH)z-” and VO(OH)2. The highest sensitivities for methods based on ternary systems with PV, in comparison with CAS or ECR, were obtained for yttrium, a comparatively basic metal. In the optimum pH ranges for the formation of ternary complexes with a TABLE 2 Spectrophotometric determination of scandium (2.0 pg) with ECR and CP in the presence of yttrium and lanthanum Y added (rg) 5 20 50 ‘5 rg Y also added.
Sc found (pg) 2.0, 1.9, 2.1,2.0, 2.3, 2.1, 2.4, 2.5, 3.1, 3.0,
1.9 1.9 2.2 2.5 3.2
La added (rg)
SC found (rcg)
20 50 100 208
2.0, 2.6, 3.1, 2.5,
2.0, 2.1 2.5, 2.6 3.1, 3.2 2.5,2.5
316
Pyrocatechol violet
Eriochrome cyanine R
Chrome azurol S
cationic surfactant, CAS and ECR are present in the forms HzR2- * HR3with the equilibrium tending to the right, whereas PV appears in the forms HSR- * H2R2-. Higher sensitivities of methods based on PV are obtained at lower pH values, at which the H3R- form prevails. The much greater sensitivity of spectrophotometric methods based on these ternary systems with a cationic surfactant, compared with the analogous binary systems, is due to the bigger mole ratio of triphenylmethane dye to metal in the ternary complexes. The ternary complexes discussed absorb at longer wavelengths than the chromophoric reagents themselves or their binary complexes with metals. In the series of metals V(IV), Y, SC, Al, the wavelengths of maximum absorption change from 600 nm for vanadium(IV) through 610 nm for Y and SC) to 630 nm for aluminium for the complexes with CAS and a surfactant. Bathochromic shifts result not only from bonding between the metal ion and the phenolic oxygen atom affecting the conjugated system of the triphenylmethane reagent, but also from interactions of the cationic surfactants with n-electrons of these bonds. The metal-phenolic oxygen bonding is of a mixed ionic-covalent character [ 191. The greater is the participation of the ionic bond, the longer is the maximum wavelength; therefore, the smaller the size of the metal ion and the greater its charge, the longer is the wavelength at which the absorption maximum of the ternary complex is situated. The influence of ionic bonds on the location of the absorption maximum of ternary complexes is also observed when the excess of cationic surfactant with respect to metal is changed. Below the turbidity range, the complexes, which are formed mainly with ionic forms of the surfactant, absorb at much longer wavelengths (Fig. 1) than the complexes formed at greater concentrations of the surfactant, which is usually then present as micelles. Cationic surfactants cause strong bathochromic shifts because of electrostatic and hydrophobic interactions with the chromophoric reagent ions. The difference in position of the maximum wavelength with different cationic surfactants is usually small (5-10 nm). The ternary complexes of aluminium and scandium with CAS absorb at the longest wavelength when CP is the third component. For complexes of these metals with ECR or PV, zephir-
317
amine causes the strongest bathochromic shift. When the binary complexes of a metal ion with a triphenyhnethane reagent are solubilized with a micellar cationic surfactant, the role played by electrostatic interactions predominates in the case of chrome azurol S, whereas hydrophobic interactions predominate with eriochrome cyanine R or pyrocatechol violet. REFERENCES 1 V. N. Tikhonov, Zh. Anal. Khim., 32 (1977) 1436. 2 S. B. Sawin, Crit. Rev. Anal. Chem., 8 (1979) 65. 3 Z. Marczenko, Crit. Rev. Anai. Chem., ll(l981) 196. 4 V. N. Tikhonov and S. N. Fedotova, Zh. Anal. Khim., 37 (1982) 1888. 