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Spectrophotometric determination of acidity constants of Alizarine Red S in water, water-Brij-35 and water-SDS micellar media solutions
Spectrochimica Acta Part A 64 (2006) 660–664
Spectrophotometric determination of acidity constants of Alizarine Red S in water, water-Brij-35 and water-SDS micellar media solutions Ali Niazi a,∗ , Mohammad Ghalie a , Ateesa Yazdanipour a , Jahanbakhsh Ghasemi b a b
Department of Chemistry, Faculty of Sciences, Azad University of Arak, Arak, Iran Department of Chemistry, Faculty of Sciences, Razi University, Kermanshah, Iran
Received 6 May 2005; received in revised form 18 July 2005; accepted 1 August 2005
Abstract The acidity constants of Alizarine Red S in water, water-Brij-35 and water-SDS micellar media solutions at 25 ◦ C and an ionic strength of 0.1 M have been determined spectrophotometrically. To evaluate the pH-absorbance data, a resolution method based on the combination of soft- and hard-modeling is applied. The acidity constants of all related equilibria are estimated using the whole spectral fitting of the collected data to an established factor analysis model. DATAN program applied for determination of acidity constants. Results show that the pKa values of Alizarine Red S are influenced as the percentages of a neutral and an anionic surfactant such as Brij-35 and SDS, respectively, added to the solution of this reagent. Effect of surfactant on acidity constants and pure spectrum of each component are also discussed. © 2005 Elsevier B.V. All rights reserved. Keywords: Alizarine Red S; Acidity constants; Brij-35; SDS; DATAN; Spectrophotometric
1. Introduction Aqueous micellar media are widely used in different areas of analytical chemistry and several reviews concerning their analytical applications have been published [1–4]. One important property of micelles is their ability to solubilize a wide variety of compounds which are insoluble or slightly soluble in water. The incorporation of a solute into micellar systems can lead to important changes in its molecular properties. Another important effect of micellar systems is that they can modify reaction rates and, to some extent, the nature of the products. Micelles can inhibit or accelerate reaction rates (by up to several orders of magnitude) and also shift equilibria (acid–base). Surfactants usually affect spectral parameters: the intensity and shifts in the absorption bands can be increased and shifts in the absorption maxima of reagents are observed [5,6]. Micelles can affect the apparent pKa values of the reagents due to a combination of electrostatic and microenviromental effects of the micelle [7–9]. Moreover, the acid–base equilibria involved in these systems are also influenced by surfactants [10–12].
∗
Corresponding author. Tel.: +98 8613663041x370; fax: +98 8613670017. E-mail address: [email protected] (A. Niazi).
1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2005.08.002
Acid dissociation constants (i.e. pKa values) can be a key parameter for understanding and quantifying chemical phenomena such as reaction rates, biological activity, biological uptake, biological transport and environmental fate [13]. There have been several methods of the determination of acidity constants, including the use of potentiometric titration, spectrophotometry, capillary electrophoresis, and so on. Spectroscopic methods are in general highly sensitive and as such are suitable for studying chemical equilibria in solution. When the components involved in the chemical equilibrium have distinct spectral responses, their concentrations can be measured directly, and the determination of the equilibrium constant is trivial [14]. For many systems, particularly those with similar components, this is not the case, and these have been difficult to analyze. Therefore, to overcome this problem we have to employ the graphical and computational methods. Up to the middle of the 1960s, the evaluation of equilibrium measurements was based on the different graphical methods. These methods were reviewed in considerable details by Rossotti and Rossotti [15]. Starting from middle of the 1960s, computers acquired ever-greater importance in the evaluation of equilibrium measurement data using multiple wavelengths or full spectrum to determining the stability and acidity constants. The most relevant reports are on SPECFIT [16], SQUAD [17] and HYPERQUAD [18]. All these computational approaches are based on an initial proposal of
A. Niazi et al. / Spectrochimica Acta Part A 64 (2006) 660–664
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2.2. Instrumentation
Scheme 1. Chemical structure of Alizarine Red S.