5 S. P. Onosova and G. K. Kuncevitch, Zh. Anal. Khim., 20 (1966) 802. 6 J. P. Young, I. C. White and R. G. BaU, Anal. C&em., 32 (1960) 1264. 7 T. Takano, Bunseki Kagaku, 15 (1966) 1087. 8 V. N. Tikhonov and S. G. Danilova, Zh. Anal. Khim., 35 (1980) 1264. 9 Y. Horiuchi and H. Nishida, Bunseki Kagaku, 17 (1968) 756. 10 J. Jurkevfitute and M. MaIat, Collect. Czech. Chem. Commun, 44 (1979) 3236. 11 V. N. Tikhonov and V. T. Samarkina, Khim. Khim. Tekhnol., 21(1978) 1116. 12 V. N. Tikhonov and 0. K. Pavlova, Zh. Anal. Khim., 37 (1982) 1809. 13 Z. Marczenko and M. Jarosz, AnaIyst (London), 107 (1982) 1431. 14 M. Jarosz and Z. Marczenko, Analyst (London), 109 (1984) 35. 15 F. J. Langmyhr and K. S. Klausen, Anal. Chim. Acta, 29 (1963) 149. 16 Z. Marczenko, Spectrophotometric Determination of Elements, Ellis Horwood, Chichester, 1976. 17 E. B. SandeIl and H. Or&hi, Photometric Determination of Traces of Metals. General Aspects. Wiley, New York, 1978. 18 V. A. Nazarenko, V. P. Antcnovich and E. M. Nevskaa, Metal Ions Hydrolysis in Dilute Solutions (in Russian), Atomizdat, Moskva, 1979. 19 S. Murakami and T. Yoshino, Talanta, 28 (1981) 623.
in The Netherlands
SPECTROPHOTOMETRIC STUDY OF REACTIONS OF SCANDIUM, YTTRIUM AND LANTHANUM IONS WITH SOME TRIPHENYLMETHANE DYES IN THE PRESENCE OF CATIONIC SURFACTANTS
M. JAROSZ and 2. MARCZENKO*
Department
of Analy tical Chemistry, Technical University, 00-664 Warsaw (Poland)
(Received 23rd September 1983)
SUMMARY Optimum conditions for the formation of ternary complexes of scandium, yttrium and lanthanum ions with chrome azurol S, eriochrome cyanine R and pyrocatechol violet in the presence of cetyltimethylammonium, cetylpyridinium and tetradecyldimethylbenzylammonium (zephhamine) ions are described. The spectrophotometric determination of scandium with chrome azurol S and zephiramine exhibits the greatest sensitivity (E = 1.50 x lo* 1 mol-’ cm-i at 610 nm). In the spectrophotometric determination of scandium with eriochrome cyanine R and cetylpyridinium ion (E = 9.2 X 10’ at 600 nm), the interference caused by yttrium is the least. In the best method for yttrium (with pyrocatechol violet and zephiramine), the molar absorptivity is 3.3 X 10’ at 660 nm. Lanthanum does not form ternary complexes of analytical interest in these systems. Some aspects of the formation of ternary complexes with cationic surfactants are discussed.
Ternary systems containing metals that hydrolyze in weakly acidic or neutral medium, chelating triphenylmethane reagents and cationic surfactar&, form the basis of very sensitive spectrophotometric methods [l-3]. Methods involving binary systems (without the surfactant) provide considerably smaller sensitivities. Table 1 shows a comparison of molar absorptivities for the spectrophotometric determinations of scandium, yttrium and lanthanum based on binary complexes with chrome azurol S (CAS), eriochrome cyanine R (ECR) and pyrocatechol violet (PV), as well as those based on ternary complexes with these chromophoric reagents and cetyltrimethylammonium (CTA), cetylpyridinium (CP) or tetradecyldimethylbenzylammonium (zephiramine; zeph) ions, which are cationic surfactants. The data are taken from the literature or were obtained in this work. The work described here was designed to examine the conditions for the formation of ternary complexes of SC, Y and La with the above-mentioned reagents in order to establish optimum systems for spectrophotometric purposes, and also to try to explain some aspects of the formation mechanism of such ternary complexes. Previous papers have dealt with similar ternary complexes of aluminium [ 131 and vanadium [ 141.