a chemical equilibrium model defining species stoichiometries and based on mass-action law and mass balance equations (hardmodeling methods) and also involve least-squares curve-fitting procedures. The starting point of using soft-modeling was in 1971 that Lawton and Sylvestre [19] introduced chemometrics based method for spectral analysis. These approaches are free from the restriction of the mass-action law and do not require an initial model of species to be set up. Data analysis was carried out by DATAN package that developed by Kubista group [20–22], called the physical constraints approach, which provides a unique solution by requiring that the calculated concentrations obey an assumed equilibrium expression and demonstrates its applicability by determining the acidity constants of two and four protolytic forms of fluorescein. A possible advantage of the Kubista et al. method is that it mixes a soft-modeling approach with a hard-modeling approach. This might be best and more general strategy, since it can handle different situations, with only a partial knowledge of the chemistry of the system. The physical constraints method calculates spectral profiles, concentrations and equilibrium constants by utilizing equilibrium expressions that related the components. The theory and application of physical constraints method was discussed by Kubista et al. in several papers [23–34]. In this work, we applied the physical constraints approach to determine the acidity constants of Alizarine Red S (Scheme 1) in pure water, water-Brij-35 and water-SDS micellar media solutions at 25 ◦ C and an ionic strength of 0.1 M spectrophotometrically. The effects of polyoxyethylenaurylether (Brij-35) as nonionic surfactant and sodium n-dodecyl sulfate (SDS) as anionic surfactant were studied on the dissociation constants and pure spectrum of Alizarine Red S. The analysis is readily performed with the computer program DATAN [20].
A Perkin-Elmer (Lambda 25) spectrophotometer controlled by a computer and equipped with a 1 cm path length quartz cell was used for UV–vis spectra acquisition. Spectra were acquired between 370 and 700 nm. A HORIBA M-12 pH-meter furnished with a combined glass-saturated calomel electrode was calibrated with at least two buffer solutions at pH 3.00 and 9.00 (which pH adjustment contains error with respect to the direct use of the buffer solutions). 2.3. Computer hardware and software All absorption spectra where digitized at five data points per nanometer in the wavelength 370–700 nm and transferred (in ASCII format) to an AMD 2000 XP (256 Mb RAM) computer for subsequent analysis by MATLAB software, version 6.5 (The MathWorks) or for processing by using DATAN package. 2.4. Spectrophotometric titrations For the Alizarine Red S (2 × 10−4 M) in pure water, waterBrij-35 and water-SDS mixtures titrations, absorption spectra were measured with a titration set-up consisting of a computer interfaced to a spectrophotometer. After each pH adjustment, solution is transferred into the cuvette and the absorption spectra are recorded. Ionic strength was maintained at 0.1 M by adding appropriate amounts of KNO3 . All measurements were carried out at the temperature (25 ± 0.5 ◦ C). 3. Results and discussion The absorption spectra of Alizarine Red S in pure water at various pH values at 370–700 nm intervals were recorded. In order to determine the influence of the nonionic surfactant (Brij-35) and the anionic surfactant (SDS) in acidity constants, a series of experiments were run at different Brij-35 and SDS concentrations, above the cmc. Sample spectrum of Alizarine Red S at different pH values in pure water, water-Brij-35 and waterSDS are shown in Figs. 1–3. The principal component analysis of all absorption data matrices obtained at various pH values
2. Experimental 2.1. Materials Alizarine Red S, Brij-35, SDS, hydrochloric acid, sodium hydroxide and potassium nitrate were analytical grade commercial products from Merck. These reagents were used without further purification. Standard stock solution of 8.0 × 10−4 M of Alizarine Red S was prepared by dissolving appropriate amounts of Alizarine Red S in water. The stock solutions of surfactants prepared by dissolving weighted amounts of substances in appropriate amounts of water. All the solutions were prepared in deionized water.
Fig. 1. Absorption spectra of Alizarine Red S in pure water at different pH values: (1) 0.85, (2) 1.49, (3) 1.91, (4) 2.42, (5) 2.90, (6) 3.40, (7) 3.90, (8) 4.40, (9) 4.95, (10) 5.42, (11) 5.92, (12) 6.46, (13) 6.94, (14) 7.30, (15) 7.85, (16) 8.36, (17) 8.94, (18) 9.41, (19) 9.70, (20) 10.06, (21) 10.32, (22) 10.49, (23) 10.79, (24) 11.00, (25) 11.31, (26) 11.55, (27) 12.02.
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A. Niazi et al. / Spectrochimica Acta Part A 64 (2006) 660–664 Table 2 Acidity constants of Alizarine Red S in pure water and at different percentage of SDS (w/v) at 25 ◦ C and constant ionic strength (0.1 M KNO3 )
Fig. 2. Absorption spectra of Alizarine Red S in 0.1% (w/v) Brij-35 to water at different pH values: (1) 1.48, (2) 2.53, (3) 3.54, (4) 4.01, (5) 4.48, (6) 4.95, (7) 5.50, (8) 6.04, (9) 6.50, (10) 6.95, (11) 7.99, (12) 8.48, (13) 8.99, (14) 9.51, (15) 9.99, (16) 10.49, (17) 11.00, (18) 11.51, (19) 12.01, (20) 12.48.