310 TABLE 1 Some data on spectrophotometric systems Binary system
E (10’ 1 mol-’ cm+)
M:R*
SFECR S?PV Y-ECR
5.2 1.7 2.9
Y-PV La-ECR La-PV
2.6 2.4 0.9
methods for SC, Y and La based on binary and ternary Ref. Ternary system
;104 1 mol-’ cm-l)
1:2 1:l 1:2
4 5 4
SdAS-CTA Sc-CAS-zeph
NDb 1:2 1:l
6 4 7
S&AS-CP Sc-ECR-CTA SFECR-CP SFPV-CTA Y-CAS-CTA Y-PV-zeph
*R is the triphenylmethane
M:Ra
Ref.
13.2 13.7 15.0
1:2 NDb 1:2
8
14.2 15.0 9.2
1:2 1:2 1:2
6.9 3.2 3.3
1:2 1:2 1:2
9
This work 10 11 This work 12 8 This work
dye. bND means no data.
EXPERIMENTAL
Reagents and apparatus For the scandium standard solution (1 mg SC ml-‘), 0.1530 g of scandium oxide was dissolved in 10 ml of hot 2 M hydrochloric acid and the solution was diluted with water to exactly 100 ml. For the yttrium standard solution (1 mg Y ml-‘), 0.1270 g of yttrium oxide was dissolved in 5 ml of hot hydrochloric acid (1 + 1) and the solution was diluted with water to exactly 100 ml. For the lanthanum standard solution (1 mg La ml-‘), 0.1170 g of lanthanum oxide was dissolved in 5 ml of hot hydrochloric acid (1 + 1) and the solution was diluted with water to exactly 100 ml. The oxides used were first dehydrated and freed from CO2 by ignition. Chrome azurol S (BDH), eriochrome cyanine R (Loba) and pyrocatechol violet (POCh) were purified as described earlier [15]. Aqueous solutions (5 X lo4 M) of the reagents were used. Cetyltrimethylammonium bromide (International Enzymes), cetylpyridinium chloride (Loba) and zephiramine (ICN) were used as aqueous solutions (5 X lo3 M). The absorbances were measured with a VSUB-P spectrophotometer and the absorption spectra were recorded on a Specord UV-VIS spectrophotometer. The pH was measured with an ELPO N-517 pH-meter. Geneml procedure A solution containing an appropriate amount of scandium, yttrium or lanthanum was placed in a 25-ml beaker. The solutions of triphenylmethane dye and cationic surfactant were added and the pH was adjusted to the
311
optimum value for the system examined. The solution was transferred to a 25ml volumetric flask and diluted with water to volume. After 5 min, the absorbance of the solution was measured at the appropriate wavelength against a reagent blank. In the examination of the ternary complexes with chelating triphenylmethane reagents and cationic surfactants, the metal ion concentrations were 7.5 X 10” M (SC) and 1.5 X lo* M (Y and La). Recommended procedure for scandium with ECR and CP A solution containing not more than 10 pg of scandium is placed in a 25-ml beaker and 5 ml of the ECR solution and 5 ml of the CP solution are added. The pH is adjusted to 5.5 f 0.1, and the solution is transferred to a 25-ml volumetric flask and diluted with water to volume. After 5 min, the absorbance of the solution is measured at 600 nm against a reagent blank. RESULTS
The ternary systems of yttrium with ECR and a cationic surfactant cannot be regarded as a basis for spectrophotometric methods. When CTA or zephiramine is used, the complexes form only to a small extent at about pH 5.5. In the Y-ECR-CP system, no ternary complex is formed in the pH range 4-9. It was shown that lanthanum does not form ternary complexes with CAS or ECR and a cationic surfactant in the pH range 2-11. Introduction of a cationic surfactant into the La-PV system at about pH 9 causes a shift of the peak maximum of the absorption spectra from 600 to 670 nm, which proves the formation of ternary complexes; but the absorbances measured are so small that none of these ternary systems can be regarded as the basis of a sensitive spectrophotometric method for lanthanum. Ternary systems of scandium and yttrium with chrome azurol S and a ca tionic surfac tan t The optimum pH ranges for the formation of ternary complexes of scandium and yttrium with CAS and a cationic surfactant do not differ substantially. Scandium complexes form at pH 5.2 + 0.4 (zeph), 5.5 f 0.2 (CTA) and 5.5 * 0.1 (CP); yttrium complexes form at 5.1 + 0.1 (CTA) and 5.2 f 0.1 (CP, zeph). The molar excesses of CAS with respect to metal that guarantee the greatest absorbances are relatively narrow: for scandium, the optimal molar excesses are 2-3 (zeph), 4-6 (CP) and 4-8 (CPA) ; and for yttrium, the optimal molar excesses are 3-4 (CP), 7-8 (CTA) and 9-10 (zeph) (all at a loo-fold molar excess of the surfactant with respect to the metal). Any increase in the excess of chromophoric reagent above the optimum leads to the occurrence of hypsochromic effects and lower absorbances. Cationic surfactants ensure the greatest absorbances if they are used in excesses above the turbidity ranges [13]. The optimum molar excesses of
312
cationic surfactants with respect to metal are: for scandium 50-100 (CP), loo-125 (zeph) and 100-150 (CTA), and for yttrium 25-30 (CTA, CP) and 100-120 (zeph). The influence of increasing amounts of CP on the absorption spectra in the ternary scandium-CAS-CP system is shown in Fig. 1. The absorption maximum of the ternary complexes of scandium and yttrium with CAS and the cationic surfactants examined is situated at 610 nm. The complexes with CTA and zeph are stable with time; the absorbances reach their maximum 5 min after mixing of the reagents and establishing the pH, and then remain unchanged for 30 min. If CP is used, the absorbances reach their maximum in 5 min and then decrease. The mole ratios CAS:Sc and CAS:Y in the ternary complexes, evaluated by Job’s method of isomolar series are 1.5:1 for the Sc-CAS-CTA and YCAS-zeph systems, slightly below 2:l for the systems of both these metals with CP, and 2:l for the Sc-CAS-zeph and Y-CAS-CTA systems. The mole ratio of scandium to the cationic surfactant was not evaluated because the complexes with maximum absorbance are formed at a large excess of the cationic surfactant and undoubtedly micelles rather than simple ions participate in the reaction. Curves showing the dependence of absorbance on scandium concentration based on the systems with CAS and a cationic surfactant satisfy Beer’s law up to a concentration of 0.4 ,ug SC ml -l. Molar absorptivities (at 610 nm) are: 1.50 X lo5 (zeph), 1.42 X lo5 (CP) and 1.22 X lo5 1mol-1 cm-’ (CTA). As far as yttrium is concerned, the spectrophotometric methods based on the CAS ternary systems are considerably less sensitive. The linear calibration ranges and molar absorptivities are: 0.5-1.6 c(g Y ml-’ and 8.3 X lo3 1 mol-’ cm-’ at 610 nm (zeph), and 0.5-1.6 fig Y ml-’ and 8.0 X lo3 1 mol-’ cm-’ at 610 nm (CTA). The standard curve for the Y-CAS-CP system does not satisfy Beer’s law.
Fig. 1. Effect of the excess of CP with respect to scandium on absorption spectra in the Sc-CAS-CP system. ca = ‘7.5 X lo4 M, CC&? = 3.8 x 10-I M, pH 5.5 f 0.1. Molar excess of CP: (1) without CP; (2) 3:l; (3) 1O:l; (4) 25:l; (5) 5O:l; (6) 15O:l. (A turbidity appears between the molar ratio 1O:l and 25:l.)