SDS (w/v) (%)
pKa1
pKa2
0.00 0.005 0.01 0.025 0.05 0.10
4.70 ± 0.10 4.77 ± 0.11 4.84 ± 0.11 5.02 ± 0.12 5.11 ± 0.13 5.24 ± 0.12
10.09 ± 0.14 10.51 ± 0.15 11.33 ± 0.15 11.39 ± 0.14 11.41 ± 0.16 11.42 ± 0.15
evidence the presence of three spectroscopically distinguishable components. Their shapes, however, are clearly unphysical and cannot be directly related to the spectral response of the three protolytic forms. The output of the program are pKa values and their standard deviation, the number of principal components, projection vectors (loadings), concentration distribution diagrams, and the pure spectrum of each assumed species. The obtained pKa values are listed in Tables 1 and 2. The previous reported values of acidity constants are mainly in pure water and in mixtures of dioxane with water [35,36]. The obtained values in pure water are in good agreement with previous values [35], which are listed in Table 1 for comparison. The differences observed between the pKa values are due to proba-
Fig. 3. Absorption spectra of Alizarine Red S in 0.1% (w/v) SDS to water at different pH values: (1) 1.35, (2) 2.54, (3) 3.50, (4) 4.02, (5) 4.49, (6) 5.03, (7) 5.58, (8) 6.03, (9) 6.43, (10) 6.92, (11) 7.34, (12) 7.94, (13) 8.61, (14) 9.11, (15) 9.74, (16) 10.13, (17) 10.52, (18) 11.09, (19) 11.55, (20) 12.01, (21) 12.54.
shows at least three significant factors that also supported by the statistical indicators of Elbergali et al. [26]. These factors could be attributed to the two dissociation equilibria of a diprotic acid such Alizarine Red S. The pKa values of Alizarine Red S were investigated in pure water and five different water-Brij-35 and water-SDS mixtures spectrophotometrically at 25 ◦ C and an ionic strength of 0.1 M. Acidity constants of Alizarine Red S in several mixtures were evaluated using the DATAN program using the corresponding spectral absorption-pH data. From inspection of the experimental spectra, it is hard to guess even the number of protolytic species involved. The three calculated most significant projection vectors with clear spectral features (as compared to noise) Table 1 Acidity constants of Alizarine Red S in pure water and at different percentage of Brij-35 (w/v) at 25 ◦ C and constant ionic strength (0.1 M KNO3 ) Brij-35 (w/v) (%)
pKa1
pKa2
0.00 0.005 0.01 0.025 0.05 0.10
5.50a 5.49a 6.10b 4.70 ± 0.10 4.68 ± 0.11 4.68 ± 0.09 4.67 ± 0.15 4.66 ± 0.11 4.65 ± 0.12
11.00a 10.85a 10.80b 10.09 ± 0.14 10.34 ± 0.13 10.75 ± 0.16 10.78 ± 0.14 10.81 ± 0.15 10.85 ± 0.14
a b
Reference [35] (pure water). Reference [36] (75% dioxane).
Fig. 4. The pure spectra of different form of Alizarine Red S in (I) pure water, (II) 0.1% (w/v) Brij-35 to water and (III) 0.1% (w/v) SDS to water.
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Fig. 6. Distribution of major species of Alizarine Red S, as a function of pH for the spectral data of Figs. 1–3, in (I) pure water, (II) 0.1% (w/v) Brij-35 to water and (III) 0.1% (w/v) SDS to water. Fig. 5. Variation of acidity constants values of Alizarine Red S with percentages of surfactants.