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Ternary systems of scandium with eriochrome cyanine R and a cationic surfac tan t The optimum pH ranges for the formation of ternary complexes of scandium with ECR and a surfactant are: 5.5 f 0.1 (CP), 5.5 f 0.2 (CTA) and 5.5 f 0.2 (zeph). Maximum and reproducible absorbances are ensured by making the molar excess of ECR with respect to scandium 8-10 with zephiramine, lo-15 with CTA, and 15-20 with CP. As in the case of the CAS systems, increases in the amounts of the chromophoric reagent lead to the hypsochromic shift of the absorption maximum. Absorbances are maximal for the following excesses of cationic surfactants with respect to scandium: 40-100 (CTA), 50-150 (CP), and 50-200 (zeph). At these concentrations of the surfactant, there is no turbidity. The changes in the absorption spectra of the complexes caused by increasing amounts of CP are shown in Fig. 2. The stability with time of the ternary complexes of scandium with ECR and a surfactant is similar to that found for the CAS complexes. The mole ratios ECR:Sc in ternary complexes evaluated by Job’s method are 2:l in the presence of CTA and a little over 2:l when CP or zephiramine is used. The spectrophotometric methods of measuring scandium based on these ternary systems with ECR exhibit lower sensitivity than those based on CAS. The absorption maximum of the complexes is situated at 600 mn. Molar absorptivities are: 9.8 X lo4 (zeph), 9.3 X lo4 (CTA) and 9.2 X lo4 1 mol-’ cm-’ (CP). Standard curves satisfy Beer’s law up to the concentration of 0.4 /.Ig SC ml-‘. Ternary systems of scandium and yttrium with pyrocatechol violet and a cationic surfactant The optimum pH ranges for the formation of the ternary complexes of scandium and yttrium with PV and the surfactant lie at higher pH than in
Fig. 2. Effect of the excess of CP with respect to scandium on absorption spectra in the SC-ECR-CP system. csc = 7.5 X lo* M, CECR = 1.2 x lOa M, pH 5.5 2 0.1. Molar excess of CP:(l) without CP;(2) 3:1;(3) 10:1;(4) 50:1;(5) 2OO:l. Fig. 3. Effect of the excess of CP with respect to scandium on absorption spectra in the SC-PV-CP system. csc = 1.5 x 10” M, cpv = 4.5 x 10” M, pH 8.5 * 0.2. Molar excess of CP: (1) without CP;(2) 5:1;(3) 10:1;(4) 25:1;(5) 100:1;(6) 150:l.
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the case of CAS and ECR. The ranges are: for scandium, 8.0 k 0.3 (zeph), 8.5 + 0.2 (CP) and 8.5 + 0.3 (CTA), and for yttrium, 9.0 If:0.1 (CTA, zeph) and 9.0 +_ 0.2 (CP). Maximum absorbances are obtained when the molar excesses of PV with respect to metal are as follows: 3-4 (SC, CTA or CP), 4-5 (SC, zeph), 5-10 (Y, CTA) and 8-10 (Y, CP or zeph). As in the ternary complexes with CAS and ECR, increasing amounts of PV cause hypsochromic shifts of the maximum wavelength. Molar excesses of the surfactant must be greater than the turbidity ranges for the formation of these ternary complexes. The optimum excesses are as follows: for scandium, 40-50 (CTA), 40-125 (CP) and 125-175 (zeph), and for yttrium, 50-75 (CTA), 50-100 (CP) and 75-100 (zeph). With only small excesses of the surfactant, the ternary complexes with PV are not formed at all (in contrast to CAS and ECR). Increasing amounts of the surfactant do not lead to hypsochromic shifts (Fig. 3) such as are characteristic of the complexes with CAS and ECR. The stability with time of these coloured ternary complexes of scandium and yttrium is similar to that found in the CAS systems. Maximum absorbantes appear 5 min after the system reaches the optimum PH. The absorbantes of the solutions of the complexes with CP decrease after 5 min. The mole ratios of PV to metal ion evaluated by Job’s method are slightly over 2:l for the ternary complexes of both scandium and yttrium. The spectrophotometric methods of measuring scandium based on PV and a surfactant exhibit considerably smaller sensitivity than the methods based on CAS and ECR. Standard curves satisfy Beer’s law up to a scandium concentration of 0.7 pg ml-’ (CTA, CP) or in the range 0.2-0.7 pg ml-’ (if zephiramine is the third component), Molar absorptivities are as follows: 5.8 X lo4 at 670 nm (CI’A), 4.7 X lo4 at 690 nm (zeph) and 4.5 X lo4 1 mol-’ cm-’ at 670 nm (CP). Pyrocatechol violet in the presence of a cationic surfactant produces yttrium complexes that show stronger absorbances than the analogous ternary complexes of this metal with CAS. These complexes absorb at 660 nm. If zephiramine is used, the standard curve satisfies Beer’s law up to an yttrium concentration of 1.2 I-(g ml-‘. The range of measurement for the methods based on CTA and CP is 0.1-1.0 pg Y ml-‘. Molar absorptivities are as follows: 3.3 X lo4 (zeph), 3.0 X lo4 (CP) and 2.4 X lo4 (CTA) 1 mol-’ cm-‘. Determination of scandium with ECR and CP in the presence of yttrium and lanthanum As has been said above, yttrium forms ternary complexes with CAS and a cationic surfactant. With ECR and CP, yttrium does not form ternary complexes. Therefore the system with ECR and CP provides the most selective method for determining scandium. The procedure recommended is given under Experimental. The influence of yttrium and lanthanum on this spectrophotometric determination of scandium was examined. The results are presented in
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Table 2. Above certain limits, both metals cause an increase in the results obtained for microgram quantities of scandium. In these cases, a preliminary separation of scandium by extraction or ion-exchange methods becomes necessary [ 16,171. DISCUSSION
The work described above shows that the most sensitive spectrophotometric methods of determination are as follows: for scandium in the system of Sc-CAS-zeph (E = 1.50 X lo5 1mol-’ cm-’ at 610 nm, pH 5.2 + 0.4) and for yttrium in the system of Y-PV-zeph (E = 3.3 X lo4 1 mol-’ cm-’ at 660 nm, pH 9.0 -+ 0.1). The determination of scandium with ECR and CP is of analytical interest, because the influence of yttrium is the least in this system. Lanthanum does not form ternary complexes with CAS, ECR or PV in the pH range 2-11 in the presence of CTA, CP or zephiramine. This is due to the high basicity of this metal; its reactive forms, hydrolyzed ions, appear only at pH >9. The sensitivity of these spectrophotometric methods based on the ternary systems with CAS, ECR or PV depends mainly on the triphenylmethane reagent. In a weakly acidic or neutral medium (pH 3--8), ECR makes it possible to achieve very high sensitivities for determinations of easily hydrolyzed metals such as aluminium [ 131 and vanadium(IV) [ 141; this reagent reacts with highly hydrolyzed metal forms, mainly VO(OH)2 and A1(OH)S [18]. Methods based on CAS show the highest molar absorptivities for scandium, a metal of stronger basicity. Weakly hydrolyzed ions SC(OH)~* [ 18 ] take part in the formation of the ternary complexes of scandium with CAS. Pyrocatechol violet reacts with more hydrolyzed metal ions such as Y(OH)z-” and VO(OH)2. The highest sensitivities for methods based on ternary systems with PV, in comparison with CAS or ECR, were obtained for yttrium, a comparatively basic metal. In the optimum pH ranges for the formation of ternary complexes with a TABLE 2 Spectrophotometric determination of scandium (2.0 pg) with ECR and CP in the presence of yttrium and lanthanum Y added (rg) 5 20 50 ‘5 rg Y also added.