ble experimental errors of old methods, against chemometrics based methods, by using the whole spectral domain, reduce considerably the level of noise. So the obtained acidity constants are more reliable and precise than previous methods. The pKa values correspond to the pH dependent variation of absorption spectra in all micelle media systems. One of the very important outputs of the DATAN program is the calculated spectrum of different forms of Alizarine Red S in each micelle media. Sample spectrum of the calculated spectra of all species in pure water and 0.1% (w/v) Brij-35 and 0.1% (w/v) SDS to water are shown in Fig. 4. It is interesting to note that the nature of the surfactant has a fundamental effect on each pure spectrum. As it is clear from Fig. 4, this effect is apparently observed in different species of Alizarine Red S. The surfactant effect on this spectrum is very interesting. Many papers and reviews have discussed the effect of micelle on the apparent pKa values of the acids [3,4,8,11]. In the present work we observed the shifts of spectrum in Brij-35 and SDS micelle media systems and then we calculated the pKa values of this reagent in these media. As is clear from Fig. 4, when the Brij-35 surfactant is used, it caused spectra of HL− to shift to shorter wavelengths. The pKa1 is changed slightly in different percentages of Brij-35, but results show that for up to 0.01% (w/v) of the micelle, pKa2 values increase and, beyond that, pKa2 values almost not changed. Also, when SDS surfactant is used, spectra of HL− shifts to longer wavelengths. The pKa1 values are increased with increase of SDS percentages and for up to
0.01% (w/v) of the SDS, pKa2 values increase and, beyond that, pKa2 values almost not changed. However, results show that, the effect of surfactant is seen in the spectra of HL− than H2 L and L2− species, pKa values also shown in Fig. 5. These changes are due to the hydrophobic and electrostatic interactions of reactants with micellar aggregates. The most important features of distribution diagrams are the pH limit of evolving and disappearance of components. So, according to distribution diagrams it is concluded that the spectra at smaller pH than 3 assigned to H2 L form because this form is dominated at this range. At pH 3.5–10.5 interval the HL− form is dominated and hence the spectra mostly attributed to this form. The L2− form appeared at pH > 10.5. Samples of obtained distribution diagrams are shown in Fig. 6. 4. Conclusion In this work, we distinguish the behavior of acidity constants of Alizarine Red S in pure water, water-Brij-35 and water-SDS systems at 25 ◦ C and an ionic strength of 0.1 M that are studied by multiwavelength spectrophotometric method. Results show that the pKa values of Alizarine Red S are influenced as the percentages of a neutral and an anionic surfactant such as Brij35 and SDS, respectively, added to the solution of this reagent. DATAN is a useful tool for resolution of the different species present in equilibria systems. By using this method and without any prior knowledge about the system, we can obtain concentration profiles and pure spectra from the experimental data. In conclusion, interaction with micellar aggregates induces signifi-
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cant pKa shifts which can be rationalized in terms of partitioning of species and electrostatic contribution. References [1] W.L. Hinze, in: W.L. Mittal (Ed.), Solution Chemistry of Surfactants, vol. 1, Plenum Press, New York, 1979, p. 79. [2] E. Pellezzeti, E. Pramaura, Anal. Chim. Acta 117 (1980) 403–406. [3] D. Myers, Surfactant Science and Technology, VCH Publishers, New York, 1988, p. 17 (Chapter 1). [4] E. Pellezzeti, E. Pramaura, Anal. Chim. Acta 128 (1981) 273–275. [5] E. Pramaura, E. Pellezzeti, Anal. Chim. Acta 126 (1981) 253–257. [6] J.L. Beltran, R. Codony, M. Granados, A. Izquierdo, M.D. Prat, Talanta 40 (1993) 157–165. [7] G.S. Hartely, J.W. Roe, Trans. Faraday Soc. 36 (1940) 101–109. [8] E. Pellizzetti, E. Pramauro, Anal. Chim. Acta 169 (1985) 1–29. [9] D.G. Hall, J. Phys. Chem. 91 (1987) 4287–4297. [10] Z. Yuanqin, L. Fan, L. Xiaoyan, L. Jing, Talanta 56 (2002) 705–710. [11] A.L. Underwood, Anal. Chim. Acta 140 (1982) 89–97. [12] A. Abbaspour, M.A. Kamyabi, J. Chem. Eng. Data 46 (2001) 623–625. [13] D. Kara, M. Alkan, Spectrochim. Acta Part A 56 (2000) 2753–2761. [14] A. Safavi, H. Abdollahi, Talanta 53 (2001) 1001–1007. [15] F.J.C. Rossotti, H.S. Rossoti, The Determination of Stability Constants, McGraw-Hill, New York, 1961, p. 40. [16] H. Gamp, M. Maeder, C.J. Mayer, A. Zuberbuhler, Talanta 32 (1985) 1133–1139. [17] D.J. Legget, in: D.J. Legget (Ed.), Computational Methods for the Determination of Formation Constants, Plenum Press, New York, 1985, p. 159 (Chapter 6). [18] P. Gans, A. Sabbatini, A. Vacca, Talanta 43 (1996) 1739–1753.
[19] W. Lawton, E. Sylvestre, Technometrics 13 (1971) 617–619. [20] M. Kubista, R. Sjoback, B. Albinsson, Anal. Chem. 65 (1993) 994– 998. [21] M. Kubista, R. Sjoback, J. Nygren, Anal. Chim. Acta 302 (1995) 121–125. [22] M. Kubista, J. Nygren, A. Elbergali, R. Sjoback, Crit. Rev. Anal. Chem. 29 (1999) 1–28. [23] I. Sacrminio, M. Kubista, Anal. Chem. 65 (1993) 409–416. [24] R. Sjoback, J. Nygren, M. Kubista, Spectrochim. Acta Part A 51 (1995) L7–L21. [25] J. Nygren, J.M. Andrade, M. Kubista, Anal. Chem. 68 (1996) 1706–1710. [26] A. Elbergali, J. Nygren, M. Kubista, Anal. Chim. Acta 379 (1999) 143–158. [27] L. Antonov, G. Gergov, V. Petrov, M. Kubista, J. Nygren, Talanta 49 (1999) 99–106. [28] J. Nygren, N. Svanvik, M. Kubista, Biopolymers 46 (1998) 39–51. [29] N. Svanvik, J. Nygren, G. Westman, M. Kubista, J. Am. Chem. Soc. 123 (2001) 803–809. [30] M. Kubista, I.H. Ismail, A. Forootan, B. Sjogreen, J. Fluorescence 14 (2004) 139–144. [31] J. Ghasemi, A. Niazi, M. Kubista, A. Elbergali, Anal. Chim. Acta 455 (2002) 335–342. [32] J. Ghasemi, Sh. Ahmadi, M. Kubista, A. Forootan, J. Chem. Eng. Data 48 (2003) 1178–1182. [33] J. Ghasemi, A. Niazi, G. Westman, M. Kubista, Talanta 62 (2004) 831–841. [34] J. Ghasemi, A. Niazi, M. Kubista, Spectrochim. Acta Part A, in press. [35] E. Bishop, Indicators, Pergamon Press, Oxford, 1972, p. 362. [36] H. Kido, W.C. Fernelius, C.G. Hass, Anal. Chim. Acta 23 (1960) 116–123.
Spectrophotometric determination of acidity constants of Alizarine Red S in water, water-Brij-35 and water-SDS micellar media solutions Ali Niazi a,∗ , Mohammad Ghalie a , Ateesa Yazdanipour a , Jahanbakhsh Ghasemi b a b
Department of Chemistry, Faculty of Sciences, Azad University of Arak, Arak, Iran Department of Chemistry, Faculty of Sciences, Razi University, Kermanshah, Iran
Received 6 May 2005; received in revised form 18 July 2005; accepted 1 August 2005
Abstract The acidity constants of Alizarine Red S in water, water-Brij-35 and water-SDS micellar media solutions at 25 ◦ C and an ionic strength of 0.1 M have been determined spectrophotometrically. To evaluate the pH-absorbance data, a resolution method based on the combination of soft- and hard-modeling is applied. The acidity constants of all related equilibria are estimated using the whole spectral fitting of the collected data to an established factor analysis model. DATAN program applied for determination of acidity constants. Results show that the pKa values of Alizarine Red S are influenced as the percentages of a neutral and an anionic surfactant such as Brij-35 and SDS, respectively, added to the solution of this reagent. Effect of surfactant on acidity constants and pure spectrum of each component are also discussed. © 2005 Elsevier B.V. All rights reserved. Keywords: Alizarine Red S; Acidity constants; Brij-35; SDS; DATAN; Spectrophotometric
1. Introduction Aqueous micellar media are widely used in different areas of analytical chemistry and several reviews concerning their analytical applications have been published [1–4]. One important property of micelles is their ability to solubilize a wide variety of compounds which are insoluble or slightly soluble in water. The incorporation of a solute into micellar systems can lead to important changes in its molecular properties. Another important effect of micellar systems is that they can modify reaction rates and, to some extent, the nature of the products. Micelles can inhibit or accelerate reaction rates (by up to several orders of magnitude) and also shift equilibria (acid–base). Surfactants usually affect spectral parameters: the intensity and shifts in the absorption bands can be increased and shifts in the absorption maxima of reagents are observed [5,6]. Micelles can affect the apparent pKa values of the reagents due to a combination of electrostatic and microenviromental effects of the micelle [7–9]. Moreover, the acid–base equilibria involved in these systems are also influenced by surfactants [10–12].