Sc found (pg) 2.0, 1.9, 2.1,2.0, 2.3, 2.1, 2.4, 2.5, 3.1, 3.0,
1.9 1.9 2.2 2.5 3.2
La added (rg)
SC found (rcg)
20 50 100 208
2.0, 2.6, 3.1, 2.5,
2.0, 2.1 2.5, 2.6 3.1, 3.2 2.5,2.5
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Pyrocatechol violet
Eriochrome cyanine R
Chrome azurol S
cationic surfactant, CAS and ECR are present in the forms HzR2- * HR3with the equilibrium tending to the right, whereas PV appears in the forms HSR- * H2R2-. Higher sensitivities of methods based on PV are obtained at lower pH values, at which the H3R- form prevails. The much greater sensitivity of spectrophotometric methods based on these ternary systems with a cationic surfactant, compared with the analogous binary systems, is due to the bigger mole ratio of triphenylmethane dye to metal in the ternary complexes. The ternary complexes discussed absorb at longer wavelengths than the chromophoric reagents themselves or their binary complexes with metals. In the series of metals V(IV), Y, SC, Al, the wavelengths of maximum absorption change from 600 nm for vanadium(IV) through 610 nm for Y and SC) to 630 nm for aluminium for the complexes with CAS and a surfactant. Bathochromic shifts result not only from bonding between the metal ion and the phenolic oxygen atom affecting the conjugated system of the triphenylmethane reagent, but also from interactions of the cationic surfactants with n-electrons of these bonds. The metal-phenolic oxygen bonding is of a mixed ionic-covalent character [ 191. The greater is the participation of the ionic bond, the longer is the maximum wavelength; therefore, the smaller the size of the metal ion and the greater its charge, the longer is the wavelength at which the absorption maximum of the ternary complex is situated. The influence of ionic bonds on the location of the absorption maximum of ternary complexes is also observed when the excess of cationic surfactant with respect to metal is changed. Below the turbidity range, the complexes, which are formed mainly with ionic forms of the surfactant, absorb at much longer wavelengths (Fig. 1) than the complexes formed at greater concentrations of the surfactant, which is usually then present as micelles. Cationic surfactants cause strong bathochromic shifts because of electrostatic and hydrophobic interactions with the chromophoric reagent ions. The difference in position of the maximum wavelength with different cationic surfactants is usually small (5-10 nm). The ternary complexes of aluminium and scandium with CAS absorb at the longest wavelength when CP is the third component. For complexes of these metals with ECR or PV, zephir-
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amine causes the strongest bathochromic shift. When the binary complexes of a metal ion with a triphenyhnethane reagent are solubilized with a micellar cationic surfactant, the role played by electrostatic interactions predominates in the case of chrome azurol S, whereas hydrophobic interactions predominate with eriochrome cyanine R or pyrocatechol violet. REFERENCES 1 V. N. Tikhonov, Zh. Anal. Khim., 32 (1977) 1436. 2 S. B. Sawin, Crit. Rev. Anal. Chem., 8 (1979) 65. 3 Z. Marczenko, Crit. Rev. Anai. Chem., ll(l981) 196. 4 V. N. Tikhonov and S. N. Fedotova, Zh. Anal. Khim., 37 (1982) 1888. 5 S. P. Onosova and G. K. Kuncevitch, Zh. Anal. Khim., 20 (1966) 802. 6 J. P. Young, I. C. White and R. G. BaU, Anal. C&em., 32 (1960) 1264. 7 T. Takano, Bunseki Kagaku, 15 (1966) 1087. 8 V. N. Tikhonov and S. G. Danilova, Zh. Anal. Khim., 35 (1980) 1264. 9 Y. Horiuchi and H. Nishida, Bunseki Kagaku, 17 (1968) 756. 10 J. Jurkevfitute and M. MaIat, Collect. Czech. Chem. Commun, 44 (1979) 3236. 11 V. N. Tikhonov and V. T. Samarkina, Khim. Khim. Tekhnol., 21(1978) 1116. 12 V. N. Tikhonov and 0. K. Pavlova, Zh. Anal. Khim., 37 (1982) 1809. 13 Z. Marczenko and M. Jarosz, AnaIyst (London), 107 (1982) 1431. 14 M. Jarosz and Z. Marczenko, Analyst (London), 109 (1984) 35. 15 F. J. Langmyhr and K. S. Klausen, Anal. Chim. Acta, 29 (1963) 149. 16 Z. Marczenko, Spectrophotometric Determination of Elements, Ellis Horwood, Chichester, 1976. 17 E. B. SandeIl and H. Or&hi, Photometric Determination of Traces of Metals. General Aspects. Wiley, New York, 1978. 18 V. A. Nazarenko, V. P. Antcnovich and E. M. Nevskaa, Metal Ions Hydrolysis in Dilute Solutions (in Russian), Atomizdat, Moskva, 1979. 19 S. Murakami and T. Yoshino, Talanta, 28 (1981) 623.