∗
Corresponding author. Tel.: +98 8613663041x370; fax: +98 8613670017. E-mail address: [email protected] (A. Niazi).
1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2005.08.002
Acid dissociation constants (i.e. pKa values) can be a key parameter for understanding and quantifying chemical phenomena such as reaction rates, biological activity, biological uptake, biological transport and environmental fate [13]. There have been several methods of the determination of acidity constants, including the use of potentiometric titration, spectrophotometry, capillary electrophoresis, and so on. Spectroscopic methods are in general highly sensitive and as such are suitable for studying chemical equilibria in solution. When the components involved in the chemical equilibrium have distinct spectral responses, their concentrations can be measured directly, and the determination of the equilibrium constant is trivial [14]. For many systems, particularly those with similar components, this is not the case, and these have been difficult to analyze. Therefore, to overcome this problem we have to employ the graphical and computational methods. Up to the middle of the 1960s, the evaluation of equilibrium measurements was based on the different graphical methods. These methods were reviewed in considerable details by Rossotti and Rossotti [15]. Starting from middle of the 1960s, computers acquired ever-greater importance in the evaluation of equilibrium measurement data using multiple wavelengths or full spectrum to determining the stability and acidity constants. The most relevant reports are on SPECFIT [16], SQUAD [17] and HYPERQUAD [18]. All these computational approaches are based on an initial proposal of
A. Niazi et al. / Spectrochimica Acta Part A 64 (2006) 660–664
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2.2. Instrumentation
Scheme 1. Chemical structure of Alizarine Red S.
a chemical equilibrium model defining species stoichiometries and based on mass-action law and mass balance equations (hardmodeling methods) and also involve least-squares curve-fitting procedures. The starting point of using soft-modeling was in 1971 that Lawton and Sylvestre [19] introduced chemometrics based method for spectral analysis. These approaches are free from the restriction of the mass-action law and do not require an initial model of species to be set up. Data analysis was carried out by DATAN package that developed by Kubista group [20–22], called the physical constraints approach, which provides a unique solution by requiring that the calculated concentrations obey an assumed equilibrium expression and demonstrates its applicability by determining the acidity constants of two and four protolytic forms of fluorescein. A possible advantage of the Kubista et al. method is that it mixes a soft-modeling approach with a hard-modeling approach. This might be best and more general strategy, since it can handle different situations, with only a partial knowledge of the chemistry of the system. The physical constraints method calculates spectral profiles, concentrations and equilibrium constants by utilizing equilibrium expressions that related the components. The theory and application of physical constraints method was discussed by Kubista et al. in several papers [23–34]. In this work, we applied the physical constraints approach to determine the acidity constants of Alizarine Red S (Scheme 1) in pure water, water-Brij-35 and water-SDS micellar media solutions at 25 ◦ C and an ionic strength of 0.1 M spectrophotometrically. The effects of polyoxyethylenaurylether (Brij-35) as nonionic surfactant and sodium n-dodecyl sulfate (SDS) as anionic surfactant were studied on the dissociation constants and pure spectrum of Alizarine Red S. The analysis is readily performed with the computer program DATAN [20].
A Perkin-Elmer (Lambda 25) spectrophotometer controlled by a computer and equipped with a 1 cm path length quartz cell was used for UV–vis spectra acquisition. Spectra were acquired between 370 and 700 nm. A HORIBA M-12 pH-meter furnished with a combined glass-saturated calomel electrode was calibrated with at least two buffer solutions at pH 3.00 and 9.00 (which pH adjustment contains error with respect to the direct use of the buffer solutions). 2.3. Computer hardware and software All absorption spectra where digitized at five data points per nanometer in the wavelength 370–700 nm and transferred (in ASCII format) to an AMD 2000 XP (256 Mb RAM) computer for subsequent analysis by MATLAB software, version 6.5 (The MathWorks) or for processing by using DATAN package. 2.4. Spectrophotometric titrations For the Alizarine Red S (2 × 10−4 M) in pure water, waterBrij-35 and water-SDS mixtures titrations, absorption spectra were measured with a titration set-up consisting of a computer interfaced to a spectrophotometer. After each pH adjustment, solution is transferred into the cuvette and the absorption spectra are recorded. Ionic strength was maintained at 0.1 M by adding appropriate amounts of KNO3 . All measurements were carried out at the temperature (25 ± 0.5 ◦ C). 3. Results and discussion The absorption spectra of Alizarine Red S in pure water at various pH values at 370–700 nm intervals were recorded. In order to determine the influence of the nonionic surfactant (Brij-35) and the anionic surfactant (SDS) in acidity constants, a series of experiments were run at different Brij-35 and SDS concentrations, above the cmc. Sample spectrum of Alizarine Red S at different pH values in pure water, water-Brij-35 and waterSDS are shown in Figs. 1–3. The principal component analysis of all absorption data matrices obtained at various pH values
2. Experimental 2.1. Materials Alizarine Red S, Brij-35, SDS, hydrochloric acid, sodium hydroxide and potassium nitrate were analytical grade commercial products from Merck. These reagents were used without further purification. Standard stock solution of 8.0 × 10−4 M of Alizarine Red S was prepared by dissolving appropriate amounts of Alizarine Red S in water. The stock solutions of surfactants prepared by dissolving weighted amounts of substances in appropriate amounts of water. All the solutions were prepared in deionized water.
Fig. 1. Absorption spectra of Alizarine Red S in pure water at different pH values: (1) 0.85, (2) 1.49, (3) 1.91, (4) 2.42, (5) 2.90, (6) 3.40, (7) 3.90, (8) 4.40, (9) 4.95, (10) 5.42, (11) 5.92, (12) 6.46, (13) 6.94, (14) 7.30, (15) 7.85, (16) 8.36, (17) 8.94, (18) 9.41, (19) 9.70, (20) 10.06, (21) 10.32, (22) 10.49, (23) 10.79, (24) 11.00, (25) 11.31, (26) 11.55, (27) 12.02.
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A. Niazi et al. / Spectrochimica Acta Part A 64 (2006) 660–664 Table 2 Acidity constants of Alizarine Red S in pure water and at different percentage of SDS (w/v) at 25 ◦ C and constant ionic strength (0.1 M KNO3 )
Fig. 2. Absorption spectra of Alizarine Red S in 0.1% (w/v) Brij-35 to water at different pH values: (1) 1.48, (2) 2.53, (3) 3.54, (4) 4.01, (5) 4.48, (6) 4.95, (7) 5.50, (8) 6.04, (9) 6.50, (10) 6.95, (11) 7.99, (12) 8.48, (13) 8.99, (14) 9.51, (15) 9.99, (16) 10.49, (17) 11.00, (18) 11.51, (19) 12.01, (20) 12.48.
SDS (w/v) (%)
pKa1
pKa2
0.00 0.005 0.01 0.025 0.05 0.10
4.70 ± 0.10 4.77 ± 0.11 4.84 ± 0.11 5.02 ± 0.12 5.11 ± 0.13 5.24 ± 0.12
10.09 ± 0.14 10.51 ± 0.15 11.33 ± 0.15 11.39 ± 0.14 11.41 ± 0.16 11.42 ± 0.15
evidence the presence of three spectroscopically distinguishable components. Their shapes, however, are clearly unphysical and cannot be directly related to the spectral response of the three protolytic forms. The output of the program are pKa values and their standard deviation, the number of principal components, projection vectors (loadings), concentration distribution diagrams, and the pure spectrum of each assumed species. The obtained pKa values are listed in Tables 1 and 2. The previous reported values of acidity constants are mainly in pure water and in mixtures of dioxane with water [35,36]. The obtained values in pure water are in good agreement with previous values [35], which are listed in Table 1 for comparison. The differences observed between the pKa values are due to proba-
Fig. 3. Absorption spectra of Alizarine Red S in 0.1% (w/v) SDS to water at different pH values: (1) 1.35, (2) 2.54, (3) 3.50, (4) 4.02, (5) 4.49, (6) 5.03, (7) 5.58, (8) 6.03, (9) 6.43, (10) 6.92, (11) 7.34, (12) 7.94, (13) 8.61, (14) 9.11, (15) 9.74, (16) 10.13, (17) 10.52, (18) 11.09, (19) 11.55, (20) 12.01, (21) 12.54.
shows at least three significant factors that also supported by the statistical indicators of Elbergali et al. [26]. These factors could be attributed to the two dissociation equilibria of a diprotic acid such Alizarine Red S. The pKa values of Alizarine Red S were investigated in pure water and five different water-Brij-35 and water-SDS mixtures spectrophotometrically at 25 ◦ C and an ionic strength of 0.1 M. Acidity constants of Alizarine Red S in several mixtures were evaluated using the DATAN program using the corresponding spectral absorption-pH data. From inspection of the experimental spectra, it is hard to guess even the number of protolytic species involved. The three calculated most significant projection vectors with clear spectral features (as compared to noise) Table 1 Acidity constants of Alizarine Red S in pure water and at different percentage of Brij-35 (w/v) at 25 ◦ C and constant ionic strength (0.1 M KNO3 ) Brij-35 (w/v) (%)
pKa1
pKa2
0.00 0.005 0.01 0.025 0.05 0.10
5.50a 5.49a 6.10b 4.70 ± 0.10 4.68 ± 0.11 4.68 ± 0.09 4.67 ± 0.15 4.66 ± 0.11 4.65 ± 0.12
11.00a 10.85a 10.80b 10.09 ± 0.14 10.34 ± 0.13 10.75 ± 0.16 10.78 ± 0.14 10.81 ± 0.15 10.85 ± 0.14
a b
Reference [35] (pure water). Reference [36] (75% dioxane).
Fig. 4. The pure spectra of different form of Alizarine Red S in (I) pure water, (II) 0.1% (w/v) Brij-35 to water and (III) 0.1% (w/v) SDS to water.
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Fig. 6. Distribution of major species of Alizarine Red S, as a function of pH for the spectral data of Figs. 1–3, in (I) pure water, (II) 0.1% (w/v) Brij-35 to water and (III) 0.1% (w/v) SDS to water. Fig. 5. Variation of acidity constants values of Alizarine Red S with percentages of surfactants.
ble experimental errors of old methods, against chemometrics based methods, by using the whole spectral domain, reduce considerably the level of noise. So the obtained acidity constants are more reliable and precise than previous methods. The pKa values correspond to the pH dependent variation of absorption spectra in all micelle media systems. One of the very important outputs of the DATAN program is the calculated spectrum of different forms of Alizarine Red S in each micelle media. Sample spectrum of the calculated spectra of all species in pure water and 0.1% (w/v) Brij-35 and 0.1% (w/v) SDS to water are shown in Fig. 4. It is interesting to note that the nature of the surfactant has a fundamental effect on each pure spectrum. As it is clear from Fig. 4, this effect is apparently observed in different species of Alizarine Red S. The surfactant effect on this spectrum is very interesting. Many papers and reviews have discussed the effect of micelle on the apparent pKa values of the acids [3,4,8,11]. In the present work we observed the shifts of spectrum in Brij-35 and SDS micelle media systems and then we calculated the pKa values of this reagent in these media. As is clear from Fig. 4, when the Brij-35 surfactant is used, it caused spectra of HL− to shift to shorter wavelengths. The pKa1 is changed slightly in different percentages of Brij-35, but results show that for up to 0.01% (w/v) of the micelle, pKa2 values increase and, beyond that, pKa2 values almost not changed. Also, when SDS surfactant is used, spectra of HL− shifts to longer wavelengths. The pKa1 values are increased with increase of SDS percentages and for up to
0.01% (w/v) of the SDS, pKa2 values increase and, beyond that, pKa2 values almost not changed. However, results show that, the effect of surfactant is seen in the spectra of HL− than H2 L and L2− species, pKa values also shown in Fig. 5. These changes are due to the hydrophobic and electrostatic interactions of reactants with micellar aggregates. The most important features of distribution diagrams are the pH limit of evolving and disappearance of components. So, according to distribution diagrams it is concluded that the spectra at smaller pH than 3 assigned to H2 L form because this form is dominated at this range. At pH 3.5–10.5 interval the HL− form is dominated and hence the spectra mostly attributed to this form. The L2− form appeared at pH > 10.5. Samples of obtained distribution diagrams are shown in Fig. 6. 4. Conclusion In this work, we distinguish the behavior of acidity constants of Alizarine Red S in pure water, water-Brij-35 and water-SDS systems at 25 ◦ C and an ionic strength of 0.1 M that are studied by multiwavelength spectrophotometric method. Results show that the pKa values of Alizarine Red S are influenced as the percentages of a neutral and an anionic surfactant such as Brij35 and SDS, respectively, added to the solution of this reagent. DATAN is a useful tool for resolution of the different species present in equilibria systems. By using this method and without any prior knowledge about the system, we can obtain concentration profiles and pure spectra from the experimental data. In conclusion, interaction with micellar aggregates induces signifi-
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